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 13 C labeled CH 4 to measure total methane turnover by isotope dilution, and with 14 C‐radiotracers to measure specifically the gross hydrogenotrophic and acetoclastic methane production. Highest CH 4 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 CH 4 production was only detected in incubations amended with TMA. Our results show that methylotrophic methanogenesis dominated the CH 4 production in these sulfate‐rich marine surface sediments.