Methylotrophic methanogenesis fuels cryptic methane cycling in marine surface sediment

Methylotrophic methanogenesis is often proposed to be responsible for methane production in sulfate-rich environments, yet the magnitude of this process remains elusive. In this study, we incubated sediment from Aarhus Bay (Denmark) with C-13 labeled CH4 to measure total methane turnover by isotope dilution, and with C-14-radiotracers to measure specifically the gross hydrogenotrophic and acetoclastic methane production. Highest CH4 production rates (> 200 pmol cm(-3)d(-1)) were found in the top 0-2 cm. Most of this production was via methylotrophic pathways. Methanogenesis via the hydrogenotrophic pathway accounted for less than 20 pmol cm(-3) d(-1) throughout the surface sediment (0-10 cm), and there was no apparent contribution from acetoclastic methanogenesis. To further assess potentials for methanogenesis from hydrogen, acetate, or trimethylamine (TMA), sediment slurry incuabtions with excess substrate addition were performed. A high and accelarating CH4 production was only detected in incubations amended with TMA. Our results show that methylotrophic methanogenesis dominated the CH4 production in these sulfate-rich marine surface sediments.