Controls and contributions of methylotrophic methanogenesis to methane production in coastal sediments

Abstract Methylotrophic methanogenesis has recently been recognized as a key process driving cryptic methane cycling within sulfate‐reducing sediments. Here we conducted biogeochemical analyses of methanogenic substrates, activity, and communities in two sediment cores (4–5 m) from the East China Sea, to constrain the dynamics and controls of methylotrophic methanogenesis in coastal sediments. We detected micromolar concentrations of methane in the presence of sulfate and high methane concentrations (up to 4.2 mM) below the sulfate–methane transition zone (~ 150–170 cm). Stable isotope composition of methane was strongly depleted (−77‰ to −91‰), indicating its biological production. Methanogenic substrates including H 2 /CO 2 , acetate, and methylated compounds were detected in the porewaters and/or sediments. Radiotracer experiments indicated methane production from various substrates, and the presence of sulfate did not inhibit methanogenesis at either site. At the coastal site with the dominance of marine organic matter (total organic carbon: 0.6%; C/N ratio: ~ 6.4; δ 13 C‐total organic carbon: −22‰), methane was primarily produced from hydrogenotrophic methanogenesis, consistent with the progressive enrichment of 13 C in dissolved inorganic carbon with depth below the sulfate–methane transition zone. However, methylotrophic methanogenesis from methanol and trimethylamine contributed significantly to methane production (up to 30.2%) at the estuarine site (total organic carbon: 0.5%; C/N ratio: 7.4, δ 13 C‐total organic carbon: −23‰) with elevated terrestrial organic matter input, also reflected from the predominance of long chain odd carbon n‐alkanes. These findings suggested that organic carbon source and composition, instead of sulfate, control methanogenic activity, providing evidence that high terrestrial organic inputs could significantly enhance methylotrophic methanogenesis in coastal sediments.