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 an ...

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 acetic ...

. The different bacterial species that are capable of acetogenesis are collectively termed

acetogen An acetogen is a microorganism that generates acetate (CH3COO−) as an end product of anaerobic respiration or fermentation. However, this term is usually employed in a narrower sense only to those bacteria and archaea that perform anaerobic resp ...

s. Reduction of CO₂ to acetate by anaerobic bacteria occurs via the

Wood–Ljungdahl pathway The Wood–Ljungdahl pathway is a set of biochemical reactions used by some bacteria. It is also known as the reductive acetyl-coenzyme A (Acetyl-CoA) pathway. This pathway enables these organisms to use hydrogen as an electron donor, and ca ...

and requires an electron source (e.g., H₂, CO, formate, 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 Work ...

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 component ...

. 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'', 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

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 fo ...

. The key aspects of the acetogenic pathway are several reactions that include the reduction of carbon dioxide to

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 simpl ...

and the attachment of the carbon monoxide to a methyl group. The first process is catalyzed by enzymes called

carbon monoxide dehydrogenase In enzymology, carbon monoxide dehydrogenase (CODH) () is an enzyme that catalyzes the chemical reaction :CO + H2O + A \rightleftharpoons CO2 + AH2 The chemical process catalyzed by carbon monoxide dehydrogenase is similar to the water-gas shift ...

. The coupling of the methyl group (provided by

methylcobalamin Methylcobalamin (mecobalamin, MeCbl, or MeB) is a cobalamin, a form of vitamin B. It differs from cyanocobalamin in that the cyano group at the cobalt is replaced with a methyl group. Methylcobalamin features an octahedral cobalt(III) centre and c ...

) 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 carbon dioxide. This loss is an issue with an increased requirement of minimization of CO₂ emissions, as well as successful competition for fossil fuels with

biofuel Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. According to the United States Energy Information Administration (EIA ...

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 Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH) ...

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