The Stable Carbon Isotope Fractionation of Methanogenesis Products at Complete Carbon Consumption

The stable carbon isotope signature (δ13C) of methane (CH4) is used to discriminate between biological, thermogenic, and abiotic sources. Methanogens, or methane-producing archaea, inhabit a broad range of chemical conditions, including extreme environments. Many of these environments are replete in dissolved inorganic carbon (DIC), giving rise to isotopically depleted 13C biogenic CH4. Serpentinizing systems are an extreme environment inhabited by methanogens that can exhibit low carbon-dioxide (CO2) availability and isotopically enriched 13C CH4 that is outside the known biogenic range. However, systematic examination of methanogenesis under extreme carbon-limitation is lacking. We measured the δ13C of carbon-dioxide, biomass, lipids, and methane during hydrogenotrophic methanogenesis with a limited carbon pool to investigate carbon isotope dynamics at complete DIC consumption. We found that final methane δ13C values closely reflect the δ13C of the initial DIC supply, and that methane is more isotopically enriched in 13C than biomass and lipids. This data shows that biogenic methane δ13C values predicted for natural environments needs to be expanded to include heavier δ13C values resulting from the impact of extreme carbon-limitation in suppressing isotope fractionation.