Evidence for cable bacteria inhabiting deep in anoxic sediment reveals a novel ecological niche

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Nascimento, Christian Stranne, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6914568/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 14 Apr, 2026 Read the published version in Environmental Microbiome → Version 1 posted You are reading this latest preprint version Abstract Background Cable bacteria are filamentous sulfide oxidizers capable of electron transport over cm-scale distances. Traditionally, they are thought to inhabit only the upper few cm of sediment, where they couple sulfide oxidation to oxygen or nitrate reduction. Despite their influence on redox gradients, trace metal mobility, and nutrient cycling, their presence and activity in deep anoxic sediments remain undocumented. We investigated the presence and activity of marine cable bacteria ( Candidatus Electrothrix) at four stations in Sweden and Finland, including deep vertical profiles of anoxic sediments. Results Using metatranscriptomic data for both rRNA-based community profiling and gene expression analysis, along with porewater geochemistry data from four stations in Sweden and Finland, we detected metabolically active Ca . Electrothrix in both regions. In Koljö Fjord (Sweden West Coast), Ca . Electrothrix was unexpectedly abundant deep in anoxic layers, with peak abundance below 20 cm. Phylogenetic analyses revealed a diverse assemblage spanning multiple Ca . Electrothrix clades, suggesting that novel lineages adapted to these conditions. Genes for nitrate respiration ( napA ), sulfide oxidation ( sqr ), and nickel uptake were highly expressed, indicating in-situ activity. Gene expression patterns were aligned with a sulfide-rich zone and a sharp nitrate peak below 20 cm. This nitrate peak likely results from sulfammox (i.e., anaerobic oxidation of ammonium by sulfate), driven by associated sulfammox bacteria such as Bacillus benzoevorans , Ca . Anammoxoglobus, and Bacillus cereus . Conclusions Our findings reveal a previously unrecognized niche for cable bacteria deep in anoxic sediment layers, where local nitrate production via sulfammox and sulfide availability may sustain their activity, independent of electron acceptors near the surface. This discovery challenges existing models of cable bacteria ecology and suggests alternative physiological modes. Furthermore, the results expand the ecological scope of marine cable bacteria, highlighting potential syntrophic relationships deep in anoxic sediment layers, offering insights into both modern biogeochemical processes and analogs of early Earth microbial ecosystems. Cable bacteria Candidatus Electrothrix sulfur bacteria sulfur oxidation nitrate reduction sulfammox novel niche anoxic sediments Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Background Cable bacteria are multicellular filamentous organisms characterized by a distinct metabolism that spatially couples the oxidation of sulfide (H 2 S) with the reduction of oxygen (O 2 ) or nitrate (NO 3 − ) by channeling electrons through their filaments over cm-scale distances (Nielsen et al., 2010; Pfeffer et al., 2012; Marzocchi et al., 2014). These bacteria belong to the Desulfobulbaceae family and are traditionally classified into two genera: Ca. Electrothrix and Ca. Electronema for marine and freshwater sediments, respectively (Trojan et al., 2016). However, recent findings indicate a substantially greater diversity of potentially up to ninety species divided among six genera (Ley et al., 2024). Cable bacteria possess a metabolic strategy based on long-distance electron transport, giving them a competitive advantage over other sulfur (S)-oxidizing bacteria (Meysman et al., 2019). Their ecological importance is rooted in regulating redox gradients, trace metals (Van De Velde et al., 2017), nutrient cycling (Hermans et al., 2021), generating a firewall against euxinia (Seitaj et al., 2015), and influencing the behavior of other microbial communities (Bjerg et al., 2023; Dong et al., 2024). The consensus is that cable bacteria are restricted to the upper few cm of surface sediments, where O 2 or NO 3 − and H 2 S are abundant (Marzocchi et al., 2014; Risgaard-Petersen et al., 2014). Environmental parameters, such as bottom water redox conditions, bioturbation (Hermans et al., 2019; Malkin et al., 2022), salinity and temperature (Dam et al., 2021), control the activity, abundance, and diversity of cable bacteria. Cable bacteria have been discovered in a wide range of aquatic environments (i.e., freshwater–brackish–marine), including rivers, estuaries, coasts, and salt marshes (Risgaard-Petersen et al., 2015; Burdorf et al., 2017; Hermans et al., 2019). So far, their presence deep in anoxic sediment layers has not been documented. This study reveals that active cable bacteria, hereafter referred specifically to as marine cable bacteria ( Candidatus Electrothrix), thrive deep in anoxic sediment layers and are sustained by NO 3 − pockets, challenging the conventional view that they are restricted to surface sediments. These findings offer new insights into their ecological and biogeochemical significance, particularly in removing H 2 S from deeper layers. Methods Study area and sediment collection Sediments from Kristineberg Bay and Koljö Fjord on the Swedish West Coast (Fig. 1A) were collected using a Gemini Twin Corer aboard R/V Alice . For comparison, data from two contrasting stations in the Tvärminne Archipelago in southern Finland (Fig. 1B) from another study were used due to the availability of a comparable dataset (Hermans et al., 2024). These four stations differ in terms of their bottom water redox conditions and salinity (Fig. 2 and Table S1). Commonly used thresholds to classify bottom water redox conditions are (Algeo and Li, 2020): oxic (O 2 > 2 mL L -1 ), hypoxic (O 2 = 0.2–2 mL L -1 ), anoxic (O 2 = 0 mL L -1 ), and euxinic (O 2 = 0 mL L -1 ; ΣH 2 S > 0 mL L -1 ). Kristineberg Bay is fully marine and has oxic bottom waters. Koljö Fjord is part of an open-ended fjord system encompassing the Orust and Tjörn Islands (Fig. 1C). It is restricted by three shallow sills connected to the adjacent Havsten Fjord: (S1) at a water depth of 12 m, Skagerrak (S2) at a water depth of 8 m, and Gullmar Fjord (S3) at a water depth of 5 m (Fig. 1D). The hydrography is controlled by the inflow of brackish surface water through Kattegat–Skagerrak, originating from the Baltic Sea. Freshwater input is of minor importance, as no major riverine inputs discharge into the fjord. The deep waters of Koljö Fjord undergo renewal. However, reoxygenation may not be guaranteed due to its variable frequency, ranging from annual to several years (Paul et al., 2023). At the time of sampling, the bottom waters in Koljö Fjord were hypoxic/anoxic. The two stations in the Tvärminne Archipelago exhibit relatively lower salinities, although they remain brackish. However, they share similar hydrographical characteristics and sediment structures as the two stations situated on the Swedish West Coast. These stations are designated as Nearshore and Offshore (Fig. 1B). The Nearshore station has oxic bottom waters and is located close to the outflow of the Pojo Bay estuary, whereas the Offshore station has hypoxic bottom waters, as it is situated in a small enclosed bay further offshore the archipelago. While the Offshore station is not situated completely offshore but rather farther out at sea, the term "offshore" is used to distinguish this station from the nearshore station. Sediment cores were collected in triplicate and meticulously sliced at depths of 0–1, 1–3, 7–9, 9–11, 20–22, and 30–32 cm (Kristineberg Bay was sampled 0–2 and 2–3, for cores 2 and 3) for metatranscriptomic (RNA sequencing) and porewater analyses (Fig. S1). The separated sediment layers were then placed into 215 mL polypropylene containers (Noax Laboratory, Product No. 207.0215PP). After each layer was homogenized, a 2–3 mL sample of the mixed sediment was transferred to a 15 mL centrifuge tube. These samples were promptly flash-frozen in liquid nitrogen and stored at − 80°C until RNA extraction was performed. RNA extraction and sequencing Total RNA was extracted from 2 g of each homogenized sediment sample using the RNeasy PowerSoil Total Kit (Qiagen). A TURBO DNA-free kit (Invitrogen) was used to eliminate residual DNA contamination in the eluate. At SciLifeLab in Stockholm, libraries were prepared for 36 samples (Supplementary Data 1), 18 for Kristineberg Bay and Koljö Fjord (three independent biological replicates per sediment depth from each station, as described above and Fig. S1) using TruSeq Stranded mRNA polyA (Illumina). The purified libraries were subsequently sequenced at SciLifeLab utilizing a NovaSeqXPlus platform (NovaSeqXSeries Control Software 1.2.0.28691) with 2 × 150 bp configuration. The sequencing libraries are available in the NCBI repository under BioProject accession number PRJNA1253322. Porewater collection and treatment Bottom and porewater samples were extracted using Rhizons™ (0.12–0.18 μm pore size) and collected into 10 mL polyethylene syringes as described in Hermans et al. (2024) and references therein. Two parallel porewater series were extracted from each core. The first series was designated for sulfate (SO 4 2 − ), inorganic nutrients (NH 4 + , NO 3 − , and NO 2 − ), and trace metals (Fe, Mn, and Ni), whereas the second series was reserved for total sulfide analysis (ΣH 2 S = H 2 S + HS − + S 2 − ). Subsamples for trace metal analysis were acidified with distilled nitric acid. Syringes for ΣH 2 S analysis were prefilled with 1 mL zinc acetate (10%) to trap ΣH 2 S as ZnS. Subsamples for SO 4 2 − , ΣH 2 S, and trace metal analyses were stored at 4°C, whereas subsamples for inorganic nutrients were frozen at − 20°C. Porewater analyses Dissolved NH 4 + concentrations were determined after the indophenol method using Segmented Flox Analysis (Alpkem SFA, O. I. Analytical Flow Solution IV). Porewater concentrations of NO 3 − and NO 2 − were determined using a carrier stream that facilitates continuous mixing with a buffer solution. First, NO 3 − was reduced to NO 2 − using cadmium. Following the addition of phosphoric acid, NO 2 − , both initially present and formed through the reduction of NO 3 − , was used to diazotize sulfanilamide in an acidic solution (Swedish Institute for Standards, 1997). In a separate batch, NO 2 − concentrations were determined using the same method as described above but without the cadmium reduction step. The concentration of NO 3 − was derived by subtracting NO 2 − from the total NO 3 − + NO 2 − . Acidified samples were analyzed using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS; Thermo Scientific, XSERIES 2 or Agilent 7800) to determine Fe, Mn, and Ni contents. Here, dissolved Fe and Mn are considered as Fe 2+ and Mn 2+ ; however, some Mn 3+ or colloidal and nanoparticulate Fe and Mn might also be present (Paul et al., 2023). Concentrations of sodium (Na), potassium (K), magnesium (Mg), and chloride (Cl) in Koljö Fjord were determined using ion chromatography (IC). Bioinformatics and statistics The libraries from Kristineberg Bay and Koljö Fjord yielded an average of 101 million reads (Supplementary Data 1). Illumina adapters were removed via targeted primer sequencing (St. John, 2011) through the SeqPrep 1.2 program with default parameters. The absence of PhiX control sequences was verified by aligning the reads to the PhiX genome (NCBI Reference Sequence: NC_001422.1) using Bowtie 2 V 2.3.5.1 (Langmead and Salzberg, 2012). The reads were subjected to quality control using the programs FastQC V 0.11.9 (Andrews, 2010) and MultiQC V 1.12 (Ewels et al., 2016) before and after the trimming step. Trimming was carried out using Trimmomatic 0.39 (Bolger et al., 2014) with the following settings: LEADING:20, TRAILING:20, MINLEN:80, and Cutadapt V 4.5 (Martin, 2011). The trimmed reads were merged using FLASH V 1.2.11 (Magoč and Salzberg, 2011) . The average number of merged reads was 54% (ranging from 42% to 73%), resulting in a read length of 220 bp per sample. Following the SAMSA2 pipeline recommendations (Westreich et al., 2018), the paired reads were merged with nonpaired forward reads. The methods used for the Tvärminne libraries are explained in detail by Hermans et al. (2024). Differential bacterial abundance was measured using the edgeR analysis script (run_DE_analysis.pl) with the default parameters provided by Trinity 2.14.0 software (Robinson et al., 2010; Haas et al., 2013). Additionally, the difference in abundance was assessed using Kruskal-Wallis and D uncan’s tests, adjusting p-values with the Benjamini-Hochberg method. Pearson correlations between bacterial abundances, depths and variables such as NO 3 − and H 2 S were tested and plotted using the function corrplot in the R package corrplot (Wei and Simko, 2021) and the Microbe-to-sample-data correlation heatmap from the microViz R package (Barnett et al., 2021). Furthermore, SparCC (parse Correlations for Compositional data) co-abundance correlations (Friedmand and Alm, 2012) were inferred with 100 iterations and two-sided pseudo-P values using a 100-round bootstrap procedure. Taxonomic classification by rRNA Following Broman et al. (2024) and Hermans et al. (2024), rRNA (SSU rRNA) reads were extracted from the quality-trimmed reads with SortMeRNA 4.3.6 using the SSU sequences in the SILVA database (silva-bac-16s-id90.fasta; Kopylova et al., 2012). To determine the taxonomic composition of active bacteria at the genus level, quality-filtered rRNA reads were classified using Kraken2 (Wood et al., 2019) with the SILVA 138 SSU (June 2024) database as a reference. Bracken was used to refine the abundance estimates by statistically reassigning reads classified at higher taxonomic levels, thereby improving the quantitative accuracy. Cable bacteria identification and abundance were further analyzed in Koljö Fjord, and the rRNA gene reads were assembled, annotated, and quantified (see details in Supplementary Material and Supplementary Data 2). Phylogeny of cable bacteria from Koljö Fjord To evaluate the evolutionary relationship between Ca . Electrothrix from Koljö Fjord, a phylogenetic tree was built using the 16S rRNA gene sequences. Following the assembly of the metatranscriptomic sequences from Koljö Fjord, 13 partial 16S rRNA gene sequences were annotated as Ca . Electrothrix. These 13 partial 16S rRNA gene sequences were aligned with 61 cable bacteria, 13 related Desulfobulbales, and Geobacter sulfurreducens as an outgroup (sequences are available in Supplementary Data 3). Multiple sequence alignment was performed using MUSCLE v5.1.0 (Edgar, 2004), and phylogenetic inference was conducted using IQ-TREE 2.3.5, employing ModelFinder Plus (MFP+MERGE) for optimal model selection. Branch support was evaluated with 5000 ultrafast bootstrap replicates, SH-aLRT (5000 replicates), and Bayesian-like transformation of aLRT (aBayes). The final tree was visualized using FigTree v1.4.4. Gene expression SortMeRNA 4.3.4 and its provided database (smr_v4.3_default_db.fasta; Kopylova et al., 2012) were used to separate rRNA from total RNA reads. Following Broman et al. (2024) and Hermans et al. (2024), DIAMOND v2.1.10.164 was utilized to categorize the non-rRNA reads against the NCBI NR database (retrieved on June 3, 2024) by applying an e-value threshold of 1e ‑10 . The daa-meganizer tool included in MEGAN 6 Ultimate Edition 6.25.10, built 27 June 2024, was applied to eliminate eukaryotic and viral data and connect the diamond results to the KEGG database (MEGAN database: megan-map-Feb2022; Huson et al., 2007; Bağcı et al., 2021). A MEGAN file containing absolute counts was generated using the MEGAN