Comparison of Unitrophic and Mixotrophic Substrate Metabolism by an Acetate-Adapted Strain of Methanosarcina barkeri

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Abstract
The unitrophic and mixotrophic metabolism of acetate and methanol was examined using detailed 14C-labeled tracer studies in a strain of M. barkeri adapted to grow on acetate as the sole C and energy source. The substrate consumption rate and methane production rate were significantly lower on acetate alone than during the unitrophic or mixotrophic metabolism of methanol. Cell yields (in g/mol substrate) were identical during exponential growth on acetate and exponential growth on methanol. During unitrophic metabolism of acetate, the methyl moiety accounted for the majority of the CH4 produced, but 14% of the CO2 generated originated from the moiety. This correlated with the concurrent reduction of equivalent amounts of the C-1 of acetate to CH4. 14CH4 was also produced from added 14CO2, although to a lesser extent than from reduction of the C-1 of acetate. During mixotrophic metabolism, methanol and acetate were catabolized simultaneously. The rates of 14CH4 and 14CO2 generation from [2-14C]acetate were logarithmic and higher in mixotrophic than in unitrophic cultures at substrate concentrations of 50 mM. A comparison of the oxidoreductase activities in cell extracts of the acetate-adapted strain grown on acetate and of strain MS grown on methanol or on H2 plus CO2 indicated that the pyruvate, .alpha.-ketoglutarate and isocitrate dehydrogenase activities remained constant, whereas the CO dehydrogenase activity was significantly higher (5000 nmol/min per mg of protein) in the acetate-adapted strain. A significant intramolecular redox pathway may be responsible for the generation of CH4 from acetate, energy metabolism from acetate by M. barkeri is not catabolite repressed by methanol, and the acetate-adapted strain is a metabolic mutant with derepressed CO dehydrogenase activity.