Acetate metabolism inMethanosarcina barkeri

Methanosarcina barkeri was grown by acetate fermentation in complex medium (N₂ gas phase). The molar growth yield was 1.6–1.9 g cells/mol methane formed. Under these conditions 63–82% of the methane produced byMethanosarcina strains was derived from the methyl carbon of acetate, indicating that some methane was derived from other media components. Growth was not demonstrated in complex media lacking acetate or mineral acetate medium containing acetate but lacking H₂/CO₂, methanol, or trypticase and yeast extract. Acetate metabolism byM. barkeri strain MS was further exmined in mineral acetate medium containing H₂/CO₂ and/or methanol, but lacking cysteine. Under these conditions, more methane was derived from the methyl carbon of acetate than from the carboxyl carbon. Methanogenesis from the methyl group increased with increasing acetate concentration. The methyl carbon contributed up to 42% of the methane formed with H₂/CO₂ and up to 5% with methanol. Methanol stimulated the oxidation of the methyl group of acetate to CO₂. The average rates of methane formation from acetate were 1.3 nomol/min ·ml/culture (0.04mg² cell dry weight) in defined media (gas phase H₂/CO₂) and complex media (gas phase N₂). Acetate contributed up to 60% of cell carbon formed under the growth conditions examined. Similar quantities of cell carbon were derived from the methyl and carboxyl carbons of acetate, suggesting incorporation of this compound as a two-carbon unit. Incorporated acetate was not preferentially localized in lipid material, as 70% of the incorporated acetate was found in the wall and protein cell fractions. Acetate catabolism was stimulated by pregrowing of cultures in media containing acetate, while acetate anabolism was not influenced. The results are discussed in terms of the differences between the mechanisms of acetate catabolism and anabolism.Abbreviations CH₃-S-CoM methyl coenzyme M - TCA trichloroacetic acid - CoM coenzyme M (2-mercaptoethane sulfonic acid) - E^o standard potential change (pH 7) - F₄₂₀ Factor 420, a low redox electron carrier - G^o standard free energy change (pH 7) - kJ kilojoules (=0.24 kilocalories) - PBBW Weimer's phosphate-buffered basal medium - X unknown C₁ carrier