Microbial Methane Oxidation Research Articles

Genomic Evidence for Simultaneous Optimization of Transcription and Translation through Codon Variants in the pmoCAB Operon of Type Ia Methanotrophs.

Understanding the interplay between genotype and phenotype is a fundamental goal of functional genomics. Methane oxidation is a microbial phenotype with global-scale significance as part of the carbon biogeochemical cycle and a sink for greenhouse gas. Microorganisms that oxidize methane (methanotrophs) are taxonomically diverse and widespread around the globe. In methanotrophic bacteria, enzymes in the methane oxidation metabolic module (KEGG module M00174, conversion of methane to formaldehyde) are encoded in four operons (pmoCAB, mmoXYZBCD, mxaFI, and xoxF). Recent reports have suggested that methanotrophs in Proteobacteria acquired methane monooxygenases through horizontal gene transfer. Here, we used a genomic meta-analysis to infer the transcriptional and translational advantages of coding sequences from the methane oxidation metabolic modules of different types of methanotrophs. By analyzing isolate and metagenome-assembled genomes from phylogenetically and geographically diverse sources, we detected an anomalous nucleotide composition bias in the coding sequences of particulate methane monooxygenase genes (pmoCAB) from type Ia methanotrophs. We found that this nucleotide bias increases the level of codon bias by decreasing the GC content in the third base of codons, a strategy that contrasts with that of other coding sequences in the module. Further codon usage analyses uncovered that codon variants of the type Ia pmoCAB coding sequences deviate from the genomic signature to match ribosomal protein-coding sequences. Subsequently, computation of transcription and translation metrics revealed that the pmoCAB coding sequences of type Ia methanotrophs optimize the usage of codon variants to maximize translation efficiency and accuracy, while minimizing the synthesis cost of transcripts and proteins.IMPORTANCE Microbial methane oxidation plays a fundamental role in the biogeochemical cycle of Earth's system. Recent reports have provided evidence for the acquisition of methane monooxygenases by horizontal gene transfer in methane-oxidizing bacteria from different environments, but how evolution has shaped the coding sequences to execute methanotrophy efficiently remains unexplored. In this work, we provide genomic evidence that among the different types of methanotrophs, type Ia methanotrophs possess a unique coding sequence of the pmoCAB operon that is under positive selection for optimal resource allocation and efficient synthesis of transcripts and proteins. This adaptive trait possibly enables type Ia methanotrophs to respond robustly to fluctuating methane availability and explains their global prevalence.

Exploring How to Use Groundwater Chemistry to Identify Migration of Methane near Shale Gas Wells in the Appalachian Basin.

Methane (CH4) enters waters in hydrocarbon-rich basins because of natural processes and problems related to oil and gas wells. As a redox-active greenhouse gas, CH4 degrades water or emits to the atmosphere and contributes to climate change. To detect if methane migrated from hydrocarbon wells (i.e., anomalous methane), we examined 20 751 methane-containing groundwaters from the Upper Appalachian Basin (AB). We looked for concentrations (mg/L) that indicated AB brine salts (chloride concentrations ([Cl]) > 30; [Ca]/[Na] < 0.52) to detect natural methane, and we looked for concentrations of redox-active species ([SO4] ≥ 6; [Fe] ≥ 0.3) to detect anomalous methane. These indicators highlight natural contamination by methane-containing brines or recent onset of microbial oxidation of methane coupled to iron- or sulfate-reduction. We hypothesized that only waters recently contaminated by methane still exhibit high iron and sulfate concentrations. Of the AB samples, 17 (0.08%) from 12 sites indicated potential contamination. All were located in areas with high densities of shale-gas or conventional oil/gas wells. In contrast, in southwestern Pennsylvania where brines are shallow and coal, oil, and gas all have been extracted extensively, no sites of recent methane migration were detectable. Such indicators may help screen for contamination in some areas even without predrill measurements.

