Methane Oxidation Rates Research Articles

Shifts in a bacterial community composition of a mesotrophic peatland after wildfire

The structural changes in the bacterial community composition of the Galitskii Mokh mesotrophic peatland (Tver oblast) after the 2010 wildfire were analyzed. Burned sites demonstrated an increase in peat water pH and mineralization, with elevated nitrate and sulfate concentrations, which resulted in doubling of the numbers of culturable sulfate reducers. Although the rate of methane oxidation by peat samples from burned sites decreased, methane emission did not increase. Enumeration of bacterial phylogenetic groups by fluorescent in situ hybridization revealed doubling of the numbers of Alphaproteobacteria and Bacteroidetes in burned areas compared to an undamaged site, while a reverse pattern was observed for Deltaproteobacteria and Planctomycetes. Comparative analysis of the 16S rRNA clone libraries revealed shifts in the bacterial community composition in the peat from burned sites. Slowly growing bacteria of the phyla Verrucomicrobia and Planctomycetes were replaced by fast-growing colonizers from the Proteobacteria. The originally diverse Actinobacterial community was replaced by the one dominated by the phylotypes related to thermophilic Aciditerrimonas ferrireducens. In the Acidobacteria, a typical group of peat-inhabiting bacteria, the subgroups 3, 6, and 17 were replaced by subgroups 7 and 10. These changes in the structure of peatland microbial communities could result in disturbances of some biospheric functions of these ecosystems.

Evidence for the cooccurrence of nitrite-dependent anaerobic ammonium and methane oxidation processes in a flooded paddy field.

Anaerobic ammonium oxidation (anammox) and nitrite-dependent anaerobic methane oxidation (n-damo) are two of the most recent discoveries in the microbial nitrogen cycle. In the present study, we provide direct evidence for the cooccurrence of the anammox and n-damo processes in a flooded paddy field in southeastern China. Stable isotope experiments showed that the potential anammox rates ranged from 5.6 to 22.7 nmol N2 g(-1) (dry weight) day(-1) and the potential n-damo rates varied from 0.2 to 2.1 nmol CO2 g(-1) (dry weight) day(-1) in different layers of soil cores. Quantitative PCR showed that the abundance of anammox bacteria ranged from 1.0 × 10(5) to 2.0 × 10(6) copies g(-1) (dry weight) in different layers of soil cores and the abundance of n-damo bacteria varied from 3.8 × 10(5) to 6.1 × 10(6) copies g(-1) (dry weight). Phylogenetic analyses of the recovered 16S rRNA gene sequences showed that anammox bacteria affiliated with "Candidatus Brocadia" and "Candidatus Kuenenia" and n-damo bacteria related to "Candidatus Methylomirabilis oxyfera" were present in the soil cores. It is estimated that a total loss of 50.7 g N m(-2) per year could be linked to the anammox process, which is at intermediate levels for the nitrogen flux ranges of aerobic ammonium oxidation and denitrification reported in wetland soils. In addition, it is estimated that a total of 0.14 g CH4 m(-2) per year could be oxidized via the n-damo process, while this rate is at the lower end of the aerobic methane oxidation rates reported in wetland soils.

Benthic foraminifera from the deep-water Niger delta (Gulf of Guinea): Assessing present-day and past activity of hydrate pockmarks

We present ecological and isotopic (δ18O and δ13C) data on benthic foraminifera sampled from 4 deep-sea stations in a pockmark field from the deep-water Niger delta (Gulf of Guinea, Equatorial Atlantic Ocean). In addition, a series of sedimentological and (bio)geochemical data are shown to back up foraminiferal observations. All stations are located within 1.2km of each other, so prevailing oceanographic conditions can be assumed to be similar at each site. Two of the sites (GMMC-01 and GMMC-02) are located in a pockmark (named “pockmark A”) where current methane seepages were recorded by ROV observations. A third station (GMMC-03) is located in the topographic depression interpreted as a collapsed pockmark (named “pockmark B”). The fourth site (GMMC-04) is a reference station, without evidence of past or present seepages. Our observations show that degraded organic matter with low bio-availability is present at all stations with a preferential burial of organic compounds in topographic depressions (GMMC-03 station). Authigenic aragonite is abundant in surface sediments at stations GMMC-01 and -02. Its precipitation is likely related to high rates of methane oxidation during past seep events in episodically active pockmark A. In contrast, the absence of anaerobic methanotrophic Archaea (ANME) during the sampling period (November 2011) suggests that only moderate sulphide and methane oxidation take place close to the sediment–water interface. Compared to the reference site GMMC-04, living foraminifera at the collapsed and episodically active pockmarks show minor changes in terms of diversity, standing stocks and faunal composition. However, the δ13C signal of living and dead (but well-preserved) foraminiferal species (Ceratobulimina contraria, Melonis barleeanus, Uvigerina peregrina) is depleted in the episodically active pockmark A compared to the other stations. Overgrowth of authigenic carbonate on altered foraminifera generates an important shift to lower δ13C values. Dead faunas carry a complex time-averaged message, integrating taphonomic gains and losses related to the temporal variability of gas emission. They reveal major faunal differences that may be useful to detect gas hydrate seepages in different pockmark stages.

