Acetogenesis is a process through which

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 ...

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 Acetic acid bacteria (AAB) are a group of Gram-negative bacteria which oxidize sugars or ethanol and produce acetic acid during fermentation. The acetic acid bacteria consist of 10 genera in the family Acetobacteraceae. Several species of acet ...

. The different bacterial species that are capable of acetogenesis are collectively termed acetogens. Reduction of CO₂ to acetate by anaerobic bacteria occurs via the Wood–Ljungdahl pathway and requires an electron source (e.g., H₂, CO,

formate Formate ( IUPAC name: methanoate) is the conjugate base of formic acid. Formate is an anion () or its derivatives such as ester of formic acid. The salts and esters are generally colorless.Werner Reutemann and Heinz Kieczka "Formic Acid" in ...

, etc.). Some acetogens can synthesize acetate

autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...

ically from carbon dioxide and hydrogen gas. Reduction of organic acids to acetate by anaerobic bacteria occurs via fermentation.

Discovery

In 1932, organisms were discovered that could convert hydrogen gas and carbon dioxide into

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

. The first acetogenic bacterium species, ''

Clostridium aceticum ''Clostridium aceticum'' is a species of bacterium in the genus Clostridium. Its name comes from the acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical for ...

'', was discovered in 1936 by Klaas Tammo Wieringa. A second species, ''

Moorella thermoacetica ''Moorella thermoacetica'', previously known as ''Clostridium thermoaceticum'', is an acetogenic, thermophilic, strictly anaerobic, endospore-forming, bacterium belonging to the phylum Bacillota. Researchers at the Lawrence Berkeley National La ...

'', attracted wide interest because of its ability, reported in 1942, to convert glucose into three moles of acetic acid.

Biochemistry

The precursor to acetic acid is the

thioester In organic chemistry, thioesters are organosulfur compounds with the functional group . They are analogous to carboxylate esters () with the sulfur in the thioester playing the role of the linking oxygen in the carboxylate ester, as implied by ...

acetyl CoA. The key aspects of the acetogenic pathway are several reactions that include the reduction 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 ...

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

and the attachment of the carbon monoxide to a

methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ma ...

. The first process is catalyzed by enzymes called carbon monoxide dehydrogenase. The coupling of the methyl group (provided by methylcobalamin) and the CO is catalyzed by acetyl CoA synthase. :2 CO₂ + 4 H₂ → CH₃COOH + 2H₂O

Applications

The unique metabolism of acetogens has significance in biotechnological uses. In carbohydrate fermentations, the

decarboxylation Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is ...

reactions involved result in the loss of carbon into

. This loss is an issue with an increased requirement of minimization of CO₂ emissions, as well as successful competition for fossil fuels with biofuel production being limited by monetary value. Acetogens can ferment glucose without any CO₂ emissions and convert it to 3 acetates, which can theoretically increase product yield by 50%. Acetogenesis does not replace glycolysis with a different pathway, but is rather used by capturing CO₂ from glycolysis and placing it through acetogenesis.

References

{{reflist, 2 Anaerobic digestion Hydrogen biology