Methane is a major greenhouse gas and a key component of global biogeochemical cycles. Its emissions are largely governed by microbial production and oxidation of methane. While the mechanisms and isotope effects of these processes have been extensively studied, important questions continue to arise. These include (1) the clumped isotope effects associated with microbial methane metabolisms in anoxic environments, specifically anaerobic oxidation of methane (AOM) and methanogenesis, and (2) the environmental controls on the electron acceptors involved in AOM.
The first question is addressed through a combination of laboratory microbial incubations and methane samples from natural environments. We found that with high metabolic reversibility of methanogenesis and AOM, the distribution of carbon and hydrogen isotopes among methane molecules is consistent with thermodynamic equilibrium. These near-equilibrium methane isotopologue signatures result from isotope exchange operating under conditions of near-threshold free energy, catalyzed by the methyl-coenzyme M reductase enzyme. When the thermodynamic driving force is elevated, methanogenesis and AOM can generate more negative and positive isotopologue signatures, respectively. We propose that clumped isotopes of methane provide a proxy for characterizing the bioenergetics of environments for methane production and consumption. Together, these observations demonstrate clumped isotopes of methane as a powerful tool to better understand the relation between methane metabolisms and the energy landscape in natural environments. We further applied this approach to track coupled hydrocarbon biodegradation and secondary methanogenesis in terrestrial mud volcanoes.
The second question is addressed through comprehensive porewater and solid-phase geochemical analyses, along with microbial radiotracer incubations, in hypersaline coastal wetland sediment. We demonstrate that, despite the high concentrations of sulfate, AOM is not associated with sulfate reduction but is instead coupled with the reduction of an unconventional electron acceptor—iron oxides—in subsurface sediment. This finding highlights the role of wetland sediments enriched in iron oxides as an effective sink for the greenhouse gas methane. Iron-dependent AOM in sulfate-free sediments has been extensively studied. Extending these observations into sulfate-rich sediments significantly advances the earlier observations and hypotheses, while suggesting that iron-AOM is an under-considered sink for methane in wetlands.
