Different methanotrophic potentials in stratified polar fjord waters

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Full MOx 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.

Introduction
Methane (CH 4 ) is a potent greenhouse gas with a global warming potential that exceeds carbon dioxide (CO 2 ) 23-fold over a 100 yr timescale and is, after water vapour and CO 2 , the most important greenhouse gas (IPCC, 2007).Substantial research efforts have consequently been made to understand its sources and sinks.A large part of oceanic CH 4 is generated under reduced conditions in anoxic marine sediments, dominantly through microbially mediated carbonate reduction and dispropornation of methylated substrates (Whiticar, 1999;Hinrichs and Boetius, 2002;Formolo, 2010).Sedimentary CH 4 is also formed by thermal breakdown of organic matter and, although of lesser importance, the Fischer-Tropsch reaction, both occurring at high temperature and pressure.In addition, conspicuous CH 4 concentrations maxima in oxic water layers provided indications for CH 4 production under oxic conditions possibly mediated by yet unknown microbes using dimethylsulfoniopropionate (DMSP) (Damm et al., 2010) or methylphosphonic acid (Karl et al., 2008;Metcalf et al., 2012) as substrate.However, despite the apparent ubiquity of methanogenesis in marine systems and the large area covered by oceans, comparably little CH 4 is liberated from the oceans into the atmosphere as a result of microbial consumption (Reeburgh, 2007;IPCC, 2007).
About 80 % of sedimentary CH 4 is consumed in reduced sediments as a result of the anaerobic oxidation of methane (AOM) with sulphate as the terminal electron acceptor (Reeburgh, 2007;Knittel and Boetius, 2009).Finally, aerobic CH 4 -oxidising bacteria at the sediment surface and/or in the water column (belonging to the alpha (Type II) or gamma (Type I and Type X) subdivision of the Proteobacteria) consume CH 4 that has Introduction

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Full by-passed the anaerobic microbial filter (Hanson and Hanson, 1996) according to the following reaction: Several techniques have been used to quantify aerobic methane oxidation (MOx) rates (Reeburgh, 2007).A common method is to incubate of water column or sediment samples with radio-labelled tracers such as 14 C-CH 4 or 3 H-CH 4 (Reeburgh et al., 1991;Valentine et al., 2001;Niemann et al., 2006;Mau et al., 2012), which has been proven highly sensitive.During the incubation, 14 C-CH 4 or 3 H-CH 4 are converted at the same rate as the natural, non-labelled CH 4 to 14 CO 2 and 14 C-biomass or 3 H 2 O.Despite the importance of water column MOx controlling oceanic CH 4 emission to the atmosphere, only a small number of water column MOx rate measurements exists, which is particularly true for high latitude environments (Ward and Kilpatrick, 1990;Griffiths et al., 1982).The available data show a large scatter of rates over several orders of magnitude (Fig. 1), but factors controlling MOx activity such as temporal variations in CH 4 availability (e.g.Mau et al., 2007a, b;Damm et al., 2007) and the rate potential, i.e. the maximum uptake rate, of the present MOx community are not well constrained.
Our aims for this study were to investigate MOx rates and rate potentials as well as the key MOx community in response to different CH 4 concentrations in a natural marine environment.As a model system, we choose the fjord Storfjorden (Svalbard), which is characterised by seasonal stratification, separating distinct water masses with different CH 4 sources during summer time.

Study site
Storfjorden is located in the Svalbard Archipelago between the islands Spitsbergen, Barentsøya, and Edgeøya (Fig. 2).CH 4 concentrations in the fjord water exceed Introduction

