Some facultatively chemoautotrophic anaerobes are able to reduce carbon dioxide with H, to acetate in place of methane, e.g. Clostridium thermoaceticum and Acetobacterium woodii, the reaction for which can be represented as:

2CO2 + 4H2 —> CH2COOH + 2H2O (ΔG0 = — 25.6 kcal/mol = — 107.5 kJ/mol) …(ii)

In the acetogenesis shown in the above equation numbered (ii) the energy yield of the reaction is less favorable than the methanogenesis. Besides acetogenesis from carbon dioxide and hydrogen these organisms can also ferment CO2 formate and methanol to acetate and have many metabolic features in common with methanogenic bacteria.

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