Methane Cycling Communities in Arctic coastal and submarine permafrost environments 

Thermokarst lagoons form at the terrestrial-marine interface when thermokarst lakes, hotspots for Arctic methane emissions, erode into the ocean. These lagoons are dynamic environments with seasonal ice build-up and potential alternations between freshwater and marine discharge and make an excellent natural laboratory for studying methane cycling communities as they shift from a terrestrial to marine environment. Our study site encompassed two thermokarst lakes and one thermokarst lagoon on the Bykovsky Peninsula in NE Siberia. In-situ methane concentrations, methane-carbon isotopic signatures, analysis of amplicon sequencing variants (ASVs), metagenomics, and pore-water geochemistry point towards efficient communities of anaerobic methane oxidizers (AOM) in a sulfate-methane transition zone 2-3 meters below the sediment surface of the lagoon. The methanogenic community in the sediment was dominated by methylotrophic methanogens. This is potentially the first known example of dominance of these often-ignored methanogens in a terrestrial/semi-terrestrial environment. Further molecular analyses also revealed an unusual co-occurrence of terrestrial/freshwater ANMEs, specifically of Candidatus Methanoperedens, with typical marine ANME2 a/b in the sulfate-methane transition zones of both systems. Our studies suggest that AOM can locally efficiently reduce sediment methane concentrations of subaquatic permafrost environments, especially of those with marine influence.