Evaluating radioisotope‐based approaches to measure anaerobic methane oxidation rates in lacustrine sediments

AbstractThe microbial anaerobic oxidation of methane (AOM) is the dominant sink for methane in anoxic sediments. AOM rate measurements are essential for assessing the efficacy of the benthic methane filter to mitigate the evasion of this potent greenhouse gas to the atmosphere. Incubation techniques with trace amounts of radiolabeled substrate (typically 14CH4) represent the most sensitive approach for methane oxidation rate measurements. Yet, radiotracer application can be performed in different ways, rendering the comparability of AOM rate measurements in field and laboratory investigations problematic. We compared four different 14CH4‐based short‐term incubation approaches to quantify methane turnover rates in lake sediments. Three of the applied methods yielded similar and reliable downcore rate profiles. They provided clear evidence for AOM with maximum rates of 15 nmol cm−3 d−1 at ~ 17 cm sediment depth. Using the short‐term slurry incubation (SL) method, however, we were unable to detect the AOM activity maximum that we observed with the other approaches. We hypothesize that changes in the microbial structure and disruption of physical interactions due to mixing of sediments negatively affected the activity of AOM communities and longer incubation times are necessary to enhance the sensitivity of this approach. Minor variabilities in rate measurement that we found in the non‐SL incubations may be related to small‐scale sediment heterogeneity, differential partial methane loss during sample handling, and/or an uneven application of the radiotracer. Whole‐core incubations interfere the least with the in situ conditions, but the ultimate choice of the AOM rate measurement method will depend on the individual sampling requirements.

Dredging Influence on Methane Concentration in the Taganrog Bay

The effects of dredging and dumping on methane concentration in water and bottom sediments of the Taganrog Bay (the Sea of Azov) are studied. It is shown that dredging operations are accompanied by the inflow of methane deposited in soil to the water column. This inflow is associated with the resuspension of soils as a result of their loosening, lifting, and loading by the dredger to the barge as well as during their discharge to underwater piles and further erosion and separation of dumping material by waves and seabed currents. In addition to the increase in methane emission to the atmosphere, this is accompanied by a negative impact on the oxygen regime due to the microbial oxidation of methane and other organic and inorganic substances which arrived during soil resuspension. However, due to a relatively small area of the seabed affected by dredging and occupied by underwater dumps as well as due to the active hydrodynamics of waters in general, their influence on methane concentration in water and bottom sediments of the Taganrog Bay is insignificant.

Perchlorate bio-reduction in a methane-based membrane biofilm reactor in the presence and absence of oxygen

Perchlorate has been widely detected in various water environments and could cause serious health problems. Methane has been proposed as a promising electron donor to remove perchlorate from contaminated water, yet it is unclear whether and how microbial methane oxidation couples with perchlorate reduction, in particular under anoxic conditions. Here, the feasibility and performance of perchlorate reduction driven by methane in the presence and absence of oxygen were investigated and compared in a lab-scale methane-based membrane biofilm reactor. Long-term operational performance suggested that perchlorate was reduced to chloride, with 4 mg Cl/L/d of perchlorate removal rate under anoxic conditions. Differently, perchlorate removal rate increased to 16 mg Cl/L/d, and volatile fatty acids (VFAs) were produced from methane partial oxidation when a limited oxygen (10 mg/L/d) was externally supplied. Regardless of oxygen conditions, microbial perchlorate reduction driven by methane might be mediated through synergistic interactions by a microbial consortium, but with different key microbial members under both oxygen regimes. Under anoxic conditions, aerobic methanotrophs (likely Methylocystaceae and Methylococcaceae) might micro-aerobically oxidize methane by utilizing internal oxygen from microbial perchlorate reduction, which might be mediated by Rhodocyclaceae. In contrast, under oxygen-limiting conditions, methanogens (e.g., Methanosarcina) and fermenters (e.g., Veillonellaceae) likely jointly converted methane into VFAs, then dissimilatory perchlorate-reducing bacteria (e.g., Rhodocyclaceae) utilized the produced VFAs to reduce perchlorate to chloride. Our findings provide evidence to link methane oxidation with perchlorate reduction under both oxygen regimes, which could be facilitated to design a process to remove perchlorate from groundwater.

