Methylotrophic methanogenesis fuels cryptic methane cycling in marine surface sediment

Abstract

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 13C labeled CH4 to measure total methane turnover by isotope dilution, and with 14C-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.