Exchange catalysis during anaerobic methanotrophy revealed by 12CH2D2 & 13CH3D in methane

The anaerobic oxidation of methane (AOM) is a crucial component of the methane cycle, but its enzymatic versatility under environmental conditions remains poorly understood. We use sediment samples collected during IODP Expedition 347 to the Baltic Sea to show that relative abundances of 12CH2D2 and 13CH3D molecules in methane gas trace the reversibility of methyl-coenzyme M reductase during AOM by driving methane towards internal, thermodynamic isotopic equilibrium. These data suggest that 12CH2D2 and 13CH3D together can identify the influence of methanotrophy in environments where conventional bulk isotope ratios are ambiguous, and these findings may lead to new insights regarding the global significance of enzymatic back-flux in the methane cycle.