ALBERT

Description: Microbial communities of the ocean can consume methane dissolved in seawater before it has a chance to escape to the atmosphere and contribute to greenhouse warming. Seawater over the shallow Arctic shelf is characterized by excess methane compared to the atmospheric equilibrium originating in sediments, permafrost and hydrates. Particularly high concentrations are found beneath sea ice. We studied the structure and methane oxidation potential of the microbial communities from seawater collected close to Utqiagvik, Alaska, in April 2016. The in situ methane concentrations were 16.3 ± 7.2 nmol L−1, approximately 4.8 times oversaturated compared to the atmospheric equilibrium. The group of methane oxidizing bacteria (MOB) in the natural seawater and seawater incubations was 〉 97 % dominated by Methylococcacales (γ-Proteobacteria). Incubations of seawater under a range of methane concentrations led to a loss of diversity in the bacterial community. The abundance of MOB was low with maximal fractions of 2.5 % at 200 times elevated methane concentration, while sequence reads of non-MOB methylotrophs were four times more abundant than MOB in most incubations. The abundances of MOB as well as non-MOB methylotrophs correlated tightly with the rate constant (kox) for methane oxidation, indicating that non-MOB methylotrophs might be coupled to MOB and involved in community methane oxidation. In sea ice, where methane concentrations of 82 ± 35.8 nmol kg−1 were found, Methylobacterium (α-Proteobacteria) was the dominant MOB with a relative abundance of 80 %. MOB abundances were very low in sea ice, with maximal fractions found at the ice-snow interface (0.1 %), while non-MOB-methlylotrophs were present in abundances compared to natural seawater communities. The differences in MOB taxa and an offset in methane concentration and stable isotope ratios between the ice and the water column point toward different methane cycling processes in both habitats.