EXTRACELLULAR ELECTRON TRANSFER IN PHOTOAUTOTROPHS AND THEIR ROLE IN ELECTROGENIC ACTIVITY

EXTRACELLULAR ELECTRON TRANSFER IN PHOTOAUTOTROPHS AND THEIR ROLE IN ELECTROGENIC ACTIVITY | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 18 September 2025 V1 Latest version Share on EXTRACELLULAR ELECTRON TRANSFER IN PHOTOAUTOTROPHS AND THEIR ROLE IN ELECTROGENIC ACTIVITY Authors : Srividya Polaki and Ganesh Mahidhara 0009-0009-8359-6308 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.175819174.45660050/v1 374 views 121 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Anoxygenic phototrophic bacteria are diverse group of microorganisms, such as Chloroflexus, Rubrivivax, Rhodopseudomonas. These photoautotrophs are defined by their ability to grow using light with distinctive metabolic properties enabling them to extracellular electron transfer (EET) without producing oxygen as byproduct. The mechanisms of EET and their ability to obtain electrons from organic molecules or reduced compounds and transfer them to external acceptors such as iron, sulphur or any other oxidised compounds will be explored in this chapter. The key Molecular interactions and metabolic pathways that support electrogenic activity are discussed and additionally, their applications in bioelectrochemical technologies, bioremediation and wastewater treatment are discussed. EXTRACELLULAR ELECTRON TRANSFER IN PHOTOAUTOTROPHS AND THEIR ROLE IN ELECTROGENIC ACTIVITY Srividya Polaki 1 , Ganesh Mahidhara 1,* 1 GITAM University, Department of Life Sciences, GSS,2 Visakhapatnam- 530 045, India * Corresponding author. [email protected] Abstract: Anoxygenic phototrophic bacteria are diverse group of microorganisms, such as Chloroflexus, Rubrivivax, Rhodopseudomonas. These photoautotrophs are defined by their ability to grow using light with distinctive metabolic properties enabling them to extracellular electron transfer (EET) without producing oxygen as byproduct. The mechanisms of EET and their ability to obtain electrons from organic molecules or reduced compounds and transfer them to external acceptors such as iron, sulphur or any other oxidised compounds will be explored in this chapter. The key Molecular interactions and metabolic pathways that support electrogenic activity are discussed and additionally, their applications in bioelectrochemical technologies, bioremediation and wastewater treatment are discussed. INTRODUCTION: Anoxygenic photoautotrophs are a group of organisms thatperform photosynthesis, they use light energy to produce organic compounds. Anoxygenic photoautotrophs include diverse groups like Purple Sulfur Bacteria (PSB), green sulfur bacteria (GSB) and Green non-sulfur bacteria.GSB are capable of oxidizing H2S into elemental sulfur, they possess special structures called Chlorosomes where the photosynthetic pigments are located. These specialized structures are vesicles, surrounded by a lipid monolayer that function as light-collecting antennas.(Kushkevych et al., 2024). Purple sulfur bacteria on the other hand similar to GSB grow under low light intensity and exhibits sensitivity to oxidizing environments, has great tolerance to H2S, anoxygenic condition is required for PSB to grow phototrophically as the molecular oxygen represses the biosynthesis of pigment and complexes (Alarcon et al., 2024). The GSB stores sulfur extracellularly while PSB can storesulfur in intracellular globules(Kushkevych et al., 2024).Non sulfur bacteria are most versatile metabolically hence they can adapt to any nutritional mode, depending on the environmental condition they grow as photoautotrophs(Dhar et al., 2023).Microbial activity drives the biochemical cycling of organic and inorganic compounds in the biosphere(Rumora et al., 2023). Unlike photoautotrophs (e.g., cyanobacteria), anoxygenic photoautotrophs do not produce oxygen as byproduct.The reaction centers in the photosynthetic bacteria covert the light energy in electrochemical energy and reaction center is a protein that spans the membrane containing many cofactors which are involved in energy conversion (Brzezinski et al., 1997). The anoxygenic photosynthetic apparatus transforms light energy into an electrochemical gradient of protons across the photosynthetic membrane (PM). This gradient can be utilized for ATP production, active transport, motility, and other energy-consuming processes(Yurkov and Beatty, 1998). Figure.1. oxygenic and anoxygenic Photoautotrophs as become global issue, which could be addressed by renewable energy technologies as a source to provide a safe alternative to develop more sustainable society(Mahidhara et al., 2017; Rumora et al., 2023), the rapid expansion of industries and large scale of urbanization lead to inappropriate waste disposal and exploitation of environment by depositing enormous amount of pollutants(Dhar et al., 2023). it is well known that microorganisms have the ability to degrade variety of pollutants, they are capable of bioremediation of hazardous substances are that are deposited and restore the ecosystem (Dhar et al., 2023). while detoxifying the pollutants,the microorganism can also produce fuels, like hydrogen, ethanol and methane from organic matter, the most interesting part is they can convert the organic matter into electricity in devices known as microbial fuel cells (MFC)(Lovley, 2006). the energy shortage and need for treatment of wastewater released untreated to water resources which pose a serious threat to water bodies which have pointed the spotlight to MFC, gaining attention for itscapability in waste water treatment and simultaneously harvesting electricity from organic waste(Haddadi et al., 2014). MFCs are attractive sources of energy because of renewable energy production,bioremediation and they are carbon-neutral(Lovley, 2006)(Haddadi et al., 2014). Microorganisms in these systems act as catalysts to oxidize organic compounds into protons, electrons, and CO2(Chen et al., 2019). The aim of this chapter is to provide a general overview of the literature available on anoxygenic photoautotrophs, about their molecular mechanism of EET, molecular mechanisms, process of anoxygenic photosynthesis, metabolic pathways, and structure of thephotosynthetic units.Finally, we will discuss on biotechnology applications of the phototrophic bacteria. General