Stable isotope signals of methane produced by methanogens under incubation conditions simulating subsurface environments

Abstract

Natural gas (methane) is classified into two types based on its origin: biogenic methane, produced by methanogenic archaea, and thermogenic methane, generated by geothermal degradation of organic matter in deep subsurface environments. Stable isotope signals have been used as key indicators for identifying the origin of methane. Biogenic methane from the subsurface is characterized by isotope signals that are close to isotopic equilibrium. However, this isotope signal has never been reproduced in laboratory cultures of methanogens, revealing a significant gap between interpretations in isotope geochemistry and microbiology. Here, we developed a high-pressure incubation system simulating subsurface conditions. By co-culturing a hydrogen-producing bacterium with a hydrogenotrophic methanogen, we reproduced isotope signals with near-equilibrium. Furthermore, we discovered that in subsurface environments, hydrogenotrophic methanogens can overwrite (re-equilibrate) the isotope signals of thermogenic methane, transforming it into a signal indistinguishable from biogenic methane. These findings highlight the need to reconsider current interpretations regarding the origin of natural gas reservoirs.