Abstract. Marine microbial communities can consume dissolved methane
before it can escape to the atmosphere and contribute to global warming.
Seawater over the shallow Arctic shelf is characterized by excess methane
compared to atmospheric equilibrium. This methane originates in sediment,
permafrost, and hydrate. 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
relative to atmospheric equilibrium. The group of methane-oxidizing bacteria
(MOB) in the natural seawater and incubated seawater was
> 97 % dominated by Methylococcales (γ-Proteobacteria). Incubations of seawater under a range of methane
concentrations led to 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 4 times more abundant than MOB in most incubations. The abundances of
MOB as well as non-MOB methylotroph sequences 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 %. Total MOB abundances were very low in sea ice, with maximal
fractions found at the ice–snow interface (0.1 %), while
non-MOB methylotrophs were present in abundances similar to natural seawater
communities. The dissimilarities in MOB taxa, methane concentrations, and
stable isotope ratios between the sea ice and water column point toward different
methane dynamics in the two environments.
Seasonal shifts of microbial methane oxidation in Arctic shelf waters above gas seeps
