Reversibility controls on extreme methane clumped isotope signatures from anaerobic oxidation of methane

Microbial anaerobic oxidation of methane (AOM) substantially mitigates atmospheric methane emissions on Earth and is a process to consider for astrobiological targets where methane has been detected. The measurement of doubly substituted, or “clumped”, methane isotopes has proven useful in tracing processes of methane formation and oxidation. Both near-equilibrium and extreme disequilibrium methane clumped isotope signatures can be attributed to AOM, but, to date, understanding the mechanistic and environmental controls on those signatures has been lacking. We report measurements of methane clumped isotope compositions of residual methane in AOM-active microbial incubations using sediment slurries from Svalbard and Santa Barbara Channel methane seeps. Incubation experiments of Svalbard sediment slurries resulted in residual methane with very high Δ¹³CH₃D and Δ¹²CH₂D₂ values up to 19.5‰ and 65.1‰, respectively. We found similarly high Δ¹³CH₃D and Δ¹²CH₂D₂ values in fluid samples from the Chamorro Seamount, a serpentinite mud volcano in the Mariana forearc, suggesting that minimal reversibility of AOM intracellular reactions leads to kinetic fractionation of clumped isotopologues. When conditions were consistent with a low thermodynamic drive for AOM, however, methane isotopologues approached intra-species quasi-equilibrium. This was clearly observed in isotope exchange experiments with methyl-coenzyme M reductase (Mcr) and in microbial incubations of the Santa Barbara Channel sediment slurries. Using an isotopologue fractionation model, we highlight the critical role of reversibility in controlling the trajectory of gases in Δ¹³CH₃D vs. Δ¹²CH₂D₂ space during AOM. The near-equilibrium methane isotopologue signatures are generalized as a result of the Mcr-catalyzed intracellular isotope exchange operating under near-threshold free energy conditions, as shown in the deep-biosphere incubations. Our results show that the reversibility of the Mcr-catalyzed reaction is central to understanding the meaning of methane isotopologue ratios affected by microbial production and oxidation.