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

[摘要] The bacterially mediated aerobic methane oxidation (MO_x) is a keymechanism in controlling methane (CH₄) emissions from the world's oceansto the atmosphere. In this study, we investigated MO_x in theArctic fjord Storfjorden (Svalbard) by applying a combination ofradio-tracer-based incubation assays (³H-CH₄ and¹⁴C-CH₄), stable C-CH₄ isotope measurements, and moleculartools (16S rRNA gene Denaturing Gradient Gel Electrophoresis (DGGE)fingerprinting, pmoA- and mxaF gene analyses). Storfjordenis stratified in the summertime with melt water (MW) in the upper 60 m ofthe water column, Arctic water (ArW) between 60 and 100 m, andbrine-enriched shelf water (BSW) down to 140 m. CH₄ concentrations weresupersaturated with respect to the atmospheric equilibrium (about 3–4 nM)throughout the water column, increasing from ∼20 nM at the surface toa maximum of 72 nM at 60 m and decreasing below. MO_x ratemeasurements at near in situ CH₄ concentrations (here measured with³H-CH₄ raising the ambient CH₄ pool by <2 nM) showed asimilar 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 deeperArW/BSW. In contrast, rate measurements with ¹⁴C-CH₄ (incubationswere spiked with ∼450 nM of ¹⁴C-CH₄, providing an estimateof the CH₄ oxidation at elevated concentration) showed comparably lowturnover rates (<1 nM day⁻¹) at 60 m, and peak rates were foundin ArW/BSW at ∼100 m water depth, concomitant with increasing¹³C values in the residual CH₄ pool. Our results indicate that theMO_x community in the surface MW is adapted to relatively lowCH₄ concentrations. In contrast, the activity of the deep-waterMO_x community is relatively low at the ambient, summertimeCH₄ concentrations but has the potential to increase rapidly in responseto CH₄ availability. A similar distinction between surface anddeep-water MO_x is also suggested by our molecular analyses. TheDGGE banding patterns of 16S rRNA gene fragments of the surface MW and deepwater were clearly different. A DGGE band related to the known type IMO_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 setspmoA and mxaF showed, in contrast to those of the surfaceMW, additional products besides the expected one of 530 base pairs (bp).Apparently, different MO_x communities have developed in thestratified water masses in Storfjorden, which is possibly related to thespatiotemporal variability in CH₄ supply to the distinct water masses.

[学科分类] 地球化学与岩石