characteristics and structural features of anoxygenic photoautotrophic bacteria Anoxygenic photoautotrophs are photosynthetic bacteria; they belong to a diverse phylogenetic group of bacteria that use a variety of organic or inorganic electron donors to perform anoxygenic photosynthesis.They are present in soil, fresh water resources like ponds or lakes, and in other habitats like sea and wastewaters and sulfur springs; They are gram negativeprokaryotic bacteria (Kushkevych et al., 2024, 2021). Anoxygenic phototrophic bacteria differ from oxygenic phototrophs by the type of chlorophyll and carotenoid pigments they possess. These bacteria thrive in anaerobic conditions and can carry out photosynthesis without producing oxygen(George et al., 2020; Kushkevych et al., 2021).Anoxygenic phototrophs distinguish from oxygenic phototrophs based on two common traits, first they do not rely on chlorophyll as the primary photopigment but rely on the bacteriochlorophylls. Second, they do not oxidize water; instead, they utilize sulfide, thiosulfate, hydrogen, or similar electron donors to grow as photoautotrophic bacteria (Figure 1) +(Gregersen et al., 2011; George et al., 2020). The anoxygenic photoautotrophs use both soluble and insoluble electron donors like Fe(II) and donot solely depend upon water splitting for photosynthesis and this process is called photoferrotrophy(Gupta et al., 2020). Anoxygenic photoautotrophic bacteria can be categorized into two primary groups based on their pigmentationand on the ability to oxidize reduced sulfur compounds: green sulfur bacteria (GSB) and purple sulfur bacteria (PSB).The two sulfur bacteria differ in their ways of storing sulfur, which can be a product of H2S oxidation, H2S can be toxic to other group of microbial community (Černý et al., 2018; Dordević et al., 2021)Although the “ green non-sulfur ” bacteria come under anoxygenic phototrophic bacteria they do not use sulfur as electron source, and they can oxidize other substrates and grow as aerobic chemoheterotrophs in dark and anaerobic photoheterotrophs in light. Green non-sulfur bacteria are now commonly classified as filamentous anoxygenic phototrophic bacteria (FAPs). This group is also known as Chloroflexales, which includes species like Chloroflexusaurantiacus, and differs from green sulfur bacteria both phylogenetically and physiologically(George et al., 2020; West-Roberts et al., 2021). Green Sulfur Bacteria ( Chlorobiaceae ) GSB belong to the Chlorobiaceae family, they are diverse chlorophototrophic organisms and are strictly anaerobic photosynthetic bacteria, that are characterized by the ability to oxidize reduced sulfur compounds or molecular hydrogen as electron donors for photoautotrophic growth(Bryant et al., 2012; Gregersen et al., 2011).GSB comprises of only four genera, which include: Chlorobium , Chlorobaculum , Prosthecochloris and Chloroherpeton (Imhoff, 2003). Genus Chloroherpethon have special structures, the cells are long flexible fiber shaped and can move by gliding(Kushkevych et al., 2024).The presence of bacteriochlorophylls, the photosynthetic pigments of green sulfur bacteria, and their degradation products in subfossil sediments indicate that photic anoxia zones existed more than 6,000 years ago(Manske et al., 2005).They mostly occur in anoxic aquatic environment and majority are mesophilic, the most common habitats include lagoons, seas, freshwater lakes and marine sediments, where light and sulfide coincide (Gregersen et al., 2011; Kushkevych et al., 2021). The only exception to this family is Chlorobiumtepidum , which is thermophilic and was isolated from sulfide hot springs in New Zealand grows typically at 46-48⁰C temperature in anoxic condition(Wahlund et al., 1991). Chlorobiaceae familymembers are Gram-negative, immobile and can either appear as rod-shaped, spherical, oval or cocci in structure. Some species produce gas pockets, and these bacteria store elemental sulfur granules outside the cells, unlike members of the Chromatiaceae family (purple sulfur bacteria). Some green sulfur bacteria, like Cbi. thiosulfatophilum, form polyphosphate granules(Hughes et al., 1963). The genus Prosthecochloris have prosthecae, protrusions of cytoplasmic membrane which provies large area for photosynthesis(Kushkevych et al., 2024).The Chromatiaceae grows as a layer of bacteria and under that Chlorobiaceae family grows asone or more layers of bacteriadeeper in the water columns. Their coexistence is relevant as the Chlorobiaceae requires less light intensity compared to Chromatiaceae. The Chromatiaceae family bacteria layers protect Chlorobiaceae family from oxygen as they are almost not tolerant to O2(Kushkevych et al., 2021). Light Harvesting complexes of GSB Chlorosomes are unusual light harvesting antennae present in GSB, Chlorosomes are natural optical antenna complex, which convert the absorbed light energy into molecular excitations and transfer to reaction center and a electrochemical gradient is generated across cytoplasmic membrane (Chlorosomes, 2014; Tang et al., 2013). These are the most efficient light-harvesting complexes known, as they can work even in low light intensity. Chlorosomes are ovoid, 70-180 nm in length and 30-60 nm in width which are attached to inner side of Transmembrane(Kushkevych et al., 2021). It is documented that Chlorosomes differ from species to species by atleast a factor of five in their volume and shape, they can be Ellipsoid shaped, conical shaped or irregularly shaped (Oostergetel et al., 2010). Chlorosomes are large structures that contain hundreds of thousands of bacteriochlorophyll (BChl) molecules. Depending on the species, a single chlorosome can have approximately 200,000 to 250,000 BChl c and BChl d molecules, around 2,500 BChl a molecules, 20,000 carotenoid molecules, 15,000 chlorobiumquinone molecules, 3,000 menaquinone-7 molecules, 5,000 proteins of 10 different types, and about 20,000 lipid molecules(Oostergetel et al., 2010). The BChls aggregates are organized as mixture of tubular and lamellar shapes (Tang et al., 2013). Chlorosome is enveloped by protein lipid monolayer on the side connected to plasma membrane, Chlorosomes are attached to the plasma membrane through a structure called the basal plate, which contains bacteriochlorophyll a. This pigment is organized by the