An acetogen is a

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 ...

that generates

acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic, or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...

(CH₃COO⁻) as an end product of anaerobic respiration or

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

. However, this term is usually employed in a narrower sense only to those

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 perform anaerobic respiration and

carbon fixation Biological carbon fixation, or сarbon assimilation, is the Biological process, process by which living organisms convert Total inorganic carbon, inorganic carbon (particularly carbon dioxide, ) to Organic compound, organic compounds. These o ...

simultaneously through the reductive acetyl coenzyme A (

acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidation, o ...

) pathway (also known as the Wood-Ljungdahl pathway). These genuine acetogens are also known as "homoacetogens" and they can produce acetyl-CoA (and from that, in most cases, acetate as the end product) from two molecules of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(CO₂) and four molecules of

molecular hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

(H₂). This process is known as

acetogenesis Acetogenesis is a process through which acetyl-CoA or acetic acid is produced by anaerobic bacteria through the Redox, reduction of Carbon dioxide, via the Wood–Ljungdahl pathway. Other microbial processes that produce acetic acid (like certain ...

, and is different from acetate fermentation, although both occur in the absence of molecular oxygen (O₂) and produce acetate. Although previously thought that only bacteria are acetogens, some

can be considered to be acetogens. Acetogens are found in a variety of habitats, generally those that are anaerobic (lack oxygen). Acetogens can use a variety of compounds as sources of energy and carbon; the best studied form of acetogenic metabolism involves the use of

as a carbon source and

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

as an energy source. Carbon dioxide reduction is carried out by the key enzyme acetyl-CoA synthase. Together with methane-forming archaea, acetogens constitute the last limbs in the anaerobic

food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Position in the food web, or trophic level, is used in ecology to broadly classify organisms as autotrophs or he ...

that leads to the production of methane from polymers in the absence of oxygen. Acetogens may represent ancestors of the first bioenergetically active cells in evolution.

Metabolic roles

Acetogens have diverse metabolic roles, which help them thrive in different environments. One of their metabolic products is acetate which is an important nutrient for the host and its inhabiting microbial community, most seen in termite's guts. Acetogens also serve as "hydrogen sinks" in termite's GI tract. Hydrogen gas inhibits biodegradation and acetogens use up these hydrogen gases in the anaerobic environment to favor the biodegradative capacity of the host by reacting hydrogen gas and carbon dioxide to make acetate. Acetogens have the ability to use variety of substrates in an event where another competitor such as a

methanogen Methanogens are anaerobic archaea that produce methane as a byproduct of their energy metabolism, i.e., catabolism. Methane production, or methanogenesis, is the only biochemical pathway for Adenosine triphosphate, ATP generation in methanogens. A ...

makes hydrogen gas a limiting substrate. Acetogens can use and convert alcohols, lactates and fatty acids, which are usually restricted to syntrophs, instead of just carbon dioxide and hydrogen. This enables them to take on the roles of important contributors of food chain such as primary fermenters. Acetogens can work together with methanogens, as exemplified by the conversion of carbohydrates by a

Methanosarcina barkeri ''Methanosarcina barkeri'' is the type species of the genus ''Methanosarcina'', characterized by its wide range of substrates used in methanogenesis. While most known methanogens produce methane from H2 and CO2, ''M. barkeri'' can also dismutate ...

and coculture of A. woodii. The methanogen takes up acetate to favor the acetogen. Sometimes the interspecies transfer of hydrogen gas between A. woodii and an H₂-consuming methanogen results in hydrogen gas being released from the acetogen instead of going toward acetogenesis by the Wood–Ljungdahl pathway. Acetogens are also one of the contributors to corrosion of steel. Acetobacterium woodii utilize hydrogen gas and CO₂ to make the acetate that is used as carbon source for many of the

sulfate-reducing bacteria Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...

growing with hydrogen gas and sulfate.

References

{{Bacteria Anaerobic digestion Bacteria