Bottom water methane sources along the high latitude eastern Canadian continental shelf and their effects on the marine carbonate system

Measurements of dissolved methane (CH4) and carbonate system parameters were conducted in shelf and slope waters off Baffin Island and northern Labrador during September 2012 and August 2016 in order to investigate potential cold seeps and their influence on the marine carbonate system. Evidence of a strong near-bottom methane source was found at two sites along the Baffin Island Shelf (Scott Inlet and Cape Dyer) and above a pair of reported sea floor mounds in the Saglek Basin between Baffin Island and northern Labrador. Stable carbon isotope measurements of dissolved methane made at the Cape Dyer site give an estimated δ13C value of −71.2‰ for the source, consistent with microbial methanogenesis. The distributions of pCO2, pH and aragonite and calcite saturation states showed no evidence of enhanced ocean acidification in the vicinity of these sites, so it would seem that these seeps were not significant local sources of carbon dioxide, either as a component in the seep fluid or produced subsequent to release by water column microbial methane oxidation, nonetheless, aragonite saturation states of one or less were seen along the Baffin Island Shelf suggesting that ecosystems here may be vulnerable to further acidification. Spectrophotometric pH measurements of fresh samples in 2012 revealed a bottom water spike in pH at two seep sites that were not replicated by measurements of preserved seawater samples. We hypothesise that this was due to an unstable anionic compound present in seep fluid that can locally increase total alkalinity, pH and calcium carbonate saturation with HS− being a likely candidate, possibly to the benefit of calcifying organisms in an environment bordering on aragonite under-saturation.

Carbonate formation in salt dome cap rocks by microbial anaerobic oxidation of methane

Major hydrocarbon accumulations occur in traps associated with salt domes. Whereas some of these hydrocarbons remain to be extracted for economic use, significant amounts have degraded in the subsurface, yielding mineral precipitates as byproducts. Salt domes of the Gulf of Mexico Basin typically exhibit extensive deposits of carbonate that form as cap rock atop salt structures. Despite previous efforts to model cap rock formation, the details of subsurface reactions (including the role of microorganisms) remain largely unknown. Here we show that cap rock mineral precipitation occurred via closed-system sulfate reduction, as indicated by new sulfur isotope data. 13C-depleted carbonate carbon isotope compositions and low clumped isotope-derived carbonate formation temperatures indicate that microbial, sulfate-dependent, anaerobic oxidation of methane (AOM) contributed to carbonate formation. These findings suggest that AOM serves as an unrecognized methane sink that reduces methane emissions in salt dome settings perhaps associated with an extensive, deep subsurface biosphere.

Effects of biochar on bacterial communities in a newly established landfill cover topsoil

Recent studies revealed the benefits of applying biochar in landfill final cover soil, such as adsorbing odorous compounds and promoting microbial methane oxidation. Most of these processes are related to the soil bacterial communities. However, the effects of biochar application on the overall bacterial community in newly established landfill cover soil are not yet understood, especially in field condition. The objective of the present field study is to investigate the effects of biochar on the diversity of soil bacterial community 3 months after incubation (short–term). Landfill final cover topsoil (0.6 m) was amended with 0 (control), 5, and 10% (w/w) of biochar derived from peanut–shell and wheat straw. Soil bacterial communities were analysed using the 16S rRNA–based T–RFLP approach. Biochar application significantly (p < 0.05) increased the diversity of soil bacterial communities. The Shannon diversity index of bacterial communities in soil amended with 5 and 10% of biochar was increased from 3.34 to 3.85 and 3.92, respectively. There were four bacterial phyla recorded found at both control and amended soils, namely Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. In addition, Gemmatimonadetes was found only in soil amended with 10% biochar. The interactions between soil bacterial communities and measured soil parameters including moisture content, electrical conductivity, total organic matter, total nitrogen and total phosphorus were found to be statistically non-significant (p > 0.05), according to the canonical correspondence analysis (CCA). This may be due to the highly heterogeneous nature of landfill soil. Results from this study revealed that short-term biochar application already altered the soil physicochemical properties and increased the diversity of soil bacterial communities.

