Dissimilatory Metal Reduction

Precise definition

Dissimilatory metal reduction (also called dissimilatory metal(loid) reduction or metal respiration) is respiratory metabolism in which a microorganism couples the oxidation of an electron donor to the reduction of an oxidized metal or metalloid that serves as the terminal electron acceptor. The metal is reduced for energy conservation, not incorporated into biomass — this distinguishes it from assimilatory metal reduction, in which small amounts of metal are reduced for incorporation into cellular components.

The most common electron acceptors are ferric iron, Fe(III), and manganese, Mn(IV), which are abundant in soils and sediments, but the same enzymatic machinery can reduce a wide range of other metals and radionuclides, including uranium U(VI), technetium Tc(VII), chromium Cr(VI), vanadium V(V), cobalt, gold, and selenium and arsenic oxyanions. The process is typically anaerobic, dominating in the anoxic zones of sediments, soils, and aquifers below the depth where oxygen and nitrate are exhausted.

How it works mechanistically

A central challenge is that the most important acceptors — Fe(III) and Mn(IV) oxide minerals — are insoluble and cannot cross the outer membrane. Metal-reducing microbes therefore export respiratory electrons from the inner-membrane quinone pool, across the periplasm, and onto the cell exterior through a chain of multiheme c-type cytochromes, a process termed extracellular electron transfer (EET).

In Shewanella oneidensis MR-1, electrons flow from the inner-membrane quinol dehydrogenase CymA to periplasmic cytochromes and then through the Mtr pathway: the trans-outer-membrane porin–cytochrome complex MtrCAB, with the surface-exposed decaheme cytochromes MtrC and OmcA delivering electrons to the mineral surface. Shewanella also secretes flavins that act as diffusible electron shuttles, and, like Geobacter, can produce conductive appendages.

In Geobacter sulfurreducens, electrons pass through inner-membrane and periplasmic cytochromes to outer-surface multiheme cytochromes such as OmcS, OmcE, and OmcZ. Geobacter can make direct electrical contact with minerals and electrodes, and its 'microbial nanowires' were shown to be micrometre-scale filaments built from polymerized multiheme cytochromes (notably OmcS) that conduct electrons over long distances. This direct, contact-based electron transfer is the basis of current generation in microbial fuel cells and of interspecies electron exchange.

Concrete examples and organisms

The two textbook model organisms are Geobacter (e.g., G. metallireducens strain GS-15, the first organism shown to conserve energy from Fe(III) reduction while completely oxidizing organic carbon, and G. sulfurreducens) and Shewanella (S. oneidensis MR-1). Both are Gram-negative facultative or obligate anaerobes intensively studied for their EET machinery.

Dissimilatory metal reduction is phylogenetically widespread beyond these models. Thermophilic and hyperthermophilic examples include the bacterium Thermus and the archaeon Pyrobaculum islandicum, while other Fe(III)- and U(VI)-reducers span the Deltaproteobacteria, Firmicutes (e.g., Desulfotomaculum, Thermincola), and additional lineages. Many sulfate reducers (Desulfovibrio) and some fermentative bacteria can also reduce metals such as U(VI) and Cr(VI).

Key molecular players named in this metabolism include the outer-membrane decaheme cytochromes MtrC and OmcA of Shewanella; the CymA/MtrA/MtrB conduit of the Mtr pathway; and the Geobacter cytochromes OmcS, OmcE, and OmcZ that assemble into conductive nanowires.

Why it matters

Dissimilatory metal reduction is a major driver of the biogeochemical cycling of iron and manganese and controls the redox chemistry, mineralogy, and trace-element behavior of anoxic sediments, soils, and groundwater. By reducing Fe(III) oxides, these microbes release sorbed contaminants and nutrients (including phosphate and arsenic) and generate reduced iron minerals such as magnetite and siderite.

It is the mechanistic basis of metal bioremediation. Because reduced forms of several toxic and radioactive metals are far less soluble than their oxidized forms, stimulating dissimilatory metal-reducers can immobilize contaminants in place: soluble, mobile U(VI) is reduced to insoluble U(IV) (uraninite), and Tc(VII), Cr(VI), and related species are similarly precipitated, a strategy field-tested at uranium-contaminated U.S. Department of Energy sites.

The same electron-transfer capability underpins microbial electrochemical technologies. Metal-reducers colonize electrodes and power microbial fuel cells and electrosynthesis systems, and their extracellular electron transfer enables direct interspecies electron transfer in anaerobic communities, linking this metabolism to methane cycling and to the broader environmental metallome. Understanding how organisms route electrons onto metals connects dissimilatory metal reduction to the wider study of metallomics and metal-microbe interactions.