Abstract
Methanosarcina barkeri was cultured on methanol, H(2)-CO(2), and acetate, and the C/C ratios of the substrates and the methane produced from them were determined. The discrimination against C in methane relative to substrate decreased in the order methanol > CO(2) > acetate. The isotopic fractionation for methane derived from acetate was only one-third of that observed with methanol as the substrate. The data presented indicate that the last enzyme of methanogenesis, methylreductase, is not the primary site of isotopic discrimination during methanogenesis from methanol or CO(2). These results also support biogeochemical interpretations that gas produced in environments in which acetate is the primary methane precursor will have higher C/C ratios than those from environments where other substrates predominate.