Drivers of methane-cycling archaeal abundances, community structure, and catabolic pathways in continental margin sediments

Marine sediments contain the largest reservoir of methane on Earth, with most of this methane being produced and consumed in situ by methane-cycling archaea. While numerous studies have investigated communities of methane-cycling archaea in hydrocarbon seeps and sulfate-methane transition zones, little is known about how these archaea change from the seafloor downward in the far more common diffusion-dominated marine sediments. Focusing on four continental margin sites of the North Sea-Baltic Sea transition, we here investigate the in situ drivers of methane-cycling archaeal community structure and metabolism based on geochemical and stable carbon-isotopic gradients, functional gene (mcrA) copy numbers and phylogenetic compositions, as well as thermodynamic calculations. We observe major vertical and lateral changes in community structure that largely follow changes in organic matter reactivity and content, sulfate concentration, and bioturbation activity. While methane-cycling archaeal communities in bioturbation and sulfate reduction zones are dominated by known methyl-dismutating taxa within the Methanosarcinaceae and putatively CO2-reducing Methanomicrobiaceae, the communities change toward dominance of known methane-oxidizing taxa (ANME-2a-b, ANME-2c, ANME-1a-b) in sulfate-methane transitions. Underlying methanogenesis zones were characterized by a change toward mainly physiologically uncharacterized groups, including ANME-1d and several new genus-level groups of putatively CO2-reducing Methanomicrobiaceae and methyl-reducing Methanomassiliicoccales. Notably, group-specific increases in mcrA copy numbers by 2 to 4 orders of magnitude from the sulfate reduction zone into the sulfate-methane transitions or methanogenesis zones indicate the thriving of several major methane-cycling archaeal taxa. Together our study provides insights into the community and pathway shifts vertically along the geochemical gradients and horizontally along the different sedimentary settings and their underlying drivers in continental margin sediments.