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Full atmospheric equilibrium concentration throughout the water column by a factor of 2-16, although surface waters CH 4 is of a different origin compared to the CH 4 in subsurface waters (Damm et al., 2008).Surface waters contain recently produced, 14 C-depleted CH 4 , which was proposed to result from a summer phytoplankton bloom producing methylated compounds such as DMSP, which is a potential substrate for methylotrophic methanogenesis.A CH 4 production-removal cycle appears to be established in the surface water as reflected by varying CH 4 concentrations and 13 C-CH 4 values (Damm et al., 2008).In contrast, deeper water contains CH 4 that is mixed into the bottom water as a result of brine-enriched shelf water (BSW) formation during wintertime causing enhanced turbulence and repeatedly occurring re-suspension of sediments releasing CH 4 (Damm et al., 2007) (Loeng, 1991).The residence time of the high-salinity water is longer in deeper layers (90-246 d) compared to the fjord's surface waters (51-141 d) (Geyer et al., 2009).1).The Storfjorden stations were aligned along the cyclonic coastal current flowing into Storfjorden along Edgeøya and out along Spitsbergen (Loeng, 1991;Skogseth et al., 2005) (Fig. 2).We intended to sample and compare the fjord's upper and lower water column because of the different CH 4 sources and water residence times.We sampled vertical profiles throughout the water column thus recovering samples from MW, ArW, and BSW.All water masses were subsampled for chemical/biogeochemical analyses (method 2.3 and 2.4), but we focused on the MW and Introduction

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Full BSW for molecular analyses (method 2.5).Specific water depths were sampled with a CTD/rosette sampler equipped with 12 five-litre Niskin bottles, a Seabird SBE 911 plus CTD and an SBE 43 oxygen sensor for online monitoring of salinity, temperature, pressure and dissolved oxygen.

CH 4 concentrations and stable isotope composition
Aliquots of sea water were immediately subsampled from the Niskin bottles using 1 L glass bottles for CH 4 concentration measurements.CH 4 was extracted from the water by vacuum-ultrasonic treatment within a few hours after sampling (Schmitt et al., 1991).Hydrocarbon concentrations were measured with a Chrompack 9003 gas chromatograph (GC) equipped with a flame ionization detector (FID).Duplicate analyses indicate an error of 5-10 % (Lammers and Suess, 1994).After GC analyses, an aliquot of the extracted CH 4 gas was transferred into pre-evacuated glass containers for stable carbon isotope analysis performed with an isotope ratio mass spectrometer (IRMS; Finnigan Delta XP plus) in our onshore laboratories.The extracted gas was purged and trapped with PreCon equipment (Finnigan) to pre-concentrate the sample.All isotopic ratios have an analytical error ≤ 1 ‰ and are presented in the δ-notation against the Vienna Pee Dee Belemnite (VPDB) standard.

Methane oxidation rates
MOx rates were determined from ex situ incubations of water samples in 100 mL serum vials.The vials were filled bubble-free from Niskin bottles and crimped with rubber stoppers (halogenated butyl elastomer).One set of samples was then incubated with 50 µl gas mixture comprised of 3 H-labelled CH 4 (160-210 kBq) and a second set was incubated with 10 µl of 14 C-labelled CH 4 (12-15 kBq).assuming first order kinetics (Reeburgh et al., 1991;Valentine et al., 2001): where k is the effective first order rate constant calculated as the fraction of labelled CH 4 oxidised per unit time and [CH 4 ] is the ambient CH 4 concentration.
In order to determine a suitable incubation time period, we performed parallel time series incubations with samples collected from the fjord (Station 2 and 18) and from an open water station (reference station -RS).During each incubation series, tracer consumption was measured in duplicates after 0.5, 1, 2, 3 and 4 or 5 days.In the CH 4 rich waters of the fjord, our results showed a linear tracer consumption of about 5-15 % over the first 3 days of incubation (Fig. 3).A potential bias due to substrate limitation and/or variations in reaction velocity thus seemes negligible at least over a time period, of 3 days, which we chose for our ex situ incubations.Just as the time series incubations, vertical distribution of MOx was determined in duplicates.
Incubations with 3 H-CH 4 and measurements of 3 H-CH 4 and 3 H-H 2 O was carried out according to Valentine et al. (2001) and Mau et al. (2012).Briefly, total activity ( 3 H-CH 4 + 3 H-H 2 O) was measured in 1 mL of sample aliquot by wet scintillation counting and activity of 3 H-H 2 O was measured after sparging the sample for ≥ 30 min with nitrogen gas to remove remaining 3 H-CH 4 .
Incubations with 14 C-CH 4 were terminated by injecting 0.5 mL of 10 M NaOH and adding a 5 mL headspace so that the remaining 14 C-CH 4 accumulated in the 3 were carried out analogous to previous measurements of CH 4 turnover in sediments (Treude et al., 2003;Niemann et al., 2005).In short, 14 C-CH 4 in the headspace was combusted to 14 C-CO 2 , while 14 C-CO wet scintillation counting.We also measured remaining radioactivity (presumably org. 14C) in the sample after 14 C-CH 4 and 14 CO 2− 3 removal.

