Clumped isotopologue fractionation by microbial cultures performing the anaerobic oxidation of methane

Methane is abundant in marine subsurface sediments, sourced from microbial or thermocatalytic production. The relative composition of its isotopologues ((CH4)-C-12, (CH4)-C-13, (CH3D)-C-12 and (CH3D)-C-13) is used to infer its sources and sinks. The anaerobic oxidation of methane (AOM) is an important methane sink reaction carried out by consortia of anaerobic methanotrophic archaea (ANME) and partner bacteria in the presence of methane and sulfate. We investigated the methane isotopologue fractionations during AOM in experiments with cultures of ANME-1 archaea and partner bacteria obtained from hydrothermally heated gas-rich sediments of the Guaymas Basin. During partial methane consumption in four sets of experiments, residual methane became enriched in (CH4)-C-13 and (CH3D)-C-12, following kinetic fractionations from 11.1 to 18.3 parts per thousand and from 117 to 180 parts per thousand, respectively. Results from one set of experiments with D-depleted medium water (delta D = -200 parts per thousand, whereas the control was 55 parts per thousand) suggest the potential reversibility during the methane activation step, which would contribute to equilibrium as opposed to kinetic fractionations. The value of Delta(CH3D)-C-13 (the abundance of (CH3D)-C-13 with respect to that expected from stochastic distribution) increased toward and beyond (up to 8.4 parts per thousand) the value expected for isotopologue equilibrium (5.3 parts per thousand at 37 degrees C). The kinetic clumped isotopologue fractionation (difference between (CH3D)-C-13/(CH3D)-C-12 and (CH4)-C-13/(CH4)-C-12 fractionations) of 4.8 to 12.8 parts per thousand is in contrast with our previous observation of little to no clumped isotopologue effect during aerobic methane oxidation. Our results demonstrate that AOM can contribute to near-equilibrium Delta(CH3D)-C-13 values observed in marine sediments and (CH3D)-C-13 systematics can be used to distinguish aerobic versus anaerobic methanotrophic processes in nature. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Methane is abundant in marine subsurface sediments, and its isotopologue composition is used to infer sources and sinks. Experiment results show significant isotopologue fractionations during AOM, suggesting its impact on methane dynamics in marine environments.