Evolution of the molybdenum and vanadium cycles through time and their impact on ancient nitrogen fixation

Evolution of the molybdenum and vanadium cycles through time and their impact on ancient nitrogen fixation | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Evolution of the molybdenum and vanadium cycles through time and their impact on ancient nitrogen fixation Kurt Konhauser, Weiduo Hao, Leslie Robbins, Betül Kaçar, Holly Rucker, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7237238/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Biological nitrogen (N 2 ) fixation is crucial for incorporating atmospheric N 2 into the biosphere. However, the evolutionary trajectory of this process remains enigmatic, particularly regarding the paradoxical relationship between the presumed scarcity of molybdenum (Mo) in the Archean ocean—before oxidative weathering on land—and the apparent early emergence of Mo-based nitrogenase over the vanadium (V)-based alternative. Here, we integrate partitioning experiments with an extensive analysis of iron (Fe)-rich sediments through time to elucidate the behavior of Mo and V in ancient seawater. Our findings suggest that in the Archean to Paleoproterozoic, dissolved V was more efficiently removed from the water column than Mo, primarily due to the preferential incorporation of V into banded iron formations (BIF). Our data also suggest that dissolved Mo was more available in seawater than commonly assumed and that the ratio Mo/V was higher in the Archean than it is today. Higher Mo abundances and greater availability of Mo relative to V offers a compelling explanation for the earlier evolution of Mo-based nitrogenase over the alternative nitrogenases. Earth and environmental sciences/Biogeochemistry/Element cycles Earth and environmental sciences/Ocean sciences/Marine chemistry Figures Figure 1 Figure 2 Figure 3 Introduction Nitrogen is a vital macronutrient essential to all living organisms. During the early Archean, production of bioavailable N species, such as ammonium (NH 4 + ) and nitrate (NO 3 - ), was likely dominated by abiotic processes including lightning discharges 1,2 , submarine hydrothermal fluid emissions 3 , and photochemical reactions 4 . However, these sources were limited in supply, potentially constraining the expansion of the early biosphere 5 . The evolution of biological N 2 -fixation, a metabolic process whereby enzymes convert atmospheric N 2 into bioavailable N (i.e., NH 3 ), helped alleviate N scarcity, paving the way for the global expansion of life 6 . Geological evidence indicates that biological N 2 -fixation dates back at least 3.2-3.1 billion years 7,8 , although the exact timing of its origin remains unknown 9 . It is possible that this process began even earlier, potentially linked to thermophilic methanogenic archaea 10 , which are amongst the oldest life forms 11 . Nitrogenase is the enzyme responsible for biological N 2 -fixation. This process uses eight electrons to reduce N 2 to 2NH 3 , accompanied by the release of one H 2 molecule 12 . The nitrogenase family is a group of isoenzymes that contain a metallocofactor composed of Fe, sulfur (S), and either Mo, V, or another Fe atom. The most efficient variant, Mo-nitrogenase, encoded by nif genes, incorporates Mo at the active site 13 . In contrast, vanadium (V)-nitrogenase (encoded by vnf genes) and Fe-nitrogenase (encoded by anf genes) are less efficient 14 and rarer (only 10% and 4% of nif -containing organisms contain anf and vnf genes, respectively 15-17 . Our interpretation of the evolutionary history of nitrogenase enzymes has been shaped largely by studies examining temporal changes in marine trace metal reservoirs 18 , informed by both thermodynamic approaches (e.g., Ref. 19,20 ) and chemical analyses of sedimentary archives, such as the shale record (e.g., Ref. 21,22 , and references within). Such approaches suggest the history of paleo-marine Mo concentrations (hereafter denoted Mo (d) ) tracks closely to changes in the oxygenation of Earth’s surface environments, and hence points to a dynamic history of paleo-marine Mo concentrations. Broadly, in the Archean, Mo (d) was assumed to be low and variable — and potentially limiting for N 2 fixation — due to a lack of oxidative continental weathering (e.g., Ref. 21,23,24 ). Scott et al. (2008) proposed an increase of Mo (d) between 2.2 and 2.0 Ga, approximately 200 Ma after O 2 accumulated in the atmosphere (the Great Oxidation Event, hereafter the GOE), after which Mo (d) may have declined. Mo (d) then rose as much as 100-fold to near modern levels (105 nM) during the Neoproterozoic and early Paleozoic, coincident with the Neoproterozoic Oxygenation Event (NOE). Based on this reconstruction of Mo (d) over time, it was suggested that early forms of nitrogenase did not rely on Mo due to the low Mo (d) levels in ancient seawater juxtaposed with extremely high Fe availability, or at least they struggled to do so with major ecological impact 18 . Mo (d) scarcity could have favored the prevalence of alternative V- and Fe-nitrogenases in early N 2 -fixation, with Mo-nitrogenase emerging with ecological significance only after an increase in seawater Mo (d) due to the rise of oxygenated environments. A Mo threshold of 5 – 10 nM has often been cited, based on limited experimental data 25 . DNA and RNA sequencing of microbial biomass in low-Mo environments have shown the prevalence of Mo-based nitrogenase in diverse environments even when V >> Mo and Mo ≤ 5 nM 26-28 . Recent reconstructions of Mo (d) using mass balance modeling constrained by low-oxygen (O 2 ) weathering experiments and the black shale record point to late Archean concentrations < 5 nM 24 , although data are sparse for the entirety of the Archean. However, this hypothesis is at odds with emerging biogeochemical and molecular biology evidence that indicates Mo-nitrogenase evolved prior to its V- and Fe-nitrogenase counterparts. Biogeochemists have been unable to find N isotope evidence of alternative nitrogenases in Archean sedimentary rocks that otherwise indicate the occurrence of biological N 2 -fixation 5,7,29-31 . Further, phylogenetic analyses of nitrogenase sequences indicate that V- and Fe-nitrogenases stem from Mo-nitrogenase, suggesting that Mo utilization is ancestral and predates the use of V and Fe in N 2 -fixation 8,32 . Recent ancestral sequence reconstructions have confirmed this hypothesis 33 and inferred the emergence of V- and Fe-nitrogenases following the GOE 34 . Most recently, phylogenetic analysis of Mo utilization in biological systems suggests that Mo- molybdoenzymes originated in the Paleoarchean (~ 3.4 Ga) 35 . Hence, a conundrum remains as to why Mo-nitrogenase would have evolved and proliferated when Mo (d) was seemingly very low. In an attempt to resolve this “Mo paradox”, we investigated Mo concentrations in ancient sediments. Reconstructions of Mo and other metal abundances in ancient oceans rely heavily on analyses of metal contents in the authigenic fraction of carbonaceous shales (e.g., Ref. 21,36 ). Ancient chemical sedimentary rocks are comparatively less studied but provide good candidates to further interrogate the record of marine Mo availability through Earth’s history. Specifically, the geochemical records of Fe-rich sediments, including Precambrian BIF, Proterozoic and Phanerozoic ironstones, and modern hydrothermal Fe-rich deposits, may reveal first-order trends in Mo (d) over geological time. BIFs are chemically precipitated sediments that formed on the continental shelves of ancient cratons when deep upwelling seawater — enriched in hydrothermally derived Fe 2+ — was biologically oxidized to a primary Fe(III)-oxyhydroxide mineral phase, such as ferrihydrite (see Ref. 37 for review). Note, alternate views have suggested that some primary BIF minerals were greenalite 38 but for reasons outlined in the SI, we consider greenalite to be a deep-sea precipitate and ferrihydrite a precipitate formed in the photic zone 39 . Thus, the chemical composition of Fe-rich sediments (greenalite and/or ferrihydrite) provides a record of ocean geochemistry that complements the shallower coastal waters captured by much of the shale record. Iron sedimentation record We compiled published geochemical data for BIF, ironstones, and hydrothermal Fe-rich sediments, supplementing a previously published BIF database 40 , to create a comprehensive geochemical record of Fe-rich sediments through time (see Supplemental Table S1). Samples predating 1.8 Ga are primarily BIF, while a significant gap exists between 1.8 Ga and 0.8 Ga reflecting a well-documented hiatus in BIF deposition. Samples younger than 0.6 Ga are mainly ironstones 37 . To provide a modern perspective, hydrothermal Fe-rich deposits were included in the dataset. Our results show a general trend of increasing maximum concentrations of Mo in Fe-rich sediments (Fig. 1A), from the oldest samples analyzed (the 3.8 Isua Greenstone Belt, Greenland) to modern hydrothermal deposits. The Archean itself shows relatively lower Mo values in BIFs compared to Proterozoic and Phanerozoic counterparts, with the highest values typically associated with Archean Algoma-type BIFs (average at 1.7 ppm) versus Archean Superior-type BIFs (average at 1.3 ppm). The former are stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits, while the latter developed in relatively shallower passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks 37 . There is a subsequent peak observed at 2.43 Ga (Cauê Formation, Brazil), with Mo concentrations reaching 53.7 ppm 41 . After this peak, BIF samples are scarce until the Neoproterozoic, with Mo concentrations typically remaining below 10 ppm between 2.43 Ga and the Neoproterozoic. Ironstones and hydrothermal deposits display a progressive rise in Mo concentrations from 0.7 Ga to the modern, with values that reach 26-52 ppm in core top sediments enriched in Fe-dominated hydrothermal plume fall-out along the western flank of the superfast-spreading East Pacific Rise (EPR), 17-21°S 42 . Vanadium concentrations more clearly follow a gradually increasing trend over time (Fig. 1A), with corresponding core-top concentrations of 700-995 ppm in the same modern Sth EPR sediments 42 . Similar to Mo, the Archean Algoma-type BIF tend to have higher V (average at 17.7 ppm) than Superior-type BIF (average at 7.6 ppm). To distinguish between detrital riverine inputs and seawater signatures, Mo and V concentrations were normalized to titanium (Ti) content (Fig. 1B), as in previous studies (e.g., Ref. 40,43,44 ). This treatment corrects for changes in Mo and V abundances resulting from contributions by detrital-associated trace elements (e.g., Ref. 45 ). The Mo/Ti ratios for our Fe-rich deposits exhibit a gradual increase between 3.0 and 2.5 Ga, indicating an elevated supply of Mo (d) during the late Archean relative to earlier. Unlike Mo/Ti, V/Ti peaks between 2.0 and 1.5 Ga, notably in the 1.82 Ga Sokoman Formation 46 . An important distinction between Mo (or Mo/Ti) and V (or V/Ti) concentrations is their magnitude: V concentrations often exceed those of Mo by two orders of magnitude. To further account for variable post-depositional processes that might affect the concentration of Mo or V, we analyzed Mo/Fe and V/Fe in all the Fe-rich sediments (Fig. 1C). We posit that Mo and V were both initially sedimented with Fe(III)-oxyhydroxides on the shelf, rather than with Fe(II)-silicates like greenalite in the deeper ocean (see the SI for discussion, as well as Ref. 39 for justifications). Therefore, instances where the Mo/Ti (V/Ti) and Mo/Fe (V/Fe) are offset relative to one another may correspond to gains or losses of Mo or V relative to Fe due to secondary alteration. Crucially, the trends observed in the Mo/Ti and V/Ti records of Fe-rich sediments remain intact when normalized to Fe, except for a minimum in Mo between 2.7 and 2.6 