Microbial dissimilatory reduction of Fe(III) and Mn(IV) significantly influences carbon and metal biogeochemical cycles. Fe(III) reduction is crucial in anaerobic soils and sediments, and plays a key role in oxidizing organic matter to CO₂. It also affects the distribution of iron, manganese, and trace metals in aquatic environments. Microbial Fe(III) and Mn(IV) reduction is important for environmental processes like methane inhibition, sulfate reduction, and contaminant oxidation. Various microorganisms, including fermentative, sulfur-oxidizing, hydrogen-oxidizing, and organic-acid-oxidizing bacteria, reduce Fe(III) and Mn(IV). Fermentative microorganisms, such as B. polymyxa and V. spp., reduce Fe(III) and Mn(IV) during anaerobic metabolism, but their efficiency is low. Sulfur-oxidizing bacteria like T. thiooxidans and T. ferrooxidans reduce Fe(III) and Mn(IV) under aerobic conditions. Hydrogen-oxidizing bacteria, such as Pseudomonas sp. and S. putrefaciens, reduce Fe(III) and Mn(IV) using hydrogen as an electron donor. Organic-acid-oxidizing bacteria, like GS-15, can completely oxidize organic compounds to CO₂ with Fe(III) or Mn(IV) as the electron acceptor. Aromatic compound-oxidizing bacteria, such as GS-15, can also reduce Fe(III) and Mn(IV) using aromatic compounds as substrates. Microbial Fe(III) and Mn(IV) reduction is often enzymatic, with specific enzymes involved in electron transport. Nonenzymatic reduction is less efficient and occurs under certain conditions. The redox model suggests that microbial metabolism lowers the redox potential, leading to Fe(III) and Mn(IV) reduction. However, direct evidence for nonenzymatic reduction is limited. Enzymatic reduction is more efficient and leads to greater Fe(III) and Mn(IV) reduction. Organic compounds, such as acetate and aromatic compounds, are often oxidized by microbial Fe(III) and Mn(IV) reducers, but nonenzymatic reduction is less effective. Reduced sulfur compounds, such as sulfide, can also reduce Fe(III) and Mn(IV) in marine sediments. Microbial Fe(III) and Mn(IV) reduction has significant environmental implications, including the oxidation of organic contaminants, the release of trace metals, and the formation of iron and manganese minerals. It also plays a role in controlling global methane fluxes and in the biogeochemical cycling of metals. The metabolism of Fe(III) and Mn(IV) reducers is important for understanding and managing environmental processes in aquatic and terrestrial systems.Microbial dissimilatory reduction of Fe(III) and Mn(IV) significantly influences carbon and metal biogeochemical cycles. Fe(III) reduction is crucial in anaerobic soils and sediments, and plays a key role in oxidizing organic matter to CO₂. It also affects the distribution of iron, manganese, and trace metals in aquatic environments. Microbial Fe(III) and Mn(IV) reduction is important for environmental processes like methane inhibition, sulfate reduction, and contaminant oxidation. Various microorganisms, including fermentative, sulfur-oxidizing, hydrogen-oxidizing, and organic-acid-oxidizing bacteria, reduce Fe(III) and Mn(IV). Fermentative microorganisms, such as B. polymyxa and V. spp., reduce Fe(III) and Mn(IV) during anaerobic metabolism, but their efficiency is low. Sulfur-oxidizing bacteria like T. thiooxidans and T. ferrooxidans reduce Fe(III) and Mn(IV) under aerobic conditions. Hydrogen-oxidizing bacteria, such as Pseudomonas sp. and S. putrefaciens, reduce Fe(III) and Mn(IV) using hydrogen as an electron donor. Organic-acid-oxidizing bacteria, like GS-15, can completely oxidize organic compounds to CO₂ with Fe(III) or Mn(IV) as the electron acceptor. Aromatic compound-oxidizing bacteria, such as GS-15, can also reduce Fe(III) and Mn(IV) using aromatic compounds as substrates. Microbial Fe(III) and Mn(IV) reduction is often enzymatic, with specific enzymes involved in electron transport. Nonenzymatic reduction is less efficient and occurs under certain conditions. The redox model suggests that microbial metabolism lowers the redox potential, leading to Fe(III) and Mn(IV) reduction. However, direct evidence for nonenzymatic reduction is limited. Enzymatic reduction is more efficient and leads to greater Fe(III) and Mn(IV) reduction. Organic compounds, such as acetate and aromatic compounds, are often oxidized by microbial Fe(III) and Mn(IV) reducers, but nonenzymatic reduction is less effective. Reduced sulfur compounds, such as sulfide, can also reduce Fe(III) and Mn(IV) in marine sediments. Microbial Fe(III) and Mn(IV) reduction has significant environmental implications, including the oxidation of organic contaminants, the release of trace metals, and the formation of iron and manganese minerals. It also plays a role in controlling global methane fluxes and in the biogeochemical cycling of metals. The metabolism of Fe(III) and Mn(IV) reducers is important for understanding and managing environmental processes in aquatic and terrestrial systems.