Acetogenesis

Acetogenesis is a process through which acetate is produced by prokaryote microorganisms 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.^[1]

The different bacterial species 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, etc.). Some acetogens can synthesize acetate autotrophically from carbon dioxide and hydrogen gas.^[2] Reduction of organic acids to acetate by anaerobic bacteria occurs via fermentation.

In 1932, organisms were discovered that could convert hydrogen gas and carbon dioxide into acetic acid. The first acetogenic bacterium species, Clostridium aceticum, 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.^[3]

The precursor to acetic acid is the thioester acetyl CoA. The key aspects of the acetogenic pathway are several reactions that include the reduction of carbon dioxide (CO₂) to carbon monoxide (CO) and the attachment of CO to a methyl group (–CH₃). The first process is catalyzed by enzymes called carbon monoxide dehydrogenase. The coupling of the methyl group (provided by methylcobalamin) and CO is catalyzed by acetyl-CoA synthase.^[4]

The global reduction reaction of CO₂ into acetic acid by H₂ is the following:

2 CO₂ + 4 H₂ → CH₃COOH + 2 H₂O         ΔG° = −95 kJ/mol^[3]

while the conversion of one mole of glucose into 3 moles of acetic acid corresponds to a ~ 3 times more exothermic reaction:

C₆H₁₂O₆ → 3 CH₃COOH                           ΔG° = −310.9 kJ/mol^[3]

However, the energy released by mole of acetic acid produced by each reaction is about the same: −95 kJ/mol for the reduction of CO₂ by H₂, and ~ 9 % more for the conversion of glucose into acetic acid (−104 kJ/mol).

The unique metabolism of acetogens has significant applications in biotechnology. In carbohydrate fermentations, the decarboxylation reactions end up in the conversion of organic carbon into carbon dioxide, the main greenhouse gas. This release is no longer compatible with the need to minimize the world CO₂ emissions. It is not only an environmental concern but also not economically profitable in the frame of the biofuel competition with fossil fuels. Acetogens can ferment glucose without CO₂ emission and convert one glucose molecule into three molecules of acetic acid, increasing the production yield of this latter by 50%. Acetogenesis does not replace glycolysis with a different pathway, but rather captures the CO₂ from glycolysis and uses it to produce acetic acid. Three molecules of acetic acid can be produced in this way, while the production of three molecules of ethanol would require an additional reducing agent such as hydrogen gas.^[5]