Ga. In particular, the V/Fe ratio consistently exceeds the Mo/Fe ratio by two orders of magnitude, and the characteristic peak values in the Ti-normalized data, along with the increase observed in modern hydrothermal sediments, are preserved. Indeed, the core-top sediments from the modern SEPR also match closely to the V/Fe and Mo/Fe ratios recorded from Fe(III)-oxyhydroxide material collected from non-buoyant hydrothermal plume samples (4.5±0.3x10 -3 mol/mol and 0.03±0.03x10 -3 mol/mol, respectively) 47 . This provides reassurance that any post-depositional alteration of Mo or V abundances in Fe-rich sediments likely occurred to a similar extent for both elements, especially since Mo (V) over Ti ratios also show similar trends with Mo (V) over Fe ratio throughout the record. This indicates that global first-order trends should be faithfully recorded, and that post-depositional alteration did not substantially affect the retention of trends in the Mo and V records. Extrapolating seawater Mo and V through time To establish a more direct correlation between BIF sedimentary concentrations and the relative availabilities of Mo (d) and V (d) (dissolved V) in contemporaneous seawater, we conducted Fe(II) (aq) oxidation and Fe(III)-oxyhydroxide precipitation experiments in the presence of Mo (d) and V (d) . This approach enables us to semi-quantitatively relate Mo and V concentrations in BIF to seawater Mo (d) and V (d) through experimentally derived partitioning coefficients (K D ), assuming that Mo and V were dissolved as molybdate (MoO 4 2- ) and a protonated vanadate (VO 4 3- ) species as in our experiments (see Figures S3 and S4). The oxyanions were chosen for reasons outlined below and are consistent with relevant conditions as depicted in the Mo aqueous speciation diagram provided in the SI. The methods used to calculate Mo (d) and V (d) are detailed in the SI and Supplemental Table S2). These coefficients are defined as: Mo (s) /Fe (s) or V (s) /Fe (s) represent the molar ratio of Mo or V over Fe in BIF, with the subscript ‘s’ referring to Mo or V incorporated into the precipitate. Previous studies have demonstrated that SiO 2(d) passivates the surface reactivity of Fe(III)-oxyhydroxides for elements like phosphorous (P) 48 and nickel (Ni) 49 . Precambrian seawater is believed to have been roughly saturated with respect to amorphous SiO 2 , preceding the evolution of widespread biological removal of SiO 2 by radiolarians in the Cambrian 50 . Therefore, the possible effects of SiO 2 on Mo and V adsorption were tested here as well; co-precipitation experiments simulated either Precambrian (2.2 mM) or Phanerozoic (0 mM) SiO 2(d) concentrations based on predictions by Ref. 51 ). All experiments were performed at pH = 8 and 0.56 M NaCl to mimic seawater pH and ionic strength. The behaviors of Mo (d) and V (d) were assessed across a wide range of initial conditions (see Methods; Supplemental Table S2). A linear relationship was observed between Mo/Fe in the solid and Mo (d) at equilibrium—and analogously between V/Fe and V (d) —which represent K D(Mo) and K D(Mo) , respectively. The linear relationships indicate that there is an abundance of available sites relative to the metals adsorbing on the Fe(III)-oxyhydroxide surfaces (i.e., site saturation has not yet been achieved). In the case of Mo, during Fe(II) oxidation and hydrolysis to Fe(III)-oxyhydroxides, significant co-precipitation occurs when the initial Mo concentration is <5 ppm (52.1 µM) under SiO 2(d) -free (0 mM) conditions (Fig. 2A). At higher initial Mo (d) , co-precipitation plateaus, with a maximum 42.5% removal of the initial Mo (d) . In contrast, almost 100% of V (d) present in the initial solution was removed by Fe(III)-oxyhydroxides across initial V (d) concentrations ranging from 0.1 ppm to 18 ppm under 0 mM SiO 2(d) (Fig. 2B). The presence of SiO 2(d) significantly impacted the co-precipitation behavior of Mo relative to V. At 2.2 mM SiO 2(d) , Mo partitioning to the precipitate is greatly reduced, with less than 4% of initial Mo (d) being removed to Fe(III)-oxyhydroxide (Fig. 2A). By contrast, V showed minimal sensitivity to SiO 2(d) at 2.2 mM SiO 2(d) (Fig. 2B). While the mechanisms leading to this differential behavior are yet to be studied, we speculate a connection to the prevailing charge of the anion, i.e., the adsorption of V is classically modeled by considering V as the trivalent vanadate, while Mo can be modelled as the divalent molybdate which shows competitive adsorption with divalent SiO 2(d) 52 . The important outcome of the results is that Mo scavenging by Fe(III)-oxyhydroxide precipitation was less effective under Precambrian (more Si) relative to Phanerozoic (less Si) marine conditions. By contrast, V exhibited a consistently high affinity for Fe(III)-oxyhydroxides regardless of silica concentrations, indicating effective sequestration of V from seawater into BIF during both eons. In short, the metal trends in BIF reflects sorptive behavior. Using our derived partition coefficients, we reconstructed the relative abundances of Mo (d) and V (d) in seawater across Earth’s history. Our analysis shows significant variability in average Mo (d) and V (d) values, with Mo (d) consistently exceeding V (d) . It also suggests that average Mo (d) values for the Archean may have approached or even exceeded those in the Proterozoic and Phanerozoic oceans, although extracting an exact concentration is difficult given the uncertainties inherent in using a K D approach (Fig. 2C). Conversely, average V (d) are suggested as lower in the Archean compared to the Proterozoic and Phanerozoic (Fig. 2D). Ultimately, when plotted as Mo (d) /V (d) through geological time (Fig. 2E), we find that mean Mo (d) /V (d) shows some fluctuation, but the overall trend is Mo (d) > V (d) in the Archean, with the Mo (d) /V (d) ratio decreasing by two orders of magnitude from the Archean to the Phanerozoic. We are not aware of other major effects that might have fractionated Mo from V during incorporation into Fe(III)-oxyhydroxides. Therefore, we propose that Mo (d) was more available than V (d) in the Archean. The surprisingly high Archean Mo (d) inferred from our approach is intriguing (see SI for reconciliation with the shale record). However, we caution that while the K D value approach is grounded in empirical observations, multiple factors affect the extent to which these simple laboratory experiments can be extrapolated to ancient natural systems about which there are many uncertainties. Possible complications include the competitive adsorption of other ions, the effects of organic complexes, redox sensitivity of Mo and V speciation, and diagenesis. Nevertheless, unless these effects account for orders of magnitude of error, it is likely that Archean Mo (d) was equal to or greater than 100 nM. Moreover, many of these effects likely apply to both Mo and V. As such, our results provide an important first-order approximation of the relative abundances of Mo (d) and V (d) in ancient seawater over Earth’s history, and we propose that the Mo (d) in the Archean was not as low as previously suggested. Although previous studies have explored the adsorption, co-precipitation, and behavior of Mo and V during Fe-mineral transformations (e.g., Ref. 53-62 ), the specific coordination environments of Mo and V with Fe, particularly under marine conditions (pH 8, 0.56 M NaCl) remain poorly constrained. Using synchrotron-based EXAFS, we found that Mo(VI) forms stable octahedral inner-sphere complexes with six oxygen atoms, engaging in mono- or binuclear interactions with Fe, while V(V) adopts tetrahedral coordination through bidentate binuclear corner-sharing bonds (Fig. S1-S2). These inner-sphere complexes are structurally robust, suggesting that once Mo and V bind to Fe(III)-oxyhydroxides, their mobility is significantly limited during sedimentation and burial — the exception being dissimilatory Fe(III) reduction — consistent with their eventual incorporation into more stable oxides 53 . Marine geochemical cycling Molybdenum delivery to the modern oceans primarily originates from the oxidative weathering of volcanic glass, Ti-bearing minerals and molybdenite (MoS 2 ) grains in continental crust as well as from authigenic sulfides in shales 63 , while V is largely derived from the oxidative weathering of vanadiferous magnetite (VFe 2 O 4 ) and organic-rich shales 64 . Rivers thus serve as the main transport mechanism today, delivering Mo to the oceans in the form of the molybdate anion and V in the form of the vanadate anion 65,66 . If oxidative weathering is the primary source of Mo and V, we would expect lower Mo/Ti and V/Ti in the anoxic Archean compared to younger BIF and Fe-rich sediments. However, the opposite trend is observed (Fig.1). Focusing on Mo and its early availability for nitrogenase, how, then, could Mo (d) have been 100 nM in the Archean, as our analyses suggest? Recent studies at modern hydrothermal vents using advanced fluid sampling equipment—where samples are protected from mixing and precipitation/adsorption effects—offer a plausible Mo source independent of surface redox conditions. Chemical analyses of vent fluids from the Main Endeavour Field (Juan de Fuca Ridge) have reported Mo (d) concentrations of 29-220 nM 67 , often exceeding the Mo (d) concentration of average riverine fluxes (4.4 nM) by over an order of magnitude 68 . These findings are supported by hydrothermal basalt alteration experiments that similarly yield high Mo (d) 69 , as well as geological evidence from the 3.24 Ga Panorama district where oceanic crust shows evidence of hydrothermal Mo leaching 70 . It is notable that the Mo (d) /Fe (d) ratios of these undiluted vent-fluids (0.30±0.22) are much closer to V (d) /Fe (d) ratios (1.25±0.58) than the two order of magnitude offsets recorded in both the Fe-rich sediment record and, indeed, modern plume fall-out. In the modern ocean this Mo (d) fractionation presumably arises from preferential incorporation into iron-sulfide nano-particles that incorporated Mo, or as molybdenite (MoS 2 ), which is the stable phase primary mineral phase in hydrothermal systems 68,71,72 . However, some uncertainty exists regarding the speciation of Mo following hydrothermal venting in the Archean where Fe (d) /H 2 S ratios were significantly higher. In the case of MoS 2 , solubility increases significantly with pH (by four orders of magnitude from pH 4 to 8) and under lower sulfide concentrations. Therefore, upon discharge from presumed ultramafic-dominated (komatiitic) seafloor hydrothermal systems into a low sulfate ocean, MoS 2 could convert to molybdate via the reaction 69 : Indeed, it has been demonstrated using x-ray adsorption spectroscopy that molybdate is stable under a wide range of conditions in hydrothermal fluids, particularly at circumneutral to basic pH conditions and over a range of temperature conditions 71 . Moreover, molybdate is the dominant Mo aqueous species under both oxic and anoxic conditions in the absence of dissolved sulfide (see SI for Mo aqueous speciation); the latter was likely low in the Archean due to minimal oxidative weathering of land or release from submarine volcanic vents when hydrothermal fluids had Fe/H 2 S ratios >>1 73 . This is significant because, in the modern ocean, the family of “oxy-anion” or “nutrient” elements (P, V, As, Cr, Mo) that are taken up into Fe-rich hydrothermal plume particles only do so in close proximity within the buoyant portions of hydrothermal plumes, within the first hour or less following release into the oceans 47 . Further, while recent work has highlighted the long-range dispersal that is possible for a sub-set of ridge-crest hydrothermal fluxes over entire ocean basins 74,75 , it remains the case that even in those studies more than 90% of the Fe (d) released through venting is removed in particulate form to underlying sediments