Diversity of MOx community
The diversity of the natural bacterioplankton assemblages was examined by denaturing gradient gel electrophoresis (DGGE) based on the 16S rRNA gene.Immediately after sampling, bacterial cells were concentrated on nuclepore filters (0.2 µm pore size) and the filters were stored frozen at −20 • C until DNA extraction.Total community DNA was extracted using the Ultraclean soil DNA kit (MoBio Laboratories, USA). 1-5 µL DNA extract was applied as template in the 16S rRNA gene specific PCR with GM5 plus GC-clamp as forward primer and 907RM as reverse primer (Muyzer et al., 1993).
PCR conditions were as described by Gerdes et al. (2005).PCR-products (ca.500 bp) were analysed by DGGE, based on the protocol of Muyzer et al. (1993) using a gradient-chamber.Clearly visible bands of the DGGE-pattern were excised from the gel and re-amplified by PCR as described by Gerdes et al. (2005) and sequenced.The 16S rDNA gene sequences were then assigned to the new higher-order taxonomy proposed in Bergey's taxonomic outline of the "Prokaryotes" by the "Ribosomal Database Project (RDP) Classifier" (Wang et al., 2007).The sequences were further compared with those deposited in GenBank using the BLAST algorithm.The presence of CH 4 oxidising bacteria in the communities was screened by two functional primer sets "pmoA" and "mxaF", targeting the genes encoding subunits of the particulate methane monooxygenase (pMMO) and the methanol dehydrogenase (MDH), respectively.Both enzymes are key enzymes for methanotrophs (e.g.McDonald et al., 2008).However, the mxaF gene is also present in almost all other methylotrophic bacteria.The primer sets and amplification conditions employed in the gene specific PCR-reaction are described in Holmes et al. (1995) and McDonald and Murrell (1997), respectively.Introduction

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The MW extended from the surface to ∼ 60 m water depth; this is the depth range were the thermocline is located and temperature decreased by ∼ 4 • C (Fig. 4a).In the MW, CH 4 concentrations increased from ∼ 20 nM at the surface to 72.3 nM at 60 m water depth (Fig. 5a).All concentrations were high and oversaturated with respect to the atmospheric equilibrium concentration of 3.3-3.9nM (at the relevant T/S conditions, Wiesenburg and Guinasso, 1979).Similar to concentrations, microbial oxidation rates determined with 3 H-and 14 C-tracer increased with depth to 2.3 nM d −1 and 0.77 nM d −1 , respectively (Fig. 5b and c).In the MW, rates measured with 14 C-tracer were consistently lower than those determined with 3 H-tracer. 13C-CH 4 values in this water mass ranged between −43.5 and −53.6 ‰ (Fig. 5d).
In the ArW, (60 to ∼ 100 m water depth) oxygen concentrations decreased from 350 to 320 µM (Fig. 4c) and CH 4 concentrations from 42 to 6.5 nM (Fig. 5a).Both, MOx rates determined with 3 H and 14 C-tracer show a maximum at ∼ 80 m in this water mass (Fig. 5b and c).The stable carbon isotopic signature of CH 4 showed a strong shift from −46 ‰ to about −32 ‰ at the MW/ArW interface (80 m, Fig. 5d).
The BSW (> 100 m water depth) was characterised by oxygen concentrations below 320 µM (Fig. 4c).CH 4 concentrations decreased slightly with depth, but were stable below 120 m (8-9 nM, Fig. 5a).MOx rates determined with 3 H-labelled CH 4 show a similar trend as the CH 4 concentrations.However, while 3 H-MOx rates were low, rates determined with 14 C-labelled CH 4 were comparably higher with a maximum of 1.9 nM d −1 at ∼ 100 m water depth (Fig. 5b and c).The carbon isotopic signature of Introduction

