Controls on the isotopic composition of microbial methane

Abstract Microbial methane production (methanogenesis) is responsible for more than half of the annual emission of this major greenhouse gas to the atmosphere. Though the stable isotopic composition of methane is often used to characterize its sources and sinks, strictly empirical descriptions of the isotopic signature of methanogenesis currently limit such attempts. We developed a biochemical-isotopic model of methanogenesis by CO 2 reduction, which predicts carbon and hydrogen isotopic fractionations, and clumped isotopologue distributions, as functions of the cell’s environment. We mechanistically explain multiple-isotopic patterns in laboratory and natural settings and show that such patterns constrain the in-situ energetics of methanogenesis. Combining our model with environmental data, we infer that in almost all marine environments and gas deposits, energy-limited methanogenesis operates close to chemical and isotopic equilibrium.