Seafloor chemosynthetic habitats and AOM-influenced sediment microbiome at a cold-water coral site off the Vesterålen coast, northern Norway

Cold seeps associated with cold-water corals have been reported worldwide. Yet, there are still knowledge gaps regarding ecological relationships due to contrasting observations. As a part of a larger multidisciplinary project on cold seeps off the Vesterålen coast (northern Norway), we analysed sediment (carbon-nitrogen systematics, foraminifera) and pore fluid (sulfate, dissolved inorganic carbon, methane) geochemistry and seafloor habitats (orthomosaics and habitat maps). Microbial mats are the dominant seep-related community, forming white patches of a few ten cm in diameter, mostly distributed along the edges of methane-derived authigenic carbonates and on sedimented spots. Mats are associated with sulfate-methane transition zones as shallow as 5 cm and high production of 13C-depleted dissolved inorganic carbon, an evidence of ongoing anaerobic oxidation of methane, also recorded in foraminiferal tests. We also report the discovery of a macroscopic white biofilm, observed while slicing a pushcore onboard. Organic matter analyses indicated that the sediment interval hosting this biofilm is associated with a sharp drop in δ13C values, as negative as -43.4 ‰. Results from 16S rRNA gene analyses on the uppermost 10 cm in the same core showed a significant shift in microbial community. Protebacteria-dominated communities near the seafloor transition to a Halobacterota-dominated composition in correspondence of the biofilm interval, with amplicon sequence variants pointing to ANME-1b anaerobic methanotrophs. The presence of a methane-charged sediment substrate leading to carbonate crust formation, the heterogeneous topography and food supply by high-energy currents appears to be a prerequisite for cold-water corals development and their spatial distribution in this area. Our results provide biogeochemical and seafloor habitat context to cold-water corals, contributing to the understanding of potential influence of methane seepage on corals. Furthermore, the produced seafloor orthomosaics and habitat maps will facilitate the monitoring of habitat changes in response to future climate-driven environmental disturbances.