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Full the CH 4 decreased steadily from its maximum of −30 ‰ at 100 m to −39 ‰ in the lowermost sample (136 m, Fig. 5d).
The MW samples showed strong DGGE-bands that we could assign to eukaryoticchloroplast DNA (#3, #4) and to Alphaproteobacteria of the genera Phaeobacter and Sulfitobacter (# 7, #8).The affiliation to the genus Phaeobacter was, however, relatively weak (0.51 confidence value, Table 2) indicating a possibly yet undescribed bacteria type.Additional bands (#5, #9, and #11) could be assigned to the genera Flavicola within the phylum Bacteroidetes, Haliea within the Gammaproteobacteria, and Ilumatobacter within the phylum Actinobacteria.Although we could measure CH 4 oxidation in the surface waters the DGGE based on the16S rRNA gene did not reveal known methanotrophs.
In contrast to the diverse MW community, all deep-water samples (Sta.12, 127 m, Sta. 2, 138 m, Sta.18, 136 m) showed a quite low diversity with only two strong (# 6 and #7) and one weaker DGGE band (#10) (Fig. 6).Band #7 was also common in the upper water masses while band #6 was only found in the BSW samples.This band could be affiliated to Methylosphaera, which is a known type I aerobic methanotrophic bacterium (Bowman et al., 1997).However, the confidence value of 0.38 (Table 2) was relatively low.The deep water specific band #10 could be assigned to the sulphatereducer Desulfobacca, also with a relatively low confidence level (0.19, Table 2).Introduction

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Molecular marker genes of methanotrophs
The pmoA gene that encodes the alpha subunit of the particulate methane monooxygenase is a molecular marker gene of methanotrophs (McDonald et al., 1997).In contrast to the 16S rRNA based survey, the pmoA based PCR yielded amplicons within all surface-and deep-water samples (Fig. 7) attesting the ubiquitous presence of MOx communities in waters of Storfjorden.However, besides the expected product of 530 bp, all deep-water samples showed a further, longer amplicon.Nevertheless, the sequences of all these amplicons could not be affiliated to known pmoA genes.A similar distinction of the water masses was also apparent from the distribution of the mxaF gene (Fig. 7) that encodes the enzyme methanol dehydrogenase, which catalyses the second step in CH 4 oxidation.The mxaF gene was also found in all samples, but deep water samples showed several additional, weak, and shorter mxaF bands.

Water column stratification and methane sources
Storfjorden water column mixing regimes were the subject of several previous publications (e.g.Haarpaintner et al., 2001;Skogseth et al., 2005;Fer, 2006).The fjord is a deep semi-enclosed basin in the Svalbard archipelago characterised by brine formation as a result of ice formation in latent heat polynyas during wintertime (Haarpaintner et al., 2001).Descending brines induce strong vertical mixing (Jardon et al., 2011) and turbulence at the sediment -water interface.However, accumulation of brine in bottom waters also leads to a stabilisation of the water column, which is further enhanced through a ∼ 60 m thick surface layer of relatively ion-depleted MW in summertime (Fig. 4).The residence time of the deep BSW is with 90-246 d relatively long compared to 51-141 d of the surface water (Geyer et al., 2009), so that on-going oxygen Introduction

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Full consumption leads to the comparably low oxygen levels that were detected previously (Anderson et al., 1988) and in this study.CH 4 concentrations in Storfjorden are generally high with 6-72 nM.These elevated concentrations originate from microbial methanogenesis in the sediments and enhanced transport from sediments into the water column as a result of the descending brines inducing turbulence at the sediment -water interface (Damm et al., 2007).However, CH 4 concentrations indicate a second CH 4 source at 40-60 m water depth (Fig. 5a).Here O 2 concentrations were high as well (Fig. 4c), possibly indicating a maximum of phytoplankton.The second CH 4 source is probably related to water column in situ production by yet unidentified microorganisms utilising the phytoplankton metabolite DMSP as a carbon source (Damm et al., 2008).While a significant fraction of the CH 4 is consumed (see Sect. 4.2), Storfjorden is apparently a CH 4 source to the atmosphere (Damm et al., 2007) as indicated by CH 4 concentrations of up to 30 nM in the well mixed surface layer.These concentrations are highly supersaturated with respect to the atmospheric equilibrium (3.3-3.9 nM, Wiesenburg and Guinasso, 1979).

