Control and contribution of methylotrophic methanogenesis to methane production in coastal sediments

Methylotrophic methanogenesis was recently recognized as a key process in driving cryptic methane cycling within sulfate-reducing sediments. In this study, 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 control 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 (SMTZ, ~150-170 cm). The stable isotope composition of methane was strongly depleted (−77 to −91‰), indicating the biological production. Methanogenic substrates including H₂/CO₂, 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 (TOC: 0.4%; C/N ratio: ~6; δ¹³C-TOC: −22‰), methane was primarily produced from hydrogenotrophic methanogenesis, consistent with the progressive enrichment of ¹³C in dissolved inorganic carbon with depth below the SMTZ. However, methylotrophic methanogenesis from methanol and trimethylamine contributed significantly to methane production (up to 30.2%) at the estuarine site (TOC: 0.5%; C/N ratio: 7.4, δ¹³C-TOC: −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.