Carbon and hydrogen isotope fractionation during methanogenesis: A laboratory study using coal and formation water

The high δ13C-CH4 values (≥ 30 ‰) and the dominance of Methanosaeta over Methanosarcina indicate that methanogens within the SSB cultures were operating at low substrate concentrations. An unusually positive δ13C-CH4 suggests a substrate depletion effect, which is thought to be related to a decrease in the relative abundance of key bacterial coal degraders with formation water inoculum storage time. Closer observation of δ13C-CH4 values during the growth of cultures within a single experiment also showed a 13C-enrichment trend over time. At log phase of growth, the CH4 produced was 13C-depleted when compared to the stationary phase that also indicates substrate depletion effects. Finally, the δ13C-CH4 values encountered in this study (as high as − 20 ‰) highlight the possible positive extension of δ13C-CH4 values of acetoclastic methanogenesis from those currently reported in the literature for natural and experimental samples (as high as − 30 ‰).