An acetogen is a

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in old ...

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 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 r ...

fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food p ...

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

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

and

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeba ...

that perform anaerobic respiration and

carbon fixation Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms. The compounds are then used to store energy and as ...

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 oxidized for ...

) 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 ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

(CO₂) and four molecules of molecular hydrogen (H₂). This process is known as

acetogenesis Acetogenesis is a process through which acetate is produced either by the reduction of CO2 or by the reduction of organic acids, rather than by the oxidative breakdown of carbohydrates or ethanol, as with acetic acid bacteria. The different bact ...

, and is different from acetate fermentation, although both occur in the absence of

molecular oxygen There are several known allotropes of oxygen. The most familiar is molecular oxygen (O2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O3). Others are ...

(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 Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

(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 the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

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. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one o ...

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 microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain Archaea. All known methanogens are members of the archaeal phylum Euryarchaeota. Methanogens are c ...

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 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 growing with hydrogen gas and sulfate.

References

{{Bacteria Anaerobic digestion Bacteria