CsmA protein into a paracrystalline structure. Chlorosomes are unique in their organization; rather than being stabilized by protein-pigment interactions, they are held together through direct interactions between pigment molecules. These pigments self-assemble into supramolecular units without the need for protein stabilization.Chlorosomesstructrure is influenced by its environment. The solar energy absorbed by Chlorosome is transferred to reaction center (RC) by a water-soluble trimer, called Fenna-Matthews-Olson protein (FMO protein) ( Figure.2 ). Figure.2. Structure of Chlorosome It binds to eight molecules of bacteriochlorophyll a, facilitating charge transfer between the baseplate complex and the reaction center. The light-excited electrons are allowed to leave the Chlorosome by basal plate and transferred to photosynthetic reaction center. (Fujita et al., 2014; Kushkevych et al., 2024, 2021; Pšenčík et al., 2004; Van Dorssen and Amesz, 1988). Figure.3. organization of the photosynthetic apparatus in Chlorobiaceae.Cyt-cytochrome; Fx, FA, FB are the FeS clusters that are bound to Reaction center. Fd-ferrodoxin Purple Sulfur Bacteria ( Chromatiaceae ) Purple Sulfur Bacteria (PSB) belong to Chromatiaceae family, a diverse group of anoxygenic photoautotrophic Gamma-proteobacteria(Kushkevych et al., 2021; Madigan et al., 2024). These bacteria require anoxic conditions for them to grow phototrophically as molecular oxygen represses the biosynthesis of pigments and complexes (Alarcon et al., 2024). PSB thrive or occur in various sulfidic aquatic environments such as lakes, ponds, estuaries, and intertidal and subtidal ocean waters. Other common habitats of purple sulfur bacteria (PSB) include sulfur springs, which provide a constant supply of sulfur compounds, as well as wastewater and coastal areas, such as those found in the Black Sea. Thermochromatiumtepidum, the only thermophilic genus within this family, can be found in extreme environments like hot sulfur springs. Additionally, it is speculated that the Chromatiaceae family may even exist in sea ice; the discovery of bacteria related to the genera Rhabdochromatium and Thiorhodovibrio lends support to this hypothesis(Kushkevych et al., 2021; Madigan et al., 2024). Purple sulfur bacteria (PSB) are Gram-negative prokaryotes that are mesophilic, motile, and can be either rod-shaped or coccoid-shaped. These bacteria contain bacteriochlorophyll (BChl) a or b and specific carotenoids, such as okenone and lycopene. The essential pigments in PSB enable them to absorb near-infrared and green light, which they utilize for anoxygenic photosynthesis. PSB can store elemental sulfur granules within their cells and are capable of reoxidizing hydrogen sulfide. They use reduced sulfur compounds as electron donors for anoxygenic photosynthesis. The reoxidation of sulfide by PSB results in the production of non-toxic forms of sulfur, such as elemental sulfur and sulfate. Most PSB are anaerobic photolithoautotrophs, although some exceptions can perform photolithoheterotrophy, chemolithotrophy, or chemoorganoheterotrophy in environments with low molecular oxygen (O2). PSB play a crucial role in the anoxic cycling of carbon, acting as primary producers that fix CO2. These bacteria require specific growth conditions: they have low to no tolerance for O2, need light for photosynthesis, and require reduced sulfur compounds in their environment to thrive(Alarcon et al., 2024; Kushkevych et al., 2021; Madigan et al., 2024). Light Harvesting complexes of PSB The Purple Sulfur Bacteria consists of two types of light-harvesting complexes, light-harvesting complex 1 (LH-1) and light-harvesting complex 2 (LH-2) ( Figure.4 ). The LH-1 surrounds the RCs directly, whereas the LH-2 is not in direct contact with RC, it transfers the absorbed energy to the LH-1 complex, which passes the energy on to the RC ( Figure.5 )(Kushkevych et al., 2021).Athird type of light-harvesting complex, LH-3 exists in some Purple Non-Sulfur Bacteria (PNSB), Such as Rhodopseudomonasacidophila and Rhodospirillum molischianum strain DSM120, which possess a absorption maxima at 800 and 820nm (Boonstra et al., 1994). Figure.4. Orientation of BChls in the LH-I–RC complex B875 BChls of LH-I shown in puple, and the special pair (PA and PB) and accessory BChls (BA and BB) of the RC shown in red and yellow. The LH complexes absorb various incident photons within the plasma membrane. These complexes are made up of proteins containing BChl and carotenoid pigments(Alarcon et al., 2024).In the model organism Allochromatiumvinosum (A. vinosum), the peripheral light-harvesting complex is composed of 12 copies of two short polypeptides, referred to as α and β. These α and β polypeptides form two concentric protein cylinders within the membrane. Between these cylinders are the light-harvesting bacteriochlorophylls and carotenoids. Specifically, there are 24 molecules of bacteriochlorophyll a, with one molecule of each α and β polypeptide firmly attached to the polypeptide ring. Additionally, 12 other bacteriochlorophyll molecules are weakly attached to the complex. This structure also contains light-harvesting carotenoids from the spirilloxanthin series(Kushkevych et al., 2021). The pigments and carotenoids can absorb the light energy by transforming their bonding and electronic states, funneling it down an energy gradient to the central RC’s, in which the charge separation occurs across the membrane and a series of redox reactions are driven involving other protein complexes such as quinone/quinol, cytochrome b/c, and cytochrome c complexed bound within their membrane (Alarcon et al., 2024). It was identified that A.vinosum has three subunits in the RC, pufC, pufM, and pufL, which are clustered and co-transcribed with three sets of pufA and pufB genes encoding LH1 complex apoproteins(Nagashima et al., 2002; Weissgerber et al., 2011).Six potential puc gene pairs were identified as coding for α and β apoproteins found in various types of LH2 complexes(Niedzwiedzki et al., 2012). Figure.5. configuration of pigment–protein complexes in the modeled bacterial PSU of Rb. sphaeroides. Metabolism Microbial metabolism consists a set of chemical reactions that allow life to exist, they allow an equivalence between the breakdown of molecules (catabolism) and synthesis of