Vertical distribution of methane oxidation and methanotrophic response to elevated methane concentrations in stratified waters of the Arctic fjord Storfjorden (Svalbard, Norway)

Abstract. The bacterially mediated aerobic methane oxidation (MO_x) is a key mechanism in controlling methane (CH₄) emissions from the world's oceans to the atmosphere. In this study, we investigated MO_x in the Arctic fjord Storfjorden (Svalbard) by applying a combination of radio-tracer-based incubation assays (³H-CH₄ and ¹⁴C-CH₄), stable C-CH₄ isotope measurements, and molecular tools (16S rRNA gene Denaturing Gradient Gel Electrophoresis (DGGE) fingerprinting, pmoA- and mxaF gene analyses). Storfjorden is stratified in the summertime with melt water (MW) in the upper 60 m of the water column, Arctic water (ArW) between 60 and 100 m, and brine-enriched shelf water (BSW) down to 140 m. CH₄ concentrations were supersaturated with respect to the atmospheric equilibrium (about 3–4 nM) throughout the water column, increasing from ∼20 nM at the surface to a maximum of 72 nM at 60 m and decreasing below. MO_x rate measurements at near in situ CH₄ concentrations (here measured with ³H-CH₄ raising the ambient CH₄ pool by <2 nM) showed a similar trend: low rates at the sea surface, increasing to a maximum of ∼2.3 nM day⁻¹ at 60 m, followed by a decrease in the deeper ArW/BSW. In contrast, rate measurements with ¹⁴C-CH₄ (incubations were spiked with ∼450 nM of ¹⁴C-CH₄, providing an estimate of the CH₄ oxidation at elevated concentration) showed comparably low turnover rates (<1 nM day⁻¹) at 60 m, and peak rates were found in ArW/BSW at ∼100 m water depth, concomitant with increasing ¹³C values in the residual CH₄ pool. Our results indicate that the MO_x community in the surface MW is adapted to relatively low CH₄ concentrations. In contrast, the activity of the deep-water MO_x community is relatively low at the ambient, summertime CH₄ concentrations but has the potential to increase rapidly in response to CH₄ availability. A similar distinction between surface and deep-water MO_x is also suggested by our molecular analyses. The DGGE banding patterns of 16S rRNA gene fragments of the surface MW and deep water were clearly different. A DGGE band related to the known type I MO_x bacterium Methylosphaera was observed in deep BWS, but absent in surface MW. Furthermore, the Polymerase Chain Reaction (PCR) amplicons of the deep water with the two functional primers sets pmoA and mxaF showed, in contrast to those of the surface MW, additional products besides the expected one of 530 base pairs (bp). Apparently, different MO_x communities have developed in the stratified water masses in Storfjorden, which is possibly related to the spatiotemporal variability in CH₄ supply to the distinct water masses.