Gammaproteobacterial methanotrophs dominate cold methane seeps in floodplains of West Siberian rivers.

A complex system of muddy fluid-discharging and methane (CH4)-releasing seeps was discovered in a valley of the river Mukhrinskaya, one of the small rivers of the Irtysh Basin, West Siberia. CH4 flux from most (90%) of these gas ebullition sites did not exceed 1.45 g CH4 h(-1), while some seeps emitted up to 5.54 g CH4 h(-1). The δ(13)C value of methane released from these seeps varied between -71.1 and -71.3‰, suggesting its biogenic origin. Although the seeps were characterized by low in situ temperatures (3.5 to 5°C), relatively high rates of methane oxidation (15.5 to 15.9 nmol CH4 ml(-1) day(-1)) were measured in mud samples. Fluorescence in situ hybridization detected 10(7) methanotrophic bacteria (MB) per g of mud (dry weight), which accounted for up to 20.5% of total bacterial cell counts. Most (95.8 to 99.3%) methanotroph cells were type I (gammaproteobacterial) MB. The diversity of methanotrophs in this habitat was further assessed by pyrosequencing of pmoA genes, encoding particulate methane monooxygenase. A total of 53,828 pmoA gene sequences of seep-inhabiting methanotrophs were retrieved and analyzed. Nearly all of these sequences affiliated with type I MB, including the Methylobacter-Methylovulum-Methylosoma group, lake cluster 2, and several as-yet-uncharacterized methanotroph clades. Apparently, microbial communities attenuating methane fluxes from these local but strong CH4 sources in floodplains of high-latitude rivers have a large proportion of potentially novel, psychrotolerant methanotrophs, thereby providing a challenge for future isolation studies.

The rise and fall of methanotrophy following a deepwater oil-well blowout

The blowout of the Macondo oil well in the Gulf of Mexico in April 2010 injected up to 500,000 tonnes of natural gas, mainly methane, into the deep sea. Spatially extensive measurements of methane dynamics in the months following the spill reveal a rapid rise and fall in the microbial consumption of methane. The blowout of the Macondo oil well in the Gulf of Mexico in April 2010 injected up to 500,000 tonnes of natural gas, mainly methane, into the deep sea1. Most of the methane released was thought to have been consumed by marine microbes between July and August 20102,3. Here, we report spatially extensive measurements of methane concentrations and oxidation rates in the nine months following the spill. We show that although gas-rich deepwater plumes were a short-lived feature, water column concentrations of methane remained above background levels throughout the rest of the year. Rates of microbial methane oxidation peaked in the deepwater plumes in May and early June, coincident with a rapid rise in the abundance of known and new methane-oxidizing microbes. At this time, rates of methane oxidation reached up to 5,900 nmol l−1 d−1—the highest rates documented in the global pelagic ocean before the blowout4. Rates of methane oxidation fell to less than 50 nmol l−1 d−1 in late June, and continued to decline throughout the remainder of the year. We suggest the precipitous drop in methane consumption in late June, despite the persistence of methane in the water column, underscores the important role that physiological and environmental factors play in constraining the activity of methane-oxidizing bacteria in the Gulf of Mexico.

Bacterial bloom and crash

Microbes quickly consumed much of the methane released in the 2010 Gulf of Mexico oil spill. Time-series measurements now suggest that, after a steep rise, methane oxidation rates crashed while hydrocarbon discharge was still continuing at the wellhead.