new molecules (anabolism). One significant, microbial metabolism that some microorganisms can perform is Extracellular Electron Transfer (EET), a type of anaerobic respiration that allows an electron to effectively make a direct or indirect transfer between a cell to an extracellular solid via an electron donor or an electron acceptor. This metabolic capability provides microbes with an energy source from electron donors or acceptors that do not enter the cell, which is distinct adaptation for life in an anoxic, mineral rich environment. Carbon fixation Anoxygenic phototrophs fundamentally use different pathways for carbon fixation, which is similar to that in plants (reductive pentose phosphate cycle) (Sirevåg and Ormerod, 1970). rTCA Cycle in GSB Green Sulfur Bacteria (GSB) assimilate inorganic carbon (CO2) autotrophically through a unique reverse tricarboxylic acid (rTCA) cycle ( Figure.7 ). The rTCA cycle, when compared to the oxidative TCA cycle of respiration, could be viewed as reverse pathway, CO2 fixation using energy to reduce carbon dioxide and produce organic matter, such as pyruvate and acetate(Tang and Blankenship, 2010). rTCA requires four important enzymes to catalyse the energetically unfavourable reverse steps in oxidative cycle: Pyruvate:ferredoxin (Fd) oxidoreductase acetyl-CoA + CO 2 + 2Fd red + 2H + ⇌ pyruvate + CoA + 2Fd ox ATP citrate lyase (ACL) acetyl-CoA + oxaloacetate + ADP + P i ⇌ citrate + CoA + ATP α-ketoglutarate:ferredoxin oxidoreductase succinyl-CoA + CO 2 + 2Fd red + 2H + ⇌ α-ketoglutartate + CoA + 2Fd ox Fumarate reductase succinate + acceptor ⇌ fumarate + reduced acceptor. Chlorobiumthiosulfatophilum provides the evidence for the rTCA cycle in GSB, by the observation that CO2 fixation in GSB was inhibited by fluoroacetate, a compound that inhibits forward TCA cycle in GSB (Sirevåg and Ormerod, 1970; Tang and Blankenship, 2010). Figure.7. rTCA cycle Calvin-Benson-Bassham Cycle in PSB The Calvin-Benson-Bassham cycle (CBB) is the principal pathway for carbon fixation in PSB. It is a common carbon fixation pathway in plants and cyanobacteria, which are oxygenic photosynthesizers, which suggests that either PSB has historical evolutionary connection to plants or cyanobacteria, or due to shared environmental niche, they must have evolved with similar characteristics which made them capable of existing in various environments(Kusian and Bowien, 2006). The CCB is the primary method for the assimilation of inorganic carbon in PSB. It uses a key enzyme Ribulose biphosphte carboxylase/oxygenase (RubisCO), which is responsible for the incorporation of inorganic carbon into biological molecule(Kusian and Bowien, 2006). The CBB cycle comprises of three phases(Farineau et al., 2011): 1. Carbon fixation 2. Reduction of phosphoglyceric acid 3. Regeneration of the CO2 acceptor. Table 1: different mechanisms of electron transport dynamics between GSB and PSB Extracellular Electron Transport in Photoautotrophs and their mechanisms Extracellular electron transport (EET) is a fundamental process in anaerobic respiration that facilitates the reduction or oxidation of molecules and minerals (metal species) outside the cell. EET is unique among other respiratory activities because of its essential role in transferring electrons across and beyond the cell membrane(Gralnick and Bond, 2023; Partipilo et al., 2022). Over the past decade, there is a significant progress in resolving the well-conserved strategies that facilitate electron conduction from the inner membrane to the outer surface, into the extracellular environment(Gralnick and Bond, 2023; Kato, 2015). EET couples the carbon oxidation to the reduction of various extracellular electron acceptors, which can include metal species, electrodes and even other organisms (Kato, 2015). This process is vital for the energy generation, allowing microbes to convert redox potential differences into bioavailable energy, which is in the form of ATP (Partipilo et al., 2022). This metabolic flexibility is advantageous in anoxygenic environment where there is little or no availability of soluble electron acceptors, which enables the microorganisms to acquire energy by interacting with insoluble solid materials (Paquete et al., 2022). Anoxygenic photoautotrophs have the ability to transfer electrons beyond their cell boundaries to interact with the insoluble electron acceptors or donors, which represents their evolution and adaptation. Microbial EET Mechanisms Microbial electron transfer (EET) is typically categorized into two primary mechanisms: Direct Electron Transfer (DET) and Indirect Electron Transfer, also referred to as Mediated Electron Transfer (MET • Direct Electron Transfer (DET): In this mechanism, microorganisms attach themselves to solid surfaces, such as minerals or electrodes, and transfer electrons through specialized membrane-bound proteins. This process involves c-type cytochromes, which are redox-active proteins that create conductive pathways extending from the inner to the outer cell surface. This facilitates electron transfer from the inner to the outer cell membrane, involving multiple redox-active proteins that lead to external materials (Paquete et al., 2022). Well-studied electroactive bacteria, such as Geobactersulfurreducens and Shewanella oneidensis , are known to produce conductive filamentous structures, including pili and outer membrane extensions known as microbial nanowires. These structures extend beyond the cell, enabling electron transfer to solid materials located at a distance (Hou et al., 2025; Kato, 2015). • Indirect Electron Transfer: METrelies on soluble electron shuttles, also known as electron mediators, to facilitate electron transfer between microbial cells and external surfaces. These mediators can be small organic molecules, such as flavins (for example, flavin mononucleotide) or phenazines, which are either synthesized or excreted by microorganisms (Bose and Wang, 2024; Kato, 2015). The process starts with microbial cells either reducing or oxidizing specific mediators, these mediators diffuse to a solid surface, where they either donate or accept electrons. After this electron exchange takes place, the re-oxidized mediators return to the microbial cell for regeneration and reuse, thereby completing the shuttle cycle (Bose and Wang, 