tool for computer comparison. This file was then imported into MEGAN software to extract all KEGG KO classifications. Sequence counts were normalized to counts per million (CPM; calculated as relative proportion × 1 million). A comprehensive matrix of all KEGG classifications can be found in Supplementary Data 4. Results Abundance of cable bacteria Analysis of 16S rRNA gene sequences identified 3,119 bacterial genera (370,937,895 sequences) in Koljö Fjord, 3,055 (453,889,324 sequences) in Kristineberg Bay, 3,339 (871,131,658 sequences) at the Offshore station, and 3,124 (564,362,145 sequences) at the Nearshore station in the Tvärminne Archipelago (Supplementary Data 2). Proteobacteria and Desulfobacterota were the dominant phyla across the data, accounting for 36% and 19% of the total abundance, respectively (Fig. S2). Within the Desulfobulbaceae family (the most abundant family and genera in Fig. S3 and S4), undescribed forms were dominant across all stations (1.34%), followed by the genera Desulfubulbus (0.09%) and cable bacteria; Ca . Electrotrix and Ca . Electronema (0.08 and 0.01%). However, the abundance of Ca . Electrothrix at the Offshore station and Koljö Fjord (Fig. 3A and Fig. S5) accounted for 0.11% of the total abundance, corresponding to 416,616 and 937,608 sequences, respectively. In contrast, Kristineberg Bay and the Nearshore station (with decreasing abundance toward deeper layers) exhibited negligible abundances of 0.02% (115,717 sequences) and 0.05% (316,115 sequences), respectively. The vertical distribution of marine cable bacteria at the Offshore station in the Tvärminne Archipelago clearly decreased with depth. The relative abundances (within the Desulfobulbaceae family) ranged from 32.3–40.4% at 0–1 cm and 23.6–62.3% at 1–3 cm, dropping sharply to 0.7–1.4% at 20–22 cm. In contrast, at Koljö Fjord, the abundance of cable bacteria increased with depth, from average relative abundances of 2.3% at 0–1 cm and 5.0% at 1–3 cm to 28.2% at 20–22 cm and 12.6% at 30–32 cm. The maximum observed abundance of Ca . Electrothrix reached 76% in core #2 (edgeR p-value of 0.05 for the difference between the 0–1 cm and 20–22 cm layers). Thus, cable bacteria were highly abundant in core #2 and, to a lesser extent, in core #1 from the same cast. In contrast, core #3 from a different cast exhibited low abundance. To confirm our findings, SSU rRNA sequences from Koljö Fjord were assembled, annotated, and quantified. Thirteen partial 16S rRNA gene sequences were obtained (Supplementary Data 2), displaying the same abundance pattern (Fig. 3B and Fig. S6) as the analysis of the shorter 16S rRNA gene fragments (Fig. 3A), with averages of 12 and 18 transcripts per million (TPM) for 0–1 and 1–3 cm depths, respectively, and a maximum average abundance at 20–22 cm depth of 141 TPM (maximum: 328 TPM for Ca . Electrothrix Koljö_8). While the Kruskal‒Wallis test indicated significant differential abundances between depths (p- value <0.05), the pairwise Dunn test revealed significant differences between 1–3 and 30–32 cm depth (p-value <0.05) (Table S3). The comparison between 1–3 cm and 20–22 cm depth yielded a p-value of 0.01 (adjusted p-value = 0.07). The results revealed that various cable bacteria types were present deep in the anoxic sediment layers of Koljö Fjord, with the highest abundance found at depths of 20–22 cm. Several species of Ca. Electrothrix reside deep within anoxic sediment layers A phylogenetic tree based on the 13 partial 16S rRNA genes revealed several Ca. Electrothrix types in Koljö Fjord (Fig. 4). According to the recent cable bacteria phylogeny (Ley et al., 2024), nine sequences were assigned to Cluster VI, which included species such as Ca. Electrothrix communis, aarhusiensis, marina, japonica, laxa, rattekaaiensis, and antwerpensis. These species are known from Baltic Sea sediments and other brackish or marine environments. The 16S rRNA gene sequences of Ca. Electrothrix from Koljö Fjord exhibited a dispersed distribution within Cluster VI. Most appeared to represent novel species, although Ca. Electrothrix Koljö_9 aligned closely with Ca. Electrothrix marina, as evidenced by two high-quality BLASTN alignments with 99% identity (219 bp and 196 bp, E-values of 6e -10 and 7e -94 ), suggesting a close evolutionary relationship. Three sequences, Koljö_1, Koljö_2, and Koljö_10, belong to Cluster IV, which includes cable bacteria from the North Sea and a mud volcano in Costa Rica. One sequence, Koljö_6, was placed within Clade V and was associated with marine habitats. These data suggest that the cable bacteria community in Koljö Fjord consists of multiple evolutionary lineages, some likely novel. Co-abundance of cable bacteria with S-oxidizers in Koljö Fjord Previous studies have identified large colorless S-oxidizing bacteria, such as members of the Beggiatoaceae family, as potential competitors of cable bacteria owing to their overlapping metabolic pathways for energy production and respiration. In Koljö Fjord, Beggiatoaceae accounted for 0.32% of the total microbial community, and Ca . Thiomargarita was the most abundant genus (0.07%). SparCC co-abundance network analysis of Ca . Electrothrix, Ca . Thiomargarita, and Desulfobulbus (Fig. 5A) revealed distinct ecological niches, such as Ca . Electrothrix shared fewer associations with the other two taxa, which were more strongly interconnected. Notably, four of the seven significant (p < 0.05) co-abundance correlations involving Ca . Electrothrix were with S-oxidizing bacteria, including Sulfurovum , Sulfurimonas , Thiohalophilus , and an unclassified member of Beggiatoaceae, with R² values ranging from 0.56 to 0.63, suggesting potential functional interactions deep in sediments (Fig. 5A). Most Beggiatoaceae genera identified in Koljö Fjord (Fig. 5B) and the Offshore station (Fig. 5C) were associated with surface and subsurface layers (1–3 cm), as was also observed in Kristineberg Bay (Beggiatoaceae abundance of 0.09%) and the Nearshore station (Beggiatoaceae abundance of 0.3%) (Fig. S7). In contrast, Ca . Electrothrix was correlated with deeper sediments at depths of 20–22 cm in Koljö Fjord (R² = 0.5, p < 0.05; Fig. 5B). Two low-abundance Beggiatoaceae genera ( Ca . Parabeggiatoa and Ca . Marithioploca) also appeared at this depth, but their abundance was negligible (0.03% and 0.007%, respectively). Similarly, undescribed bacteria of the Beggiatoaceae family appear to be correlated with depths of 20–22 cm in Koljö Fjord. A key question is what environmental conditions in Koljö Fjord enable cable bacteria to colonize deep sediment layers. Gene expression associated with NO 3 − reduction, H 2 S oxidation, and Ni The expression of the marker gene for periplasmic NO 3 − reduction napA was highest (~500 CPM) at the surface (0–1 cm) and below 20 cm depth in Koljö Fjord (Fig. 6A). Conversely, no strong signal of gene expression was detected at other stations at depths less than 20 cm (Fig. 6B–D). This elevated gene expression at 20–22 cm in Koljö Fjord (gene count matrix in Supplementary Data 4), observed only in core #2, is consistent with the 16S rRNA gene abundance pattern of cable bacteria (Fig. 3A and B). High expression of sulfide:quinone oxidoreductase ( sqr ), involved in ΣH 2 S oxidation, was observed at Koljö Fjord at 20 cm depth (Fig. 6F). The highest expression of the sqr gene was detected at 0–1 cm (~2000 CPM) and 20–22 cm depth (~1000 CPM) in core #2. Similarly, the Offshore station showed an increasing sqr gene expression at 20 cm depth (Fig. 6H). In contrast, Kristineberg Bay and the Nearshore station showed a decreasing sqr gene expression toward deeper sediment layers (Fig. 6G and I). The expression of genes involved in Ni metabolism, such as the ABC.PS.S transporter, coupled with the high Ni peak below 20 cm in Koljö Fjord and tended to increase with depth (Fig. S8). However, similar patterns were observed at the other stations. Porewater profiles At Koljö Fjord, a porewater NO 3 − peak of ~23 µM was found between 20–30 cm depth, similar to the concentration at the surface (~22.5 µM). The zonation of this peak was considerably lower than the typical depth of oxic nitrification (Fig. 6E). Kristineberg Bay had higher background NO 3 − concentrations than Koljö Fjord, with a slight increase in deeper layers. In Koljö Fjord, porewater ΣH 2 S concentrations (Fig. 6J) increased with depth, reaching a maximum of 1036.8 µM at 27–28 cm, with an average concentration of 475.2 µM. Similar concentrations were found at the Offshore station, with a maximum of 963.5 µM (4–5 cm) and an average of 487.7 µM, albeit having different patterns (Fig. 6K). In contrast, Kristineberg Bay presented a lower concentration (average of 68.1 µM and a maximum of 499.5 µM), similar to the Nearshore station (average of 171.3 and a maximum of 402.9 µM). Porewater concentrations of SO 4 2− were much higher in Koljö Fjord (average of ~18.2 mM and maximum of ~19.9 mM) and in Kristineberg Bay (average of ~27.5 mM and maximum of ~30.6 mM) (Fig. 7A) than in the Tvärminne stations (Fig. 7D), where the concentration decreased in deeper layers (average of ~2.3–3.3 mM and maximum of ~5.4–6.6 mM). Strikingly, the depth at which the highest abundance of cable bacteria in Koljö Fjord was detected (20–22 cm) exhibited an Ni peak that reached 0.17 μM (Fig. 7B). This concentration was higher than the average background level of ~0.03 μM at all other depths. Kristineberg Bay (Fig. 7B) and the Tvarminne Archipelago stations (Fig. 7E) have similar concentrations with straight uniform profiles. The NH 4 + concentration profiles were measured only in Koljö Fjord and Kristineberg Bay. These profiles were similar, increasing toward deeper sediment layers, with an average of 206.7–206.0 and maximums of 476.2–505.6 μM (Fig. 7C). The porewater data are available in Supplementary Data 5. Discussion Evidence for active cable bacteria deep in anoxic sediment layers Unexpectedly, a high abundance of cable bacteria was detected in the anoxic Koljö Fjord (Paul et al., 2023). Even more strikingly, the Ca. Electrothrix abundance was relatively high at 7–9 cm and increased toward deeper sediment layers, reaching its maximum between 20–22 cm sediment depth, although it remained quite high between 30–32 cm as well. The discovery of Ca. Electrothrix deep in anoxic sediment layers in Koljö Fjord challenges the consensus that cable bacteria are restricted to the oxic–anoxic interface of surface sediments (Dong et al., 2024). Cable bacteria have not been extensively studied in anoxic environments. However, recently, they have been found in O 2 -deficient systems (Fonseca et al., 2022; Wu et al., 2024; Slomp et al., 2025), where cable bacteria might survive when NO 3 − is used as the main electron acceptor. Notably, the Ca. Electrothrix abundance in this study increased toward deeper sediment layers, contrary to the typical decrease with sediment depth. The abundances found here align with previously reported abundance ranges found in other natural environments, such as the seasonally anoxic Chesapeake Bay, ranging from 0.04 to 3.6% (Malkin et al., 2022). We studied the presence of marine cable bacteria using filtered rRNA derived from metatranscriptomic sequences. The presence of Ca. Electrothrix was further confirmed by larger 16S rRNA gene sequences obtained by assembling, annotating, and quantifying the total SSU rRNA from metatranscriptomic samples. A strong advantage of 16S rRNA analysis using metatranscriptomics is that it indicates ribosomal activity, thereby supporting its presence but also ensures that the bacteria are metabolically active (Singer et al., 2017), although nothing can be said regarding their biomass. The patchy distribution of cable bacteria, with high abundance in core #2 but not in core #3, points toward microspatial heterogeneity in terms of environmental conditions or colonization dynamics. This distribution has also been observed in other benthic microbial communities and may reflect sediment features such as porewater chemistry, particle size, or bioturbation. Phylogeny reveals a community with several phylotypes of cable bacteria The phylogenetic diversity of cable bacteria from Koljö Fjord spans several phylotypes of Ca . Electrothrix into different clusters (IV, V, and VI) according to recent phylogenetic analysis (Ley et al., 2024). Most of those cable bacteria fell within Cluster VI. This cluster encompasses previously described genera of Ca. Electrothrix, including gigas (Geelhoed et al., 2023), communis, aarhusiensis, marina, japonica (Trojan et al., 2016), laxa (Sereika et al., 2023), rattekaaiensis (Plum-Jensen et al., 2024), and antwerpensis (Hiralal et al., 2024), among others. The species Ca . Electrothrix communis, aarhusiensis and marina are derived from sulfidic sediments located near the Baltic Sea shore (Trojan et al., 2016). Notably, Ca . Electrothrix communis and aarhusiensis are present in both brackish and marine sediments, including salt marshes. The phylogenetic analysis of Ca . Electrothrix from Koljö Fjord indicates that it constitutes a community, likely representing novel species with diverse evolutionary phylotypes within cable bacteria. This suggests that cable bacteria have broader diversity, occupying more ecological niches than previously recognized, possibly involving NO 3 − reduction or other cryptic electron transfer processes in deeper, sulfidic, and anoxic sediments. Interaction between cable bacteria and S-oxidizing competitors Co-abundance analysis revealed that cable bacteria in Koljö Fjord had few links with the Beggiatoaceae family, which had more links with the Desulfubulbus genus, indicating that cable bacteria occupy a distinct niche. Beggiatoaceae family members, which include large colorless S-oxidizing bacteria, have been identified as competitors of cable bacteria. On the other hand, observations suggest that certain S-oxidizers may form mutually beneficial relationships with cable bacteria in anoxic environments rather than competing with them (Liau et al., 2022) Some factors, such as bioturbation, can mediate their interplay and performance in shared niches. In Koljö Fjord, infauna and bioturbation are absent. Previous studies have reported that heavy bioturbation inhibits cable bacteria proliferation, thereby allowing other bacteria, including Beggiatoaceae, to outcompete them (Hermans et al., 2019; Malkin et al., 2022). However, other factors, such as a more efficient enzymatic system for NO 3 − or sulfite uptake and utilization, may also contribute to cable bacteria proliferation, particularly in the absence of bioturbation, such as in Koljö Fjord. In contrast, we found that Beggiatoaceae dominated the upper layers, outcompeting cable bacteria in those zones, although a few members correlated with deeper sediments as well. In this sense, some large S-oxidizers from Beggiatoaceae, such as Beggiatoa and Ca . Marithioploca, are known to store NO 3 − in large vacuoles, allowing them to migrate downward into deep sulfidic sediments (Jørgensen and Gallardo, 1999; Salman et al., 2011), explaining their positive correlation with the surface and deeper layers. The unusual occurrence of Ca . Electrothrix in deep anoxic layers, alongside correlations with other S-oxidizers, indicates a potentially more widespread role for H 2 S oxidation in anoxic sediments. Locally produced NO 3 − may sustain cable bacteria deep in anoxic sediments The high NO 3 − peak found below 20 cm in Koljö Fjord is unusual, and similar findings under these conditions are scarce. However, a comparable NO 3 − peak in deep anoxic sediment layers of Loch Duich, an organic-rich marine fjord, has been reported (Mortimer et al., 2004). This NO 3 − peak may supply electron acceptors for supporting cable bacteria respiration in deep sediment