Salinity and vegetation shape methylotrophic methanogen diversity and potential activity in coastal sediments

Anoxic coastal sediments are significant natural sources of the potent greenhouse gas methane where, methylotrophic methanogenesis may predominate. However, the underlying methanogen community composition as well as the impact of salinity levels and vegetation on their diversity and potential activity remain poorly resolved. To assess methane production and characterize methanogen diversity underlying methanol and trimethylamine (TMA) degradation, we incubated anoxic sediments from three sites along the Medway Estuary (UK)-a high-salinity marine site and two brackish sites, one vegetated (saltmarsh with cordgrass) and one non-vegetated. Saltmarsh sediments exhibited the highest methane yields (46% from TMA, 27% from methanol), whereas we did not observe methanogenesis from methanol in the high-salinity marine samples. Methylotrophic methanogen genera Methanolobus, Methanomethylophilus, and Methanofastidiosum were abundant in all original sediments, where metabolic generalist Methanosarcinales were also present. Methanoculleus were the most abundant taxon (31.5%) in original brackish sediments, while they were only at 1% in saltmarsh sediments. Following the amendment with TMA or methanol, Methanolobus dominated all the sediments with methane production (80%-99.5% relative abundance), however Methanoculleus were still in high relative abundance in brackish incubations with methanol or TMA (4.6% and 10%, respectively). These findings indicate that both salinity and vegetation shape the community composition of methylotrophic methanogens in coastal sediments, yet key methylated substrates are primarily metabolized by Methanolobus, regardless of differences in salinity and vegetation.