Vertical distribution of methane oxidation
Our results indicate two regimes of CH 4 oxidation when comparing deep BSW (> 100 m) and surface MW (< 60 m).The ArW (60-100 m) appears to be an intermediate between the two regimes.This distinction is apparent from the vertical distribution of MOx rates (Fig. 5b and c).We incubated parallel samples with 3 H-and 14 C-labelled CH 4 .While absolute rate measurements with 3 H-CH 4 were moderate in ArW and BSW, rates with 14 C-CH 4 were elevated in these water masses.We suggest that this is related to the different amounts of CH 4 that were added as a result of 3 H-CH 4 compared to 14 C-CH 4 application.While in incubations with 3 H-CH 4 , the final CH 4 concentrations were only raised by < 2 nM, 14 C-CH 4 amendments lead to a CH 4 increase of ∼ 450 nM.
It is therefore reasonable to assume that the activity of the deep water MOx community was stimulated as a result of elevated CH 4 concentrations (Pack et al., 2011).This is Introduction

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Full most likely related to enzyme kinetics (Ward and Kilpatrick, 1990;Bender and Conrad, 1993;Smith et al., 1997), which can be described with the Michaelis-Menten model (Button, 1985;Translation of the 1913 Michaelis-Menten paper;Johnson and Goody, 2011).The Michaelis-Menten relation shows that enzyme activity, expressed by the reaction rate, increases hyperbolically with substrate concentration but levels off once the enzymatic machinery involved in in the metabolic pathway is saturated with substrate.Similar relations were found between cell-or community-specific rates and substrate concentrations (Button, 2010 and references therein).For a stable community, a maximum rate thus exists, which may only increase as a result of enzyme concentration increase (e.g.population growth) and/or optimisation of cytoarchitectural components relevant for substrate metabolism (e.g.transporter system).We could show that substrate turnover rates were linear over the incubation time of 3 d (Fig. 3), so that it seems unlikely that the CH 4 amendments induced an increase in enzyme concentration or optimisation of other parameters relevant for substrate metabolism, at least over the time period of our incubation experiments.
The derivative of the Michaelis-Menton function (for low substrate concentrations) yields the first order rate constant (k ), which, multiplied with the substrate concentration, defines the actual rate (r ox ; see Eq. 1).Consequently, under substrate limiting conditions, k -values are high but decrease if substrate concentrations approach enzyme saturation level.This relationship is depicted in Fig. 8.In MW (the fjord's surface layers) k -values were high during 3 H-CH 4 incubations, i.e. without substantial CH 4 amendments, but the addition of CH 4 in the 14 C-CH 4 incubations led to a substantial decrease (5-10 fold) in k , which suggests enzyme saturation.On the other hand, the deep water community in ArW and particularly in BSW appeared to operate at CH 4 concentrations below saturation because the addition of CH 4 through 14 C-CH 4 tracer application led to an increase in k compared to parallel incubations with 3 H-CH 4 .
The question remains as to why the MOx communities in deep and surface waters were apparently adapted to high and low CH 4 concentrations, respectively.Relatively low CH 4 concentrations in deeper water layers seem to be a regular feature of Introduction

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Full Storfjorden, at least during summertime (Damm et al., 2008).However, during winter time, CH 4 export from the sediments is enhanced leading to strongly elevated CH 4 concentration of up to ∼ 60 nM with a 14 C-signature of −40 to −50 ‰ in deeper water layers of Storfjorden (Damm et al., 2007).It thus appears reasonable to assume that the deep-water community is adapted to comparably high wintertime CH 4 concentrations.In summertime, on-going CH 4 oxidation leads to decreasing CH 4 concentrations and an increase in 13 C in the residual CH 4 (Fig. 5).In contrast, surface CH 4 seems only to increase strongly during summer (to ∼ 50 nM), potentially as a result of phytoplankton induced DMSP production, which fuels methanogenesis in the oxic water column (Damm et al., 2008).However, we cannot explain why surface-water methanotrophs appear not to have adapted to the high summertime CH 4 concentrations or possibly lack the ability to adapt.