within ≤100km of their point of origin 76 . Importantly, Evans et al. (2023b) 69 have proposed that while hydrothermal processes have limited impact on today’s Mo-rich ocean, it could have played a significant role in supplying Mo to the Archean’s Mo-poor ocean because of higher heat flow on the earlier Earth 77 . Such an inference arises because, in this system, the Fe (d) /H₂S ratios were sufficiently high—and thus free H₂S concentrations sufficiently low—that significant removal of dissolved Mo from vent fluids as sulfidic phases was inhibited. At the same time, the Archean ocean was not oxidizing enough at depth (where hydrothermal fluids entered) for Mo to be rapidly scavenged from solution by co-precipitation with Fe(III)-oxyhydroxides in the rising buoyant hydrothermal plume. Interestingly, if Archean Mo was primarily sourced from submarine-emplaced hydrothermal vents, this might explain its higher concentrations in Algoma-type BIF compared to Superior-type BIF, since the former have higher associated volcanic activity. The contrasting trends observed in BIF records—declining Mo (d) over time versus increasing V (d) — probably reflect the coupled evolution of their respective sinks. Today, the major Mo sinks are co-precipitation with Fe(III)- and Mn(IV)-oxyhydroxides in the ambient oceans and removal in waters bearing hydrogen sulfide, H 2 S 78 , in sulfide-rich (euxinic) restricted basins, and in porewaters. Under euxinic conditions with H 2 S reaching 10 −3 to 10 −4 M, MoO 4 2- is converted into particle-reactive oxythiomolybdates and efficiently scavenged to the seafloor, making black shales significant Mo sinks 79 . As a result, euxinic sediments for much of the Phanerozoic often contain Mo concentrations >100 ppm versus ~1 ppm in average crust 80,81 . In the Archean, with high Fe 2+ and minimal seawater sulfate (SO 4 2- ) concentrations 73,82,83 , euxinic conditions were likely scarcer than in the modern. Moreover, in a setting where Fe 2+ upwelling onto early Archean continental shelves was oxidized to Fe(III), the high concentrations of SiO 2(d) in ancient oceans 84 would have further inhibited Mo (d) sorption onto sinking Fe(III)-oxyhydroxides particles. The possible combination of high hydrothermal Mo input and hindered Mo sinks could have sustained Mo (d) concentrations in Archean seawater that were more elevated than commonly considered. During the Proterozoic the source and sinks changed. This was reflected in the onset of oxidative weathering on land, which initiated a strong riverine Mo (d) flux and the expansion of euxinic conditions in the ocean to approximately 1–10% of modern seafloor area 85 , possibly leading to enhanced Mo (d) sequestration from the water column despite enhanced continental weathering after the Proterozoic. In addition, the deposition of Mn(IV)-oxyhydroxides increased in the rock record (e.g., the 2.4 Ga Hotazel Formation, South Africa; see Ref. 86 ), and then further in the Phanerozoic through the formation of Mn(IV)-rich nodules in the deep-sea, leading to the widespread removal of Mo (d) from seawater. Collectively, the balance between the increased source from continental weathering and expanded sinks, may have kept Mo (d) at low to intermediate concentrations until the more intense oxidative weathering and a decreasing euxinic sink in the Phanerozoic. Under oxygenated conditions, V (d) exists as highly-soluble VO 4 3- , and its primary sink in oxic marine environments is adsorption onto Fe(III)- and Mn(IV)-oxyhydroxides and clay minerals in pelagic sediments 87 . Unlike Mo (d) , which competes with SiO 2(d) for sorption sites on Fe(III)-oxyhydroxides, V (d) is effectively removed regardless of SiO 2(d) concentrations. In this regard, extensive deposition of BIF before 1.8 Ga would have facilitated the removal of V (d) from seawater despite higher hydrothermal inputs than today. Then, as BIF deposition waned after 1.8 Ga, this major sink diminished, likely allowing seawater V (d) concentrations to increase over time. Although Mn(IV)-oxyhydroxide sinks eventually became important, it is unlikely that they were ever as large in magnitude as BIF were in the Precambrian 86 . As discussed above, the relative abundances of Mo (d) and V (d) in seawater reflect a dynamic balance between the sources and sinks of these elements. During the Archean, submarine hydrothermal vents may have released significant amounts of Mo. It is estimated that, because of the higher heat and volume fluxes predicted for submarine venting at that time 77 , the gross Mo flux at that time may have been as high as 2 x 10 7 mol/yr—an increase of four-fold over modern day gross fluxes 69 . Because of the reducing conditions in the deep oceans where those fluids were emitted, however, the net increase in supply of dissolved Mo to the deep-water column, absent any precipitation/removal as sulfides and/or oxyhydroxides close to the point of origin would have been even higher. Subsequently, upon upwelling of that Fe-, V- and Mo-rich seawater to shallower more oxygenated depths, high seawater SiO 2(d) would have hindered Mo removal to Fe(III)-oxyhydroxides, with much of that Mo (d) remaining in solution, while V (d) was preferentially sequestered into BIF because of its higher affinity for Si-rich Fe(III)-oxyhydroxides (Fig. 3A). Following the GOE, as continental exposure and oxidative weathering increased, the riverine supply of MoO 4 2- and VO 4 3- to seawater would have increased. As BIF deposition waned at 1.8 Ga and the extent of marine euxinia increased (as well as the deposition of Mn(IV)-oxyhydroxides at 2.4 Ga), the trends in Mo and V would have been reversed, with Mo sequestered into sediments while V became more bioavailable (Fig 3B). This represents an inflection point in the cycling of Mo and V in Earth’s surface environments and the fundamental role that the balance of sources and sinks play in governing elemental bioavailability over geological timescales. By the Phanerozoic, SiO 2(d) declined due to the evolution of silica-secreting eukaryotes, the bulk oceans became increasingly oxygenated, leading to a Mo cycle like today (Fig 3C). Estimates suggest that modern submarine hydrothermal Mo fluxes are no higher than 4.5 x 10 6 mol/yr; for comparison the modern continental flux of Mo to seawater is 3.1 x 10 8 mol/yr 88 , dwarfing all other known sources. Bioavailability of Mo and its relationship with N fixation It has been proposed that low availability of Mo (d) played a significant role in influencing the evolution of the biosphere 18,89 . However, our findings suggest that Archean seawater was characterized by elevated Mo/V despite low continental weathering inputs. It is thus not surprising that recent molecular data reveal that the earliest nitrogenases utilized Mo and that N isotope evidence of alternative nitrogenases has not been found. While our results suggest that Mo (d) was probably high in the Archean, we also acknowledge that Mo (d) in modern anoxic waters rarely reaches levels low enough to limit N 2 -fixation, and that Mo-based nitrogenase activity continues when <5 nM, as suggested by recent studies 26-28 . Moreover, even when dissolved Mo is low, mineral-bound Mo, V, and Fe remain bioavailable. For example, recent work has shown that Methanococcus maripaludis can utilize thiolated Mo species, such as MoS 2 or MoS 4 2− , to meet its Mo demands 90 . These findings align with recent reports that the anaerobe Clostridium kluyveri and the facultative anaerobe Rhodopseudomonas palustris can extract Mo from MoS 2 using metal-chelating compounds to express nitrogenase genes and fix N 2 91,92 . Similarly, the N 2 -fixing bacterium Azotobacter vinelandii secretes a high-affinity molybdophore to mobilize Mo from MoS 2 , enabling nif gene expression even under Mo (d) -depleted conditions 93 . While the evolutionary history of molybdophores is not well understood, these observations collectively point toward the ability of the biosphere to access Mo in a variety of forms that may be present in the Archean oceans or proximal to hydrothermal vents. Our findings support the early evolution of molybdoenzymes, indicating that the Mo (d) concentrations in the Archean may have rendered Mo more bioavailable than previously assumed. The expansion of Mo sinks under the more widespread sulfidic conditions in the Proterozoic likely exerted selective pressure for organisms to develop Mo storage systems to mitigate the decreasing Mo (d) bioavailability. Taken together, in the light of previous phylogenetic and isotopic studies 5,33,35 , our study suggests that the evolution of Mo-nitrogenase and the availability of Mo (d) were closely linked, resolving the “Mo paradox”. Methods The co-precipitation experiments of Mo and V with Fe(III)-oxyhydroxides were performed in 0.56 M NaCl solution – exposed to the atmosphere - to mimic the major cation and anion contributions to seawater salinity. Different concentrations of Mo and V, in the form of their respective oxyanions, were prepared by diluting 1,000 ppm Mo and V stock solutions into several 50 mL polypropylene tubes containing 0.56 M NaCl to generate Mo and V solutions with concentrations ranging from 0.1 ppm to 18 ppm, respectively. Our Mo and V concentrations are significantly higher than modern ocean dissolved Mo and V concentrations (around 10 ppb and 1 ppb, respectively), which is because our initial Mo and V concentrations represent the amount of Mo and V before adsorption. The same concentration range for Mo and V was used to allow for a more direct comparison of their behavior on Fe(III)-oxyhydroxides surfaces. The total volume of each solution is 50 ml. Then, a 5 ml of each solution was removed using a syringe for measuring the initial concentrations of Mo and V. After that, 0.1 mL of 1 M FeCl 2 was added to the 0.56 M NaCl solutions, and the solutions were left to reach equilibrium with the atmosphere for three days. During this period, the solution pH was maintained at 8 by adding small aliquots of diluted NaOH and HCl solution to mimic seawater pH. Note: we chose this pH because recent work on BIF sedimentation velocities have argued that a pH range between 7 and 8 was required for the deposition of major BIF deposits 94 . The coprecipitation experiments were conducted at both 0 mM and 2.2 mM Si conditions to allow for comparisons between a Precambrian Si-rich and Phanerozoic Si-depleted ocean. After three days, solutions were filtered through 0.2 µm nylon filters. The concentrations of Mo and V in the initial solutions and final solutions after filtration were measured by ICP-MS. Synchrotron-based extended X-ray adsorption fine structure (EXAFS) analysis was performed on solid samples from systems where Mo and V were coprecipitated with Fe(III)-oxyhydroxides. The protocol for the preparation of Mo and V bearing Fe(III)-oxyhydroxides was the same as in the above coprecipitation experiments. The Mo, and V containing Fe(III)-oxyhydroxides samples were air-dried (until no further weight loss was detected), sealed in airtight zip-lock bags using a heat-sealer and transported for EXAFS analysis. The EXAFS experiments were performed at the Hard X-ray MicroAnalysis (HXMA) beamline of the Canadian Light Source (CLS) in Saskatoon, Canada. EXAFS spectra were processed and fitted using ATHENA in the DEMETER software package and WinXAS. FEFF 7 was used to generate backscattering phase and amplitude functions for the corresponding scattering paths. Detailed EXAFS analytical methods are provided in SI. Declarations Acknowledgements W.H. was supported by the National Key Research and Development Program of China (2024YFF0810200). This work was supported by National Science and Engineering Research Council of Canada (NSERC) Discovery Grants to K.O.K. (RGPIN-165831) and L.J.R. (RGPIN-2021-02523). The authors would also like to acknowledge comments provided on earlier versions of the text by Drs. William Seyfried, Guy Evans and Jennifer Glass. Author contributions W.H., K.O.K., and L.J.R. conceived the study; W.H. performed all the co-precipitation experiments and data analysis and EXAFS experiments; W.H., K.O.K., and L.J.R. compiled the BIF database compilation; N.C., and W.S. helped with the analysis of EXAFS data. 