Sedimentary biofilms comprising microbial communities mediating the anaerobic oxidation of methane are rare. Here, we describe two biofilm communities discovered in sediment cores recovered from Arctic cold seep sites (gas hydrate pingos) in the north-western Barents Sea, characterized by steady methane fluxes. We found macroscopically visible biofilms in pockets in the sediment matrix at the depth of the sulphate-methane-transition zone. 16S rRNA gene surveys revealed that the microbial community in one of the two biofilms comprised exclusively of putative anaerobic methanotrophic archaea of which ANME-1 was the sole archaeal taxon. The bacterial community consisted of relatives of sulphate-reducing bacteria (SRB) belonging to uncultured Desulfobacteraceae clustering into SEEP-SRB1 (i.e. the typical SRB associated to ANME-1), and members of the atribacterial JS1 clade. Confocal laser scanning microscopy demonstrates that this biofilm is composed of multicellular strands and patches of ANME-1 that are loosely associated with SRB cells, but not tightly connected in aggregates. Our discovery of methanotrophic biofilms in sediment pockets closely associated with methane seeps constitutes a hitherto overlooked and potentially widespread sink for methane and sulphate in marine sediments.

Cold seeps are characterized by high biomass, which is supported by the microbial oxidation of the available methane by capable microorganisms. The carbon is subsequently transferred to higher trophic levels. South of Svalbard, five geological mounds shaped by the formation of methane gas hydrates, have been recently located. Methane gas seeping activity has been observed on four of them, and flares were primarily concentrated at their summits. At three of these mounds, and along a distance gradient from their summit to their outskirt, we investigated the eukaryotic and prokaryotic biodiversity linked to 16S and 18S rDNA. Here we show that local methane seepage and other environmental conditions did affect the microbial community structure and composition. We could not demonstrate a community gradient from the summit to the edge of the mounds. Instead, a similar community structure in any methane-rich sediments could be retrieved at any location on these mounds. The oxidation of methane was largely driven by anaerobic methanotrophic Archaea-1 (ANME-1) and the communities also hosted high relative abundances of sulfate reducing bacterial groups although none demonstrated a clear co-occurrence with the predominance of ANME-1. Additional common taxa were observed and their abundances were likely benefiting from the end products of methane oxidation. Among these were sulfide-oxidizing Campilobacterota, organic matter degraders, such as Bathyarchaeota, Woesearchaeota, or thermoplasmatales marine benthic group D, and heterotrophic ciliates and Cercozoa.

The Arctic Ocean subseabed holds vast reservoirs of the potent greenhouse gas methane (CH 4 ), often seeping into the ocean water column. In a continuously warming ocean as a result of climate change an increase of CH 4 seepage from the seabed is hypothesized. Today, CH 4 is largely retained in the water column due to the activity of methane-oxidizing bacteria (MOB) that thrive there. Predicted future oceanographic changes, bottom water warming and increasing CH 4 release may alter efficacy of this microbially mediated CH 4 sink. Here we investigate the composition and principle controls on abundance and activity of the MOB communities at the shallow continental shelf west of Svalbard, which is subject to strong seasonal changes in oceanographic conditions. Covering a large area (364 km 2 ), we measured vertical distribution of microbial methane oxidation (MOx) rates, MOB community composition, dissolved CH 4 concentrations, temperature and salinity four times throughout spring and summer during three consecutive years. Sequencing analyses of the pmoA gene revealed a small, relatively uniform community mainly composed of type-Ia methanotrophs (deep-sea 3 clade). We found highest MOx rates (7 nM d −1 ) in summer in bathymetric depressions filled with stagnant Atlantic Water containing moderate concentrations of dissolved CH 4 (< 100 nM). MOx rates in these depressions during spring were much lower (< 0.5 nM d −1 ) due to lower temperatures and mixing of Transformed Atlantic Water flushing MOB with the Atlantic Water out of the depressions. Our results show that MOB and MOx in CH 4 -rich bottom waters are highly affected by geomorphology and seasonal conditions.