Microbial community
Similar to the MOx regimes, the diversity of the bacterial assemblage was different when comparing surface MW to the deep BSW.Our DGGE analyses indicate a higher microbial diversity in surface-compared to the deep water (Fig. 6, Table 2).Nevertheless, we only found one band in the surface water (#9) and one band in the deep water (#6) that could be related to CH 4 oxidisers.Band #9 could be affiliated to the genus Haliea of which novel isolates were found to oxidize ethylene and to possess genes similar to particulate methane monooxygenases (pMMO) (Suzuki et al., 2012).Band #6 could be assigned to a known aerobic methanotroph of the genus Methylosphaera (yet with a relatively low confidence value of 0.38).Species of the order Methylosphaera were previously found in Antarctic marine-salinity, meromictic lakes (Bowman et al., 1997).The different patterns of MOx-related bands in surface-and deep water thus indicate the presents of different MOx-communities in these water masses.
Similar to the 16S rRNA based survey, the pmoA and mxaF gene analyses indicated differences between surface-and deep water masses (Fig. 7).Although, both genes

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Full were detected in all samples analysed (attesting an ubiquitous presence of MOx communities in Strofjorden), the deep water samples showed an additional, longer pmoA band and several weak, shorter mxaF bands suggesting the presence of different pmoA and mxaF related gene sequences.In addition to the 16S banding pattern and rate potentials, this further indicates that surface-and deep waters comprise different MOx communities.
The question remains as to what are the driving mechanisms for the development of the MOx communities in the different water masses.Here, we suggest that resuspension of sediments as a result of turbulent mixing during wintertime could have inoculated the deeper water masses with sediment microbes including benthic MOx communities.These are often distinct from planktonic communities (Bowman et al., 1997;He et al., 2012;Tavormina et al., 2008) and probably adapted to higher CH 4 concentrations.This scenario would also explain the presence of the sulphate reducer Desulfobacca in the oxic deep waters.Sulphate reducing bacteria are usually adapted to an anoxic environment (e.g.sediments) and may tolerate only low O 2 levels, yet resting cells of sulphate reducers were also found in fully oxygenated waters (Hastings and Emerson, 1988;Teske et al., 1996).The comparably short residence time of surface waters and the rather rapid exchange with the Barents Sea argues for a planktonic source of MOx communities in this water mass.Introduction

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Full  Full      2. MW and BSW samples are framed by a light blue and dark blue rectangle, respectively.Dendrogramm derived from UPGMA cluster analysis with the similarity coefficient of Jaccard.
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 H-CH 4 tracer addition raised ambient CH 4 concentrations by 1-2 nM and 14 C-CH 4 addition by 440-540 nM.The samples were subsequently shaken for ∼ 10 min on an orbital shaker to facilitate tracer dissolution and then incubated in the dark at 2 • C. CH 4 oxidation rates (r ox ) were calculated Discussion Paper | Discussion Paper | Discussion Paper | acidification with HCl.In either case, 14 C-CO 2 was then trapped in a solution of methoxyethanol : penylethylamine and the radioactivity was measured by Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 Results Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Ward, B. B. and Kilpatrick, K. A.: Relationship between substrate concentration and oxidation of ammonium and methane in a stratified water column, Cont.Shelf Res., 10, 1193-1208, 1990.Ward, B. B. and Kilpatrick, K. A.: Methane oxidation associated with mid-depth methane maxima in the Southern California Bight, Cont.Shelf Res., 13, 1111-1122, 1993Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
. The winter-released CH 4 is then trapped by increasing water stratification during warmer seasons and on-going CH 4 consumption leads to a 13 Cenriched isotopic signature of the residual CH 4 .During summer time the water column is stratified where surface melt water (MW) and intermediate Arctic water (ArW) occupy the upper water column, whereas denser BSW is restricted to deep basins CH 4 , these ex situ rates rather provide an estimate for the rate potential of the MOx community.Rate measurements typically provide a temporal snapshot, which is difficult to upscale particularly in environments with spatiotemporal varying CH 4 fluxes.Knowledge on the MOx rate potential, on the other hand, provides a mean to estimate the response in MOx activity in relation to changing CH 4 fluxes.Introduction

Table 1 .
Locations of stations and performed analyses.

Table 2 .
(Wang et al., 2007)artial 16S rRNA sequences to bacterial taxa performed with the RDP Classifier(Wang et al., 2007).The confidence value (0-1) for assignment at the level of class and genus is given in brackets.