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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-7237238","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":507499683,"identity":"e2bf2cc8-1e5b-432e-9b8c-140355253ef0","order_by":0,"name":"Kurt Konhauser","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzUlEQVRIiWNgGAWjYBACCSBmbKhgYOCTIEELY2PDGQYGNpCWA0RraWwjRYvkjNznD2fOOyzPJt187POHGgZ5/gYCWqQl0g0bN247bNgmcyx5xoFjDIYzCFklJ5HG2Phw22HGNokcY4YDbAwJBF0H0TLnsH2bRP5nhgP/GBLkCWmRBmnZ2HA4EWgLM8PBNoYEA0JaJHueMc6ccSw9GegXY4azfRKGGwlpkTiexvCxp8batl+6+TFDxTcbeTlCWjCMIFH9KBgFo2AUjAKsAABQcUDWeGBIMwAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-7722-7068","institution":"University of Alberta","correspondingAuthor":true,"prefix":"","firstName":"Kurt","middleName":"","lastName":"Konhauser","suffix":""},{"id":507499684,"identity":"72d62f9a-a7bb-4527-9f2d-b69cefcca1bd","order_by":1,"name":"Weiduo Hao","email":"","orcid":"https://orcid.org/0000-0002-0486-424X","institution":"Northwest University","correspondingAuthor":false,"prefix":"","firstName":"Weiduo","middleName":"","lastName":"Hao","suffix":""},{"id":507499685,"identity":"ef806424-dc10-4dfd-b217-0fc8bf592703","order_by":2,"name":"Leslie Robbins","email":"","orcid":"https://orcid.org/0000-0002-6931-5743","institution":"University of Regina","correspondingAuthor":false,"prefix":"","firstName":"Leslie","middleName":"","lastName":"Robbins","suffix":""},{"id":507499686,"identity":"49815ca1-8ff2-4e72-a991-d6350ade3e69","order_by":3,"name":"Betül Kaçar","email":"","orcid":"https://orcid.org/0000-0002-0482-2357","institution":"University of Wisconsin–Madison","correspondingAuthor":false,"prefix":"","firstName":"Betül","middleName":"","lastName":"Kaçar","suffix":""},{"id":507499687,"identity":"a7385a45-22d9-43d2-88a5-741d9ebe4633","order_by":4,"name":"Holly Rucker","email":"","orcid":"","institution":"University of Wisconsin - Madison","correspondingAuthor":false,"prefix":"","firstName":"Holly","middleName":"","lastName":"Rucker","suffix":""},{"id":507499688,"identity":"f6941b92-3f1e-40c3-9d2a-0fa5736e8e0c","order_by":5,"name":"Ning Chen","email":"","orcid":"","institution":"Canadian Light Source Inc","correspondingAuthor":false,"prefix":"","firstName":"Ning","middleName":"","lastName":"Chen","suffix":""},{"id":507499689,"identity":"01f793a6-c455-48e2-acb4-6300e0785ca0","order_by":6,"name":"Wenyuan Sun","email":"","orcid":"","institution":"China University of Petroleum","correspondingAuthor":false,"prefix":"","firstName":"Wenyuan","middleName":"","lastName":"Sun","suffix":""},{"id":507499690,"identity":"03890fe5-ba5a-4437-b74e-4bc54114a4a9","order_by":7,"name":"Daniel Alessi","email":"","orcid":"https://orcid.org/0000-0002-8360-8251","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Daniel","middleName":"","lastName":"Alessi","suffix":""},{"id":507499691,"identity":"29b275b4-e4be-47fe-8e15-c4666fd05ccf","order_by":8,"name":"Hailiang Dong","email":"","orcid":"","institution":"China University of Geosciences","correspondingAuthor":false,"prefix":"","firstName":"Hailiang","middleName":"","lastName":"Dong","suffix":""},{"id":507499692,"identity":"bd9811b6-c2ca-45f7-a904-4cfafc2a6aa5","order_by":9,"name":"Eva Stüeken","email":"","orcid":"https://orcid.org/0000-0001-6861-2490","institution":"University of St Andrews","correspondingAuthor":false,"prefix":"","firstName":"Eva","middleName":"","lastName":"Stüeken","suffix":""},{"id":507499693,"identity":"1335eec9-c741-47ef-aaaf-58c221d2ac12","order_by":10,"name":"Christopher German","email":"","orcid":"https://orcid.org/0000-0002-3417-6413","institution":"WHOI","correspondingAuthor":false,"prefix":"","firstName":"Christopher","middleName":"","lastName":"German","suffix":""},{"id":507499694,"identity":"ac9e1883-bab0-449f-baaf-6d3c86808f3b","order_by":11,"name":"Ariel Anbar","email":"","orcid":"https://orcid.org/0000-0002-6015-7750","institution":"Arizona State University","correspondingAuthor":false,"prefix":"","firstName":"Ariel","middleName":"","lastName":"Anbar","suffix":""},{"id":507499695,"identity":"46135efb-0521-4d61-859c-ec81ebe47b9f","order_by":12,"name":"Timothy Lyons","email":"","orcid":"https://orcid.org/0000-0001-8674-6775","institution":"University of California, Riverside","correspondingAuthor":false,"prefix":"","firstName":"Timothy","middleName":"","lastName":"Lyons","suffix":""}],"badges":[],"createdAt":"2025-07-28 21:10:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7237238/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7237238/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":90295598,"identity":"5a83b9e3-6c0d-414a-b562-e0a7ee45f039","added_by":"auto","created_at":"2025-09-01 08:14:41","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1016971,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7237238/v1/5a61220aa2032aa5c77d2548.png"},{"id":90295596,"identity":"8d7da2d2-9137-4693-86ee-41b93c3e56a6","added_by":"auto","created_at":"2025-09-01 08:14:35","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1730628,"visible":true,"origin":"","legend":"\u003cp\u003eThe co-precipitation and adsorption of Mo and V with Fe(III) oxyhydroxide as a function of initial Mo and V concentration, and estimations of dissolved Mo and V concentrations in the ocean (Mo\u003csub\u003e(d) \u003c/sub\u003eand V\u003csub\u003e(d)\u003c/sub\u003e, respectively)\u003csub\u003e \u003c/sub\u003ethroughout Earth’s history. (A-B) The coprecipitation and adsorption of Mo and V onto ferrihydrite. The blue dots represent the adsorption behavior of Mo and V in the absence of Si, mimicking modern low Si oceans; yellow dots represent the adsorption behavior of Mo and V in the presence of 2.2 mM Si, reflecting conditions likely for a silica-saturated Precambrian ocean; purple straight lines represent 100% removal of Mo/V through co-precipitation/adsorption onto the ferrihydrite surfaces. The inset diagram in panel (A) shows the molar Mo/Fe ratios versus equilibrium Mo concentration (mM) and the corresponding linear regression equation, while the inset diagram in panel (B) is the plot of the molar V/Fe ratios against the equilibrium V concentration (mM) and the corresponding linear regression equation. The purple stippled lines represent the linear regression of selected experimental data (see below for the criteria of selection of Mo/Fe and V/Fe ranges), while the K\u003csub\u003eD\u003c/sub\u003e values were derived by linear regression of the scatterplots with the y-intercept set to zero. The Mo/Fe range used for the extrapolation of the K\u003csub\u003eD\u003c/sub\u003e value is 0-0.004, and the V/Fe range used for extrapolation of K\u003csub\u003eD\u003c/sub\u003e is 0-0.1, this is because the Mo/Fe and V/Fe ranges in the BIF database is 0-0.004 and 0-0.1, respectively. (C-D) The scatterplots and bar plots for the calculated Mo\u003csub\u003e(d) \u003c/sub\u003eand V\u003csub\u003e(d)\u003c/sub\u003e concentrations (mM) as a function of geological time. The purple lines represent the variation of average concentrations. The calculation is based on the K\u003csub\u003eD\u003c/sub\u003e values in Fig. 2A-B. Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e are calculated by applying the K\u003csub\u003eD\u003c/sub\u003e values to the BIF database (Fig. 1). We apply the K\u003csub\u003eD\u003c/sub\u003e values calculated for 0 mM Si conditions for ages younger than 0.5 Ga, while we use K\u003csub\u003eD\u003c/sub\u003e values at the 2.2 mM Si condition for ages older than 0.5 Ga. (E) The calculated molar ratio of Mo\u003csub\u003e(d) \u003c/sub\u003eover V\u003csub\u003e(d) \u003c/sub\u003ethrough geological time. The average Mo\u003csub\u003e(d)\u003c/sub\u003e/V\u003csub\u003e(d) \u003c/sub\u003eratio is generally higher in the Archean than in the Proterozoic and Phanerozoic, indicating that the Archean ocean likely provides more Mo than V for nitrogenase. Boxes in (C-E) show the mean value (horizontal line) and 25%/75% quantiles (edges), whiskers in (C-E) show the full range. Superimposed dots in (C-E) denote individual samples. The red stars in (C-E) represent the modern ocean Mo\u003csub\u003e(d) \u003c/sub\u003eand V\u003csub\u003e(d)\u003c/sub\u003e concentrations.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7237238/v1/0f441c21bf97dd69f129dbe1.png"},{"id":90295594,"identity":"6b10f1d1-a035-4b52-8a16-a94110157646","added_by":"auto","created_at":"2025-09-01 08:14:35","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1980542,"visible":true,"origin":"","legend":"\u003cp\u003eA schematic model illustrating the variation in sources and sinks for Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e from the Archean to the Phanerozoic. During the Archean, low fluxes of Mo and V were transported from land to the ocean due to the limited exposure of continental land mass compared to the Proterozic and Phanerozoic. Hydrothermal vents likely served as the primary source of Mo and V during this period in the form of metal-iron-sulfide particles. The deposition of BIFs fractionated marine Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e concentrations, with Mo remaining in seawater while V was incorporated into the sediment pile due to sorption to S-rich ferric oxyhydroxide mineral precipitation. Following the GOE, increased continental exposure and oxidative weathering enhanced the flux of Mo and V, as Mo\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e and VO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e, into the ocean via riverine input. As BIF deposition waned after 1.8 Ga and euxinic conditions expanded, Mo was sequestered into sediments while V became more bioavailable. By the Phanerozoic, SiO\u003csub\u003e2(d)\u003c/sub\u003e declined due to the evolution of silica-secreting eukaryotes, the bulk oceans became increasingly oxygenated, and rivers emerged as the primary source of Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7237238/v1/1e7ee08e74aa7126d29504b4.png"},{"id":106961766,"identity":"da71619a-3c06-47a4-aef0-da7a989df649","added_by":"auto","created_at":"2026-04-15 09:26:57","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6637003,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7237238/v1/bfc91cfd-bb08-405c-9e08-4ae536593e0c.pdf"},{"id":90295595,"identity":"0ff1d8ba-26b2-4bed-9102-f2ad943f01da","added_by":"auto","created_at":"2025-09-01 08:14:35","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2931516,"visible":true,"origin":"","legend":"Supplemental Information","description":"","filename":"MoVSupplementaryInformation1stsubmissionfinal.docx","url":"https://assets-eu.researchsquare.com/files/rs-7237238/v1/021125d4d85aa8db82772082.docx"},{"id":90295599,"identity":"ee1322a5-2632-4f67-8d9c-08cf28fb941c","added_by":"auto","created_at":"2025-09-01 08:14:42","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":131528,"visible":true,"origin":"","legend":"Table S1","description":"","filename":"MoVSupplementaryTableS11stsubmissionfinal.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7237238/v1/4eb3564957645b948eda52cd.xlsx"},{"id":90295600,"identity":"28b3496e-a51b-4880-9033-eef665d035f7","added_by":"auto","created_at":"2025-09-01 08:14:44","extension":"xlsx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":40799,"visible":true,"origin":"","legend":"Table S2","description":"","filename":"MoVSupplementalTableS21stsubmissionfinal.