A centrifuge tube reactor for the determination of bacterial methane oxidation enrichment factors without influence of diffusion related isotope fractionation

Biotransformation of methane at landfill sites can be estimated by applying compound specific stable isotope analysis of methane from the anaerobic and the cover layer surface zone. Next to these two input parameters, merely the knowledge of the carbon isotopic fractionation of the bacterial methane oxidation in terms of the enrichment factor (ε) is required. However, many factors and conditions have been described to affect ε. These include temperature, the applied landfill cover, the type of expressed methane monooxygenase (MMO), and cell density. In this work we investigated the microbial methane oxidation with respect to temperature and type of methanotrophic enrichment culture. A newly designed setup was used to overcome potential CH4-substrate limitations such as diffusion that could affect the determined values of ε by improper and inhomogeneous mixing. The isotopic fractionation was determined based on the stable carbon isotope analysis of methane and carbon dioxide. The obtained value for isotopic fractionation was ε22°C = −0.0136 ± 0.0036. Also for the first time, bulk stable isotope analysis of bacterial cell mass was performed by flow injection analysis isotope ratio mass spectrometry.

Seasonal Dynamics of Microbial Processes in Bottom Sediments of the Sevastopol Coastal Area

A seasonal study of microbial methane oxidation (MO) and sulfate reduction (SR) processes in bottom sediments was carried out at five stations in the estuarian Sevastopol coastal area, which differed in their sedimentation and geochemical conditions. High methane concentrations in bottom sediments, as well as accordingly high MO rates, were obtained at stations marked by high organic carbon (Corg) concentrations. The dependence of these values was found to be exponential, whereas for SR, the amount of organic carbon was not the limiting factor. For the studied stations, the integral SR rates ranged from 2.9 to 11.3 mmol m–2 day–1. The trends of the SR rate have opposite directions for the open and closed parts of the sea: SR decreased during study period at the center of the bay, while in the open sea area it increased. The MO rates varied in a wide range from 0.0001 to 3.6 mmol m–2 day–1; the highest were obtained in Corg-rich sediments of the central part of the bay. Only at one station, where the mean annual Corg content in sediments reached 3.39%, did the methane concentration in sediment pore water exceed the calculated solubility.

Improvements in Drill-Core Headspace Gas Analysis for Samples from Microbially Active Depths

The IsoJar™ container is widely used in headspace gas analysis for gases adsorbed on cuttings or bore cores from oil and gas fields. However, large variations in the carbon isotopic ratios of CH4and CO2are often reported, especially for data obtained from depths of &lt;1000 m. The IsoJar™ method leaves air in the headspace that allows microbial oxidation of CH4to CO2, meaning that isotopic fractionation occurs during storage. This study employed the IsoJar™ method to investigate the causes of differences inδ13C data reported by previous studies in the Horonobe area of Japan. It was found that after 80 d storage,δ13CCO2values decreased by ~2‰, whileδ13CCH4values increased by &gt;30‰, whereas samples analyzed within a week of collection showed no such fluctuations. The conventional amount of microbial suppressant (~0.5 ml of 10% benzalkonium chloride (BKC) solution) is insufficient to suppress microbial activity if groundwater is used as filling water. The significant variations in carbon isotopic compositions previously reported were caused by microbial methane oxidation after sampling and contamination by groundwater from different depths. To avoid these problems, we recommend the following: (1) if long-term sample storage is necessary, &gt;10 ml of 10% BKC solution should be added or &gt;0.3% BKC concentration is required to suppress microbial activity; (2) analyses should be performed within one week of sampling; and (3) for CO2analyses, it is important that samples are not contaminated by groundwater from different depths.