Ethylene rather than dissolved organic carbon controls methane uptake in upland soils

Sullivan et al. (2013) reported that there was a significantly positive relationship (P < 0.01, r2 = 0.58) between dissolved organic carbon (DOC) using salt extraction methods and potential methane oxidation rates in an arid region across a substrate age gradient. The authors observed that during the wet season rates of methane oxidation were higher, in opposition to trends in other ecosystems where increased soil moisture limits methane oxidation. Furthermore, DOC was more closely correlated with potential methane oxidation rates than other relevant parameters such as soil moisture content, pore space and texture. After considering alternative options, the authors indicated that DOC may be an important regulator of methane oxidation rates in these arid soils. The authors indicated that this conclusion was supported by observations that incubation with 13C-glucose enriched the methaneoxidizing bacteria (MOB) biomarker 18:1x7c, suggesting that DOC was a facultative substrate for MOB, and also explaining the observed correlation.

The more, the merrier: heterotroph richness stimulates methanotrophic activity.

Although microorganisms coexist in the same environment, it is still unclear how their interaction regulates ecosystem functioning. Using a methanotroph as a model microorganism, we determined how methane oxidation responds to heterotroph diversity. Artificial communities comprising of a methanotroph and increasing heterotroph richness, while holding equal starting cell numbers were assembled. We considered methane oxidation rate as a functional response variable. Our results showed a significant increase of methane oxidation with increasing heterotroph richness, suggesting a complex interaction in the cocultures leading to a stimulation of methanotrophic activity. Therefore, not only is the methanotroph diversity directly correlated to methanotrophic activity for some methanotroph groups as shown before, but also the richness of heterotroph interacting partners is relevant to enhance methane oxidation too. In this unprecedented study, we provide direct evidence showing how heterotroph richness exerts a response in methanotroph-heterotroph interaction, resulting in increased methanotrophic activity. Our study has broad implications in how methanotroph and heterotroph interact to regulate methane oxidation, and is particularly relevant in methane-driven ecosystems.

Molecular and biogeochemical evidence for methane cycling beneath the western margin of the Greenland Ice Sheet.

Microbial processes that mineralize organic carbon and enhance solute production at the bed of polar ice sheets could be of a magnitude sufficient to affect global elemental cycles. To investigate the biogeochemistry of a polar subglacial microbial ecosystem, we analyzed water discharged during the summer of 2012 and 2013 from Russell Glacier, a land-terminating outlet glacier at the western margin of the Greenland Ice Sheet. The molecular data implied that the most abundant and active component of the subglacial microbial community at these marginal locations were bacteria within the order Methylococcales (59-100% of reverse transcribed (RT)-rRNA sequences). mRNA transcripts of the particulate methane monooxygenase (pmoA) from these taxa were also detected, confirming that methanotrophic bacteria were functional members of this subglacial ecosystem. Dissolved methane ranged between 2.7 and 83 μM in the subglacial waters analyzed, and the concentration was inversely correlated with dissolved oxygen while positively correlated with electrical conductivity. Subglacial microbial methane production was supported by δ(13)C-CH4 values between -64‰ and -62‰ together with the recovery of RT-rRNA sequences that classified within the Methanosarcinales and Methanomicrobiales. Under aerobic conditions, >98% of the methane in the subglacial water was consumed over ∼30 days incubation at ∼4 °C and rates of methane oxidation were estimated at 0.32 μM per day. Our results support the occurrence of active methane cycling beneath this region of the Greenland Ice Sheet, where microbial communities poised in oxygenated subglacial drainage channels could serve as significant methane sinks.

Effects of 17β‐estradiol pollution on water microbial methane oxidation activity

17β-estradiol (17β-E2), a widespread and natural estrogen in the environment, has imposed a serious threat to the safety and function of aquatic ecosystems because of worsening pollution and high potential toxicity. In the present study, the authors focus on the impact of 17β-E2 pollution on water microbial methane oxidation function. The authors investigated the mechanism of its influence on water microbial activity and discuss the growth rate of methane-oxidizing bacteria. The results showed that 17β-E2 could significantly inhibit the function of water microbial methane oxidation. When 17β-E2 concentration was ≥ 5 ng L(-1) , the methane oxidation rate increased with increasing 17β-E2 and finally settled to a constant value. Furthermore, the authors found no significant linear correlation between 17β-E2 concentrations and its methane oxidation rate. However, increasing 17β-E2 dramatically improved water microbial community activity, because a significant or highly significant promotion in the generation rate of CO2 was measured. Moreover, within a certain period of time and at certain concentrations, positive linear correlation existed between water CO2 generation rate and 17β-E2 concentrations. In addition, the growth rate of culturable methane-oxidizing bacteria was promoted when 17β-E2 pollution concentration from 2 ng L(-1) to 20 ng L(-1) . Therefore, 17β-E2 pollution can inhibit microbial methane oxidation function in water, which indirectly promotes the release of water methane and directly contributes to the rate of water-generated and released CO2 . Specifically, 17β-E2 pollution can promote water emissions of greenhouse gases. Environ