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7237238/v1/e67c3e6a0f11a6742dc11f20.xlsx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Evolution of the molybdenum and vanadium cycles through time and their impact on ancient nitrogen fixation","fulltext":[{"header":"Introduction","content":"\u003cp\u003eNitrogen is a vital macronutrient essential to all living organisms. During the early Archean, production of bioavailable N species, such as ammonium (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) and nitrate (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e), was likely dominated by abiotic processes including lightning discharges\u003csup\u003e1,2\u003c/sup\u003e, submarine hydrothermal fluid emissions\u003csup\u003e3\u003c/sup\u003e, and photochemical reactions\u003csup\u003e4\u003c/sup\u003e. However, these sources were limited in supply, potentially constraining the expansion of the early biosphere\u003csup\u003e5\u003c/sup\u003e. The evolution of biological N\u003csub\u003e2\u003c/sub\u003e-fixation, a metabolic process whereby enzymes convert atmospheric N\u003csub\u003e2\u003c/sub\u003e into bioavailable N (i.e., NH\u003csub\u003e3\u003c/sub\u003e), helped alleviate N scarcity, paving the way for the global expansion of life\u003csup\u003e6\u003c/sup\u003e. Geological evidence indicates that biological N\u003csub\u003e2\u003c/sub\u003e-fixation dates back at least 3.2-3.1 billion years\u003csup\u003e7,8\u003c/sup\u003e, although the exact timing of its origin remains unknown\u003csup\u003e9\u003c/sup\u003e. It is possible that this process began even earlier, potentially linked to thermophilic methanogenic archaea\u003csup\u003e10\u003c/sup\u003e, which are amongst the oldest life forms\u003csup\u003e11\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Nitrogenase is the enzyme responsible for biological N\u003csub\u003e2\u003c/sub\u003e-fixation. This process uses eight electrons to reduce N\u003csub\u003e2\u003c/sub\u003e to 2NH\u003csub\u003e3\u003c/sub\u003e, accompanied by the release of one H\u003csub\u003e2\u003c/sub\u003e molecule\u003csup\u003e12\u003c/sup\u003e. The nitrogenase family is a group of isoenzymes that contain a metallocofactor composed of Fe, sulfur (S), and either Mo, V, or another Fe atom. The most efficient variant, Mo-nitrogenase, encoded by\u003cem\u003e\u0026nbsp;nif\u003c/em\u003e genes, incorporates Mo at the active site\u003csup\u003e13\u003c/sup\u003e. In contrast, vanadium (V)-nitrogenase (encoded by \u003cem\u003evnf\u0026nbsp;\u003c/em\u003egenes) and Fe-nitrogenase (encoded by \u003cem\u003eanf\u0026nbsp;\u003c/em\u003egenes) are less efficient\u003csup\u003e14\u003c/sup\u003e and rarer (only 10% and 4% of \u003cem\u003enif\u003c/em\u003e-containing organisms contain \u003cem\u003eanf\u0026nbsp;\u003c/em\u003eand \u003cem\u003evnf\u003c/em\u003e genes, respectively\u003csup\u003e15-17\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOur interpretation of the evolutionary history of nitrogenase enzymes has been shaped largely by studies examining temporal changes in marine trace metal reservoirs\u003csup\u003e18\u003c/sup\u003e, informed by both thermodynamic approaches (e.g., Ref.\u003csup\u003e19,20\u003c/sup\u003e) and chemical analyses of sedimentary archives, such as the shale record (e.g., Ref.\u003csup\u003e21,22\u003c/sup\u003e, and references within). Such approaches suggest the history of paleo-marine Mo concentrations (hereafter denoted Mo\u003csub\u003e(d)\u003c/sub\u003e) tracks closely to changes in the oxygenation of Earth\u0026rsquo;s surface environments, and hence points to a dynamic history of paleo-marine Mo concentrations. Broadly, in the Archean, Mo\u003csub\u003e(d)\u003c/sub\u003e was assumed to be low and variable\u003cstrong\u003e\u0026mdash;\u003c/strong\u003eand potentially limiting for N\u003csub\u003e2\u003c/sub\u003e fixation\u003cstrong\u003e\u0026mdash;\u003c/strong\u003edue to a lack of oxidative continental weathering (e.g., Ref.\u003csup\u003e21,23,24\u003c/sup\u003e). Scott et al. (2008) proposed an increase of Mo\u003csub\u003e(d)\u003c/sub\u003e between 2.2 and 2.0 Ga, approximately 200 Ma after O\u003csub\u003e2\u003c/sub\u003e accumulated in the atmosphere (the Great Oxidation Event, hereafter the GOE), after which Mo\u003csub\u003e(d)\u003c/sub\u003e may have declined. Mo\u003csub\u003e(d)\u003c/sub\u003e then rose as much as 100-fold to near modern levels (105 nM) during the Neoproterozoic and early Paleozoic, coincident with the Neoproterozoic Oxygenation Event (NOE).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBased on this reconstruction of Mo\u003csub\u003e(d)\u003c/sub\u003e over time, it was suggested that early forms of nitrogenase did not rely on Mo due to the low Mo\u003csub\u003e(d)\u003c/sub\u003e levels in ancient seawater juxtaposed with extremely high Fe availability, or at least they struggled to do so with major ecological impact\u003csup\u003e18\u003c/sup\u003e. Mo\u003csub\u003e(d)\u003c/sub\u003e scarcity could have favored the prevalence of alternative V- and Fe-nitrogenases in early N\u003csub\u003e2\u003c/sub\u003e-fixation, with Mo-nitrogenase emerging with ecological significance only after an increase in seawater Mo\u003csub\u003e(d)\u003c/sub\u003e due to the rise of oxygenated environments. A Mo threshold of 5 \u0026ndash; 10 nM has often been cited, based on limited experimental data\u003csup\u003e25\u003c/sup\u003e. \u0026nbsp;DNA and RNA sequencing of microbial biomass in low-Mo environments have shown the prevalence of Mo-based nitrogenase in diverse environments even when V \u0026gt;\u0026gt; Mo and Mo \u003ccode\u003e\u0026le;\u003c/code\u003e 5 nM\u003csup\u003e26-28\u003c/sup\u003e. Recent reconstructions of Mo\u003csub\u003e(d)\u003c/sub\u003e using mass balance modeling constrained by low-oxygen (O\u003csub\u003e2\u003c/sub\u003e) weathering experiments and the black shale record point to late Archean concentrations \u0026lt; 5 nM\u003csup\u003e24\u003c/sup\u003e, although data are sparse for the entirety of the Archean.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHowever, this hypothesis is at odds with emerging biogeochemical and molecular biology evidence that indicates Mo-nitrogenase evolved prior to its V- and Fe-nitrogenase counterparts. Biogeochemists have been unable to find N isotope evidence of alternative nitrogenases in Archean sedimentary rocks that otherwise indicate the occurrence of biological N\u003csub\u003e2\u003c/sub\u003e-fixation\u003csup\u003e5,7,29-31\u003c/sup\u003e. Further, phylogenetic analyses of nitrogenase sequences indicate that V- and Fe-nitrogenases stem from Mo-nitrogenase, suggesting that Mo utilization is ancestral and predates the use of V and Fe in N\u003csub\u003e2\u003c/sub\u003e-fixation\u003csup\u003e8,32\u003c/sup\u003e. Recent ancestral sequence reconstructions have confirmed this hypothesis\u003csup\u003e33\u003c/sup\u003e and inferred the emergence of V- and Fe-nitrogenases following the GOE\u003csup\u003e34\u003c/sup\u003e. Most\u0026nbsp;recently, phylogenetic analysis of Mo utilization in biological systems suggests that Mo- molybdoenzymes originated in the Paleoarchean (~ 3.4 Ga)\u003csup\u003e35\u003c/sup\u003e.\u0026nbsp;Hence, a conundrum remains as to why Mo-nitrogenase would have evolved and proliferated when Mo\u003csub\u003e(d)\u003c/sub\u003e was seemingly very low.\u003c/p\u003e\n\u003cp\u003eIn an attempt to resolve this \u0026ldquo;Mo paradox\u0026rdquo;, we investigated Mo concentrations in ancient sediments. Reconstructions of Mo and other metal abundances in ancient oceans rely heavily on analyses of metal contents in the authigenic fraction of carbonaceous shales (e.g.,\u0026nbsp;Ref.\u003csup\u003e21,36\u003c/sup\u003e). Ancient chemical sedimentary rocks are comparatively less studied but\u0026nbsp;provide good candidates to further interrogate the record of marine Mo availability through Earth\u0026rsquo;s history. Specifically, the geochemical records of\u0026nbsp;Fe-rich sediments, including Precambrian BIF, Proterozoic and Phanerozoic ironstones, and modern hydrothermal\u0026nbsp;Fe-rich deposits, may reveal first-order trends in Mo\u003csub\u003e(d)\u003c/sub\u003e over geological time. BIFs are chemically precipitated sediments that formed on the continental shelves of ancient cratons when deep upwelling seawater\u003cstrong\u003e\u0026mdash;\u003c/strong\u003eenriched in hydrothermally derived Fe\u003csup\u003e2+\u003c/sup\u003e\u003cstrong\u003e\u0026mdash;\u003c/strong\u003ewas biologically oxidized to a primary Fe(III)-oxyhydroxide mineral phase, such as ferrihydrite (see\u0026nbsp;Ref.\u003csup\u003e37\u003c/sup\u003e for review). Note, alternate views have suggested that some primary BIF minerals were greenalite\u003csup\u003e38\u003c/sup\u003e but for reasons outlined in the SI, we consider greenalite to be a deep-sea precipitate and ferrihydrite a precipitate formed in the photic zone\u003csup\u003e39\u003c/sup\u003e. Thus, the chemical composition of Fe-rich sediments (greenalite and/or ferrihydrite) provides a record of ocean geochemistry that complements the shallower coastal waters captured by much of the shale record.\u0026nbsp;\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eIron sedimentation record\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe compiled published geochemical data for BIF, ironstones, and hydrothermal Fe-rich sediments, supplementing a previously published BIF database\u003csup\u003e40\u003c/sup\u003e, to create a comprehensive geochemical record of Fe-rich sediments through time (see Supplemental Table S1). Samples predating 1.8 Ga are primarily BIF, while a significant gap exists between 1.8 Ga and 0.8 Ga reflecting a well-documented hiatus in BIF deposition. Samples younger than 0.6 Ga are mainly ironstones\u003csup\u003e37\u003c/sup\u003e. To provide a modern perspective, hydrothermal Fe-rich deposits were included in the dataset.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOur results show a general trend of increasing maximum concentrations of Mo in Fe-rich sediments (Fig. 1A), from the oldest samples analyzed (the 3.8 Isua Greenstone Belt, Greenland) to modern hydrothermal deposits. The Archean itself shows relatively lower Mo values in BIFs compared to Proterozoic and Phanerozoic counterparts, with the highest values typically associated with Archean Algoma-type BIFs (average at 1.7 ppm) versus Archean Superior-type BIFs (average at 1.3 ppm). The former are stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits, while the latter developed in relatively shallower passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks\u003csup\u003e37\u003c/sup\u003e. There is a subsequent peak observed at 2.43 Ga (Cau\u0026ecirc; Formation, Brazil), with Mo concentrations reaching 53.7 ppm\u003csup\u003e41\u003c/sup\u003e. After this peak, BIF samples are scarce until the Neoproterozoic, with Mo concentrations typically remaining below 10 ppm between 2.43 Ga and the Neoproterozoic. Ironstones and hydrothermal deposits display a progressive rise in Mo concentrations from 0.7 Ga to the modern, with values that reach 26-52 ppm in core top sediments enriched in Fe-dominated hydrothermal plume fall-out along the western flank of the superfast-spreading East Pacific Rise (EPR), 17-21\u0026deg;S\u003csup\u003e42\u003c/sup\u003e. Vanadium concentrations more clearly follow a gradually increasing trend over time (Fig. 1A), with corresponding core-top concentrations of 700-995 ppm in the same modern Sth EPR sediments\u003csup\u003e42\u003c/sup\u003e. Similar to Mo, the Archean Algoma-type BIF tend to have higher V (average at 17.7 ppm) than Superior-type BIF (average at 7.6 ppm).\u003c/p\u003e\n\u003cp\u003eTo distinguish between detrital riverine inputs and seawater signatures, Mo and V concentrations were normalized to titanium (Ti) content (Fig. 1B), as in previous studies (e.g., Ref.\u003csup\u003e40,43,44\u003c/sup\u003e). This treatment corrects for changes in Mo and V abundances resulting from contributions by detrital-associated trace elements (e.g., Ref.