2024; Hou et al., 2025). • Pivotal Role of Biofilms in EET: Biofilms are complex, structured communities of microorganisms that play a crucial role in enhancing electron transfer processes (EET). They create a conductive network that facilitates electron transfer through the localized production and retention of electron shuttles. The density and attachment of biofilms to surfaces are critical for generating electricity efficiently in bioelectrical systems(Hou et al., 2025; Kato, 2015). The interaction between direct and indirect electron transfer pathways demonstrates a more synergistic relationship than mere competition. Evidence of accelerated direct electron transfer and the consumption of flavins supports this idea. The functioning of biofilm networks suggests a collective approach to electron transfer rather than a series of discrete, adaptivestrategies.This system allows for variability among microorganisms, enabling them to optimize electron transfer in response to frequently changing physicochemical conditions, such as the distance to electron acceptors, the availability of dissolved redox-active compounds, and the structure of multicellular arrangements. This striking example of metabolic adaptability highlights the strategies that microbes have developed for effective electron transfer in diverse and fluctuating redox environments(Bose and Wang, 2024). EET in GSB In the family Chlorobiaceae, the type I reaction center, also known as the iron-sulfur type reaction center, utilizes ferredoxin and flavodoxin as the initial electron acceptors. The primary donor of this reaction center (P840) is a special pair of bacteriochlorophylls, which is located on the core homodimer protein PscA. Its immediate electron donor is RC-bound cytochrome c-551 (PscC, the product of the CT1639 gene)(Sakurai et al., 2010). The P840 is excited by light (P840*) and, in turn, reduce ferredoxin (Fd), which acts as a strong reductant vital for carbon dioxide fixation and the reduction of NAD+. The oxidized reaction center possesses a redox potential of 240 mV and is re-reduced by electrons derived from various extracellular electron donors, such as H2S, Fe2+, or thiosulfate (Hauska et al., 2001). The oxidation of sulfide ions leads to the formation of elemental sulfur in extracellular globules. These sulfur globules act as intermediates that are subsequently consumed and further oxidized to sulfate when the external sulfide concentration diminishes(Sakurai et al., 2010). All GSB strains contain at least one homolog of sulfide:quinone oxidoreductase (SQR). The subsequent oxidation of sulfide products to sulfite typically involves the dissimilatory sulfite reductase (DSR) system. This system seems to have been acquired through horizontal gene transfer from both sulfide-oxidizing and sulfate-reducing bacteria(Gregersen et al., 2011). Some species of the Chlorobiaceae family, such as Chlorobiumferrooxidans and Chlorobiumphaeoferrooxidans, have a unique metabolic ability to oxidize ferrous iron, using it as their sole electron donor for photosynthesis, while assimilating sulfur as sulfate. Other species, including Chlorobium sp. N1 and Chlorobium sp. BLA1, demonstrate metabolic flexibility by being able to oxidize both ferrous iron and sulfide. The molecular basis for photoferrotrophy in the Chlorobia is partly due to genes that encode porin-cytochrome c protein complexes (PCC), such as pioAB and mtoAB, as well as outer-membrane monoheme c-type cytochromes like cyc2(Tsuji et al., 2020) . Cyc2 has been identified as a critical marker of iron oxidation and plays a specific role in the microaerophilic pathways of iron oxidation. In addition to linear electron transport mechanisms, GSB also utilize cyclic electron transport, where electrons are cycled around the photosynthetic apparatus to convert light energy into ATP. This cyclic mechanism enhances ATP production consistently, even when the reducing power for carbon fixation is not immediately needed by the cell.The identification of sulfur oxidation components such as SQR and iron oxidation components like PCCs and Cyc2 indicates that there are separate pathways for each terminal electron donor. GSB have also been shown to undergo horizontal gene transfer of certain DSR components, indicating a complex evolutionary history. This reveals a highly evolved strategy in which GSB have adapted their unique electron transfer systems to exploit a variety of inorganic electron donors, taking advantage of multiple co-occurring sources in their anoxic environments. This metabolic flexibility is facilitated by the diversity of cytochromes and protein complexes, which is crucial to the ecological success of these organisms and their historical role in regulating Earth’s biogeochemistry, particularly in the sulfur and iron cycles of ancient oceans (Tsuji et al., 2020) Molecular mechanism of EET and its key proteins and complexes in GSB The Type I Reaction Centre (P840) is a crucial component of the photosynthetic electron transfer system in green sulfur bacteria (GSB). This reaction center, composed of bacteriochlorophyll a, absorbs light at a wavelength of 840 nm, initiating a charge separation that leads to the reduction of ferredoxin. The photo-oxidized P840+ accepts electrons from extracellular inorganic donors. Chlorosomes function as highly efficient, large antenna complexes that can capture light energy across a broad range of wavelengths, particularly in the far-red range (720-750 nm), due to the high concentrations of bacteriochlorophylls c, d, or e that they contain. The Fenna-Matthews-Olson (FMO) complex acts as a vital transport bridge, physically linking the chlorosomes to the P840 reaction center ( Figure.3 ). This facilitates the transfer of absorbed light energy to the reaction center for electron transfer(Hauska et al., 2001). Cytochromes play a significant role in iron oxidation within the Chlorobiaceae family. Genes such as pioAB and mtoAB, which encode Porin-Cytochrome c Protein Complexes (PCC), have been identified in the genomes of photoferrotrophicChlorobia. These complexes are likely essential for their iron oxidation metabolism. They are thought to aid in the transfer of electrons across the outer membrane, with the Monoheme Cyc2-like protein being a critical component in this iron oxidation process (Tsuji et al., 2020) It can be inferred that this protein oxidizes ferrous iron (Fe(II)) at the cell surface. Genome and proteome studies have indicated the presence and expression of cyc2 genes in photoferrotrophicChlorobia, highlighting the importance of Cyc2 in this specialized metabolism. Although the cyc2 sequences exhibit variability, the heme-binding site and the N-terminal residues surrounding the heme are conserved among Cyc2 proteins. Cytochromes are a diverse family of redox-active proteins characterized by a heme cofactor containing an iron atom, which allows for reversible oxidation states (Fe2+ to Fe3+), resembling ferrous iron (Tsuji et al., 2020; Wang et al., 2015) C-type cytochromes are defined by covalent thioether bonds between the polypeptide chain and the vinyl side chain of heme b. These proteins are essential because they often form part of electron transport chains and facilitate redox catalysis in alternative microbial metabolism. The details provided about chlorosomes, FMO complexes, and the Type I reaction center illustrate a highly efficient system for light-assisted energy transfer. The specific association of Cyc2 with PCCs in iron oxidation implies that there are specialized molecular machines dedicated to specific extracellular electron donors (Nomenclature Committee of the International Union of Biochemistry (NC-IUB). Nomenclature of electron-transfer proteins. Recommendations 1989., 1992). EET in PSB Purple bacteria primarily utilize a cyclic electron transport pathway to generate ATP. Light energy absorbed by light-harvesting complexes excites bacteriochlorophyll pigments (either P870 or P960) within the reaction center (RC). The excited electrons are transferred from P870 to their first (QA) and second (QB) quinones, and are then passed to a cytochrome bc1 complex, onto cytochrome c2, and back to the oxidized form of P870 to complete the process. This flow of electrons pumps protons into the periplasm through the cytochrome bc1 complex, establishing a proton motive force that drives ATP synthesis by ATP synthase (Chen et al., 2020; Grattieri, 2020a). While hydrogen sulfide (H2S) is the primary electron donor for most purple sulfur bacteria (PSB), some species exhibit greater versatility. These species can utilize hydrogen (H2), elemental sulfur, thiosulfate, or ferrous iron (Fe2+) as electron donors. The oxidation of sulfide typically results in the formation of intracellular granules of elemental sulfur, which can subsequently be oxidized to sulfate(Hunter et al., 2009). The anaerobic oxidation of Fe2+ is a significant process undertaken by purple bacteria. Two strains, L7 and SW2, can oxidize colorless Fe(II) to brown ferric iron (Fe(III)) in anoxic, light-exposed environments (Ehrenreich and Widdel, 1994). Energetically, ferrous iron can donate electrons to the photosystem of anoxygenic phototrophs at neutral pH (E0’ of Fe(OH)3 + HCO3-/FeCO3 is +0.2 V), while their reaction centers typically have midpoint potentials around +0.45V(Hedrich et al., 2011). Figure.8. The arrangement of photosynthetic apparatus in Chromatiaceae. Cyt- cytochrome, B800, B850 and B875 are bacteriochlorphy;; molecules that are bound to LH complex 1 and 2.The LH2 complex is encoded by PucA and PucB; The 850 and 820 nm absorption maxima of the dimeric BChla arrangements in LHC II and LHC III, respectively, provide an efficient means to direct excitation energy toward the photosynthetic reaction center.Recent studies on Rhodovulumsulfidophilum have shown that this bacterium utilizes both photoferrotrophy and phototrophic extracellular electron uptake (pEEU) from poised electrodes, allowing it to connect external electron sources directly to the photosynthetic electron transport chain (Gupta et al., 2021).Unlike green phototrophic bacteria, purple bacteria do not have external electron carriers with sufficiently negative reduction potentials to spontaneously reduce NAD(P)+ to NAD(P)H. Consequently, they must depend on their reduced quinones to endergonically reduce NAD(P)+, utilizing the proton motive force in a process known as reverse electron flow(Microbial nutrition and basic metabolism, 2003). The focus on generating ATP through cyclic photophosphorylation and the need for reverse electron flow to produce NAD(P)H indicates that purple bacteria may experience energy limitations compared to green bacteria. This suggests that while purple bacteria can efficiently generate ATP, the energetic costs associated with producing reducing power could influence their ecological niche and growth conditions, leading to a preference for photoheterotrophy when organic carbon is available (Beaver et al., 2022; Klamt et al., 2008). These fundamental differences in electron flow and the energetic trade-offs involved in generating reducing power define the ecological niches of purple sulfur bacteria. Additionally, their ability to engage in photoferrotrophy and utilize other non-sulfur electron donors, along with their metabolic versatility in switching to chemoheterotrophic or chemoautotrophic pathways, enables them to thrive in diverse and dynamic anoxic environments by maximizing energy acquisition based on available resources. Molecular mechanism of EET and its key proteins and complexes in PSB The photosynthetic reaction center (RC), typically identified as P870 or P960, is a membrane-bound pigment-protein complex that serves as the core of the photosynthetic apparatus in purple bacteria. The RC is enveloped by light-harvesting complexes (LHCs) that efficiently absorb light energy and transfer it to the RC. When light energy is absorbed, it induces a charge separation, allowing electrons to migrate from the special pair (P870) to a monomeric bacteriochlorophyll (BChl), then to bacteriopheophytin (BPh), and finally to the primary (QA) and secondary (QB) quinone acceptors (Figure.7) . This process achieves a high quantum yield of electron transfer within the RC (Grattieri, 2020b; Nogi et al., 2000).Upon acquiring two electrons and two protons from the cytoplasm, the converted quinone (QH2) from the lipid-soluble quinone (QP) pool is exchanged for a quinone located outside the RC (Grattieri, 2020; The Evolution of the Purple Photosynthetic Bacterial Light-Harvesting System, 2013). he QH2 then interacts with the cytochrome bc1 complex, facilitating