Dissimilatory metal-reducing microorganisms are a group of

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s (both

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and

archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

) that can perform

anaerobic respiration Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. In aerobic organisms undergoing ...

utilizing a

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

as terminal

electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. Electron acceptors are oxidizing agents. The electron accepting power of an electron acceptor is measured by its redox potential. In the ...

rather than

molecular oxygen There are several known allotropes of oxygen. The most familiar is molecular oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are: * Atomic ...

(O₂), which is the terminal electron acceptor reduced to

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

(H₂O) in

aerobic respiration Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cellu ...

. The most common metals used for this end are

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

e(III)and

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

n(IV) which are reduced to Fe(II) and Mn(II) respectively, and most microorganisms that reduce Fe(III) can reduce Mn(IV) as well. But other metals and metalloids are also used as terminal electron acceptors, such as

vanadium Vanadium is a chemical element; it has Symbol (chemistry), symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an ...

(V)

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

r(VI)

molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...

o(VI)

cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...

o(III)

palladium Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...

d(II)

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

u(III) and mercury g(II)

Conditions and mechanisms for dissimilatory metal reduction

Dissimilatory metal reducers are a diverse group of microorganisms, which is reflected in the factors that affect the different forms of metal reduction. The process of dissimilatory metal reduction occurs in the absence of oxygen (O₂), but dissimilatory metal reducers include both obligate (strict) anaerobes, such as the family

Geobacter ''Geobacter'' is a genus of bacteria. ''Geobacter'' species use anaerobic respiration to alter the redox state of minerals and many pollutants, a trait that makes them useful in bioremediation. ''Geobacter'' was the first organism described wit ...

aceae, and

facultative anaerobe A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent. Some examples of facultatively anaerobic bacteria are ''Staphylococcus' ...

s, such as ''

Shewanella ''Shewanella'' is the sole genus included in the marine bacteria family Shewanellaceae. Some species within it were formerly classed as '' Alteromonas''. ''Shewanella'' consists of facultatively anaerobic Gram-negative rods, most of which are fou ...

'' spp. As well, across the dissimilatory metal reducers species, various electron donors are used in the oxidative reaction that is coupled to metal reduction. For instance, some species are limited to small organic acids and hydrogen (H₂), whereas others may oxidize aromatic compounds. In certain instances, such as Cr(VI) reduction, the use of small organic compounds can optimize the rate of metal reduction. Another factor that influences metal respiration is environmental acidity. Although acidophilic and alkaliphilic dissimilatory metal reducers exist, the neutrophilic metal reducers group contains the most well-characterized genera. In soil and sediment environments, where the pH is often neutral, metals like iron are found in their solid oxidized forms, and exhibit variable reduction potential, which can affect their use by microorganisms. Due to the impermeability of the cell wall to minerals and the insolubility of metal oxides, dissimilatory metal reducers have developed ways to reduce metals extracellularly via electron transfer. Cytochromes ''c'', which are transmembrane proteins, play an important role in transporting electrons from the cytosol to enzymes attached to the outside of the cell. The electrons are then further transported to the terminal electron acceptor via direct interaction between the enzymes and the metal oxide. In addition to establishing direct contact, dissimilatory metal reducers also display the ability to perform ranged metal reduction. For instance, some species of dissimilatory metal reducers produce compounds that can dissolve insoluble minerals or act as electron shuttles, enabling them to perform metal reduction from a distance. Other organic compounds frequently found in soils and sediments, such as humic acids, may also act as electron shuttles.Lovley, D. R., Fraga, J. L., Blunt-Harris, E. L., Hayes, L. A., Phillips, E. J. P., & Coates, J. D. (1998). Humic substances as a mediator for microbially catalyzed metal reduction. Acta hydrochimica et hydrobiologica, 26(3), 152-157. In

biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...

nanowires upright=1.2, Crystalline 2×2-atom tin selenide nanowire grown inside a single-wall carbon nanotube (tube diameter ≈1 nm). A nanowire is a nanostructure in the form of a wire with the diameter of the order of a nanometre (10−9 m). Mor ...

and multistep electron hopping (in which electrons jump from cell to cell towards the mineral) have also been suggested as methods for reducing metals without requiring direct cell contact. It has been proposed that cytochromes c are involved in both of these mechanisms. In nanowires, for instance, cytochromes c function as the final component that transfers electrons to the metal oxide.

Terminal electron acceptors

A wide range of Fe(III)-bearing minerals have been observed to function as terminal electron acceptors, including magnetite, hematite, goethite, lepidocrocite, ferrihydrite, hydrous ferric oxide, smectite, illite, jarosite, among others.

Secondary mineral formation

In natural systems, secondary minerals may form as a byproduct of bacterial metal reduction. Commonly observed secondary minerals produced during experimental bio-reduction by dissimilatory metal reducers include magnetite, siderite, green rust, vivianite, and hydrous Fe(II)-carbonate.

Genera that include dissimilatory metal reducers

* '' Albidiferax'' (Betaproteobacteria) * ''

'' (Gammaproteobacteria) * ''

'' (Deltaproteobacteria) * ''

Geothrix fermentans ''Geothrix fermentans'' is a rod-shaped, anaerobic bacterium. It is about 0.1 μm in diameter and ranges from 2-3 μm in length. Cell arrangement occurs singly and in chains. ''Geothrix fermentans'' can normally be found in aquatic sed ...

'' (Acidobacteria) * '' Deferribacter'' (Deferribacteres) * '' Thermoanaerobacter'' (Firmicutes)

References

{{reflist Bacteria Metabolism Extremophiles