Methane formation and consumption processes in Xiangxi Bay of the Three Gorges Reservoir

Indoor simulation experiment was carried out to evaluate the formation and consumption rates of methane (CH4) in Xiangxi Bay of the Three Gorges Reservoir (TGR), China. The results show that both the CH4 formation and consumption rates were significantly positively correlated with temperature. CH4 efflux decreased with rising temperature due to its potential increasing oxidation rate. CH4 oxidation in surface sediments accounted for 51.8% of the total production and it even reached to 77.4% at 35°C. The methane oxidation rate in water column ranged from 1.26 to 4.65 mg/(m2h), of which the average and greatest rate accounted for 46.7% and 73.9% of CH4 production respectively under the condition of 30 m water column and 35°C. The methane oxidation may increase by 41.04 mg/(m2h) under average water level of TGR (160 m), and most methane resulted from sediments can be oxidized in the water column.

Nitrate-dependent anaerobic methane oxidation in a freshwater sediment

Anaerobic oxidation of methane coupled to denitrification (DAOM) is a novel process of potential importance to the regulation of methane emissions from freshwater environments. We established nitrate-enriched microcosms of sediment from a freshwater pond in order to quantify the role of this process in a simulated natural redox zonation. The microcosms were allowed to acclimate to nitrate levels of 1–2mmolL−1 in the overlying water for 16months leading to a nitrate penetration of 4cm. The nitrate enrichment significantly stimulated AOM relative to controls, and based on the similar concentrations of sulfate and reactive Fe(III) in the control sediment we conclude that the observed AOM was coupled to denitrification. DAOM occurred at rates that were two orders of magnitude lower than aerobic methane oxidation rates reported in freshwater sediments, and the process appeared to be limited by nitrate or nitrite even at millimolar nitrate concentrations. By contrast, ammonium was efficiently consumed at the base of the nitrate zone, presumably by the anammox process. Although DAOM was stimulated by nitrate enrichment, there were no significant differences between the methane emission from the control and nitrate-enriched microcosms. Our results provide the first experimental evaluation of the kinetics of DAOM in whole sediment cores and indicate that AOM coupled to denitrification can consume a substantial part of the methane flux in nitrate-rich environments. Because it is much less efficient in scavenging methane than its aerobic counterpart, the anaerobic process will, however, mainly be of significance in the regulation of methane emission from oxygen-depleted systems.

Micro‐aerobic bacterial methane oxidation in the chemocline and anoxic water column of deep south‐Alpine Lake Lugano (Switzerland)

We measured seasonal variations in the vertical distribution of methane concentration, methane oxidation rates, and lipid biomarkers in the northern basin of Lake Lugano. Methane consumption below the oxic—anoxic interface co‐occurred with concentration maxima of 13C‐depleted C16 fatty acid biomarkers (with δ13C values as low as −70‰) in the anoxic water column, as well as characteristic δ13CCH4 profiles. We argue that the conspicuous methane concentration gradients are primarily driven by (micro‐)aerobic methane oxidation (MOx) below the chemocline. We measured a strong MOx potential throughout the anoxic water column, while MOx rates at in situ O2 concentration &gt; 10 nmol L−1 were undetectable. Similarly, we found MOx‐related biomarkers and gene sequences encoding the particulate methane monooxygenase in the anoxic, but not the oxic, water. The mechanism of (episodic) oxygen supply sustaining the MOx community in anoxic waters is still uncertain. Our results indicate that a bacterial methanotrophic community is responsible for the methane consumption in Lake Lugano, without detectable contribution from archaeal methanotrophs. Bacterial populations that accumulated both at the suboxic—anoxic interface and in the deeper anoxic hypolimnion, where maximum potential MOx rates were observed throughout the year (1.5–2.5 µmol L−1 d−1) were mainly related to Methylobacter sp. Close relatives are found in lacustrine environments throughout the world, and their potential to thrive under micro‐ and anoxic conditions in Lake Lugano may imply that micro‐aerobic methane oxidation is important in methane cycling and competition for methane and oxygen in stratified lakes worldwide.