the transfer of electrons and protons, which contributes to the proton gradient across the membrane. The cytochrome bc1 complex subsequently transfers one electron to the soluble electron carrier cytochrome c2, while also reducing the photo-oxidized special pair (P870+) to complete the cycle of electron flow(Mizrahi and Cusanovich, 1980; Nogi et al., 2000). In some organisms, the High-Potential Iron-Sulfur Protein (HiPIP) acts as a soluble electron carrier, donating electrons to the RC’s peripheral c-type cytochrome subunit. For example, when grown autotrophically using sulfide/thiosulfate under aerobic conditions, Chromatiumvinosum appears to rely solely on HiPIP as the direct electron donor to the heme of the RC. It is presumed that HiPIP binds to the cytochrome subunit through hydrophobic interactions in the region near heme-1, which is the heme farthest from the special pair(Mizrahi and Cusanovich, 1980; Nogi et al., 2000).A significant discovery has been made involving a previously unknown, non-photosynthetic organism from the family Chromatiaceae, found in a self-regenerating biocathode that exhibits extracellular electron transfer and CO2 fixation. This organism possesses a complete genomic locus for a MopB-containing Alternative Complex III (ACIII), which is homologous to an iron oxidation pathway found in Zetaproteobacteria. The ACIII cluster comprises genes for two multiheme c-type cytochromes, a 4Fe-4S ferredoxin, an iron-sulfur binding protein, an integral membrane polysulfide reductase (NrfD), and two quinol-cytochrome c oxidoreductases. It is believed that this ACIII may function as a replacement for the traditional cytochrome bc1 complex, accepting electrons from a transperiplasmic redox module. Additionally, the organism may have electrode oxidase proteins, including a Cyc2-like protein (56 kDa) that is related to Acidithiobacillusferrooxidans, as well as an undecaheme c-type cytochrome involved in extracellular electron transfer (EET). The presence of these proteins, along with genes for the Calvin-Benson-Bassham (CBB) cycle, suggests the potential for electrode-driven autotrophy under microaerobic conditions. In this scenario, CO2 fixation could be coupled with O2 reduction to generate a proton motive force (PMF)(Wang et al., 2015). Ecological importance and applications in Biotechnology EET plays a crucial role in the biogeochemical cycling of essential global elements, including carbon, metals (such as iron and manganese), sulfur, and nitrogen (Bose and Wang, 2024). EET is not merely an additional metabolic pathway; it is one of the foundational ecological processes that enhances and sustains ecosystem stability, productivity, and resilience. Microorganisms that are competent in EET act as biogeochemical engineers, adjusting the redox state of critical elements to support nutrient cycling and environmental processes (Kato, 2015). The practical applications of EET are diverse and can range widely, including sustainable energy generation, innovative environmental remediation, and the biosynthesis of useful products. Notable examples of EET applications include Microbial Fuel Cells (MFCs), which treat and recover wastewater while generating bioelectricity, Microbial Electrolysis Cells (MECs) for producing hydrogen gas, and Microbial Electrosynthesis (MES) for creating biofuels from carbon dioxide (Kato, 2015; Shaw et al., 2025). Additionally, EET can be utilized in bioremediation systems designed to manage highly toxic metals and organic pollutants. Future research will focus on developing more efficient EET systems by employing advanced genetic engineering techniques and creating new types of electrodes to promote a sustainable and circular bioeconomy (Kato, 2015). Driving biogeochemical cycles: Carbon cycling: oxidation of hydrocarbons EET plays a crucial role in the anaerobic oxidation of hydrocarbons in environments lacking oxygen, particularly in marine ecosystems. It is a significant consumer of natural gases, such as methane, ethane, and butane, as it couples their oxidation with sulfate reduction. A clear example of EET in nature is the anaerobic oxidation of methane (AOM), which is primarily sulfate-dependent. In this process, anaerobic methane-oxidizing archaea (ANMEs) oxidize methane to carbon dioxide, facilitating electron transfer with the help of sulfate-reducing bacteria (SRBs). This transfer utilizes one of several pathways, including direct and mediated interspecies electron transferThe ecological importance of AOM is substantial; it consumes about 90% of the methane produced at the seafloor and around half of the methane generated in freshwater wetlands. This process acts as a natural climate regulator by significantly reducing greenhouse gas emissions (Gao et al., 2022). Understanding that EET can enable the consumption of potent greenhouse gases before they are released into the atmosphere underscores its often invisible yet vital role in maintaining atmospheric climate stability. The potential enhancement of EET occurring in nature could provide alternative, sustainable strategies for mitigating climate changes (Zhuang et al., 2024). Sulfur Cycling: Sulfate reduction and oxidation EET plays a crucial role in linking sulfur cycling with other biogeochemical cycles. Sulfate-reducing bacteria (SRB) utilize extracellular electron transfer (EET) to reduce sulfate. Dissimilatory sulfate reduction by SRB mineralizes organic carbon, predominantly in marine systems, and accounts for approximately 97% of the total sulfide produced on Earth. This process is fundamental for nutrient recycling and maintaining redox balance, particularly in the sediments of anoxic environments(Jørgensen et al., 2019). Cable bacteria are a unique type of multicellular filamentous microorganism that demonstrate long-distance electron transfer (LDET) and have significant implications for sulfur cycling. They can enhance the availability of sulfate, which is often depleted in stratified environments, especially in low-oxygen waters. Additionally, cable bacteria facilitate the interaction between sedimentary sulfur and iron cycles, potentially delaying the onset of euxinia (which refers to anoxic and sulfidic conditions). This interaction helps shape the chemical and biological processes in coastal waters (Zhuang et al., 2024). Nitrogen