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

Abstract

Abstract. The bacterially mediated aerobic methane oxidation (MO x ) is a key mechanism in controlling methane (CH 4 ) 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-tracerbased incubation assays ( 3 H-CH 4 and 14 C-CH 4 ), stable C-CH 4 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. CH 4 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 CH 4 concentrations (here measured with 3 H-CH 4 raising the ambient CH 4 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 14 C-CH 4 (incubations were spiked with ∼ 450 nM of 14 C-CH 4 , providing an estimate of the CH 4 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 13 C values in the residual CH 4 pool. Our results indicate that the MO x community in the surface MW is adapted to relatively low CH 4 concentrations. In contrast, the activity of the deep-water MO x community is relatively low at the ambient, summertime CH 4 concentrations but has the potential to increase rapidly in response to CH 4 availability. A similar distinction between surface and deepwater 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 CH 4 supply to the distinct water masses.

Methane-oxidizing seawater microbial communities from an Arctic shelf

“…High concentrations of methane have been reported from the entire water column up to the surface around Svalbard (Damm et al, 2005;Mau et al, 2013;Myhre et al, 2016), the Siberian Shelf (Shakhova et al, 2010), and the Beaufort Sea (Lorenson et al, 2016). In addition, during periods of near 100 % sea-ice cover, gas exchange from the water column to the atmosphere is restricted (Loose et al, 2011).…”

“…Along with factors like oxygen and trace-metal availability (Crespo-Medina et al, 2014;Sansone et al, 2001;Semrau et al, 2010), as well as local oceanographic and geologic conditions (Schmale et al, 2015;Steinle et al, 2015), dissolved methane concentration can be a control on the community of MOB and thus methane oxidation rates (CrespoMedina et al, 2014;Kessler et al, 2011;Mau et al, 2013). Methane hotspots, promoted by limited gas exchange under sea ice, might thus be candidate locations for accumulation of methane oxidizers.…”

“…During the last few years, the first studies have determined methane oxidation rates from seawater in these regions to cover a range from 10 −4 up to 3.2 nmol L −1 d −1 (Gentz et al, 2014;Lorenson et al, 2016;Mau et al, 2013Mau et al, , 2017Steinle et al, 2015). In only two of these studies, both performed off Svalbard, oxidation rate measurements were combined with analysis of the microbial community.…”

“…Low but relatively constant cell-specific oxidation rates were determined from the oxidation rates and MOB abundance, indicating that MOB community size is an important control on the total methane oxidation rate in the system. Mau et al (2013) analyzed the bacterial community with denaturing gradient gel electrophoresis (DGGE) of the 16S gene and compared patterns of PCR products for pmoA. Different MOB communities were observed in the meltwater layer and deep water in this stratified system, also reflecting the observed differences in methane oxidation rates.…”

Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard

“…The rate of aerobic methane oxidation (MOx) in the water column varies widely in time and space [Mau et al, 2013;Steinle et al, 2015], and the rate of bubble dissolution is mainly dependent on initial bubble size, water temperature and salinity, pressure, the presence of bubble coatings (organic material, or hydrate skin), and bubble rise velocity [Leifer and Patro, 2002;Rehder et al, 2009]. In water depths greater than 200 m, bubbles less than 10 mm in diameter are expected to dissolve before they reach the surface mixed layer, preventing direct venting of methane from seafloor seeps into the atmosphere [e.g., Gentz et al, 2014;McGinnis et al, 2006].…”