\u003csup\u003e45\u003c/sup\u003e). The Mo/Ti ratios for our Fe-rich deposits exhibit a gradual increase between 3.0 and 2.5 Ga, indicating an elevated supply of Mo\u003csub\u003e(d)\u003c/sub\u003e during the late Archean relative to earlier. Unlike Mo/Ti, V/Ti peaks between 2.0 and 1.5 Ga, notably in the 1.82 Ga Sokoman Formation\u003csup\u003e46\u003c/sup\u003e. An important distinction between Mo (or Mo/Ti) and V (or V/Ti) concentrations is their magnitude: V concentrations often exceed those of Mo by two orders of magnitude.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo further account for variable post-depositional processes that might affect the concentration of Mo or V, we analyzed Mo/Fe and V/Fe in all the Fe-rich sediments (Fig. 1C). We posit that Mo and V were both initially sedimented with Fe(III)-oxyhydroxides on the shelf, rather than with Fe(II)-silicates like greenalite in the deeper ocean (see the SI for discussion, as well as Ref.\u003csup\u003e39\u003c/sup\u003e for justifications). Therefore, instances where the Mo/Ti (V/Ti) and Mo/Fe (V/Fe) are offset relative to one another may correspond to gains or losses of Mo or V relative to Fe due to secondary alteration. Crucially, the trends observed in the Mo/Ti and V/Ti records of Fe-rich sediments remain intact when normalized to Fe, except for a minimum in Mo between 2.7 and 2.6 Ga. In particular, the V/Fe ratio consistently exceeds the Mo/Fe ratio by two orders of magnitude, and the characteristic peak values in the Ti-normalized data, along with the increase observed in modern hydrothermal sediments, are preserved. Indeed, the core-top sediments from the modern SEPR also match closely to the V/Fe and Mo/Fe ratios recorded from Fe(III)-oxyhydroxide material collected from non-buoyant hydrothermal plume samples (4.5\u0026plusmn;0.3x10\u003csup\u003e-3\u003c/sup\u003emol/mol and 0.03\u0026plusmn;0.03x10\u003csup\u003e-3\u003c/sup\u003emol/mol, respectively)\u003csup\u003e47\u003c/sup\u003e. This provides reassurance that any post-depositional alteration of Mo or V abundances in Fe-rich sediments likely occurred to a similar extent for both elements, especially since Mo (V) over Ti ratios also show similar trends with Mo (V) over Fe ratio throughout the record. This indicates that global first-order trends should be faithfully recorded, and that post-depositional alteration did not substantially affect the retention of trends in the Mo and V records. \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eExtrapolating seawater Mo and V through time\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo establish a more direct correlation between BIF sedimentary concentrations and the relative availabilities of Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e (dissolved V) in contemporaneous seawater, we conducted Fe(II)\u003csub\u003e(aq)\u003c/sub\u003e oxidation and Fe(III)-oxyhydroxide precipitation experiments in the presence of Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e. This approach enables us to semi-quantitatively relate Mo and V concentrations in BIF to seawater Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u0026nbsp;\u003c/sub\u003ethrough experimentally derived partitioning coefficients (K\u003csub\u003eD\u003c/sub\u003e), assuming that Mo and V were dissolved as molybdate (MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e) and a protonated vanadate (VO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e) species as in our experiments (see Figures S3 and S4). The oxyanions were chosen for reasons outlined below and are consistent with relevant conditions as depicted in the Mo aqueous speciation diagram provided in the SI. The methods used to calculate\u0026nbsp;Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e are detailed in the SI and Supplemental Table S2). These coefficients are defined as:\u0026nbsp;\u003cimg src=\"data:image/png;base64,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\"\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eMo\u003csub\u003e(s)\u003c/sub\u003e/Fe\u003csub\u003e(s)\u003c/sub\u003e or V\u003csub\u003e(s)\u003c/sub\u003e/Fe\u003csub\u003e(s)\u003c/sub\u003e represent the molar ratio of Mo or V over Fe in BIF, with the subscript \u0026lsquo;s\u0026rsquo; referring to Mo or V incorporated into the precipitate.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ePrevious studies have demonstrated that SiO\u003csub\u003e2(d)\u0026nbsp;\u003c/sub\u003epassivates the surface reactivity of Fe(III)-oxyhydroxides for elements like phosphorous (P)\u003csup\u003e48\u003c/sup\u003e and nickel (Ni)\u003csup\u003e49\u003c/sup\u003e. Precambrian seawater is believed to have been roughly saturated with respect to amorphous SiO\u003csub\u003e2\u003c/sub\u003e, preceding the evolution of widespread biological removal of SiO\u003csub\u003e2\u003c/sub\u003e by radiolarians in the Cambrian\u003csup\u003e50\u003c/sup\u003e. Therefore, the possible effects of SiO\u003csub\u003e2\u003c/sub\u003e on Mo and V adsorption were tested here as well; co-precipitation experiments simulated either Precambrian (2.2 mM) or Phanerozoic (0 mM) SiO\u003csub\u003e2(d)\u003c/sub\u003e concentrations based on predictions by Ref.\u003csup\u003e51\u003c/sup\u003e). All experiments were performed at pH = 8 and 0.56 M NaCl to mimic seawater pH and ionic strength.\u003c/p\u003e\n\u003cp\u003eThe behaviors of Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e were assessed across a wide range of initial conditions (see Methods; Supplemental Table S2). A linear relationship was observed between Mo/Fe in the solid and Mo\u003csub\u003e(d)\u003c/sub\u003e at equilibrium\u0026mdash;and analogously between V/Fe and V\u003csub\u003e(d)\u003c/sub\u003e\u0026mdash;which represent K\u003csub\u003eD(Mo)\u0026nbsp;\u003c/sub\u003eand K\u003csub\u003eD(Mo)\u003c/sub\u003e, respectively. The linear relationships indicate that there is an abundance of available sites relative to the metals adsorbing on the Fe(III)-oxyhydroxide surfaces (i.e., site saturation has not yet been achieved). In the case of Mo, during Fe(II) oxidation and hydrolysis to Fe(III)-oxyhydroxides, significant co-precipitation occurs when the initial Mo concentration is \u0026lt;5 ppm (52.1 \u0026micro;M) under SiO\u003csub\u003e2(d)\u003c/sub\u003e-free (0 mM) conditions (Fig. 2A). At higher initial Mo\u003csub\u003e(d)\u003c/sub\u003e, co-precipitation plateaus, with a maximum 42.5% removal of the initial Mo\u003csub\u003e(d)\u003c/sub\u003e. In contrast, almost 100% of V\u003csub\u003e(d)\u003c/sub\u003e present in the initial solution was removed by Fe(III)-oxyhydroxides across initial V\u003csub\u003e(d)\u003c/sub\u003e concentrations ranging from 0.1 ppm to 18 ppm under 0 mM SiO\u003csub\u003e2(d)\u003c/sub\u003e (Fig. 2B).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe presence of SiO\u003csub\u003e2(d)\u003c/sub\u003e significantly impacted the co-precipitation behavior of Mo relative to V. At 2.2 mM SiO\u003csub\u003e2(d)\u003c/sub\u003e, Mo partitioning to the precipitate is greatly reduced, with less than 4% of initial Mo\u003csub\u003e(d)\u003c/sub\u003e being removed to Fe(III)-oxyhydroxide (Fig. 2A). By contrast, V showed minimal sensitivity to SiO\u003csub\u003e2(d)\u003c/sub\u003e at 2.2 mM SiO\u003csub\u003e2(d)\u0026nbsp;\u003c/sub\u003e(Fig. 2B). While the mechanisms leading to this differential behavior are yet to be studied, we speculate a connection to the prevailing charge of the anion, i.e., the adsorption of V is classically modeled by considering V as the trivalent vanadate, while Mo can be modelled as the divalent molybdate which shows competitive adsorption with divalent SiO\u003csub\u003e2(d)\u003c/sub\u003e\u003csup\u003e52\u003c/sup\u003e. The important outcome of the results is that Mo scavenging by Fe(III)-oxyhydroxide precipitation was less effective under Precambrian (more Si) relative to Phanerozoic (less Si) marine conditions. By contrast, V exhibited a consistently high affinity for Fe(III)-oxyhydroxides regardless of silica concentrations, indicating effective sequestration of V from seawater into BIF during both eons. In short, the metal trends in BIF reflects sorptive behavior.\u003c/p\u003e\n\u003cp\u003eUsing\u0026nbsp;our derived partition coefficients, we reconstructed the relative abundances of\u0026nbsp;Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e in seawater across Earth\u0026rsquo;s history. Our analysis shows significant variability in average Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u0026nbsp;\u003c/sub\u003evalues, with Mo\u003csub\u003e(d)\u003c/sub\u003e consistently exceeding V\u003csub\u003e(d)\u003c/sub\u003e.\u0026nbsp;It also suggests that average Mo\u003csub\u003e(d)\u0026nbsp;\u003c/sub\u003evalues for the Archean\u0026nbsp;may have approached or even exceeded those in the Proterozoic and Phanerozoic oceans, although extracting an exact concentration is difficult given the uncertainties inherent in using a K\u003csub\u003eD\u003c/sub\u003e approach (Fig. 2C). Conversely, average V\u003csub\u003e(d)\u003c/sub\u003e are suggested as lower in the Archean compared to the Proterozoic and Phanerozoic (Fig. 2D). Ultimately, when plotted as Mo\u003csub\u003e(d)\u003c/sub\u003e/V\u003csub\u003e(d)\u0026nbsp;\u003c/sub\u003ethrough geological time (Fig. 2E), we find that mean Mo\u003csub\u003e(d)\u003c/sub\u003e/V\u003csub\u003e(d)\u0026nbsp;\u003c/sub\u003eshows some fluctuation, but the overall trend is Mo\u003csub\u003e(d)\u003c/sub\u003e \u0026gt; V\u003csub\u003e(d)\u0026nbsp;\u003c/sub\u003ein the Archean,\u0026nbsp;with\u0026nbsp;the Mo\u003csub\u003e(d)\u003c/sub\u003e/V\u003csub\u003e(d)\u0026nbsp;\u003c/sub\u003eratio decreasing\u0026nbsp;by two orders of magnitude from the Archean to the Phanerozoic. We are not aware of other major effects that might have fractionated Mo from V during incorporation into Fe(III)-oxyhydroxides. Therefore, we propose that Mo\u003csub\u003e(d)\u003c/sub\u003e was more available than V\u003csub\u003e(d)\u0026nbsp;\u003c/sub\u003ein the Archean.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe surprisingly high Archean\u0026nbsp;Mo\u003csub\u003e(d)\u003c/sub\u003e inferred from our approach is intriguing (see SI for reconciliation with the shale record). However, we caution that while the K\u003csub\u003eD\u003c/sub\u003e value approach is grounded in empirical observations,\u0026nbsp;multiple factors affect the extent to which these simple laboratory experiments can be extrapolated to ancient natural systems about which there are many uncertainties. Possible complications include the competitive adsorption of other ions, the effects of organic complexes, redox sensitivity of Mo and V speciation, and diagenesis. Nevertheless, unless these effects account for orders of magnitude of error, it is likely that Archean Mo\u003csub\u003e(d)\u003c/sub\u003e was equal to or greater than 100 nM. Moreover, many of these effects likely apply to both Mo and V. As such, our results provide an important first-order approximation of the relative abundances of\u0026nbsp;Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e in ancient seawater over Earth\u0026rsquo;s history, and we propose that the\u0026nbsp;Mo\u003csub\u003e(d)\u003c/sub\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003ein the Archean was not as low as previously suggested.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAlthough previous studies have explored the adsorption, co-precipitation, and behavior of Mo and V during Fe-mineral transformations (e.g., Ref.