Cycling: EET is also a significant component of the global nitrogen cycle. Some electroactive bacteria not only exhibit electroactivity but also facilitate electron transfer with electron acceptors such as nitrate and nitrite during the process of denitrification (Bose and Wang, 2024; Shaw et al., 2025). Additionally, it is important to highlight that purple non-sulfur bacteria (PNSB) are well-known diazotrophs, as they can convert atmospheric nitrogen into forms that are usable by other organisms. This characteristic makes them crucial players in the nitrogen cycle(Morrison and Bose, 2024). Biotechnology Applications of EET MFC: Microbial fuel cells (MFCs) are bioelectrochemical devices that utilize microorganisms to oxidize organic matter, transferring the released electrons to an anode, which produces an electrical current (Grattieri, 2020). Microbes convert chemical energy from the oxidation of organic and inorganic matter into adenosine triphosphate (ATP) through a series of sequential reactions, transferring electrons to a terminal electron acceptor and generating electrical energy(Chaturvedi and Verma, 2016). A key feature of MFCs is the capability of electroactive bacteria (EAB) to utilize any source of biodegradable organic matter not only as a nutrient source but also to produce energy from that substrate(Maddalwar et al., 2024).In this sense, MFCs can be regarded as a green technology for generating renewable bioelectricity, as they can use various organic carbon-rich wastewaters(Naureen et al., 2016). MFCs effectively address both energy generation and environmental sustainability. Traditional aerobic wastewater treatment processes are typically energy-intensive; however, MFCs can integrate these processes by treating wastewater while simultaneously generating electricity and producing less waste. Most importantly, MFCs transform waste disposal costs into positive opportunities, becoming energy positive and creating beneficial waste streams. MFCs have the potential to be pioneers of circular bioeconomies, representing advancements in sustainability by providing a source of power while diverting waste from fossil fuels. This further suggests that they may become an attractive option for sustainable wastewater management and energy recovery y(Garbini et al., 2023; Wang et al., 2022). Other applications include: • Microbial electrolysis cells (MES) utilizing EET to create useful fuels like hydrogen gas from organic waste with a minimal external voltage input. MESsystems are applying EET to convert carbon dioxide into biofuels and other chemicals from electrons supplied at the cathode. • Biophotovoltaics for sustainable energy conversion, solar energy into electrical energy via EET • Wastewater treatment to reduce chemical oxygen demand (COD), nitrogen, phosphorus, colorants, and antibiotics. • Soil and sediment remediation by degrading organic pollutants or immobilizing heavy metals. • Production of Bio-based Products that include biofuels such as biohydrogen and biogas from waste streams, bioplastics (Polyhydroxyalkanoates - PHAs) as a biodegradable alternative to traditional plastics, • Single-Cell Protein (SCP) for food and feed, • biofertilizers to improve soil fertility and improve plant growth. Conclusion: Extracellular Electron Transfer (EET) is a fundamental and widespread bioelectrochemical process that underpins the metabolic versatility and ecological success of diverse microbial lineages. This process, which can be direct or mediated, allows microorganisms to exchange electrons with insoluble external materials like minerals and electrodes, effectively extending their respiratory chains beyond the cell membrane. This unique metabolic capability has profound implications, shaping both global ecosystems and the frontiers of sustainable technology. The Green Sulfur Bacteria (GSB) or Purple Sulfur Bacteria (PSB), represent a specialized group of obligate anaerobes adapted to low-light, anoxic environments. Their unique light harvesting structures likechlorosomes or LH1 and LH2, reaction centers enhance light harvesting, and they can utilize various inorganic electron donors like sulfide and ferrous iron. This specialization allows them to occupy ecological niches not used by other phototrophs and plays a key role in the cycling of sulfur and iron. Despite their ecological significance, many potential biotechnological applications for GSB and PSB remain unexamined, offering opportunities in energy production and bioremediation. Ecologically, EET is an indispensable driver of global biogeochemical cycles, particularly in anoxic environments. It plays a critical role in the cycling of carbon, metals, sulfur, and nitrogen. For instance, EET facilitates the anaerobic oxidation of methane (AOM), a process that consumes over 90% of the methane produced from the seafloor, thereby acting as a crucial natural climate regulator. In the sulfur cycle, EET is fundamental to dissimilatory sulfate reduction, which accounts for the majority of organic carbon mineralization in marine environments. Beyond elemental cycles, EET shapes microbial communities by enabling syntrophic relationships and allowing microbes to adapt to a wide array of niches, from metal-rich sediments to the depths of the ocean. In conclusion, the diverse modes of electron energy transfer (EET) used by the Chlorobiaceae and Chromatiaceae families highlight the flexibility and ingenuity of the microbial world in harnessing and transporting electrons across membranes. Understanding the complexities of these molecular processes and the ecological contexts in which they occur is essential for advancing microbiology as a scientific field. Further, this knowledge holds significant potential for developing innovative and sustainable biotechnological applications that enhance energy production, environmental remediation, and resource recovery. There is still much to investigate and learn about these remarkable microbial processes. Acknowledgements: This work received funding from the Science and Engineering Research Board (SERB), New Delhi, under the SURE Grant Scheme (SUR/2022/000582). Infrastructure support was provided by the MURTI Faculty Fellowship, GITAM Seed Grant (2022/0213), and the MURTI research facility. 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