Abundance and diversity of methanotrophic Gammaproteobacteria in northern wetlands

Numeric abundance, identity and pH preferences of methanotrophic Gammaproteobacteria (type I methanotrophs) inhabiting the northern acidic wetlands were studied. The rates of methane oxidation by peat samples from six-wetlands of European Northern Russia (pH 3.9-4.7) varied from 0.04 to 0.60 μg CH4 g(-1) peat h(-1). The number of cells revealed by hybridization with fluorochrome-labeled probes M84 + M705 specific for type I methanotrophs was 0.05-2.16 x 10(5) cells g(-1) dry peat, i.e. 0.4-12.5% of the total number of methanotrophs and 0.004-0.39% of the total number of bacteria. Analysis of the fragments of the pmoA gene encoding particulate methane monooxygenase revealed predominance of the genus Methylocystis (92% of the clones) in the studied sample of acidic peat, while the proportion of the pmoA sequences of type I methanotrophs was insignificant (8%). PCR amplification of the 16S rRNA gene fragments of type I methanotrophs with TypeIF-Type IR primers had low specificity, since only three sequences out of 53 analyzed belonged to methanotrophs and exhibited 93-99% similarity to those of Methylovulum, Methylomonas, and Methylobacter species. Isolates of type I methanotrophs obtained from peat (strains SH10 and 83A5) were identified as members of the species Methylomonaspaludis and Methylovulum miyakonense, respectively. Only Methylomonaspaludum SH10 was capable of growth in acidic media (pH range for growth 3.8-7.2 with the optimum at pH 5.8-6.2), while Methylovulum miyakonense 83A5 exhibited the typical growth characteristics of neutrophilic methanotrophs (pH range for growth 5.5-8.0 with the optimum at pH 6.5-7.5).

Microbiological and isotopic geochemical investigation of Lake Kislo-Sladkoe, a meromictic water body at the Kandalaksha Bay shore (White Sea)

Microbiological, biogeochemical, and isotopic geochemical investigation of Lake Kislo-Sladkoe (Polusolenoe in early publications) at the Kandalaksha Bay shore (White Sea) was carried out in September 2010. Lake Kislo-Sladkoe was formed in the mid-1900s out of a sea gulf due to a coastal heave. At the time of investigation, the surface layer was saturated with oxygen, while near-bottom water contained sulfide (up to 32 mg/L). Total number of microorganisms was high (12.3 x 10(6) cells/mL on average). Light CO2 fixation exhibited two pronounced peaks. In the oxic zone, the highest rates of photosynthesis were detected at 1.0 and 2.0 m. The second, more pronounced peak of light CO2 fixation was associated with activity of anoxygenic phototrophic bacteria in the anoxic layer at the depth of 2.9 m (413 μg C L(-1) day(-1)). Green-colored green sulfur bacteria (GSB) predominated in the upper anoxic layer (2.7-2.9 m), their numbers being as high as 1.12 x 10(4) cells/mL, while brown-colored GSB predominated in the lower horizons. The rates of both sulfate reduction and methanogenesis peaked in the 2.9 m horizon (1690 μg S L(-1) day(-1) and 2.9 μL CH4 L(-1) day(-1)). The isotopic composition of dissolved methane from the near-bottom water layer (δ13C (CH4) = -87.76 per thousand) was significantly lighter than in the upper horizons (δ13C (CH4) = -77.95 per thousand). The most isotopically heavy methane (δ13C (CH4) = -72.61 per thousand) was retrieved from the depth of 2.9 m. The rate of methane oxidation peaked in the same horizon. As a result of these reactions, organic matter (OM) carbon of the 2.9 m horizon became lighter (-36.36 per thousand), while carbonate carbon became heavier (-7.56 per thousand). Thus, our results demonstrated that Lake Kislo-Sladkoe is a stratified meromictic lake with active microbial cycles of carbon and sulfur. Suspended matter in the water column was mostly of autochthonous origin. Anoxygenic photosynthesis coupled to utilization of reduced sulfur compounds contributed significantly to OM production.

