Modulation of methyl–coenzyme M reductase expression alters the isotopic composition of microbial methane

The stable isotopic composition of microbial methane varies substantially, and the underlying causes are debated. In this work, we experimentally controlled the abundance of the central enzyme in methanogenesis, methyl-coenzyme M reductase (MCR), in <i>Methanosarcina acetivorans</i> and tested whether its cellular concentration alters methane isotopic compositions. We found that during growth on methanol and acetate, lowering the expression of <i>mcr</i> increases the degree of hydrogen isotope exchange between methane and water. Using an isotope-enabled model of methanogenesis, we found that these changes result from an increase in reversibility of enzymes involved in the oxidation of the substrate methyl group. This result indicates that methane produced from organic compounds can deviate from commonly assumed pathway-specific isotope effects, with implications for the interpretation of environmentally relevant methane sources.