Palaeohydrogeology using geochemical, isotopic and mineralogical analyses: Salinity and redox evolution in a deep groundwater system through Quaternary glacial cycles

Mineralogical, geochemical and isotopic analyses of secondary calcites are interpreted as part of an investigation of deep groundwaters in fractured metavolcanic rock overlain by sedimentary rock. Drillcore rock samples and groundwater samples from deep boreholes (maximum depth 1950 m) were analysed. This produces information about the evolution of salinity and redox in relation to past groundwater movements including the impacts of climatic change through the Quaternary period.Salinities of present-day groundwaters vary from dilute to brine concentrations and are related to three distinct groundwater flow regimes. Crystal morphology, stable isotopic analyses and isotopic dating, cathodoluminescence and microanalyses of Fe, Mn and REEs in the latest generation of secondary calcite, plus other analyses, have provided insights into variations of salinity over time and of redox in past groundwaters. Interpretation suggests that groundwater in the depth range of the transition from dilute to brackish/saline concentrations has been gradually diluted over time by meteoric water ingress. 230Th/234U whole-crystal ages indicate that at least part of the late-stage calcite mineralisation in the present groundwater flow system is of Quaternary age, although the mineralisation may have been initiated much earlier by meteoric invasion in the Miocene, following regional uplift. The calcites exhibit a wide range in oxygen isotope composition (δ18OPDB −2 to −22‰), although no extremely light or heavy δ13C values indicative of microbial methane oxidation or deep methanogenesis were observed. The very light δ18O values suggest that glacial or other cold-climate waters flowed to more than 700 m depth in the centre of the study area and formed a greater proportion of groundwater at that depth than at present.Fe and Ce are interpreted as semi-quantitative proxies for past redox conditions over the period when secondary calcite was deposited. Variability of Fe and Mn contents of secondary calcites in deeper rock, presently containing saline groundwater, is evidence of reducing conditions being maintained in the long term, though the strength of negative redox has probably fluctuated due to other redox-active chemistry. Depth-wise changes of groundwater redox in the past are also indicated by Ce concentrations versus other REEs in secondary calcites. Shallow calcites show a negative Ce anomaly in some growth zones due to oxidation to CeIV whilst deeper calcites do not exhibit this Ce behaviour, indicating that reducing conditions prevailed. Distribution of Fe-Mn oxyhydroxides and pyrite confirm, at a broader scale over depth and time, the findings about redox variations that secondary calcites indicate.Mineralogical and geochemical studies add further information to the understanding of past geochemical conditions in deep groundwaters in this area. Interpretations provide semi-quantitative constraints on the evolution and likely variations and directions of movement of groundwater salinity and redox over the Quaternary timescale.

Human exposure to radon radiation geohazard in Rong Cave, Dong Van Karst Plateau Geopark, Vietnam

Human exposure to radon radiation geohazard in Rong Cave, Dong Van Karst Plateau Geopark, Vietnam

Unravelling the Identity, Metabolic Potential and Global Biogeography of the Atmospheric Methane-Oxidizing Upland Soil Cluster α.

SummaryUnderstanding of global methane sources and sinks is a prerequisite for the design of strategies to counteract global warming. Microbial methane oxidation in soils represents the largest biological sink for atmospheric methane. However, still very little is known about the identity, metabolic properties and distribution of the microbial group proposed to be responsible for most of this uptake, the uncultivated upland soil cluster α (USCα). Here, we reconstructed a draft genome of USCα from a combination of targeted cell sorting and metagenomes from forest soil, providing the first insights into its metabolic potential and environmental adaptation strategies. The 16S rRNA gene sequence recovered was distinctive and suggests this crucial group as a new genus within the Beijerinckiaceae, close to Methylocapsa. Application of a fluorescently labelled suicide substrate for the particulate methane monooxygenase enzyme (pMMO) coupled to 16S rRNA fluorescence in situ hybridisation (FISH) allowed for the first time a direct link of the high‐affinity activity of methane oxidation to USCα cells in situ. Analysis of the global biogeography of this group further revealed its presence in previously unrecognized habitats, such as subterranean and volcanic biofilm environments, indicating a potential role of these environments in the biological sink for atmospheric methane.

