Iron is a metal with strong redox activity. It exists mainly in the natural environment in two forms: divalent iron (Fe(II)) and trivalent iron (Fe(III)). It is one of the most widely distributed metals on Earth. Although dissimilatory iron reduction is an anaerobic process in which Fe(III) serves as a terminal electron acceptor, it is for energy production instead of oxygen. It is a fundamental process in industrial and environmental contexts, playing a key role in processes including bioremediation, electrobiosynthesis, and biogeochemical recycling.
Iron reduction occurs through two major mechanisms: abiotic (chemical) reduction and biotic (microbial) reduction. Among these, biologically processes are particularly significant in natural environments and are increasingly leveraged in sustainable technologies for metal recovery and environmental remediation.
Overview of iron-reducing bacteria (IRB) Iron-reducing bacteria are one of the most important bacterial groups present in various environments. Their main reaction is to reduce ferric iron to ferrous iron. Microorganisms play a vital role in the natural transformation of iron from solute to precipitate. Iron-reducing metabolic pathways can be carried out in aerobic or anaerobic environments, but IRB mainly operate under anaerobic and micro-aerobic conditions. As of 2016, more than 71 facultative IRB have been identified, with morphologies ranging from cocci to comma-shaped and rod-shaped.
Under anaerobic conditions, IRB utilize ferric iron (Fe(III)) as a terminal electron acceptor to support the anaerobic degradation of various organic compounds. The consumption of these organic substrates and the production of CO2 are key indicators of their metabolic activity.
Some IRB species, such as Shewanella putrefaciens, Shewanella algae, and certain Pseudomonas spp., are facultative anaerobes capable of utilizing multiple electron acceptors, including oxygen. However, when Fe3+ is used as the terminal electron acceptor, their efficiency in oxidizing organic electron donors is significantly reduced. In such cases, compounds like lactate and pyruvate are oxidized to acetate.
Importance in Biogeochemical Cycle Iron reduction plays a key role in biogeochemical cycles, influencing a variety of environmental processes and the cycling of other elements. For example, processes such as nitrogen and sulfur cycles, anaerobic ammonium oxidation with iron reduction, and sulfur-driven iron reduction highlight the interconnectedness of these cycles in terrestrial and aquatic ecosystems.
The discovery of microbial iron reduction as a key environmental process marked a significant advancement in the understanding of subsurface biogeochemistry. Microbial iron reduction not only contributes to greenhouse gas formation but also regulates the dynamics of nutrients and contaminants in aquatic systems.
In addition, in terrestrial soils, iron redox cycles are closely linked to carbon and nitrogen cycles. By mediating essential biological and chemical reactions, iron reduction is a key driver of soil ecological functions, soil fertility, and nutrient availability.
Historical Background and Discovery Microbial iron reduction is one of the oldest metabolic processes on Earth. Direct fossil evidence of early microbial life on Earth is scarce. Some microbiological data suggest that both sulfate reduction and iron reduction are the earliest forms of microbial respiration. Furthermore, Fe isotope geochemistry may provide a new method to identify microbial iron reduction early in Earth history.
Studies have shown that the geological record of dissimilatory iron reduction (DIR) extends back to more than 560 million years ago (Myr) and confirms that microorganisms closely related to the last common ancestor could reduce Fe(III).
Metabolic process
Overall reaction Dissimilatory iron reduction is an anaerobic process catabolized by bacteria that involves the oxidation of organic acids/H2 and the reduction of extracellular iron. A representative reaction utilizing acetate as the organic acid electron donor and ferric iron ((Fe(III)) as the terminal electron acceptor, while ferrous iron ((Fe(II)), carbon dioxide, and energy are produced, is shown below. A wide variety of organic acids can be used as reductants for dissimilatory iron reduction. This process typically occurs on the cell surface or extracellularly.
CH3COO- + 8Fe3+ + 3H2O -> 8Fe2+ + HCO3- + CO2 + 8H+ + 814 kJ/reaction
Electron transport Bacteria utilize a variety of methods to transfer electrons onto solid state ferric iron. Microorganisms typically form biofilms on the surface of iron minerals due to the role extracellular polymeric substances play in mediated electron transfer. The four primary methods of electron transfer include: 1) direct contact, 2) ligand mediation, 3) electron shuttling, and 4) pili (nanowires).
Direct contact involves the physical interaction between the surface of the iron mineral and the bacteria. This process is mediated in Shewanella oneidensis by the MR-1 pathway which directly transfers electrons onto the solid state iron via cytochromes. The MR-1 pathway involves the periplasmic transfer of electrons from the cytoplasmic quinol oxidase protein CymA to an outer membrane c-type cytochrome MtrABC complex; other proteins also play an unknown role in cell surface electron transfer.
Ligand-mediated electron transfer involves the use of chelating agents or ligands produced by bacteria to solubilize ferric iron from minerals, increasing its bioavailability. These ligands, often organic acids or siderophores, bind to the iron minerals, forming soluble complexes that are reduced at the cell surface or in the extracellular environment.