layers. The NO 3 − peak in Koljö Fjord is linked to the expression of the napA gene, which encodes periplasmic NO 3 − reduction (Fig. 6A). This elevated gene expression at 20–22 cm in Koljö Fjord (gene count matrix in Supplementary Data 4) is consistent with the 16S rRNA gene abundance pattern found in core #2 (Fig. 3A). This further supports the notion of a localized, patchy distribution of cable bacteria. Periplasmic NO 3 − reductase napAB plays a crucial role in NO 3 − reduction in cable bacteria (Kjeldsen et al., 2019), coupled with ΣH 2 S oxidation (Risgaard-Petersen et al., 2014). Hence, the availability of NO 3 − in deeper sediment layers may facilitate respiration of cable bacteria in the absence of O 2 . To gain a comprehensive understanding of what drives the high porewater concentrations of NO 3 − found in the deeper sediment layers at Koljö Fjord. All potential sources were identified. Input of NO 3 − into deeper sediment layers via groundwater would have been associated with a steep decrease in the salinity gradient with depth due to the inflow of freshwater (Capone and Bautista, 1985). However, the stable brackish porewater conditions, as evident by the depth profiles of Na, K, Mg, and Cl, indicate that there is no source of groundwater discharging into the deeper sediment layer (Fig. S9). This implies that the NO 3 − peak found deep into the sediment is locally produced through anaerobic oxidation of NH 4 + , which is a microbially mediated process. A local process that can be eliminated as a potential source is the anaerobic oxidation of NH 4 + (Fig. 7C) by Fe oxides (Eq. 1). Typically, Fe oxides undergo rapid reductive dissolution upon contact with porewater ΣH 2 S (Burdige, 1993). Given the sulfidic porewater conditions at Köljö Fjord, it is highly unlikely that Fe oxides (Fig. S10) would reach the deeper sediment layers. (1) Another local process that is likely insignificant is the anaerobic oxidation of NH 4 + by Mn oxides (Eq. 2). Unlike Fe oxides, Mn oxides do not undergo rapid reductive dissolution upon contact with ΣH 2 S (Burdige, 1993). However, the low abundance of Mn oxides in the sediment from a nearby station in Köljö Fjord and the relatively low porewater concentrations of Mn 2+ (Fig. S10 and Supplementary Data 5) from potential dissolution indicate that this is likely not a significant contributor. (2) No highly expressed genes associated with Mn or Fe oxidation or with hydrazine dehydrogenase (HDH), which catalyzes the final step of NH 4 + oxidation in anammox bacteria using nitrite or nitric oxide (Liao et al., 2014), were identified. A potential explanation for the production of NO 3 − in the deeper sediment is via SO 4 2− reducing NH 4 + oxidation (sulfammox), which is a microbially mediated process in which NH 4 + oxidation is coupled to SO 4 2− reduction under anoxic conditions (see SO 4 2− in Fig. 7A). Although sulfammox was initially discovered in wastewater treatment (Fdz-Polanco, 2001), it has also been reported in marine sediments (Schrum et al., 2009; Rios-Del Toro et al., 2018). However, to date, reports of its occurrence in natural environments are scarce. In sulfammox, the NH 4 + /SO 4 2− molar ratio is regarded as a critical parameter, as it not only influences the rates of NH 4 + oxidation and SO 4 2− reduction but also controls the end product. When the NH 4 + /SO 4 2− ratio is ≥4, NH 4 + is converted into NO 2 − . However, when the NH 4 + /SO 4 2− ratio is ≤2, NH 4 + is overoxidized to NO 3 − to facilitate the reduction of SO 4 2− (Zhang et al., 2019; Wu et al., 2023). Besides controlling the end product, the NH 4 + /SO 4 2− ratio also regulates whether sulfammox or anammox is the preferred metabolic pathway. A reactor experiment demonstrated that sulfammox dominated when the NH 4 + /SO 4 2− ratio was less than 1.5. Conversely, anammox became the primary pathway when the NH 4 + /SO 4 2− ratio exceeded 1.5 (Yang et al., 2021). Considering that at Koljö Fjord, the NH 4 + /SO 4 2− ratios in the porewater in the deeper sediment were extremely low (<0.03), this could point toward sulfammox being the primary pathway with NO 3 − as end product (Eq. 3). (3) Although sulfammox was recently discovered and the exact metabolic pathways are unknown (Wu et al., 2023), various bacteria associated with this process have been identified. These include Bacillus benzoevorans (Cai et al., 2010), Bacillus , Candidatus Anammoxoglobus (Wu et al., 2023), and Bacillus cereus (Mohammed Madani et al., 2022). The Bacillus genus was highly abundant in Koljö Fjord, accounting for 0.9% of the total abundance, while the maximum was at 20–22 cm (1.5–2.5%) (Supplementary Data 2). Using 16S rRNA gene sequences of Bacillus benzoevorans and Bacillus cereus as references showed that they were present in similar abundances in all sediment layers of Kristineberg Bay and Koljö Fjord. However, the maximum abundance of Bacillus benzoevorans was found at 20–22 cm depth in Koljö Fjord (552,509 TPM) (Supplementary Data 2). Conversely, Ca . Scalindua sp., which is characterized as an anammox bacterium and has been associated with the reduction of Fe(III) to Fe(II) (Zhao et al. 2014), was abundant in Kristineberg Bay (Supplementary Data 2). In addition, bacteria such as Sulfurimonas may be related to sulfammox (Cai et al., 2010), which were abundant in Koljö Fjord, with a maximum at 20–22 cm depth (2.1%) (Supplementary Data 2). Interestingly, this bacterium presented a strong positive SparCC co-abundance correlation with Ca . Electrothrix (Fig. 5A). Given that the NH 4 + /SO 4 2− ratio was favorable for sulfammox and that various bacteria associated with this process were identified, it is reasonable to suggest that the increased concentration of NO 3 − between 20–30 cm in Koljö Fjord may have been generated by sulfammox. S-oxidation in deeper sediment layers and Ni utilization The high expression of sulfide:quinone oxidoreductase ( srq ) observed at Koljö Fjord (Fig. 6F) matches the highest cable bacteria abundance and the NO 3 − peak in Koljö Fjord. SQR facilitates the oxidation of H 2 S in the periplasm to produce polysulfides. Although the mechanisms of energy conservation in cable bacteria are not fully understood, sqr has been identified in several cable bacteria genomes and may play a role in S-oxidation (Kjeldsen et al., 2019; Hiralal et al., 2024). Other genes associated with S-oxidation, such as aprAB , soeA , and soxY , as well as polysulfide reductase ( psrA ), S-reduction genes such as sat and dsrB , and electron transfer genes such as cytB and cydA , exhibited high expression at 20–22 cm depth in core #2 in Koljö Fjord (Fig. S8, 11-12), suggesting constant H 2 S oxidation. In Koljö Fjord, the high Ni peak below 20 cm coincided with genes involved in Ni metabolism, such as the ABC.PS.S transporter, whose expression increased with depth (Fig. S8). Cable bacteria utilize Ni by incorporating it into their conductive wires, a mechanism exclusive to them (Hiralal et al., 2024). They can also thrive in sulfidic sediments harboring relatively high Ni porewater concentrations of up to ~1 μM (Van De Velde et al., 2017). While a similar gene expression pattern was observed in the other stations, the high Ni peak and high gene expression suggest possible Ni utilization by cable bacteria in Koljö Fjord. Our results revealed that Koljö Fjord is a characteristic environment in which a combination of electron acceptors (NO 3 − ) and donors (ΣH 2 S) is available in deep sediment layers, along with the expression of essential genes such as periplasmic nitrate reduction ( napAB ), sulfide oxidation ( sqr ), Ni utilization, and other metabolic markers, suggesting that metabolically active cable bacteria can persist in anoxic sediments. Proposed novel niche for cable bacteria deep in anoxic sediment layers The proposed model for cable bacteria communities deep in anoxic sediments (Fig. 8) assumes the involvement of sulfammox (Schrum et al., 2009; Rios-Del Toro et al., 2018), which serves as the primary source of NO 3 − (Fig. 6E). This model proposes a localized, transient event that generates suitable microenvironments for cable bacteria, which provide electron acceptors to patchily distributed and potentially dormant communities. The traditional conceptual framework of cable bacteria bioenergetics is based on the spatial arrangement of cells across vertical redox zones, with division of labor among the cells (Kjeldsen et al., 2019). This traditional model links cathodic O 2 reduction at the sediment surface to anodic ΣH 2 S oxidation in deeper anoxic layers via electrical currents. However, this study revealed that ΣH 2 S and NO 3 − coexist with cable bacteria within the same deep sediment layers of Koljö Fjord. In such a scenario, where the electron acceptor and donor are located within the same layer, a different physiological mode is suggested, potentially eliminating the need for long-distance electron transport mechanisms such as conductive nanowires or metabolic differentiation along the filaments. Heavy bioturbation can negatively impact the abundance of cable bacteria (Hermans et al., 2019; Malkin et al., 2022). Koljö Fjord is not bioturbated, as there are no macrofauna present due to persistent anoxia (Nordberg et al., 2001). This undisturbed sediment provides optimal conditions for maintaining the physical integrity of cable bacteria for extended periods. Research conducted at Koljö Fjord has shown extended survival of dinoflagellate cysts under anoxic conditions, with the ability to germinate after remaining in anoxic sediments for up to a century (Lundholm et al., 2011). A plausible hypothesis is that cable bacteria may also remain dormant for several years or even decades until specific events, such as anoxic nitrification via sulfammox, reactivate them. This hypothesis is further supported by a laboratory experiment that revealed that sediment containing a low abundance (14 m cm -2 ) of cable bacteria, which has been stored in anoxic conditions for several months, can rapidly develop a thriving community (724 m cm -2 ) when provided with an electron acceptor (Hermans et al., 2020). If cable bacteria deep in anoxic sediments are continuously supplied with NO 3 − through sulfammox, which has a distinct physiology, as suggested above, this community may be well-suited to these conditions and potentially comprise ancient, primordial forms of cable bacteria that predate O 2 -dependent lineages. An alternative explanation is that cable bacteria might gradually migrate downward toward layers with higher NO 3 − concentrations, as evidenced by their maximum abundance just above the NO 3 − peak (20–22 cm). In this scenario, the cable bacteria community does not reach a steady state but instead temporarily thrives in transient NO 3 − pockets that are eventually depleted. To further advance understanding of anoxic niches of cable bacteria, future research should focus on similar environments, such as Koljö Fjord, to characterize these niches. Sediment incubations could provide insights into the potential syntrophic relationships between S-oxidizers and sulfammox microorganisms, shedding light on the underlying mechanisms involved. Conclusions Our findings significantly advance the fields of microbial ecology and biogeochemistry, challenging the prevailing paradigm that cable bacteria are restricted to surface sediments. This has important implications for their potential role in removing ΣH 2 S from deeper sediment layers. Moreover, cable bacteria can form a deep, patchy community within anoxic sediments, occupying a previously overlooked ecological niche. This raises further questions about syntrophic relationships with organisms capable of anoxic nitrification via sulfammox, potentially reflecting analogs of primordial bacterial communities in early Earth marine ecosystems. Abbreviations Ca. Candidatus Ca Calcium Cl Chloride Fe Iron H 2 S Sulfide HS − Hydrosulfide IC Ion Chromatography ICP-MS Inductively Coupled Plasma-Mass Spectrometry K Potassium Mn Manganese Na Sodium napA Periplasmic nitrate reductase NH 4 + Ammonium Ni Nickel NO 2 − Nitrite NO 3 − Nitrate O 2 Oxygen S-oxidizers Sulfide-oxidizers S 2− Sulfide ion SO 4 2− Sulfate sqr Sulfide:quinone reductase ΣH 2 S Total sulfide Declarations Data availability All raw sequence data are deposited and available online at the NCBI repository under accession number PRJNA1253322 (https://www.ncbi.nlm.nih.gov/bioproject/?term=PRJNA1253322). Additional data, figures and tables from the analyses presented in this paper are available in the Supplementary Material, and the main scripts and procedures for data analysis performed in the study are publicly available in the GitHub repository: https://github.com/Alexis-Fonseca/CB_Koljo_Fjord.git. Acknowledgments We are grateful to the captain, technicians, Märta Brunberg and Stefano Bonaglia for their assistance aboard R/V Alice . We would like to express our gratitude to Hanna Reijola (IC), Juhani K. Virkanen (ICP-MS) and Tomas Thillman (SFA) for their analytical support. Funding Swedish Research Council (VR) grant no. 2021-04641 (CH). Marcus and Amalia Wallenberg Foundation. Stockholm University internal project number: 31004348 (FJN). The computations were enabled by resources in projects NAISS 2024/22-947 provided by the National Academic Infrastructure for Supercomputing in Sweden at the Uppsala Multidisciplinary Center for Advanced Computational Science (UPPMAX). Author information Authors and affiliations Baltic Sea Centre, Stockholm University, Stockholm, Sweden Alexis Fonseca, Martijn Hermans, Francisco J.A. Nascimento, Christian Stranne, Bo G. Gustafsson & Christoph Humborg Department of Ecology, Environment, and Plant Sciences (DEEP), Stockholm University, 106 91, Stockholm, Sweden Alexis Fonseca & Francisco J.A. Nascimento Department of Geological Sciences, Stockholm University, Stockholm, Sweden Christian Stranne Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden Christian Stranne Tvärminne Zoological Station, University of Helsinki, Hanko, Finland Alf Norkko Baltic Nest Institute, Stockholm University, Stockholm, Sweden Bo G. Gustafsson Contributions AF, MH, CH. Conceptualized the study; AF. & MH. planned the methodology; AF & MH. conducted the investigation; AF. performed the data analysis, bioinformatics studies and graphics production; MH. carried out the pore water analysis interpretation; CH. & FJAN. supervision; AF & MH. wrote the manuscript; and AF, MH, CH, FJAN, CS., BGG & AN revised and edited the manuscript. 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Impact of electrogenic sulfur oxidation on trace metal cycling in a coastal sediment. Chemical Geology 452, 9–23. https://doi.org/10.1016/j.chemgeo.2017.01.028 Wei, T., Simko, V., 2021. R package “corrplot”: Visualization of a Correlation Matrix, R package. Westreich, S.T., Treiber, M.L., Mills, D.A., Korf, I., Lemay, D.G., 2018. SAMSA2: a standalone metatranscriptome analysis pipeline. BMC Bioinformatics 19, 175. https://doi.org/10.1186/s12859-018-2189-z Wood, D.E., Lu, J., Langmead, B., 2019. Improved metagenomic analysis with Kraken 2. Genome Biol 20, 257. https://doi.org/10.1186/s13059-019-1891-0 Wu, B., Liu, F., Liang, Z., Wang, C., Wang, S., 2024. Spatial distribution of cable bacteria in nationwide organic-matter-polluted urban rivers in China. Science of The Total Environment 946, 174118. https://doi.org/10.1016/j.scitotenv.2024.174118 Wu, T., Ding, J., Zhong, L., Sun, H.-J., Pang, J.-W., Zhao, L., Bai, S.-W., Ren, N.-Q., Yang, S.