Methanotrophs: The Natural Way to Tackle Greenhouse Effect

Methane is considered as an important greenhouse gas which produced from a wide range of anthropogenic and natural sources. It plays a key role in global warming. Capturing and disposal of methane is technically both costly and problematic. A low-cost alternate to the conventional methods is the microbial oxidation of methane. Methanotrophs are the type of bacteria that aerobically oxidize methane as a source of energy through their key enzyme, monooxygenase (MMO), especially the soluble MMO, it is noteworthy in its broad substrate specificity. This exceptional capability, i.e., catalyzing reactions of environmental importance, has owned methanotrophs attention from applied microbiologists and biotechnologist. From about 30 years, it is observed that copper (Cu) is playing an important role in the physiology and activity of methanotrophs, but the discovery of how Cu collect by these cells, more importantly what is the role Cu playing in CH4 oxidation by the particulate CH4 monooxygenase and how Cu affects the ability of methanotrophs to oxidize different substrates has been made. In this review we summarize the morphology, phylogeny, ecology, and possible applications of methanotrophs to address the global as well as regional issues, along with the role of gene expression regulation by Cu and how it affects the cell activity of methane-oxidizing bacteria. Our focus was on two main aspects of potential value and application of methanotrophs in environmental bioremediation, namely physiology along with working of methanotrophs and methane removal from atmosphere.

Biochar-Amended Soil Cover for Microbial Methane Oxidation: Effect of Biochar Amendment Ratio and Cover Profile

AbstractA long-term soil column study was carried out to investigate the methane (CH4) removal capacity of landfill cover soil and biochar-amended cover soil under simulated landfill cover conditio...

A new occurrence of the Early Jurassic brachiopod Anarhynchia from the Canadian Cordillera confirms its membership in chemosynthesis-based ecosystems

Cold seeps, where seepage of hydrocarbon-rich fluids occurs in the sea floor, are sites that harbor highly specialized ecosystems associated with distinctive carbonate sediments. Although their Mesozoic record is scarce and patchy, it commonly includes dimerelloid rhynchonellide brachiopods. Here we report a monospecific assemblage of Anarhynchia from a limestone boulder of early Pliensbachian (Early Jurassic) age in the Inklin Formation of the Whitehorse Trough in the Stikine terrane, from a locality at Atlin Lake in northern British Columbia. Specimens are among the largest known Mesozoic brachiopods, up to 9 cm in length, and described here as Anarhynchia smithi n. sp. Early precipitated carbonate cement phases of the limestone have carbon isotopic composition highly depleted in 13C, indicative of the influence of microbial oxidation of methane derived from a cold seep. Carbonate petrography of the banded-fibrous cement and other characteristic components supports this paleoenvironmental inference. Volcanogenic detrital grains in the matrix are indistinguishable from those in the sandstone layers in the siliciclastic sequence, suggesting that the seep carbonate is broadly coeval with the enclosing conglomerate. The new record extends the geographic range and species-level diversity of the genus, but supports its endemism to the East Pacific and membership in chemosynthesis-based ecosystems. The distribution of three distinct but congeneric species suggests that allopatric speciation occurred at separate sites along the active margin of western North America and Anarhynchia was restricted to seep and vent habitats in the Early Jurassic.

Effect of salinity on microbial methane oxidation in freshwater and marine environments

ABSTRACT: Salinity is an important environmental control of aerobic methane oxidation, which reduces the emission of the potent greenhouse gas methane into the atmosphere. The effect of salinity on methane oxidation is especially severe in river estuaries and adjacent coastal waters, which are important sources of methane emission and, at the same time, are usually characterized by pronounced salinity gradients. Using methane oxidation rates determined by a radiotracer technique as a measure of methanotrophic activity, we tested the effect of immediate and gradual salinity changes on pure cultures of methanotrophic bacteria, and natural freshwater (Elbe River) and natural marine (North Sea) methanotrophic populations. According to our results, Methylomonas sp. and Methylosinus trichosporium are resistant to an increase in salinity, whereas Methylovulum sp. and Methylobacter luteus are sensitive to such an increase. Natural methanotrophic populations from freshwater are more resistant to an increase in salinity than those from marine water are to a decrease in salinity. In contrast to an immediate change of salinity, gradual change (1.25 PSU d−1) can attenuate salinity stress. Experiments with the natural populations revealed different reactions to changes in salinity; thus, we assume that the initial composition of the methanotrophic population, i.e. the ratio of sensitive versus resistant strains, also governs the community response to salinity stress.Repeated experiments with the natural populations revealed different reactions to changes of salinity; thus we assume that the initial composition of the methanotrophic population, i.e. the ratio of sensitive and resistant strains, also governs the community response to salinity stress.