Electron shuttling involves the use of redox-active compounds that act as intermediates to transfer electrons from the bacterial cell to Fe(III). These shuttles, which can be endogenous or exogenous, cycle between reduced and oxidized states. Common examples include humic substances, flavins and quinones secreted by Shewanella species. After being reduced by the microbes, the shuttle diffuses to the ferric iron mineral, donates electrons, and returns to the cell in its oxidized form to repeat the cycle. This mechanism allows the bacteria to reduce iron at a distance.
Pili (also known as nanowires), are electrically conductive appendages that facilitate long-range electron transfer and are produced by some iron-reducing bacteria, such as Geobacter sulfurreducens. These pili are protein filaments and contain aromatic amino acids and cytochromes which enable efficient electron conduction from the cell to distant iron minerals. Nanowires are particularly important in biofilms, where they form a network that increases electron transfer efficiency and supports microbial community interaction.
Phylogenetic diversity and evolutionary relationships
Distribution of iron-reducing pathways across Bacteria and Archaea Iron-reducing pathways are phylogenetically widespread, occurring in both Bacteria and Archaea. Most cultured dissimilatory iron-reducing bacteria (DIRB) are affiliated with the class Deltaproteobacteria, including genera such as Geobacter and Shewanella. However, Fe(III) reduction has also been observed in Gammaproteobacteria (e.g., Shewanella alga), Epsilonproteobacteria (e.g., Geospirillum barnesii), and Firmicutes (e.g., Desulfotomaculum reducens). In the archaeal domain, thermophilic species such as Ferroglobus placidus have also demonstrated iron-reducing capabilities. Additionally, organisms like Sinorhodobacter ferrireducens exhibit iron-reducing traits despite falling outside traditional IRB lineages, suggesting horizontal gene transfer or convergent evolution.
Evolutionary origins of ferric iron reduction Dissimilatory iron reduction is considered one of the earliest respiratory pathways to evolve, possibly predating nitrate and oxygen respiration. Ferric reductases are predicted to have evolved around 3.5 billion years ago, following the formation of Fe(III) via ultraviolet-driven oxidation and anaerobic photosynthesis using Fe2+ as an electron donor.
Comparative studies of ferric reductase enzymes suggest that iron reduction evolved independently in multiple microbial lineages. For example, the ferric reductase in the archaeon Archaeoglobus fulgidus shares structural features with bacterial flavin reductases but lacks sequence similarity, implying convergent evolution or horizontal gene transfer across domains.
Fossilized microbial mats from iron-rich hydrothermal environments also indicate that microbial iron cycling was active in early anoxic ecosystems. These ancient microbial communities may have contributed to Fe(III) accumulation and created conditions favorable for the emergence of iron-reducing pathways.
Sulfate-reducing bacteria (SRB) and Fe(III) reduction Although sulfate-reducing bacteria (SRB) are primarily associated with the reduction of sulfate (SO42-), several species have demonstrated the ability to reduce ferric iron. Organisms such as Desulfobulbus propionicus and Desulfotomaculum reducens are capable of using Fe(III) as an electron acceptor under laboratory conditions. These dual capabilities may enable SRB to adapt to fluctuating redox environments in marine sediments, where the availability of sulfate and iron can vary over time.
Evolutionary relationship of IRB and SRB Phylogenetic studies indicate that IRB and SRB are closely related. Members of the Geobacteraceae family, which includes well-studied IRB, share phylogenetic proximity to sulfate reducers such as Desulfomonile tiedjei and Desulfobulbus spp. Some SRB that reduce Fe(III) possess homologous electron transport components to those found in IRB, supporting a shared ancestry or the possibility of horizontal gene transfer of iron-reducing capabilities.
Similarities in electron transport mechanisms Both iron-reducing bacteria (IRB) and sulfate-reducing bacteria (SRB) rely on extracellular electron transfer (EET) to access insoluble electron acceptors such as ferric iron (Fe(III)) oxides or metallic iron surfaces. SRB have also been implicated in the corrosion of iron, with evidence suggesting direct electron uptake via semiconductive mineral layers like iron sulfides. These mechanistic similarities may reflect convergent strategies for extracellular respiration.
Ecological interactions and co-occurrence In natural environments such as coastal sediments and wetlands, IRB and SRB often co-exist. In high-organic-carbon environments like acid sulfate soils, both groups can function simultaneously without outcompeting each other due to the abundance of electron donors and acceptors. Under sulfate-limited conditions, iron reduction may become the dominant anaerobic respiration process. Environmental parameters such as redox potential, iron and sulfate availability, and organic matter content are key factors shaping the spatial distribution and activity of IRB and SRB in marine ecosystems.
Distribution and Environmental Conditions Iron-reducing bacteria (IRB) inhabit suboxic and anoxic regions, rich in organic matter and iron, which serve as crucial components in their metabolic processes. Despite these specific environmental conditions, IRB inhabit a wide array of terrestrial and aquatic environmental settings. These range from grasslands and riverbeds to continental shelves and deep-sea sediments. However, their primary niche is occupying marine and freshwater sediments, where the combination of nutrient availability and redox conditions support their metabolic activity.