-S., 2023. Sulfate-reducing ammonium oxidation: A promising novel process for nitrogen and sulfur removal. Science of The Total Environment 893, 164997. https://doi.org/10.1016/j.scitotenv.2023.164997 Yang, S., Zhu, Y., Liu, Han, Liu, Hongxu, 2021. Effects of N/S and ammonia concentrations on the process of sulfate reduction anaerobic ammonium oxidation. Transactions of the Chinese Society of Agricultural Engineering 37, 199–204. Zhang, D., Cui, L., Madani, R.M.A., Wang, H., Zhu, H., Liang, Ji., 2019. Effect of nitrite and nitrate on sulfate reducing ammonium oxidation. Water Science & Technology 80, 634–643. https://doi.org/10.2166/wst.2019.277 Additional Declarations No competing interests reported. Supplementary Files SupplementaryMaterial.docx Supplementary Information Supplementary Data are contained in the document in the Supplementary Material. Further material is divided into five files, available in the Figshare repository, labeled Supplementary Data: Supplementary Data 1, Supplementary Data 2, Supplementary Data 3, Supplementary Data 4 Supplementary Data 5 SupplementaryData1.xlsx SupplementaryData2.xlsx SupplementaryData3.xlsx SupplementaryData4.xlsx SupplementaryData5.xlsx Cite Share Download PDF Status: Published Journal Publication published 14 Apr, 2026 Read the published version in Environmental Microbiome → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6914568","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":477776531,"identity":"e9cbf791-36eb-4393-991d-e4af2d6b12b3","order_by":0,"name":"Alexis Fonseca","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7ElEQVRIiWNgGAWjYDACdiBOYGDgh3JtiNDCDNEi2QDhphGphQGh5TBhHfzNzMckHvxhkOCXbn/44EfNeXuDA+wPH+DTInGYLdkgsY1BQnLOGWPDnmO3Ezcc4DE2wGvNYR7DB4kNDHUGN3LYpBnYbicYHOBhk8CnQ/4w/4cDCUCHGdxIf/6b4d85kMOe/8CnxeAwD+ODBDaQlgQzZsa2A4wbDjCY4XWX4WE2Y6BfJCQkZ+QYS/b2JSfOPMxjjNdhcsebn0n++GMjwS+R/vDDj2929nzH2x9+wGsNBCAby0yE+lEwCkbBKBgF+AEAzGRF32fTrB4AAAAASUVORK5CYII=","orcid":"","institution":"Stockholm University","correspondingAuthor":true,"prefix":"","firstName":"Alexis","middleName":"","lastName":"Fonseca","suffix":""},{"id":477776532,"identity":"8856813d-4c44-45a5-9dce-31d3e43e2e15","order_by":1,"name":"Martijn Hermans","email":"","orcid":"","institution":"Stockholm University","correspondingAuthor":false,"prefix":"","firstName":"Martijn","middleName":"","lastName":"Hermans","suffix":""},{"id":477776533,"identity":"66061f86-d179-4cb4-98a1-fec7de141803","order_by":2,"name":"Francisco J.A. Nascimento","email":"","orcid":"","institution":"Stockholm University","correspondingAuthor":false,"prefix":"","firstName":"Francisco","middleName":"J.A.","lastName":"Nascimento","suffix":""},{"id":477776534,"identity":"ce1a6121-fe3d-4792-a336-e0a0da791e3d","order_by":3,"name":"Christian Stranne","email":"","orcid":"","institution":"Stockholm University","correspondingAuthor":false,"prefix":"","firstName":"Christian","middleName":"","lastName":"Stranne","suffix":""},{"id":477776535,"identity":"405f1359-aa37-4b18-9ac4-ad0b3d9f256d","order_by":4,"name":"Alf Norkko","email":"","orcid":"","institution":"University of Helsinki","correspondingAuthor":false,"prefix":"","firstName":"Alf","middleName":"","lastName":"Norkko","suffix":""},{"id":477776536,"identity":"23493159-700d-4288-96cf-5f080929d99d","order_by":5,"name":"Bo G. Gustafsson","email":"","orcid":"","institution":"Stockholm University","correspondingAuthor":false,"prefix":"","firstName":"Bo","middleName":"G.","lastName":"Gustafsson","suffix":""},{"id":477776537,"identity":"2c4b1842-4ab5-472f-a4bf-5cf9f3731a5f","order_by":6,"name":"Christoph Humborg","email":"","orcid":"","institution":"Stockholm University","correspondingAuthor":false,"prefix":"","firstName":"Christoph","middleName":"","lastName":"Humborg","suffix":""}],"badges":[],"createdAt":"2025-06-17 12:53:18","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-6914568/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6914568/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s40793-026-00895-7","type":"published","date":"2026-04-15T00:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":85817452,"identity":"6329f814-759e-4290-96b4-85897e4c2b4f","added_by":"auto","created_at":"2025-07-02 05:59:47","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":299493,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMaps of the study areas.\u003c/strong\u003e (\u003cstrong\u003eA\u003c/strong\u003e) Map of the Baltic Sea. The two research vessels designate the study areas. (\u003cstrong\u003eB\u003c/strong\u003e) Tvärminne Archipelago in southern Finland, with stations marked as Nearshore and Offshore. \u003cstrong\u003e(C)\u003c/strong\u003e Fjord system around the Orust and Tjörn Islands. \u003cstrong\u003e(D)\u003c/strong\u003e Swedish West Coast, with stations marked as Kristineberg Bay and Koljö Fjord, and the adjacent sills: Havsten Fjord (S1), Skagerrak (S2), and Gullmar Fjord (S3).\u003c/p\u003e","description":"","filename":"image1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/9da8fc74186795f1f42a3a8a.jpeg"},{"id":85816922,"identity":"838a21b8-84cd-41f6-9e9a-77d7e254ea23","added_by":"auto","created_at":"2025-07-02 05:51:47","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":76814,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eClassification of the stations by salinity and O\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e concentration.\u003c/strong\u003e Classification of stations based on bottom water characteristics: O\u003csub\u003e2\u003c/sub\u003e, salinity at the time of sampling, and physical configuration (open vs. restricted).\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/0b42f719c4ac930f7016e091.png"},{"id":85817450,"identity":"a92d38d8-929f-49b8-b7c0-2fe8bb041f01","added_by":"auto","created_at":"2025-07-02 05:59:47","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":215284,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAbundance of \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eCa\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e. Electrothrix and other members of the Desulfobulbaceae family.\u003c/strong\u003e (\u003cstrong\u003eA\u003c/strong\u003e) Relative abundance of 16S rRNA genes from total SSU rRNA identified with Kraken2 and Bracken programs in a vertical profile from 0 to 32 cm at Koljö Fjord and Kristineberg Bay and up to 42 cm at the Tvärminne Archipelago stations. (\u003cstrong\u003eB\u003c/strong\u003e) Average abundance of transcripts per million (TPM) of the 13 partial 16S rRNA gene sequences of \u003cem\u003eCa\u003c/em\u003e. Electrothrix. Partial 16S rRNA gene sequences were identified after assembling and annotating the SSU rRNA from the Koljö Fjord samples. The length of 16S rRNA gene sequences is in base pairs (bp), in parentheses.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/dc21315095e19bcb1f134355.png"},{"id":85816925,"identity":"d5788502-4aef-4b03-8a66-84b89adf4f2b","added_by":"auto","created_at":"2025-07-02 05:51:47","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":726686,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePhylogenetic tree of cable bacteria from Koljö Fjord.\u003c/strong\u003e Thirteen partial 16S rRNA gene sequences from Koljö Fjord were annotated as \u003cem\u003eCa\u003c/em\u003e. Electrothrix were aligned with 61 cable bacteria, 11 related Desulfobulbales, and \u003cem\u003eGeobacter sulfurreducens\u003c/em\u003e as an outgroup (list of sequences in Supplementary Data 3). Sequence alignment was performed using MUSCLE. The phylogenetic inference was made using IQ-TREE 2.3.5 with ModelFinder Plus (MFP+MERGE). Branch support was evaluated with 5000 ultrafast bootstrap replicates, SH-aLRT (5000 replicates), and Bayesian-like transformation of aLRT (aBayes). Roman numbers depict clusters of cable bacteria defined by Ley et al. (2024).\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/de2af933418b06f873635108.png"},{"id":85816924,"identity":"5ce1be7c-b009-41a5-9efa-adc120cbf36c","added_by":"auto","created_at":"2025-07-02 05:51:47","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":492945,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCo-abundance and correlations of cable bacteria. \u003c/strong\u003e(\u003cstrong\u003eA\u003c/strong\u003e) SparCC co-abundance correlations of \u003cem\u003eCa\u003c/em\u003e. Electrothrix, \u003cem\u003eCa.\u003c/em\u003eThiomargarita and \u003cem\u003eDesulfobulbus\u003c/em\u003e. Larger center nodes represent the main taxa in the comparison. The cluster of \u003cem\u003eCa\u003c/em\u003e. Electrothrix depicts positive (gold) and negative (blue) correlations, respectively. (\u003cstrong\u003eB\u003c/strong\u003e) Pearson correlation coefficients of the relative abundance of the ten most abundant genera in the Desulfobulbaceae and Beggiatoacea families in relation to sediment depth layers in Koljö Fjord and (\u003cstrong\u003eC\u003c/strong\u003e) Offshore station. One star represents a correlation with a p-value \u0026lt; 0.05, and two stars represent an adjusted p-value \u0026lt; 0.05 (values in Supplementary Data 2).\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/781ea1655163dac64ac97153.png"},{"id":85816927,"identity":"40bef88a-d957-4ae7-9c6d-54ab84e3170c","added_by":"auto","created_at":"2025-07-02 05:51:47","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":258549,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGene expression and porewater signatures related to NO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003csup\u003e\u003cstrong\u003e− \u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eand ΣH\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eS\u003c/strong\u003e. Results for \u003cem\u003enapA\u003c/em\u003e gene expression, which is involved in periplasmic NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e− \u003c/sup\u003ereduction, measured in counts per million (CPM) are shown in Plots \u003cstrong\u003eA\u003c/strong\u003e–\u003cstrong\u003eD\u003c/strong\u003e. Additionally, Plots \u003cstrong\u003eF\u003c/strong\u003e–\u003cstrong\u003eI\u003c/strong\u003e present the gene expression of SQR (\u003cem\u003esqr\u003c/em\u003e gene), a sulfide quinone oxidoreductase involved in H\u003csub\u003e2\u003c/sub\u003eS oxidation. Plot \u003cstrong\u003eE\u003c/strong\u003e shows the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e concentration profile, and \u003cstrong\u003eJ\u003c/strong\u003e and \u003cstrong\u003eK\u003c/strong\u003e depict the porewater profiles of ΣH\u003csub\u003e2\u003c/sub\u003eS.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/88b3b9029aa021643f740ac8.png"},{"id":85816931,"identity":"a66b3d16-5e04-4d79-bbe6-05a36f9729cc","added_by":"auto","created_at":"2025-07-02 05:51:48","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":104117,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePorewater depth profiles.\u003c/strong\u003e \u003cstrong\u003eA\u003c/strong\u003e and \u003cstrong\u003eD\u003c/strong\u003e represent the SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2− \u003c/sup\u003econcentration profiles, whereas \u003cstrong\u003eB\u003c/strong\u003e and \u003cstrong\u003eE\u003c/strong\u003e represent Ni, and \u003cstrong\u003eC\u003c/strong\u003e represents NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/8b26f7211b9c35c7f157c2c9.png"},{"id":85816929,"identity":"8aeaea5f-324a-4af2-96c9-0f367fc3d56f","added_by":"auto","created_at":"2025-07-02 05:51:47","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":172528,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eNiche for cable bacteria that inhabit deep in anoxic sediments.\u003c/strong\u003e This diagram illustrates the hypothesis that cable bacteria communities can be sustained by NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e pockets that are locally produced through sulfammox deep in anoxic sediments.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/749dbba3728d7b15b23502c0.png"},{"id":107077547,"identity":"b2228fb5-2884-4aaa-854f-4f81c34d6107","added_by":"auto","created_at":"2026-04-16 13:32:09","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3820032,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/cfb4b002-826a-4c7a-a632-2592e8dce7f6.pdf"},{"id":85816919,"identity":"4711be86-bf9c-4440-a0fb-401d1e32b34a","added_by":"auto","created_at":"2025-07-02 05:51:47","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1882530,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSupplementary Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary Data are contained in the document in the \u003cstrong\u003eSupplementary Material\u003c/strong\u003e. Further material is divided into five files, available in the Figshare repository, labeled Supplementary Data:\u003c/p\u003e\n\u003cp\u003eSupplementary Data 1, Supplementary Data 2, Supplementary Data 3, Supplementary Data 4\u003c/p\u003e\n\u003cp\u003eSupplementary Data 5\u003c/p\u003e","description":"","filename":"SupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/14b9d7f61085aab0b78572e5.docx"},{"id":85817451,"identity":"b35527ca-5baf-4ad0-95de-6257d947d707","added_by":"auto","created_at":"2025-07-02 05:59:47","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":18786,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryData1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/eb08023a90433d76dc9a32c7.xlsx"},{"id":85817454,"identity":"de6cf38b-ad3b-49c7-af2c-68d938e23b17","added_by":"auto","created_at":"2025-07-02 05:59:47","extension":"xlsx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":1883627,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryData2.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/d4b0cc25dc96054eccec008f.xlsx"},{"id":85817453,"identity":"24e9f997-0dd1-4d2a-a23f-585e1eb9a858","added_by":"auto","created_at":"2025-07-02 05:59:47","extension":"xlsx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":13869,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryData3.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/50fb6c35e0ee14808e0f2adb.xlsx"},{"id":85816932,"identity":"a53b46e6-5ab7-4f65-9111-da1b701a410c","added_by":"auto","created_at":"2025-07-02 05:51:48","extension":"xlsx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":4056797,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryData4.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/7b22657351e80fc73b00310f.xlsx"},{"id":85817455,"identity":"2f48c62d-8f42-43fa-a029-e5b05111a5c5","added_by":"auto","created_at":"2025-07-02 05:59:47","extension":"xlsx","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":21611,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"SupplementaryData5.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6914568/v1/b12f45760581aa1b1b387b7b.xlsx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Evidence for cable bacteria inhabiting deep in anoxic sediment reveals a novel ecological niche","fulltext":[{"header":"Background","content":"\u003cp\u003eCable bacteria are multicellular filamentous organisms characterized by a distinct metabolism that spatially couples the oxidation of sulfide\u0026nbsp;(H\u003csub\u003e2\u003c/sub\u003eS)\u0026nbsp;with the reduction of oxygen\u0026nbsp;(O\u003csub\u003e2\u003c/sub\u003e)\u0026nbsp;or nitrate\u0026nbsp;(NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e)\u0026nbsp;by channeling electrons through their filaments over cm-scale distances\u0026nbsp;(Nielsen et al., 2010; Pfeffer et al., 2012; Marzocchi et al., 2014).\u0026nbsp;These bacteria belong to the Desulfobulbaceae family and are traditionally classified into two genera: \u003cem\u003eCa.\u0026nbsp;\u003c/em\u003eElectrothrix and \u003cem\u003eCa.\u003c/em\u003e Electronema for marine and freshwater sediments, respectively\u0026nbsp;(Trojan et al., 2016). However, recent findings indicate a substantially greater diversity of potentially up to ninety species divided among six genera\u0026nbsp;(Ley et al., 2024).