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

Abstract. Cold seeps associated with cold-water corals have been reported worldwide. Yet, there are still knowledge gaps regarding ecological relationships due to contrasting observations. Here, we report the results from a multidisciplinary study on cold seeps off the Vesterålen coast (northern Norway) hosting coral mounds. We discuss the geochemical results from sediment (carbon-nitrogen systematics, foraminifera) and pore fluids (sulfate, dissolved inorganic carbon, methane) in relation to 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 cracks on top of them. Foraminifera tests in the sediment display negative δ¹³C values down to − 18.5 ‰, suggesting ongoing authigenic carbonate precipitation. 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 mainly consisting of ANME-1b anaerobic methanotrophs in correspondence of the biofilm interval. Corals in this area are spatially associated with seafloor chemosynthetic habitats and bubbling, but not vice versa, suggesting that seafloor emissions do not influence coral distribution. Instead, the presence of a methane-charged sediment substrate leading carbonate crust formation and food supply by high-energy currents appears to be a prerequisite for cold-water corals development in this area.