Study on the Photo-Electro-Catalytic Oxidation of Methane

Emission control of methane from landfills has received considerable attentions in recent years. Photo-electro-catalytic oxidation of methane was carried out at the mode of continuous electrolysis and intermittent illumination with TiO2-coated titanium mesh plate as pholocatalyst and electrode in combination with ionic liquid as electrolyte under UV light irradiation. The result showed that the rapid conversion of methane was achieved under ambient pressure and temperature through photocatalytic oxidation combining with electrolysis. Under the mode of intermittent illumination (on:off = 10 min : 10 min) and continuous electrolysis (II-CE), the highest oxidation rate of methane (280 mg/(m3min)) was obtained. Under fixed off time and total illumination time, the oxidation quantity of methane linearly increased with the intermittent frequency. Photo-electro-catalytic oxidation of methane can be described by the first-order kinetics, and the apparent kinetic constant increased with the intermittent frequency.

Effect of trichloroethylene and tetrachloroethylene on methane oxidation and community structure of methanotrophic consortium

The methane oxidation rate and community structure of a methanotrophic consortium were analyzed to determine the effects of trichloroethylene (TCE) and tetrachloroethylene (PCE) on methane oxidation. The maximum methane oxidation rate (Vmax ) of the consortium was 326.8 μmol·g-dry biomass−1·h−1, and it had a half-saturation constant (Km ) of 143.8 μM. The addition of TCE or PCE resulted in decreased methane oxidation rates, which were decreased from 101.73 to 5.47–24.64 μmol·g-dry biomass−1·h−1 with an increase in the TCE-to-methane ratio, and to 61.95–67.43 μmol·g-dry biomass−1·h−1 with an increase in the PCE-to-methane ratio. TCE and PCE were non-competitive inhibitors for methane oxidation, and their inhibition constants (Ki ) were 33.4 and 132.0 μM, respectively. When the methanotrophic community was analyzed based on pmoA using quantitative real-time PCR (qRT-PCR), the pmoA gene copy numbers were shown to decrease from 7.3 ± 0.7 × 108 to 2.1–5.0 × 107 pmoA gene copy number · g-dry biomass−1 with an increase in the TCE-to-methane ratio and to 2.5–7.0 × 107 pmoA gene copy number · g-dry biomass−1 with an increase in the PCE-to-methane ratio. Community analysis by microarray demonstrated that Methylocystis (type II methanotrophs) were the most abundant in the methanotrophic community composition in the presence of TCE. These results suggest that toxic effects caused by TCE and PCE change not only methane oxidation rates but also the community structure of the methanotrophic consortium.

Methane oxidation needs less stressed plants

Methane oxidation rates in soil are liable to be reduced by plant stress responses to climate change. Stressed plants exude ethylene into soil, which inhibits methane oxidation when present in the soil atmosphere. Here we discuss opportunities to use 1-aminocyclopropane-1-carboxylate deaminase to manage methane oxidation by regulating plant stress responses.

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 (MOx) is a key mechanism in controlling methane (CH4) emissions from the world's oceans to the atmosphere. In this study, we investigated MOx 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. MOx rate measurements at near in situ CH4 concentrations (here measured with 3H-CH4 raising the ambient CH4 pool by &lt;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 (&lt;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 MOx community in the surface MW is adapted to relatively low CH4 concentrations. In contrast, the activity of the deep-water MOx 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 MOx 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 MOx 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 MOx 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.

Sulfate reduction and microbial processes of the methane cycle in the sediments of the Sevastopol Bay

The rates of microbial processes of sulfate reduction and of the methane cycle were measured in the bottom sediments of the Sevastopol basin, where seeps of gaseous methane have been previously found. Typically for marine environments, sulfate reduction played the major role in the terminal phase of decomposition of organic matter (OM) in reduced sediments of this area. The rate of this process depended on the amount of available OM. The rate of methanogenesis in the sediments increased with depth, peaking in the subsurface horizons, where decreased sulfate concentration was detected in the pore water. The highest rates of sulfate-dependent anaerobic methane oxidation were found close to the methane-sulfate transition zone as is typical of most investigated marine sediments. The data on the carbon isotopic composition of gaseous methane from the seeps and dissolved CH4 from the bottom sediments, as well as on the rates of microbial methanogenesis and methane oxidation indicate that the activity of the methane seeps results from accumulation of biogenic methane in the cavities of the underlying geological structures with subsequent periodic release of methane bubbles into the water column.