\u003c/p\u003e\n\u003cp\u003eCable bacteria possess a metabolic strategy based on long-distance electron transport, giving them a competitive advantage over other sulfur (S)-oxidizing bacteria\u0026nbsp;(Meysman et al., 2019). Their ecological importance is rooted in regulating redox gradients, trace metals\u0026nbsp;(Van De Velde et al., 2017), nutrient cycling\u0026nbsp;(Hermans et al., 2021), generating a firewall against euxinia\u0026nbsp;(Seitaj et al., 2015), and influencing the behavior of other microbial communities\u0026nbsp;(Bjerg et al., 2023; Dong et al., 2024).\u003c/p\u003e\n\u003cp\u003eThe consensus is that cable bacteria are restricted to the upper few cm of surface sediments, where O\u003csub\u003e2\u003c/sub\u003e or\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e and H\u003csub\u003e2\u003c/sub\u003eS are abundant\u0026nbsp;(Marzocchi et al., 2014; Risgaard-Petersen et al., 2014). Environmental parameters, such as bottom water redox conditions, bioturbation\u0026nbsp;(Hermans et al., 2019; Malkin et al., 2022), salinity and temperature\u0026nbsp;(Dam et al., 2021), control the activity, abundance, and diversity of cable bacteria.\u003c/p\u003e\n\u003cp\u003eCable bacteria have been discovered in a wide range of aquatic environments (i.e., freshwater–brackish–marine), including rivers, estuaries, coasts, and salt marshes\u0026nbsp;(Risgaard-Petersen et al., 2015; Burdorf et al., 2017; Hermans et al., 2019).\u0026nbsp;So far, their presence deep in anoxic sediment layers has not been documented.\u003c/p\u003e\n\u003cp\u003eThis study reveals that active cable bacteria, hereafter referred specifically to as marine cable bacteria (\u003cem\u003eCandidatus\u003c/em\u003e Electrothrix), thrive deep in anoxic sediment layers and are sustained by NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e pockets, challenging the conventional view that they are restricted to surface sediments. These findings offer new insights into their ecological and biogeochemical significance, particularly in removing H\u003csub\u003e2\u003c/sub\u003eS from deeper layers.\u003c/p\u003e"},{"header":"Methods","content":"\u003ch2\u003eStudy area and sediment collection\u003c/h2\u003e\n\u003cp\u003eSediments from Kristineberg Bay and Kolj\u0026ouml; Fjord on the Swedish West Coast (Fig. 1A) were collected using a Gemini Twin Corer aboard \u003cem\u003eR/V Alice\u003c/em\u003e.\u0026nbsp;For comparison, data from two contrasting stations in the Tv\u0026auml;rminne Archipelago in southern Finland\u0026nbsp;(Fig. 1B)\u0026nbsp;from another study were used due to the availability of a comparable dataset\u0026nbsp;(Hermans et al., 2024). These four stations differ in terms of their bottom water redox conditions and salinity\u0026nbsp;(Fig. 2 and Table S1). Commonly used thresholds to classify bottom water redox conditions are\u0026nbsp;(Algeo and Li, 2020): oxic (O\u003csub\u003e2\u003c/sub\u003e \u0026gt; 2 mL L\u003csup\u003e-1\u003c/sup\u003e), hypoxic (O\u003csub\u003e2\u003c/sub\u003e = 0.2\u0026ndash;2 mL L\u003csup\u003e-1\u003c/sup\u003e), anoxic (O\u003csub\u003e2\u003c/sub\u003e = 0 mL L\u003csup\u003e-1\u003c/sup\u003e), and euxinic (O\u003csub\u003e2\u003c/sub\u003e = 0 mL L\u003csup\u003e-1\u003c/sup\u003e; \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS \u0026gt; 0 mL L\u003csup\u003e-1\u003c/sup\u003e).\u003c/p\u003e\n\u003cp\u003eKristineberg Bay is fully marine and has oxic bottom waters. Kolj\u0026ouml; Fjord is part of an open-ended fjord system encompassing the Orust and Tj\u0026ouml;rn Islands (Fig. 1C). It is restricted by three shallow sills connected to the adjacent Havsten Fjord: (S1) at a water depth of 12 m, Skagerrak (S2) at a water depth of 8 m, and Gullmar Fjord (S3) at a water depth of 5 m (Fig. 1D). The hydrography is controlled by the inflow of brackish surface water through Kattegat\u0026ndash;Skagerrak, originating from the Baltic Sea. Freshwater input is of minor importance, as no major riverine inputs discharge into the fjord. The deep waters of Kolj\u0026ouml; Fjord undergo renewal. However, reoxygenation may not be guaranteed due to its variable frequency, ranging from annual to several years\u0026nbsp;(Paul et al., 2023). At the time of sampling, the bottom waters in Kolj\u0026ouml; Fjord were hypoxic/anoxic.\u003c/p\u003e\n\u003cp\u003eThe two stations in the Tv\u0026auml;rminne Archipelago exhibit relatively lower salinities, although they remain brackish. However, they share similar hydrographical characteristics and sediment structures as the two stations situated on the Swedish West Coast. These stations are designated as Nearshore and Offshore (Fig. 1B). The Nearshore station has oxic bottom waters and is located close to the outflow of the Pojo Bay estuary, whereas the Offshore station has hypoxic bottom waters, as it is situated in a small enclosed bay further offshore the archipelago.\u003c/p\u003e\n\u003cp\u003eWhile the Offshore station is not situated completely offshore but rather farther out at sea, the term \u0026quot;offshore\u0026quot; is used to distinguish this station from the nearshore station.\u003c/p\u003e\n\u003cp\u003eSediment cores were collected in triplicate and meticulously sliced\u0026nbsp;at depths of 0\u0026ndash;1, 1\u0026ndash;3, 7\u0026ndash;9, 9\u0026ndash;11, 20\u0026ndash;22, and 30\u0026ndash;32 cm\u0026nbsp;(Kristineberg Bay was sampled 0\u0026ndash;2 and 2\u0026ndash;3, for cores 2 and 3) for metatranscriptomic (RNA sequencing) and porewater analyses\u0026nbsp;(Fig. S1). The separated sediment layers were then placed into 215 mL polypropylene containers (Noax Laboratory, Product No. 207.0215PP). After each layer was homogenized, a 2\u0026ndash;3 mL sample of the mixed sediment was transferred to a 15 mL centrifuge tube. These samples were promptly flash-frozen in liquid nitrogen and stored at\u003cstrong\u003e\u0026nbsp;\u0026minus;\u003c/strong\u003e80\u0026deg;C until RNA extraction was performed.\u003c/p\u003e\n\u003ch2\u003eRNA extraction and sequencing\u003c/h2\u003e\n\u003cp\u003eTotal RNA was extracted from 2 g of each homogenized sediment sample using the RNeasy PowerSoil Total Kit (Qiagen). A TURBO DNA-free kit (Invitrogen) was used to eliminate residual DNA contamination in the eluate. At SciLifeLab in Stockholm, libraries were prepared for 36 samples (Supplementary Data 1), 18 for Kristineberg Bay and Kolj\u0026ouml; Fjord (three independent biological replicates per sediment depth from each station, as described above and\u0026nbsp;Fig. S1)\u0026nbsp;using TruSeq Stranded mRNA polyA (Illumina). The purified libraries were subsequently sequenced at SciLifeLab utilizing a NovaSeqXPlus platform (NovaSeqXSeries\u0026nbsp;Control Software 1.2.0.28691) with 2 \u0026times; 150 bp configuration. The sequencing libraries are available in the NCBI repository under BioProject accession number PRJNA1253322.\u003c/p\u003e\n\u003ch2\u003ePorewater collection and treatment\u003c/h2\u003e\n\u003cp\u003eBottom and porewater samples were extracted using Rhizons\u0026trade; (0.12\u0026ndash;0.18 \u0026mu;m pore size) and collected into 10 mL polyethylene syringes as described in Hermans et al.\u0026nbsp;(2024)\u0026nbsp;and references therein. Two parallel porewater series were extracted from each core. The first series was designated for\u0026nbsp;sulfate\u0026nbsp;(SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e), inorganic nutrients (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, and\u0026nbsp;NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e), and trace metals (Fe, Mn, and Ni), whereas the second series was reserved for total sulfide analysis (\u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS = H\u003csub\u003e2\u003c/sub\u003eS + HS\u003csup\u003e\u0026minus;\u003c/sup\u003e + S\u003csup\u003e2\u003c/sup\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e). Subsamples for trace metal analysis were acidified with distilled nitric acid. Syringes for \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS analysis were prefilled with 1 mL zinc acetate (10%) to trap \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS as ZnS. Subsamples for SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS, and trace metal analyses were stored at 4\u0026deg;C, whereas subsamples for inorganic nutrients were frozen at \u003cstrong\u003e\u0026minus;\u003c/strong\u003e20\u0026deg;C.\u003c/p\u003e\n\u003ch2\u003ePorewater analyses\u003c/h2\u003e\n\u003cp\u003eDissolved\u0026nbsp;NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e concentrations were determined after the indophenol method using Segmented Flox Analysis (Alpkem SFA, O. I. Analytical Flow Solution IV). Porewater concentrations of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e were determined using a carrier stream that facilitates continuous mixing with a buffer solution. First, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was reduced to NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e using cadmium. Following the addition of phosphoric acid, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, both initially present and formed through the reduction of\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, was used to diazotize sulfanilamide in an acidic solution\u0026nbsp;(Swedish Institute for Standards, 1997). In a separate batch,\u0026nbsp;NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations were determined using the same method as described above but without the cadmium reduction step. The concentration of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was derived by subtracting NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e from the total NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e + NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eAcidified samples were analyzed using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS; Thermo Scientific, XSERIES 2 or Agilent 7800) to determine Fe, Mn, and Ni contents. Here, dissolved Fe and Mn are considered as Fe\u003csup\u003e2+\u003c/sup\u003e and Mn\u003csup\u003e2+\u003c/sup\u003e; however, some Mn\u003csup\u003e3+\u003c/sup\u003e or colloidal and nanoparticulate Fe and Mn might also be present (Paul et al., 2023).\u003c/p\u003e\n\u003cp\u003eConcentrations of sodium (Na), potassium (K), magnesium (Mg), and chloride (Cl) in Kolj\u0026ouml; Fjord were determined using ion chromatography (IC).\u003c/p\u003e\n\u003ch2\u003eBioinformatics and statistics\u003c/h2\u003e\n\u003cp\u003eThe libraries from Kristineberg Bay and Kolj\u0026ouml; Fjord yielded an average of 101 million reads (Supplementary Data 1). Illumina adapters were removed via targeted primer sequencing\u0026nbsp;(St. John, 2011)\u0026nbsp;through the SeqPrep 1.2 program with default parameters. The absence of PhiX control sequences was verified by aligning the reads to the PhiX genome (NCBI Reference Sequence: NC_001422.1) using Bowtie 2 V 2.3.5.1\u0026nbsp;(Langmead and Salzberg, 2012). The reads were subjected to quality control using the programs FastQC V 0.11.9\u0026nbsp;(Andrews, 2010)\u0026nbsp;and MultiQC V 1.12\u0026nbsp;(Ewels et al., 2016)\u0026nbsp;before and after the trimming step. Trimming was carried out using Trimmomatic 0.39\u0026nbsp;(Bolger et al., 2014)\u0026nbsp;with the following settings: LEADING:20, TRAILING:20, MINLEN:80, and Cutadapt V 4.5\u0026nbsp;(Martin, 2011). The trimmed reads were merged using FLASH V 1.2.11\u0026nbsp;(Magoč and Salzberg, 2011)\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003eThe average number of merged reads was 54% (ranging from 42% to 73%), resulting in a read length of 220 bp per sample. Following the SAMSA2 pipeline recommendations\u0026nbsp;(Westreich et al., 2018),\u0026nbsp;the paired reads were merged with nonpaired forward reads.\u0026nbsp;The methods used for the Tv\u0026auml;rminne libraries are explained in detail by Hermans et al.\u0026nbsp;(2024).\u003c/p\u003e\n\u003cp\u003eDifferential bacterial abundance was measured using the edgeR analysis script (run_DE_analysis.pl) with the default parameters provided by Trinity 2.14.0 software (Robinson et al., 2010; Haas et al., 2013). Additionally, the difference in abundance was assessed using Kruskal-Wallis and D\u003cstrong\u003euncan\u0026rsquo;s\u003c/strong\u003e tests, adjusting p-values with \u003cstrong\u003ethe Benjamini-Hochberg method.\u003c/strong\u003e Pearson correlations between bacterial abundances, depths and variables such as NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and H\u003csub\u003e2\u003c/sub\u003eS were tested and plotted using the function corrplot in the R package corrplot\u0026nbsp;(Wei and Simko, 2021)\u0026nbsp;and the Microbe-to-sample-data correlation heatmap from the microViz R package (Barnett et al., 2021). Furthermore, SparCC (parse Correlations for Compositional data) co-abundance correlations (Friedmand and Alm, 2012) were inferred with 100 iterations and two-sided pseudo-P values using a 100-round bootstrap procedure.\u003c/p\u003e\n\u003ch2\u003eTaxonomic classification by rRNA\u003c/h2\u003e\n\u003cp\u003eFollowing Broman et al.\u0026nbsp;(2024)\u0026nbsp;and Hermans et al.\u0026nbsp;(2024), rRNA (SSU rRNA) reads were extracted from the quality-trimmed reads with SortMeRNA 4.3.6 using the SSU sequences in the SILVA database\u0026nbsp;(silva-bac-16s-id90.fasta;\u0026nbsp;Kopylova et al., 2012). To determine the taxonomic composition of active bacteria at the genus level, quality-filtered rRNA reads were classified using Kraken2\u0026nbsp;(Wood et al., 2019)\u0026nbsp;with the SILVA 138 SSU (June 2024) database as a reference. Bracken was used to refine the abundance estimates by statistically reassigning reads classified at higher taxonomic levels, thereby improving the quantitative accuracy.\u003c/p\u003e\n\u003cp\u003eCable bacteria identification and abundance were further analyzed in Kolj\u0026ouml; Fjord, and the rRNA gene reads were assembled, annotated, and quantified (see details in\u0026nbsp;Supplementary Material and Supplementary Data 2).\u003c/p\u003e\n\u003ch2\u003ePhylogeny of cable bacteria from Kolj\u0026ouml; Fjord\u003c/h2\u003e\n\u003cp\u003eTo evaluate the evolutionary relationship between \u003cem\u003eCa\u003c/em\u003e. Electrothrix from Kolj\u0026ouml; Fjord, a phylogenetic tree was built using the 16S rRNA gene sequences. Following the assembly of the metatranscriptomic sequences from Kolj\u0026ouml; Fjord, 13 partial 16S rRNA gene sequences were annotated as \u003cem\u003eCa\u003c/em\u003e. Electrothrix. These 13 partial 16S rRNA gene sequences were aligned with 61 cable bacteria, 13 related Desulfobulbales, and \u003cem\u003eGeobacter sulfurreducens\u003c/em\u003e as an outgroup (sequences are available in Supplementary Data 3). Multiple sequence alignment was performed using MUSCLE v5.1.0 (Edgar, 2004), and phylogenetic inference was conducted using IQ-TREE 2.3.5, employing ModelFinder Plus (MFP+MERGE) for optimal model selection. Branch support was evaluated with 5000 ultrafast bootstrap replicates, SH-aLRT (5000 replicates), and Bayesian-like transformation of aLRT (aBayes). The final tree was visualized using FigTree v1.4.4.\u003c/p\u003e\n\u003ch2\u003eGene expression\u003c/h2\u003e\n\u003cp\u003eSortMeRNA 4.3.4 and its provided database\u0026nbsp;(smr_v4.3_default_db.fasta;\u0026nbsp;Kopylova et al., 2012)\u0026nbsp;were used to separate rRNA from total RNA reads. Following Broman et al.\u0026nbsp;(2024)\u0026nbsp;and Hermans et al.