Conversion of acetic acid to methane by thermophiles

Acetate is the precursor of approximately two‐thirds of the methane produced in mesophilic (30–40°C) and thermophilic (45–65°C) anaerobic bioreactors. In the past decade, several thermophilic acetotrophic methanogenic cultures have been isolated including Methanosarcina thermophila, Methanotrhix sp. strain CALS‐1, the TAM organism, and syntrophic acetate‐oxidizing co‐culture. The thermophilic Methanosarcina and Methanothrix cultures physiologically and morphologically resemble their mesophilic homologues, except that they grow two to five times as rapidly. This more rapid growth offers a partial explanation for the higher rates of methanogenesis and lower minimum retention times often found in thermophilic anaerobic bioreactors when compared with mesophilic bioreactors. The thermophilic Methanothrix shows a much lower minimum threshold for acetate utilization (12–20 μM) than does a thermophilic Methanosarcina (1–2.5 mM), consistent with ecological evidence indicating that low acetate concentrations favor Methanothrix . The thermophilic Methanotrhix cultures have a maximum growth temperature near 70°C, while that for thermophilic Methanosarcina cultures is near 60°C, and there is evidence that bioreactors in which Methanothrix predominates are more tolerant to temperatures over 60°C. The rod‐shaped TAM organism grows optimally at 60°C, and may represent a new genus. A thermophilic synthrophic coculture converts acetate to methane by a two‐step process in which acetate is first oxidized to H 2 and CO 2 by an acetate oxidizing rod (AOR) followed by methanogenesis from H 2 and CO 2 by a thermophilic Methanobacterium . The AOR has been isolated in pure culture and has been found to be an acetogen capable of converting H 2 ‐CO 2 to acetate, the opposite of the reaction it carries out in synthrophic coculture. The partial pressures of hydrogen in the acetate oxidizing coculture must be poised such that both organisms are able to conserve energy. It was found that entropy effects must be taken into account to make an accurate prediction of H 2 partial pressures at 60°C, and it is predicted that H 2 partial pressures will be generally about five to ten‐fold higher in thermophilic anaerobic bioreactors than in mesophilic ones. The presently known upper temperature limit for methanogenesis from acetate is 70°C, despite attempts by the author and others to isolate more thermophilic strains from geothermal environments.