Growth of Methanosarcina sp. strain 227 and Methanosarcina mazei on H2-CO2 and mixtures of H2-CO2 and acetate or methanol was examined. The growth yield of strain 227 on H2-CO2 in complex medium was 8.4 mg/mmol of methane produced. Growth in defined medium was characteristically slower, and cell yields were proportionately lower. Labeling studies confirmed that CO2 was rapidly reduced to CH4 in the presence of H2, and little acetate was used for methanogenesis until H2 was exhausted. This resulted in a biphasic pattern of growth similar to that reported for strain 227 grown on methanol-acetate mixtures. Biphasic growth was not observed in cultures on mixtures of H2-CO2 and methanol, and less methanol oxidation occurred in the presence of H2. In M. mazei the aceticlastic reaction was also inhibited by the added H2, but since the cultures did not immediately metabolize H2, the duration of the inhibition was much longer
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