\u003csup\u003e53-62\u003c/sup\u003e), the specific coordination environments of Mo and V with Fe, particularly under marine conditions (pH 8, 0.56 M NaCl) remain poorly constrained. Using synchrotron-based EXAFS, we found that Mo(VI) forms stable octahedral inner-sphere complexes with six oxygen atoms, engaging in mono- or binuclear interactions with Fe, while V(V) adopts tetrahedral coordination through bidentate binuclear corner-sharing bonds (Fig. S1-S2). These inner-sphere complexes are structurally robust, suggesting that once Mo and V bind to Fe(III)-oxyhydroxides, their mobility is significantly limited during sedimentation and burial \u0026mdash; the exception being dissimilatory Fe(III) reduction \u0026mdash; consistent with their eventual incorporation into more stable oxides\u003csup\u003e53\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eMarine geochemical cycling\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMolybdenum delivery to the modern oceans primarily originates from the oxidative weathering of volcanic glass, Ti-bearing minerals and molybdenite (MoS\u003csub\u003e2\u003c/sub\u003e) grains in continental crust as well as from authigenic sulfides in shales\u003csup\u003e63\u003c/sup\u003e, while V is largely derived from the oxidative weathering of vanadiferous magnetite (VFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) and organic-rich shales\u003csup\u003e64\u003c/sup\u003e. Rivers thus serve as the main transport mechanism today, delivering Mo to the oceans in the form of\u0026nbsp;the molybdate anion and V in the form of the vanadate anion\u003csup\u003e65,66\u003c/sup\u003e. If oxidative weathering is the primary source of Mo and V, we would expect lower Mo/Ti and V/Ti in the anoxic Archean compared to younger BIF and Fe-rich sediments. However, the opposite trend is observed (Fig.1).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFocusing on Mo and its early availability for nitrogenase, how, then, could Mo\u003csub\u003e(d)\u003c/sub\u003e have been 100 nM in the Archean, as our analyses suggest? Recent studies at modern hydrothermal vents using advanced fluid sampling equipment\u0026mdash;where samples are protected from mixing and precipitation/adsorption effects\u0026mdash;offer a plausible Mo source independent of surface redox conditions. Chemical analyses of vent fluids from the Main Endeavour Field (Juan de Fuca Ridge) have reported Mo\u003csub\u003e(d)\u003c/sub\u003e concentrations of 29-220 nM\u003csup\u003e67\u003c/sup\u003e, often exceeding the Mo\u003csub\u003e(d)\u003c/sub\u003e concentration of average riverine fluxes (4.4 nM) by over an order of magnitude\u003csup\u003e68\u003c/sup\u003e. These findings are supported by hydrothermal basalt alteration experiments that similarly yield high Mo\u003csub\u003e(d)\u003c/sub\u003e\u003csup\u003e69\u003c/sup\u003e, as well as geological evidence from the 3.24 Ga Panorama district where oceanic crust shows evidence of hydrothermal Mo leaching\u003csup\u003e70\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIt is notable that the Mo\u003csub\u003e(d)\u003c/sub\u003e/Fe\u003csub\u003e(d)\u003c/sub\u003e ratios of these undiluted vent-fluids (0.30\u0026plusmn;0.22) are much closer to V\u003csub\u003e(d)\u003c/sub\u003e/Fe\u003csub\u003e(d)\u003c/sub\u003e ratios (1.25\u0026plusmn;0.58) than the two order of magnitude offsets recorded in both the Fe-rich sediment record and, indeed, modern plume fall-out. In the modern ocean this Mo\u003csub\u003e(d)\u003c/sub\u003e fractionation presumably arises from preferential incorporation into iron-sulfide nano-particles that incorporated Mo, or as molybdenite (MoS\u003csub\u003e2\u003c/sub\u003e), which is the stable phase primary mineral phase in hydrothermal systems\u003csup\u003e68,71,72\u003c/sup\u003e. However, some uncertainty exists regarding the speciation of Mo following hydrothermal venting in the Archean where Fe\u003csub\u003e(d)\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eS ratios were significantly higher. In the case of MoS\u003csub\u003e2\u003c/sub\u003e, solubility increases significantly with pH (by four orders of magnitude from pH 4 to 8) and under lower sulfide concentrations. Therefore, upon discharge from presumed ultramafic-dominated (komatiitic) seafloor hydrothermal systems into a low sulfate ocean, MoS\u003csub\u003e2\u003c/sub\u003e could convert to molybdate via the reaction\u003csup\u003e69\u003c/sup\u003e:\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003eIndeed, it has been demonstrated using x-ray adsorption spectroscopy that molybdate is stable under a wide range of conditions in hydrothermal fluids, particularly at circumneutral to basic pH conditions and over a range of temperature conditions\u003csup\u003e71\u003c/sup\u003e. Moreover, molybdate is the dominant Mo aqueous species under both oxic and anoxic conditions in the absence of dissolved sulfide (see SI for Mo aqueous speciation); the latter was likely low in the Archean due to minimal oxidative weathering of land or release from submarine volcanic vents when hydrothermal fluids had Fe/H\u003csub\u003e2\u003c/sub\u003eS ratios \u0026gt;\u0026gt;1\u003csup\u003e73\u003c/sup\u003e. This is significant because, in the modern ocean, the family of \u0026ldquo;oxy-anion\u0026rdquo; or \u0026ldquo;nutrient\u0026rdquo; elements (P, V, As, Cr, Mo) that are taken up into Fe-rich hydrothermal plume particles only do so in close proximity within the buoyant portions of hydrothermal plumes, within the first hour or less following release into the oceans\u003csup\u003e47\u003c/sup\u003e. Further, while recent work has highlighted the long-range dispersal that is possible for a sub-set of ridge-crest hydrothermal fluxes over entire ocean basins\u003csup\u003e74,75\u003c/sup\u003e, it remains the case that even in those studies more than 90% of the Fe\u003csub\u003e(d)\u003c/sub\u003e released through venting is removed in particulate form to underlying sediments within \u0026le;100km of their point of origin\u003csup\u003e76\u003c/sup\u003e. Importantly, Evans et al. (2023b)\u003csup\u003e69\u003c/sup\u003e have proposed that while hydrothermal processes have limited impact on today\u0026rsquo;s Mo-rich ocean, it could have played a significant role in supplying Mo to the Archean\u0026rsquo;s Mo-poor ocean because of higher heat flow on the earlier Earth\u003csup\u003e77\u003c/sup\u003e. Such an inference arises because, in this system, the Fe\u003csub\u003e(d)\u003c/sub\u003e/H₂S ratios were sufficiently high\u0026mdash;and thus free H₂S concentrations sufficiently low\u0026mdash;that significant removal of dissolved Mo from vent fluids as sulfidic phases was inhibited. At the same time, the Archean ocean was not oxidizing enough at depth (where hydrothermal fluids entered) for Mo to be rapidly scavenged from solution by co-precipitation with Fe(III)-oxyhydroxides in the rising buoyant hydrothermal plume. Interestingly, if\u0026nbsp;Archean Mo was primarily sourced from submarine-emplaced hydrothermal vents, this might explain its higher concentrations in Algoma-type BIF compared to Superior-type BIF, since the former have higher associated volcanic activity. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe contrasting trends observed in BIF records\u0026mdash;declining Mo\u003csub\u003e(d)\u003c/sub\u003e over time versus increasing V\u003csub\u003e(d)\u003c/sub\u003e \u0026mdash; probably reflect the coupled evolution of their respective sinks. Today, the major Mo sinks are co-precipitation with Fe(III)- and Mn(IV)-oxyhydroxides in the ambient oceans and removal in waters bearing hydrogen sulfide, H\u003csub\u003e2\u003c/sub\u003eS\u003csup\u003e78\u003c/sup\u003e, in sulfide-rich (euxinic) restricted basins, and in porewaters. Under euxinic conditions with H\u003csub\u003e2\u003c/sub\u003eS reaching 10\u003csup\u003e\u0026minus;3\u003c/sup\u003e to 10\u003csup\u003e\u0026minus;4\u003c/sup\u003e M,\u0026nbsp;MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e is converted into particle-reactive oxythiomolybdates and efficiently scavenged to the seafloor, making black shales significant Mo sinks\u003csup\u003e79\u003c/sup\u003e. As a result, euxinic sediments for much of the Phanerozoic often contain Mo concentrations \u0026gt;100 ppm versus ~1 ppm in average crust\u003csup\u003e80,81\u003c/sup\u003e. In the Archean, with high Fe\u003csup\u003e2+\u003c/sup\u003e and minimal seawater sulfate (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e) concentrations\u003csup\u003e73,82,83\u003c/sup\u003e, euxinic conditions were likely scarcer than in the modern. Moreover, in a setting where Fe\u003csup\u003e2+\u003c/sup\u003e upwelling onto early Archean continental shelves was oxidized to Fe(III), the high concentrations of SiO\u003csub\u003e2(d)\u003c/sub\u003e in ancient oceans\u003csup\u003e84\u003c/sup\u003e would have further inhibited Mo\u003csub\u003e(d)\u003c/sub\u003e sorption onto sinking Fe(III)-oxyhydroxides particles. The possible combination of high hydrothermal Mo input and hindered Mo sinks could have sustained Mo\u003csub\u003e(d)\u003c/sub\u003e concentrations in Archean seawater that were more elevated than commonly considered.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDuring the Proterozoic the source and sinks changed. This was reflected in the onset of oxidative weathering on land, which initiated a strong riverine Mo\u003csub\u003e(d)\u003c/sub\u003e flux and the expansion of euxinic conditions in the ocean to approximately 1\u0026ndash;10% of modern seafloor area\u003csup\u003e85\u003c/sup\u003e, possibly leading to enhanced Mo\u003csub\u003e(d)\u003c/sub\u003e sequestration from the water column despite enhanced continental weathering after the Proterozoic. In addition, the deposition of Mn(IV)-oxyhydroxides increased in the rock record (e.g., the 2.4 Ga Hotazel Formation, South Africa; see Ref.\u003csup\u003e86\u003c/sup\u003e), and then further in the Phanerozoic through the formation of Mn(IV)-rich nodules in the deep-sea, leading to the widespread removal of Mo\u003csub\u003e(d)\u003c/sub\u003e from seawater. Collectively, the balance between the increased source from continental weathering and expanded sinks, may have kept Mo\u003csub\u003e(d)\u003c/sub\u003e at low to intermediate concentrations until the more intense oxidative weathering and a decreasing euxinic sink in the Phanerozoic.\u003c/p\u003e\n\u003cp\u003eUnder oxygenated conditions, V\u003csub\u003e(d)\u003c/sub\u003e exists as highly-soluble VO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e, and its primary sink in oxic marine environments is adsorption onto Fe(III)- and Mn(IV)-oxyhydroxides and clay minerals in pelagic sediments\u003csup\u003e87\u003c/sup\u003e. Unlike Mo\u003csub\u003e(d)\u003c/sub\u003e, which competes with SiO\u003csub\u003e2(d)\u0026nbsp;\u003c/sub\u003efor sorption sites on Fe(III)-oxyhydroxides, V\u003csub\u003e(d)\u003c/sub\u003e is effectively removed regardless of SiO\u003csub\u003e2(d)\u0026nbsp;\u003c/sub\u003econcentrations. In this regard, extensive deposition of BIF before 1.8 Ga would have facilitated the removal of V\u003csub\u003e(d)\u003c/sub\u003e from seawater despite higher hydrothermal inputs than today. Then, as BIF deposition waned after 1.8 Ga, this major sink diminished, likely allowing seawater V\u003csub\u003e(d)\u003c/sub\u003e concentrations to increase over time. Although Mn(IV)-oxyhydroxide sinks eventually became important, it is unlikely that they were ever as large in magnitude as BIF were in the Precambrian\u003csup\u003e86\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eAs discussed above, the relative abundances of Mo\u003csub\u003e(d)\u003c/sub\u003e and V\u003csub\u003e(d)\u003c/sub\u003e in seawater reflect a dynamic balance between the sources and sinks of these elements. During the Archean, submarine hydrothermal vents may have released significant amounts of Mo. It is estimated that, because of the higher heat and volume fluxes predicted for submarine venting at that time\u003csup\u003e77\u003c/sup\u003e, the \u003cem\u003egross\u0026nbsp;\u003c/em\u003eMo flux at that time may have been as high as 2 x 10\u003csup\u003e7\u003c/sup\u003e mol/yr\u0026mdash;an increase of four-fold over modern day gross