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e(2024), DIAMOND v2.1.10.164 was utilized to categorize the non-rRNA reads against the NCBI NR database (retrieved on June 3, 2024) by applying an e-value threshold of 1e\u003csup\u003e‑10\u003c/sup\u003e. The daa-meganizer tool included in MEGAN 6 Ultimate Edition 6.25.10, built 27 June 2024,\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003ewas applied to eliminate eukaryotic and viral data and connect the diamond results to the KEGG database (MEGAN database: megan-map-Feb2022; Huson et al., 2007; Bağcı et al., 2021). A MEGAN file containing absolute counts was generated using the MEGAN tool for computer comparison. This file was then imported into MEGAN software to extract all KEGG KO classifications. Sequence counts were normalized to counts per million (CPM; calculated as relative proportion \u0026times; 1 million). A comprehensive matrix of all KEGG classifications can be found in Supplementary Data 4.\u003c/p\u003e"},{"header":"Results","content":"\u003ch2\u003eAbundance of cable bacteria\u003c/h2\u003e\n\u003cp\u003eAnalysis of 16S rRNA gene sequences identified 3,119 bacterial genera (370,937,895 sequences) in Kolj\u0026ouml; Fjord, 3,055 (453,889,324 sequences) in Kristineberg Bay, 3,339 (871,131,658 sequences) at the Offshore station, and 3,124 (564,362,145 sequences) at the Nearshore station in the Tv\u0026auml;rminne Archipelago (Supplementary Data 2). Proteobacteria and Desulfobacterota were the dominant phyla across the data, accounting for 36% and 19% of the total abundance, respectively (Fig. S2). Within the Desulfobulbaceae family (the most abundant family and genera in Fig. S3 and S4), undescribed forms were dominant across all stations (1.34%), followed by the genera \u003cem\u003eDesulfubulbus\u003c/em\u003e (0.09%) and cable bacteria; \u003cem\u003eCa\u003c/em\u003e. Electrotrix and \u003cem\u003eCa\u003c/em\u003e. Electronema (0.08 and 0.01%). However, the abundance of \u003cem\u003eCa\u003c/em\u003e. Electrothrix at the Offshore station and Kolj\u0026ouml; Fjord (Fig. 3A and Fig. S5) accounted for 0.11% of the total abundance, corresponding to 416,616 and 937,608 sequences, respectively. In contrast, Kristineberg Bay and the Nearshore station (with decreasing abundance toward deeper layers) exhibited negligible abundances of 0.02% (115,717 sequences) and 0.05% (316,115 sequences), respectively.\u003c/p\u003e\n\u003cp\u003eThe vertical distribution of marine cable bacteria at the Offshore station in the Tv\u0026auml;rminne Archipelago clearly decreased with depth. The relative abundances (within the Desulfobulbaceae family) ranged from 32.3\u0026ndash;40.4% at 0\u0026ndash;1 cm and 23.6\u0026ndash;62.3% at 1\u0026ndash;3 cm, dropping sharply to 0.7\u0026ndash;1.4% at 20\u0026ndash;22 cm. In contrast, at Kolj\u0026ouml; Fjord, the abundance of cable bacteria increased with depth, from average relative abundances of 2.3% at 0\u0026ndash;1 cm and 5.0% at 1\u0026ndash;3 cm to 28.2% at 20\u0026ndash;22 cm and 12.6% at 30\u0026ndash;32 cm. The maximum observed abundance of \u003cem\u003eCa\u003c/em\u003e. Electrothrix reached 76% in core #2 (edgeR p-value of 0.05 for the difference between the 0\u0026ndash;1 cm and 20\u0026ndash;22 cm layers). Thus, cable bacteria were highly abundant in core #2 and, to a lesser extent, in core #1 from the same cast. In contrast, core #3 from a different cast exhibited low abundance.\u003c/p\u003e\n\u003cp\u003eTo confirm our findings, SSU rRNA sequences from Kolj\u0026ouml; Fjord were assembled, annotated, and quantified. Thirteen partial 16S rRNA gene sequences were obtained (Supplementary Data 2), displaying the same abundance pattern (Fig. 3B and Fig. S6) as the analysis of the shorter 16S rRNA gene fragments (Fig. 3A), with averages of 12 and 18 transcripts per million (TPM) for 0\u0026ndash;1 and 1\u0026ndash;3 cm depths, respectively, and a maximum average abundance at 20\u0026ndash;22 cm depth of 141 TPM (maximum: 328 TPM for \u003cem\u003eCa\u003c/em\u003e. Electrothrix Kolj\u0026ouml;_8).\u003c/p\u003e\n\u003cp\u003eWhile the Kruskal‒Wallis test indicated significant differential abundances between depths (p- value \u0026lt;0.05), the pairwise Dunn test revealed significant differences between 1\u0026ndash;3 and 30\u0026ndash;32 cm depth (p-value \u0026lt;0.05)\u0026nbsp;(Table S3). The comparison between 1\u0026ndash;3 cm and 20\u0026ndash;22 cm depth yielded a p-value of 0.01 (adjusted p-value = 0.07).\u0026nbsp;The results revealed that various cable bacteria types were present deep in the anoxic sediment layers of Kolj\u0026ouml; Fjord, with the highest abundance found at depths of 20\u0026ndash;22 cm.\u003c/p\u003e\n\u003ch2\u003eSeveral species of \u003cem\u003eCa.\u0026nbsp;\u003c/em\u003eElectrothrix reside deep within anoxic sediment layers\u003c/h2\u003e\n\u003cp\u003eA phylogenetic tree based on the 13 partial 16S rRNA genes revealed several \u003cem\u003eCa.\u0026nbsp;\u003c/em\u003eElectrothrix types in Kolj\u0026ouml; Fjord (Fig. 4). According to the recent cable bacteria phylogeny (Ley et al., 2024), nine sequences were assigned to Cluster VI, which included species such as \u003cem\u003eCa.\u0026nbsp;\u003c/em\u003eElectrothrix communis, aarhusiensis, marina, japonica, laxa, rattekaaiensis, and antwerpensis. These species are known from Baltic Sea sediments and other brackish or marine environments.\u003c/p\u003e\n\u003cp\u003eThe 16S rRNA gene sequences of \u003cem\u003eCa.\u0026nbsp;\u003c/em\u003eElectrothrix from Kolj\u0026ouml; Fjord exhibited a dispersed distribution within Cluster VI. Most appeared to represent novel species, although \u003cem\u003eCa.\u0026nbsp;\u003c/em\u003eElectrothrix Kolj\u0026ouml;_9 aligned closely with \u003cem\u003eCa.\u0026nbsp;\u003c/em\u003eElectrothrix marina,\u0026nbsp;as evidenced by two high-quality BLASTN alignments with 99% identity\u0026nbsp;(219 bp and 196 bp, E-values of 6e\u003csup\u003e-10\u003c/sup\u003e and 7e\u003csup\u003e-94\u003c/sup\u003e), suggesting a close evolutionary relationship.\u003c/p\u003e\n\u003cp\u003eThree sequences, Kolj\u0026ouml;_1, Kolj\u0026ouml;_2, and Kolj\u0026ouml;_10, belong to Cluster IV, which includes cable bacteria from the North Sea and a mud volcano in Costa Rica. One sequence, Kolj\u0026ouml;_6, was placed within Clade V and was associated with marine habitats. These data suggest that the cable bacteria community in Kolj\u0026ouml; Fjord consists of multiple evolutionary lineages, some likely novel.\u003c/p\u003e\n\u003ch2\u003eCo-abundance of cable bacteria with S-oxidizers in Kolj\u0026ouml; Fjord\u003c/h2\u003e\n\u003cp\u003ePrevious studies have identified large colorless S-oxidizing bacteria, such as members of the Beggiatoaceae family, as potential competitors of cable bacteria owing to their overlapping metabolic pathways for energy production and respiration. In Kolj\u0026ouml; Fjord, Beggiatoaceae accounted for 0.32% of the total microbial community, and \u003cem\u003eCa\u003c/em\u003e. Thiomargarita was the most abundant genus (0.07%). SparCC co-abundance network analysis of \u003cem\u003eCa\u003c/em\u003e. Electrothrix, \u003cem\u003eCa\u003c/em\u003e. Thiomargarita, and \u003cem\u003eDesulfobulbus\u003c/em\u003e (Fig. 5A) revealed distinct ecological niches, such as \u003cem\u003eCa\u003c/em\u003e. Electrothrix shared fewer associations with the other two taxa, which were more strongly interconnected. Notably, four of the seven significant (p \u0026lt; 0.05) co-abundance correlations involving \u003cem\u003eCa\u003c/em\u003e. Electrothrix were with S-oxidizing bacteria, including \u003cem\u003eSulfurovum\u003c/em\u003e, \u003cem\u003eSulfurimonas\u003c/em\u003e, \u003cem\u003eThiohalophilus\u003c/em\u003e, and an unclassified member of Beggiatoaceae, with R\u0026sup2; values ranging from 0.56 to 0.63, suggesting potential functional interactions deep in sediments\u0026nbsp;(Fig. 5A).\u003c/p\u003e\n\u003cp\u003eMost Beggiatoaceae genera identified in Kolj\u0026ouml; Fjord (Fig. 5B) and the Offshore station (Fig. 5C) were associated with surface and subsurface layers (1\u0026ndash;3 cm), as was also observed in Kristineberg Bay (Beggiatoaceae abundance of 0.09%) and the Nearshore station (Beggiatoaceae abundance of 0.3%) (Fig. S7). In contrast, \u003cem\u003eCa\u003c/em\u003e. Electrothrix was correlated with deeper sediments at depths of 20\u0026ndash;22 cm in Kolj\u0026ouml; Fjord (R\u0026sup2; = 0.5, p \u0026lt; 0.05; Fig. 5B). Two low-abundance Beggiatoaceae genera (\u003cem\u003eCa\u003c/em\u003e. Parabeggiatoa and \u003cem\u003eCa\u003c/em\u003e. Marithioploca) also appeared at this depth, but their abundance was negligible\u0026nbsp;(0.03% and 0.007%, respectively). Similarly, undescribed bacteria of the Beggiatoaceae family appear to be correlated with depths of 20\u0026ndash;22 cm in Kolj\u0026ouml; Fjord.\u003c/p\u003e\n\u003cp\u003eA key question is what environmental conditions in Kolj\u0026ouml; Fjord enable cable bacteria to colonize deep sediment layers.\u003c/p\u003e\n\u003ch2\u003eGene expression associated with NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction, H\u003csub\u003e2\u003c/sub\u003eS oxidation, and Ni\u003c/h2\u003e\n\u003cp\u003eThe expression of the marker gene for periplasmic NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction \u003cem\u003enapA\u003c/em\u003e was highest (~500 CPM) at the surface (0\u0026ndash;1 cm) and below 20 cm depth in Kolj\u0026ouml; Fjord (Fig. 6A). Conversely, no strong signal of gene expression was detected at other stations at depths less than 20 cm (Fig. 6B\u0026ndash;D). This elevated gene expression at 20\u0026ndash;22 cm in Kolj\u0026ouml; Fjord (gene count matrix in Supplementary Data 4), observed only in core #2, is consistent with the 16S rRNA gene abundance pattern of cable bacteria (Fig. 3A and B).\u003c/p\u003e\n\u003cp\u003eHigh expression of sulfide:quinone oxidoreductase (\u003cem\u003esqr\u003c/em\u003e), involved in \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS oxidation, was observed at Kolj\u0026ouml; Fjord at 20 cm depth (Fig. 6F). The highest expression of the \u003cem\u003esqr\u003c/em\u003e gene was detected at 0\u0026ndash;1 cm (~2000 CPM) and 20\u0026ndash;22 cm depth (~1000 CPM) in core #2. Similarly, the Offshore station showed an increasing \u003cem\u003esqr\u003c/em\u003e gene expression at 20 cm depth (Fig. 6H). In contrast, Kristineberg Bay and the Nearshore station showed a decreasing \u003cem\u003esqr\u003c/em\u003e gene expression toward deeper sediment layers (Fig. 6G and I).\u003c/p\u003e\n\u003cp\u003eThe expression of genes involved in Ni metabolism, such as the ABC.PS.S transporter, coupled with the high Ni peak below 20 cm in Kolj\u0026ouml; Fjord and tended to increase with depth (Fig. S8).\u0026nbsp;However, similar patterns were observed at the other stations.\u003c/p\u003e\n\u003ch2\u003ePorewater profiles\u003c/h2\u003e\n\u003cp\u003eAt Kolj\u0026ouml; Fjord, a porewater NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e peak of ~23 \u0026micro;M was found between 20\u0026ndash;30 cm depth, similar to the concentration at the surface (~22.5 \u0026micro;M). The zonation of this peak was considerably lower than the typical depth of oxic nitrification (Fig. 6E). Kristineberg Bay had higher background NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations than Kolj\u0026ouml; Fjord, with a slight increase in deeper layers. In Kolj\u0026ouml; Fjord, porewater \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS concentrations (Fig. 6J) increased with depth, reaching a maximum of 1036.8 \u0026micro;M at 27\u0026ndash;28 cm, with an average concentration of 475.2 \u0026micro;M.\u003c/p\u003e\n\u003cp\u003eSimilar concentrations were found at the Offshore station, with a maximum of 963.5 \u0026micro;M (4\u0026ndash;5 cm) and an average of 487.7 \u0026micro;M, albeit having different patterns (Fig. 6K).\u0026nbsp;In contrast, Kristineberg Bay presented a lower concentration (average of 68.1 \u0026micro;M and a maximum of 499.5 \u0026micro;M), similar to the Nearshore station (average of 171.3 and a maximum of 402.9 \u0026micro;M).\u003c/p\u003e\n\u003cp\u003ePorewater concentrations of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u0026nbsp;\u003c/sup\u003ewere much higher in Kolj\u0026ouml; Fjord (average of ~18.2 mM and maximum of ~19.9 mM) and in Kristineberg Bay (average of ~27.5 mM and maximum of ~30.6 mM) (Fig. 7A) than in the Tv\u0026auml;rminne stations (Fig. 7D), where the concentration decreased in deeper layers (average of ~2.3\u0026ndash;3.3 mM and maximum of ~5.4\u0026ndash;6.6 mM).\u003c/p\u003e\n\u003cp\u003eStrikingly, the depth at which the highest abundance of cable bacteria in Kolj\u0026ouml; Fjord was detected (20\u0026ndash;22 cm) exhibited an Ni peak that reached 0.17 \u0026mu;M (Fig. 7B). This concentration was higher than the average background level of ~0.03 \u0026mu;M at all other depths. Kristineberg Bay (Fig. 7B) and the Tvarminne Archipelago stations (Fig. 7E) have similar concentrations with straight uniform profiles. The NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e concentration profiles were measured only in Kolj\u0026ouml; Fjord and Kristineberg Bay. These profiles were similar, increasing toward deeper sediment layers, with an average of 206.7\u0026ndash;206.0 and maximums of 476.2\u0026ndash;505.6 \u0026mu;M (Fig. 7C). The porewater data are available in Supplementary Data 5.\u003c/p\u003e"},{"header":"Discussion","content":"\u003ch2\u003e\u003cstrong\u003eEvidence for active cable bacteria deep in anoxic sediment layers\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eUnexpectedly, a high abundance of cable bacteria was detected in the anoxic Kolj\u0026ouml; Fjord (Paul et al., 2023). Even more strikingly, the \u003cem\u003eCa.\u003c/em\u003e Electrothrix abundance was relatively high at 7\u0026ndash;9 cm and increased toward deeper sediment layers, reaching its maximum between 20\u0026ndash;22 cm sediment depth, although it remained quite high between 30\u0026ndash;32 cm as well.\u003c/p\u003e\n\u003cp\u003eThe discovery of \u003cem\u003eCa.\u003c/em\u003e Electrothrix deep in anoxic sediment layers in Kolj\u0026ouml; Fjord challenges the consensus that cable bacteria are restricted to the oxic\u0026ndash;anoxic interface of surface sediments (Dong et al., 2024). Cable bacteria have not been extensively studied in anoxic environments. However, recently, they have been found in O\u003csub\u003e2\u003c/sub\u003e-deficient\u0026nbsp;systems\u0026nbsp;(Fonseca et al., 2022; Wu et al., 2024; Slomp et al., 2025), where cable bacteria might survive when\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e is used as the main electron acceptor. Notably, the \u003cem\u003eCa.\u003c/em\u003e Electrothrix abundance in this study increased toward deeper sediment layers, contrary to the typical decrease with sediment depth. The abundances found here align with previously reported abundance ranges found in other natural environments, such as the seasonally anoxic Chesapeake Bay, ranging from 0.04 to 3.6% (Malkin et al., 2022).\u003c/p\u003e\n\u003cp\u003eWe studied the presence of marine cable bacteria using filtered rRNA derived from metatranscriptomic sequences. The presence of \u003cem\u003eCa.\u003c/em\u003e Electrothrix was further confirmed by larger 16S rRNA gene sequences obtained by assembling, annotating, and quantifying the total SSU rRNA from metatranscriptomic samples. A strong advantage of 16S rRNA analysis using metatranscriptomics is that it indicates ribosomal activity, thereby supporting its presence but also ensures that the bacteria are metabolically active (Singer et al., 2017), although nothing can be said regarding their biomass.\u003c/p\u003e\n\u003cp\u003eThe patchy distribution of cable bacteria, with high abundance in core #2 but not in core #3, points toward microspatial heterogeneity in terms of environmental conditions or colonization dynamics. This distribution has also been observed in other benthic microbial communities and may reflect sediment features such as porewater chemistry, particle size, or bioturbation.