Coupling Stable Isotope Analysis with Gas Push‐Pull Tests to Derive In Situ Values for the Fractionation Factor αox Associated with the Microbial Oxidation of Methane in Soils

Core Ideas: Methane CH4 isotopologue fraction was measured in situ for the first time. αox varied strongly with oxidation rate. αox varied between landfills and between individual points on the same landfill. αox varied with spatial and temporal variability of soil properties. Our findings limit application of stable isotopes for CH4 oxidation quantification. A prerequisite for the application of stable isotope fractionation for the quantification of the methane (CH4) oxidation efficiency of landfill covers is that the fractionation factor αox is known or can be estimated with adequate accuracy. So far, αox has only been determined in laboratory experiments. In this study, αox was determined under in situ conditions in the field by coupling two independent methods, gas push‐pull tests and stable isotope analysis, to assess biological fractionation of CH4 isotopologues in landfill cover soils. On six landfills with nine points of investigation, 22 measurements were performed, covering a wide range of environmental conditions, such as soil temperature and moisture and observed oxidation rates. Values for αox varied between near 1, indicating little fractionation, and 1.151. Correlation of αox with the CH4 oxidation rate found by gas push‐pull tests revealed a clear asymptotic relationship, with low rates being associated with high values for αox and high rates resulting in little fractionation. Values for αox varied between the different landfills and between the individual points of investigation on the same landfill. The latter is assumed to reflect the spatial variability of methanotrophic activity due to spatial variability in soil moisture and hence air‐filled porosity as well as the spatial variability of gas fluxes. Significant variation of αox was observed for the same sampling point, presumably reflecting the temporal variability of factors influencing methanotrophic activity. These effects could include seasonally changing environmental conditions (e.g., soil temperature and moisture) but also the temporal variability of gas fluxes through the landfill soil cover, changing exposure of methanotrophs to CH4 and oxygen and hence their activity. The quantification of the CH4 oxidation efficiency using fractionation of stable isotopes is very sensitive to αox. Assuming a value constant in time and space and transferring this value from laboratory experiments to field settings entails significant uncertainty regarding the quantification of CH4 oxidation.

Using stable isotopes and gas concentrations for independent constraints on microbial methane oxidation at Arctic Ocean temperatures

AbstractMicrobial oxidation of methane in oxic water bodies is an important control on the amount of dissolved methane which is released from the ocean to the atmosphere. We explored the use of stable isotope methane spikes to quantify methane oxidation rates in Arctic seawater samples. A Picarro G2201‐i cavity ring‐down spectrometer was used to determine methane concentration and isotope ratios from headspace samples in foil incubators. The methane mass balance and the change in stable isotope ratios served as independent constraints on methane oxidation. For a fractionation factor of 1.025 oxidation rate constants determined with both methods agreed within 20% for small changes in isotope ratio (e.g., 10‰). For large changes in isotope ratio (e.g., 90‰), which was outside the calibration range, methods diverged. Rate constants down to 0.01 d−1 could be resolved with high statistical support. Stable isotope infrared spectroscopy to determine methane oxidation in foil incubators (ISMOFI) was successfully tested on under ice seawater from Utqiagvik, Alaska, by repeated sampling from each incubation vessel. Depending on the amount of isotope spike added, we determined oxidation rates of 0.15 ± 0.02 nmol L−1 d−1 at in situ methane concentration and a maximal oxidation potential of 271 ± 41 nmol L−1 d−1. The ISMOFI method permits variable incubation durations from days to months in a single incubator. The method is transportable and applicable in a variety of field or seagoing laboratory environments, and it avoids the use of hazardous substances such as radioisotopes and toxic chemicals.