fluxes\u003csup\u003e69\u003c/sup\u003e. Because of the reducing conditions in the deep oceans where those fluids were emitted, however, the \u003cem\u003enet\u003c/em\u003e increase in supply of dissolved Mo to the deep-water column, absent any precipitation/removal as sulfides and/or oxyhydroxides close to the point of origin would have been even higher. \u0026nbsp;Subsequently, upon upwelling of that Fe-, V- and Mo-rich seawater to shallower more oxygenated depths, high seawater SiO\u003csub\u003e2(d)\u003c/sub\u003e would have hindered Mo removal to Fe(III)-oxyhydroxides, with much of that Mo\u003csub\u003e(d)\u003c/sub\u003e remaining in solution, while V\u003csub\u003e(d)\u003c/sub\u003e was preferentially sequestered into BIF because of its higher affinity for Si-rich Fe(III)-oxyhydroxides (Fig. 3A). Following the GOE, as continental exposure and oxidative weathering increased, the riverine supply of MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e and VO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e to seawater would have increased. As BIF deposition waned at 1.8 Ga and the extent of marine euxinia increased (as well as the deposition of Mn(IV)-oxyhydroxides at 2.4 Ga), the trends in Mo and V would have been reversed, with Mo sequestered into sediments while V became more bioavailable (Fig 3B). This represents an inflection point in the cycling of Mo and V in Earth\u0026rsquo;s surface environments and the fundamental role that the balance of sources and sinks play in governing elemental bioavailability over geological timescales. By the Phanerozoic, SiO\u003csub\u003e2(d)\u003c/sub\u003e declined due to the evolution of silica-secreting eukaryotes, the bulk oceans became increasingly oxygenated, leading to a Mo cycle like today (Fig 3C). Estimates suggest that modern submarine hydrothermal Mo fluxes are no higher than 4.5 x 10\u003csup\u003e6\u003c/sup\u003e mol/yr; for comparison the modern continental flux of Mo to seawater is 3.1 x 10\u003csup\u003e8\u003c/sup\u003e mol/yr\u003csup\u003e88\u003c/sup\u003e, dwarfing all other known sources. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eBioavailability of Mo and its relationship with N fixation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIt has been proposed that low availability of Mo\u003csub\u003e(d)\u003c/sub\u003e played a significant role in influencing the evolution of the biosphere\u003csup\u003e18,89\u003c/sup\u003e. However, our findings suggest that Archean seawater was characterized by elevated Mo/V despite low continental weathering inputs. It is thus not surprising that recent molecular data reveal that the earliest nitrogenases utilized Mo and that N isotope evidence of alternative nitrogenases has not been found.\u003c/p\u003e\n\u003cp\u003eWhile our results suggest that Mo\u003csub\u003e(d)\u003c/sub\u003e was probably high in the Archean, we also acknowledge that Mo\u003csub\u003e(d)\u003c/sub\u003e in modern anoxic waters rarely reaches levels low enough to limit N\u003csub\u003e2\u003c/sub\u003e-fixation, and that Mo-based nitrogenase activity continues when \u0026lt;5 nM, as suggested by recent studies\u003csup\u003e26-28\u003c/sup\u003e. Moreover, even when dissolved Mo is low, mineral-bound Mo, V, and Fe remain bioavailable. For example, recent work has shown that \u003cem\u003eMethanococcus maripaludis\u003c/em\u003e can utilize thiolated Mo species, such as MoS\u003csub\u003e2\u003c/sub\u003e or MoS\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, to meet its Mo demands\u003csup\u003e90\u003c/sup\u003e. These findings align with recent reports that the anaerobe \u003cem\u003eClostridium kluyveri\u003c/em\u003e and the facultative anaerobe \u003cem\u003eRhodopseudomonas palustris\u003c/em\u003e can extract Mo from MoS\u003csub\u003e2\u003c/sub\u003e using metal-chelating compounds to express nitrogenase genes and fix N\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e91,92\u003c/sup\u003e. Similarly, the N\u003csub\u003e2\u003c/sub\u003e-fixing bacterium \u003cem\u003eAzotobacter vinelandii\u003c/em\u003e secretes a high-affinity molybdophore to mobilize Mo from MoS\u003csub\u003e2\u003c/sub\u003e, enabling \u003cem\u003enif\u003c/em\u003e gene expression even under Mo\u003csub\u003e(d)\u003c/sub\u003e-depleted conditions\u003csup\u003e93\u003c/sup\u003e. While the evolutionary history of molybdophores is not well understood, these observations collectively point toward the ability of the biosphere to access Mo in a variety of forms that may be present in the Archean oceans or proximal to hydrothermal vents.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOur findings support the early evolution of molybdoenzymes, indicating that the Mo\u003csub\u003e(d)\u003c/sub\u003e concentrations in the Archean may have rendered Mo more bioavailable than previously assumed. The expansion of Mo sinks under the more widespread sulfidic conditions in the Proterozoic likely exerted selective pressure for organisms to develop Mo storage systems to mitigate the decreasing Mo\u003csub\u003e(d)\u003c/sub\u003e bioavailability. Taken together, in the light of previous phylogenetic and isotopic studies\u003csup\u003e5,33,35\u003c/sup\u003e, our study suggests that the evolution of Mo-nitrogenase and the availability of Mo\u003csub\u003e(d)\u003c/sub\u003e were closely linked, resolving the \u0026ldquo;Mo paradox\u0026rdquo;.\u0026nbsp;\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eThe co-precipitation experiments of Mo and V with Fe(III)-oxyhydroxides were performed in 0.56 M NaCl solution \u0026ndash; exposed to the atmosphere - to mimic the major cation and anion contributions to seawater salinity. Different concentrations of Mo and V, in the form of their respective oxyanions, were prepared by diluting 1,000 ppm Mo and V stock solutions into several 50 mL polypropylene tubes containing 0.56 M NaCl to generate Mo and V solutions with concentrations ranging from 0.1 ppm to 18 ppm, respectively. Our Mo and V concentrations are significantly higher than modern ocean dissolved Mo and V concentrations (around 10 ppb and 1 ppb, respectively), which is because our initial Mo and V concentrations represent the amount of Mo and V before adsorption. The same concentration range for Mo and V was used to allow for a more direct comparison of their behavior on Fe(III)-oxyhydroxides surfaces. The total volume of each solution is 50 ml. Then, a 5 ml of each solution was removed using a syringe for measuring the initial concentrations of Mo and V. After that, 0.1 mL of 1 M FeCl\u003csub\u003e2\u003c/sub\u003e was added to the 0.56 M NaCl solutions, and the solutions were left to reach equilibrium with the atmosphere for three days. During this period, the solution pH was maintained at 8 by adding small aliquots of diluted NaOH and HCl solution to mimic seawater pH. Note: we chose this pH because recent work on BIF sedimentation velocities have argued that a pH range between 7 and 8 was required for the deposition of major BIF deposits\u003csup\u003e94\u003c/sup\u003e. The coprecipitation experiments were conducted at both 0 mM and 2.2 mM Si conditions to allow for comparisons between a Precambrian Si-rich and Phanerozoic Si-depleted ocean. After three days, solutions were filtered through 0.2 \u0026micro;m nylon filters. The concentrations of Mo and V in the initial solutions and final solutions after filtration were measured by ICP-MS.\u003c/p\u003e\n\u003cp\u003eSynchrotron-based extended X-ray adsorption fine structure (EXAFS) analysis was performed on solid samples from systems where Mo and V were coprecipitated with Fe(III)-oxyhydroxides. The protocol for the preparation of Mo and V bearing Fe(III)-oxyhydroxides was the same as in the above coprecipitation experiments. The Mo, and V containing Fe(III)-oxyhydroxides samples were air-dried (until no further weight loss was detected), sealed in airtight zip-lock bags using a heat-sealer and transported for EXAFS analysis. The EXAFS experiments were performed at the Hard X-ray MicroAnalysis (HXMA) beamline of the Canadian Light Source (CLS) in Saskatoon, Canada. EXAFS spectra were processed and fitted using ATHENA in the DEMETER software package and WinXAS. FEFF 7 was used to generate backscattering phase and amplitude functions for the corresponding scattering paths. Detailed EXAFS analytical methods are provided in SI.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eW.H. was supported by the National Key Research and Development Program of China (2024YFF0810200). This work was supported by National Science and Engineering Research Council of Canada (NSERC) Discovery Grants to K.O.K. (RGPIN-165831) and L.J.R. (RGPIN-2021-02523). The authors would also like to acknowledge comments provided on earlier versions of the text by Drs. William Seyfried, Guy Evans and Jennifer Glass. \u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eW.H., K.O.K., and L.J.R. conceived the study; W.H. performed all the co-precipitation experiments and data analysis and EXAFS experiments; W.H., K.O.K., and L.J.R. compiled the BIF database compilation; N.C., and W.S. helped with the analysis of EXAFS data. W.H., K.O.K, L.J.R. wrote the manuscript with input from A.D.A., B.K., H.R.R., D.S.A, H.D., E.E.S., C.R.G., and T.W.L. \u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eYung, Y. \u0026amp; McElroy, M. 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Earth Sci.\u003c/em\u003e \u003cstrong\u003e127\u003c/strong\u003e, 359-378 (2024).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"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":"","lastPublishedDoi":"10.21203/rs.3.rs-7237238/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7237238/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eBiological nitrogen (N\u003csub\u003e2\u003c/sub\u003e) fixation is crucial for incorporating atmospheric N\u003csub\u003e2\u003c/sub\u003e into the biosphere. However, the evolutionary trajectory of this process remains enigmatic, particularly regarding the paradoxical relationship between the presumed scarcity of molybdenum (Mo) in the Archean ocean—before oxidative weathering on land—and the apparent early emergence of Mo-based nitrogenase over the vanadium (V)-based alternative. Here, we integrate partitioning experiments with an extensive analysis of iron (Fe)-rich sediments through time to elucidate the behavior of Mo and V in ancient seawater. Our findings suggest that in the Archean to Paleoproterozoic, dissolved V was more efficiently removed from the water column than Mo, primarily due to the preferential incorporation of V into banded iron formations (BIF). Our data also suggest that dissolved Mo was more available in seawater than commonly assumed and that the ratio Mo/V was higher in the Archean than it is today. Higher Mo abundances and greater availability of Mo relative to V offers a compelling explanation for the earlier evolution of Mo-based nitrogenase over the alternative nitrogenases. \u0026nbsp;\u0026nbsp;\u003c/p\u003e","manuscriptTitle":"Evolution of the molybdenum and vanadium cycles through time and their impact on ancient nitrogen fixation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-01 08:14:31","doi":"10.21203/rs.3.rs-7237238/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":"2d0c08a5-cd7b-4dff-97b8-f69532055728","owner":[],"postedDate":"September 1st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":53909698,"name":"Earth and environmental sciences/Biogeochemistry/Element cycles"},{"id":53909699,"name":"Earth and environmental sciences/Ocean sciences/Marine chemistry"}],"tags":[],"updatedAt":"2026-04-14T16:00:42+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-01 08:14:31","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7237238","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7237238","identity":"rs-7237238","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}