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003ePhylogeny reveals a community with several phylotypes of cable bacteria\u003c/h2\u003e\n\u003cp\u003eThe phylogenetic diversity of cable bacteria from Kolj\u0026ouml; Fjord spans several phylotypes of \u003cem\u003eCa\u003c/em\u003e. Electrothrix into different clusters (IV, V, and VI) according to recent phylogenetic analysis (Ley et al., 2024). Most of those cable bacteria fell within Cluster VI. This cluster encompasses previously described genera of \u003cem\u003eCa.\u003c/em\u003e Electrothrix, including gigas (Geelhoed et al., 2023), communis, aarhusiensis, marina, japonica (Trojan et al., 2016), laxa (Sereika et al., 2023), rattekaaiensis (Plum-Jensen et al., 2024), and antwerpensis (Hiralal et al., 2024), among others. The species \u003cem\u003eCa\u003c/em\u003e. Electrothrix communis, aarhusiensis and marina are derived from sulfidic sediments located near the Baltic Sea shore (Trojan et al., 2016). Notably, \u003cem\u003eCa\u003c/em\u003e. Electrothrix communis and aarhusiensis are present in both brackish and marine sediments, including salt marshes.\u003c/p\u003e\n\u003cp\u003eThe phylogenetic analysis of \u003cem\u003eCa\u003c/em\u003e. Electrothrix from Kolj\u0026ouml; Fjord indicates that it constitutes a community, likely representing novel species with diverse evolutionary\u0026nbsp;phylotypes\u0026nbsp;within cable bacteria.\u0026nbsp;This suggests that cable bacteria have broader diversity, occupying more ecological niches than previously recognized, possibly involving NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction or other cryptic electron transfer processes in deeper, sulfidic, and anoxic sediments.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003eInteraction between cable bacteria and S-oxidizing competitors\u003c/h2\u003e\n\u003cp\u003eCo-abundance analysis revealed that cable bacteria in Kolj\u0026ouml; Fjord had few links with the Beggiatoaceae family, which had more links with the \u003cem\u003eDesulfubulbus\u003c/em\u003e genus, indicating that cable bacteria occupy a distinct niche. Beggiatoaceae family members, which include large colorless S-oxidizing bacteria, have been identified as competitors of cable bacteria. On the other hand, observations suggest that certain S-oxidizers may form mutually beneficial relationships with cable bacteria in anoxic environments rather than competing with them (Liau et al., 2022)\u003c/p\u003e\n\u003cp\u003eSome factors, such as bioturbation, can mediate their interplay and performance in shared niches. In Kolj\u0026ouml; Fjord, infauna and bioturbation are absent. Previous studies have reported that heavy bioturbation inhibits cable bacteria proliferation, thereby allowing other bacteria, including Beggiatoaceae, to outcompete them\u0026nbsp;(Hermans et al., 2019; Malkin et al., 2022). However, other factors, such as a more efficient enzymatic system for\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e or sulfite uptake and utilization, may also contribute to cable bacteria proliferation, particularly in the absence of bioturbation, such as in Kolj\u0026ouml; Fjord. In contrast, we found that Beggiatoaceae dominated the upper layers, outcompeting cable bacteria in those zones, although a few members correlated with deeper sediments as well. In this sense, some large S-oxidizers from Beggiatoaceae, such as \u003cem\u003eBeggiatoa\u003c/em\u003e and \u003cem\u003eCa\u003c/em\u003e. Marithioploca, are known to store\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in large vacuoles, allowing them to migrate downward into deep sulfidic sediments (J\u0026oslash;rgensen and Gallardo, 1999; Salman et al., 2011), explaining their positive correlation with the surface and deeper layers.\u003c/p\u003e\n\u003cp\u003eThe unusual occurrence of \u003cem\u003eCa\u003c/em\u003e. Electrothrix in deep anoxic layers, alongside correlations with other S-oxidizers, indicates a potentially more widespread role for H\u003csub\u003e2\u003c/sub\u003eS oxidation in anoxic sediments.\u003c/p\u003e\n\u003ch2\u003eLocally produced NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e may sustain cable bacteria deep in anoxic sediments\u003c/h2\u003e\n\u003cp\u003eThe high NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e peak found below 20 cm in Kolj\u0026ouml; Fjord is unusual, and similar findings under these conditions are scarce. However, a comparable NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e peak in deep anoxic sediment layers of Loch Duich, an organic-rich marine fjord, has been reported (Mortimer et al., 2004). This NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e peak may supply electron acceptors for supporting cable bacteria respiration in deep sediment layers. The NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e peak in Kolj\u0026ouml; Fjord is linked to the expression of the \u003cem\u003enapA\u003c/em\u003e gene, which encodes periplasmic NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction (Fig. 6A). This elevated gene expression at 20\u0026ndash;22 cm in Kolj\u0026ouml; Fjord (gene count matrix in Supplementary Data 4) is consistent with the 16S rRNA gene abundance pattern found in core #2 (Fig. 3A). This further supports the notion of a localized, patchy distribution of cable bacteria. Periplasmic NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reductase \u003cem\u003enapAB\u003c/em\u003e plays a crucial role in NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction in cable bacteria (Kjeldsen et al., 2019), coupled with \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS oxidation\u0026nbsp;(Risgaard-Petersen et al., 2014). Hence, the availability of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in deeper sediment layers may facilitate respiration of cable bacteria in the absence of O\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eTo gain a comprehensive understanding of what drives the high porewater concentrations of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e found in the deeper sediment layers at Kolj\u0026ouml; Fjord. All potential sources were identified. Input of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e into deeper sediment layers via groundwater would have been associated with a steep decrease in the salinity gradient with depth due to the inflow of freshwater (Capone and Bautista, 1985). However, the stable brackish porewater conditions, as evident by the depth profiles of Na, K, Mg, and Cl, indicate that there is no source of groundwater discharging into the deeper sediment layer (Fig. S9). This implies that the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e peak found deep into the sediment is locally produced through anaerobic oxidation of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, which is a microbially mediated process.\u003c/p\u003e\n\u003cp\u003eA local process that can be eliminated as a potential source is the anaerobic oxidation of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (Fig. 7C) by Fe oxides (Eq. 1). Typically, Fe oxides undergo rapid reductive dissolution upon contact with porewater \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS (Burdige, 1993). Given the sulfidic porewater conditions at K\u0026ouml;lj\u0026ouml; Fjord, it is highly unlikely that Fe oxides (Fig. S10) would reach the deeper sediment layers.\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 567px;\"\u003e\n \u003cp\u003e\u003cimg width=\"359\" height=\"38\" 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\" alt=\"image\"\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 34px;\"\u003e\n \u003cp\u003e(1)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eAnother local process that is likely insignificant is the anaerobic oxidation of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e by Mn oxides (Eq. 2). Unlike Fe oxides, Mn oxides do not undergo rapid reductive dissolution upon contact with \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS (Burdige, 1993). However, the low abundance of Mn oxides in the sediment from a nearby station in K\u0026ouml;lj\u0026ouml; Fjord and the relatively low porewater concentrations of Mn\u003csup\u003e2+\u003c/sup\u003e (Fig. S10 and Supplementary Data 5) from potential dissolution indicate that this is likely not a significant contributor.\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 567px;\"\u003e\n \u003cp\u003e\u003cimg width=\"325\" height=\"38\" src=\"data:image/png;base64,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\" alt=\"image\"\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 34px;\"\u003e\n \u003cp\u003e(2)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;No highly expressed genes associated with Mn or Fe oxidation or with hydrazine dehydrogenase (HDH), which catalyzes the final step of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e oxidation in anammox bacteria using nitrite or nitric oxide (Liao et al., 2014), were identified.\u003c/p\u003e\n\u003cp\u003eA potential explanation for the production of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in the deeper sediment is via SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e reducing NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e oxidation (sulfammox), which is a microbially mediated process in which NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e oxidation is coupled to SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e reduction under anoxic conditions (see SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e in Fig. 7A). Although sulfammox was initially discovered in wastewater treatment (Fdz-Polanco, 2001), it has also been reported in marine sediments (Schrum et al., 2009; Rios-Del Toro et al., 2018). However, to date, reports of its occurrence in natural environments are scarce. In sulfammox, the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e molar ratio is regarded as a critical parameter, as it not only influences the rates of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e oxidation and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e reduction but also controls the end product. When the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ratio is \u0026ge;4, NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e is converted into NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. However, when the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ratio is \u0026le;2, NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e is overoxidized to NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to facilitate the reduction of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e (Zhang et al., 2019; Wu et al., 2023). Besides controlling the end product, the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ratio also regulates whether sulfammox or anammox is the preferred metabolic pathway. A reactor experiment demonstrated that sulfammox dominated when the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ratio was less than 1.5. Conversely, anammox became the primary pathway when the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ratio exceeded 1.5 (Yang et al., 2021). Considering that at Kolj\u0026ouml; Fjord, the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ratios in the porewater in the deeper sediment were extremely low (\u0026lt;0.03), this could point toward sulfammox being the primary pathway with NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e as end product (Eq. 3).\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 567px;\"\u003e\n \u003cp\u003e\u003cimg width=\"391\" height=\"38\" 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\" alt=\"image\"\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 34px;\"\u003e\n \u003cp\u003e(3)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eAlthough sulfammox was recently discovered and the exact metabolic pathways are unknown (Wu et al., 2023), various bacteria associated with this process have been identified. These include \u003cem\u003eBacillus benzoevorans\u003c/em\u003e (Cai et al., 2010), \u003cem\u003eBacillus\u003c/em\u003e, \u003cem\u003eCandidatus\u003c/em\u003e Anammoxoglobus (Wu et al., 2023), and \u003cem\u003eBacillus cereus\u003c/em\u003e (Mohammed Madani et al., 2022). The \u003cem\u003eBacillus\u003c/em\u003e genus was highly abundant in Kolj\u0026ouml; Fjord, accounting for 0.9% of the total abundance, while the maximum was at 20\u0026ndash;22 cm (1.5\u0026ndash;2.5%) (Supplementary Data 2). Using 16S rRNA gene sequences of \u003cem\u003eBacillus benzoevorans\u0026nbsp;\u003c/em\u003eand \u003cem\u003eBacillus cereus\u0026nbsp;\u003c/em\u003eas references showed that they were present in similar abundances in all sediment layers of Kristineberg Bay and Kolj\u0026ouml; Fjord. However, the maximum abundance of \u003cem\u003eBacillus benzoevorans\u003c/em\u003e was found at 20\u0026ndash;22 cm depth in Kolj\u0026ouml; Fjord (552,509 TPM) (Supplementary Data 2). Conversely, \u003cem\u003eCa\u003c/em\u003e. Scalindua sp., which is characterized as an anammox bacterium and has been associated with the reduction of Fe(III) to Fe(II) (Zhao et al. 2014), was abundant in Kristineberg Bay (Supplementary Data 2). In addition, bacteria such as \u003cem\u003eSulfurimonas\u003c/em\u003e may be related to sulfammox (Cai et al., 2010), which were abundant in Kolj\u0026ouml; Fjord, with a maximum at 20\u0026ndash;22 cm depth (2.1%) (Supplementary Data 2). Interestingly, this bacterium presented a strong positive SparCC co-abundance correlation with \u003cem\u003eCa\u003c/em\u003e. Electrothrix (Fig. 5A).\u0026nbsp;Given that the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ratio was favorable for sulfammox and that various bacteria associated with this process were identified, it is reasonable to suggest that the increased concentration of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e between 20\u0026ndash;30 cm in Kolj\u0026ouml; Fjord may have been generated by sulfammox.\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003eS-oxidation in deeper sediment layers and Ni utilization\u003c/h2\u003e\n\u003cp\u003eThe\u0026nbsp;high expression of sulfide:quinone oxidoreductase (\u003cem\u003esrq\u003c/em\u003e) observed at Kolj\u0026ouml; Fjord (Fig. 6F)\u0026nbsp;matches the highest cable bacteria abundance and the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003epeak in Kolj\u0026ouml; Fjord. SQR facilitates the oxidation of H\u003csub\u003e2\u003c/sub\u003eS in the periplasm to produce polysulfides. Although the mechanisms of energy conservation in cable bacteria are not fully understood, \u003cem\u003esqr\u003c/em\u003e has been identified in several cable bacteria genomes and may play a role in S-oxidation (Kjeldsen et al., 2019; Hiralal et al., 2024). Other genes associated with S-oxidation, such as \u003cem\u003eaprAB\u003c/em\u003e,\u003cem\u003e\u0026nbsp;soeA\u003c/em\u003e, and \u003cem\u003esoxY\u003c/em\u003e, as well as polysulfide reductase (\u003cem\u003epsrA\u003c/em\u003e), S-reduction genes such as \u003cem\u003esat\u003c/em\u003e and \u003cem\u003edsrB\u003c/em\u003e, and electron transfer genes such as \u003cem\u003ecytB\u003c/em\u003e and \u003cem\u003ecydA\u003c/em\u003e, exhibited high expression at 20\u0026ndash;22 cm depth in core #2 in Kolj\u0026ouml; Fjord (Fig. S8, 11-12), suggesting constant H\u003csub\u003e2\u003c/sub\u003eS oxidation.\u003c/p\u003e\n\u003cp\u003eIn Kolj\u0026ouml; Fjord, the high Ni peak below 20 cm coincided with genes involved in Ni metabolism, such as the ABC.PS.S transporter, whose expression increased with depth (Fig. S8). Cable bacteria utilize Ni by incorporating it into their conductive wires, a mechanism exclusive to them\u0026nbsp;(Hiralal et al., 2024). They can also thrive in sulfidic sediments harboring relatively high Ni porewater concentrations of up to ~1 \u0026mu;M\u0026nbsp;(Van De Velde et al., 2017). While a similar gene expression pattern was observed in the other stations, the high Ni peak and high gene expression suggest possible Ni utilization by cable bacteria in Kolj\u0026ouml; Fjord.\u003c/p\u003e\n\u003cp\u003eOur results revealed that Kolj\u0026ouml; Fjord is a characteristic environment in which a combination of electron acceptors (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) and donors (\u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS) is available in deep sediment layers, along with the expression of essential genes such as periplasmic nitrate reduction (\u003cem\u003enapAB\u003c/em\u003e), sulfide oxidation (\u003cem\u003esqr\u003c/em\u003e), Ni utilization, and other metabolic markers, suggesting that metabolically active cable bacteria can persist in anoxic sediments.\u003c/p\u003e\n\u003ch2\u003eProposed novel niche for cable bacteria deep in anoxic sediment layers\u003c/h2\u003e\n\u003cp\u003eThe proposed model for cable bacteria communities deep in anoxic sediments (Fig. 8) assumes the involvement of sulfammox\u0026nbsp;(Schrum et al., 2009; Rios-Del Toro et al., 2018), which serves as the primary source of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (Fig. 6E). This model proposes a localized, transient event that generates suitable microenvironments for cable bacteria, which provide electron acceptors to patchily distributed and potentially dormant communities.\u003c/p\u003e\n\u003cp\u003eThe traditional conceptual framework of cable bacteria bioenergetics is based on the spatial arrangement of cells across vertical redox zones, with division of labor among the cells\u0026nbsp;(Kjeldsen et al., 2019). This traditional model links cathodic O\u003csub\u003e2\u003c/sub\u003e reduction at the sediment surface to anodic \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS oxidation in deeper anoxic layers via electrical currents. However, this study revealed that \u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS and\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e coexist with cable bacteria within the same deep sediment layers of Kolj\u0026ouml; Fjord. In such a scenario, where the electron acceptor and donor are located within the same layer, a different physiological mode is suggested, potentially eliminating the need for long-distance electron transport mechanisms such as conductive nanowires or metabolic differentiation along the filaments.\u003c/p\u003e\n\u003cp\u003eHeavy bioturbation can negatively impact the abundance of cable bacteria (Hermans et al., 2019; Malkin et al., 2022). Kolj\u0026ouml; Fjord is not bioturbated, as there are no macrofauna present due to persistent anoxia\u0026nbsp;(Nordberg et al., 2001). This undisturbed sediment provides optimal conditions for maintaining the physical integrity of cable bacteria for extended periods. Research conducted at Kolj\u0026ouml; Fjord has shown extended survival of dinoflagellate cysts under anoxic conditions, with the ability to germinate after remaining in anoxic sediments for up to a century\u0026nbsp;(Lundholm et al., 2011). A plausible hypothesis is that cable bacteria may also remain dormant for several years or even decades until specific events, such as anoxic nitrification via sulfammox, reactivate them. This hypothesis is further supported by a laboratory experiment that\u0026nbsp;revealed that sediment containing a low abundance (14 m cm\u003csup\u003e-2\u003c/sup\u003e) of cable bacteria, which has been stored in anoxic conditions for several months, can rapidly develop a thriving community (724 m cm\u003csup\u003e-2\u003c/sup\u003e) when provided with an electron acceptor (Hermans et al., 2020). If cable bacteria deep in anoxic sediments are continuously supplied with NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e through sulfammox, which has a distinct physiology, as suggested above, this community may be well-suited to these conditions and potentially comprise ancient, primordial forms of cable bacteria that predate O\u003csub\u003e2\u003c/sub\u003e-dependent lineages.\u003c/p\u003e\n\u003cp\u003eAn alternative explanation is that cable bacteria might gradually migrate downward toward layers with higher NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations, as evidenced by their maximum abundance just above the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e peak (20\u0026ndash;22 cm). In this scenario, the cable bacteria community does not reach a steady state but instead temporarily thrives in transient NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e pockets that are eventually depleted.\u003c/p\u003e\n\u003cp\u003eTo further advance understanding of anoxic niches of cable bacteria, future research should focus on similar environments, such as Kolj\u0026ouml; Fjord, to characterize these niches. Sediment incubations could provide insights into the potential syntrophic relationships between S-oxidizers and sulfammox microorganisms, shedding light on the underlying mechanisms involved.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eOur findings significantly advance the fields of microbial ecology and biogeochemistry, challenging the prevailing paradigm that cable bacteria are restricted to surface sediments. This has important implications for their potential role in removing ΣH\u003csub\u003e2\u003c/sub\u003eS from deeper sediment layers. Moreover, cable bacteria can form a deep, patchy community within anoxic sediments, occupying a previously overlooked ecological niche. This raises further questions about syntrophic relationships with organisms capable of anoxic nitrification via sulfammox, potentially reflecting analogs of primordial bacterial communities in early Earth marine ecosystems.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003e\u003cem\u003eCa.\u003c/em\u003e \u003cem\u003eCandidatus\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eCa\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Calcium\u003c/p\u003e\n\u003cp\u003eCl\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Chloride\u003c/p\u003e\n\u003cp\u003eFe\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Iron\u003c/p\u003e\n\u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eS\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Sulfide\u003c/p\u003e\n\u003cp\u003eHS\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Hydrosulfide\u003c/p\u003e\n\u003cp\u003eIC\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Ion Chromatography\u003c/p\u003e\n\u003cp\u003eICP-MS\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Inductively Coupled Plasma-Mass Spectrometry\u003c/p\u003e\n\u003cp\u003eK\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Potassium\u003c/p\u003e\n\u003cp\u003eMn\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Manganese\u003c/p\u003e\n\u003cp\u003eNa\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Sodium\u003c/p\u003e\n\u003cp\u003e\u003cem\u003enapA\u003c/em\u003e Periplasmic nitrate reductase\u003c/p\u003e\n\u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Ammonium\u003c/p\u003e\n\u003cp\u003eNi\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Nickel\u003c/p\u003e\n\u003cp\u003eNO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Nitrite\u003c/p\u003e\n\u003cp\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Nitrate\u003c/p\u003e\n\u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Oxygen\u003c/p\u003e\n\u003cp\u003eS-oxidizers\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Sulfide-oxidizers\u003c/p\u003e\n\u003cp\u003eS\u003csup\u003e2\u0026minus;\u003c/sup\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Sulfide ion\u003c/p\u003e\n\u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Sulfate\u003c/p\u003e\n\u003cp\u003e\u003cem\u003esqr\u003c/em\u003e Sulfide:quinone reductase\u003c/p\u003e\n\u003cp\u003e\u0026Sigma;H\u003csub\u003e2\u003c/sub\u003eS \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Total sulfide\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eData availability\u003c/h2\u003e\n\u003cp\u003eAll raw sequence data are deposited and available online at the NCBI repository under accession number PRJNA1253322 (https://www.ncbi.nlm.nih.gov/bioproject/?term=PRJNA1253322). Additional data, figures and tables from the analyses presented in this paper are available in the Supplementary Material, and the main scripts and procedures for data analysis performed in the study are publicly available in the GitHub repository: https://github.com/Alexis-Fonseca/CB_Koljo_Fjord.git.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe are grateful to the captain, technicians, M\u0026auml;rta Brunberg and Stefano Bonaglia for their assistance aboard \u003cem\u003eR/V Alice\u003c/em\u003e. We would like to express our gratitude to Hanna Reijola (IC), Juhani K. Virkanen (ICP-MS) and Tomas Thillman (SFA) for their analytical support.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSwedish Research Council (VR) grant no. 2021-04641 (CH). Marcus and Amalia Wallenberg Foundation. Stockholm University internal project number: 31004348 (FJN). The computations were enabled by resources in projects NAISS 2024/22-947 provided by the National Academic Infrastructure for Supercomputing in Sweden at the Uppsala Multidisciplinary Center for Advanced Computational Science (UPPMAX).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors and affiliations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eBaltic Sea Centre, Stockholm University, Stockholm, Sweden\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAlexis Fonseca, Martijn Hermans,\u0026nbsp;Francisco J.A. Nascimento, Christian Stranne, Bo G. Gustafsson \u0026amp; Christoph Humborg\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDepartment of Ecology, Environment, and Plant Sciences (DEEP), Stockholm University, 106 91, Stockholm, Sweden\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAlexis Fonseca \u0026amp;\u0026nbsp;Francisco J.A. Nascimento\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDepartment of Geological Sciences, Stockholm University, Stockholm, Sweden\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eChristian Stranne\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eBolin Centre for Climate Research, Stockholm University, Stockholm, Sweden\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eChristian Stranne\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTv\u0026auml;rminne Zoological Station, University of Helsinki, Hanko, Finland\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAlf Norkko\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eBaltic Nest Institute, Stockholm University, Stockholm, Sweden\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBo G. Gustafsson\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAF, MH, CH. Conceptualized the study; AF. \u0026amp; MH. planned the methodology; AF \u0026amp; MH. conducted the investigation; AF. performed the data analysis, bioinformatics studies and graphics production; MH. carried out the pore water analysis interpretation; CH. \u0026amp; FJAN. supervision; AF \u0026amp; MH. wrote the manuscript; and AF, MH, CH, FJAN, CS., BGG \u0026amp; AN revised and edited the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding author\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCorrespondence to [email protected] and [email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAlgeo, T.J., Li, C., 2020.\u0026nbsp;Redox classification and calibration of redox thresholds in sedimentary systems. 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Water Science \u0026amp; Technology 80, 634\u0026ndash;643. https://doi.org/10.2166/wst.2019.277\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Cable bacteria, Candidatus Electrothrix, sulfur bacteria, sulfur oxidation, nitrate reduction, sulfammox, novel niche, anoxic sediments","lastPublishedDoi":"10.21203/rs.3.rs-6914568/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6914568/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCable bacteria are filamentous sulfide oxidizers capable of electron transport over cm-scale distances. Traditionally, they are thought to inhabit only the upper few cm of sediment, where they couple sulfide oxidation to oxygen or nitrate reduction. Despite their influence on redox gradients, trace metal mobility, and nutrient cycling, their presence and activity in deep anoxic sediments remain undocumented. We investigated the presence and activity of marine cable bacteria (\u003cem\u003eCandidatus\u003c/em\u003eElectrothrix) at four stations in Sweden and Finland, including deep vertical profiles of anoxic sediments.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eUsing metatranscriptomic data for both rRNA-based community profiling and gene expression analysis, along with porewater geochemistry data from four stations in Sweden and Finland, we detected metabolically active \u003cem\u003eCa\u003c/em\u003e. Electrothrix in both regions. In Koljö Fjord (Sweden West Coast), \u003cem\u003eCa\u003c/em\u003e. Electrothrix was unexpectedly abundant deep in anoxic layers, with peak abundance below 20 cm. Phylogenetic analyses revealed a diverse assemblage spanning multiple \u003cem\u003eCa\u003c/em\u003e. Electrothrix clades, suggesting that novel lineages adapted to these conditions. Genes for nitrate respiration (\u003cem\u003enapA\u003c/em\u003e), sulfide oxidation (\u003cem\u003esqr\u003c/em\u003e), and nickel uptake were highly expressed, indicating \u003cem\u003ein-situ\u003c/em\u003e activity. Gene expression patterns were aligned with a sulfide-rich zone and a sharp nitrate peak below 20 cm. This nitrate peak likely results from sulfammox (i.e., anaerobic oxidation of ammonium by sulfate), driven by associated sulfammox bacteria such as \u003cem\u003eBacillus benzoevorans\u003c/em\u003e, \u003cem\u003eCa\u003c/em\u003e. Anammoxoglobus, and \u003cem\u003eBacillus cereus\u003c/em\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOur findings reveal a previously unrecognized niche for cable bacteria deep in anoxic sediment layers, where local nitrate production via sulfammox and sulfide availability may sustain their activity, independent of electron acceptors near the surface. This discovery challenges existing models of cable bacteria ecology and suggests alternative physiological modes. Furthermore, the results expand the ecological scope of marine cable bacteria, highlighting potential syntrophic relationships deep in anoxic sediment layers, offering insights into both modern biogeochemical processes and analogs of early Earth microbial ecosystems.\u003c/p\u003e","manuscriptTitle":"Evidence for cable bacteria inhabiting deep in anoxic sediment reveals a novel ecological niche","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-02 05:51:43","doi":"10.21203/rs.3.rs-6914568/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6c008e27-dbcd-44af-9218-ca4ad832d969","owner":[],"postedDate":"July 2nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-04-16T13:32:01+00:00","versionOfRecord":{"articleIdentity":"rs-6914568","link":"https://doi.org/10.1186/s40793-026-00895-7","journal":{"identity":"environmental-microbiome","isVorOnly":false,"title":"Environmental Microbiome"},"publishedOn":"2026-04-15 00:00:00","publishedOnDateReadable":"April 15th, 2026"},"versionCreatedAt":"2025-07-02 05:51:43","video":"","vorDoi":"10.1186/s40793-026-00895-7","vorDoiUrl":"https://doi.org/10.1186/s40793-026-00895-7","workflowStages":[]},"version":"v1","identity":"rs-6914568","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6914568","identity":"rs-6914568","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}