Methane-oxidizing Bacterial Community Research Articles

Methane-oxidizing bacterial community dynamics in sub-alpine forest soil.

Methane-oxidizing bacteria are found in diverse soil and sediment environments and play an important role in mitigating flux of this potent greenhouse gas into the atmosphere. However, it is unclear how these bacteria and their associated communities are structured in the environment and how their activity ultimately influences methane flux. In this work, we examine the composition and structure of methane-oxidizing bacterial communities in sub-alpine forest soil and find soil- and time-specific differences between the stable and potentially active populations. We also find that the potentially active populations of certain methanotrophs and non-methanotrophs are positively correlated. This work provides a step toward refining our understanding of microbially mediated biogeochemical cycles.

Relief of Phosphate Limitation Stimulates Methane Oxidation

Aquatic ecosystems such as shallow lakes and wetlands are important emitters of the greenhouse gas methane (CH4). Increased phosphorus (P) loading is expected to increase CH4production in these ecosystems. This increased CH4production can potentially be mitigated by increased CH4oxidation, but how P availability affects methane-oxidizing bacterial (MOB) community composition and potential CH4oxidation remains to be tested. Here, we incubated MOB from sediments of four subtropical lakes of different trophic states for 7 days at different phosphate (PO43-) concentrations to determine the effects of P on MOB community composition and potential CH4oxidation. We measured CH4consumption daily and compared CH4oxidation during the exponential growth phase. Furthermore, we determined MOB community composition at the end of the incubations using qPCR of thepmoAgene. To test for differences in N and P uptake, we determined bacterial biomass N and P content. We found that increases in PO43-concentrations until 10 µM significantly increased CH4oxidation. PO43-also increased bacterial biomass P content, while N content was not affected. MOB community composition was not affected by PO43-but more strongly correlated to lake of origin, likely due to the short duration of the incubations. Our results show that PO43-can not only stimulate CH4oxidation indirectly through increased CH4production, but also directly by increasing MOB growth. Importantly, these effects only occur at low PO43-concentrations, indicating that at high nutrient loads the increased CH4oxidation will likely not mitigate the increased CH4production.

Distinct Co-occurrence Relationships and Assembly Processes of Active Methane-Oxidizing Bacterial Communities Between Paddy and Natural Wetlands of Northeast China.

Studies of methane-oxidizing bacteria are updating our views of their composition and function in paddy and natural wetlands. However, few studies have characterized differences in the methane-oxidizing bacterial communities between paddy and natural wetlands. Here, we conducted a 13C stable isotope-probing experiment and high-throughput sequencing to determine the structure profiling, co-occurrence relationships, and assembly processes of methanotrophic communities in four wetlands of Northeast China. There was a clear difference in community structure between paddy and natural wetlands. LEfSe analyses revealed that Methylobacter, FWs, and Methylosinus were enriched in natural wetlands, while Methylosarcina were prevailing in paddy, all identified as indicative methanotrophs. We observed distinct co-occurrence relationships between paddy and natural wetlands: more robust and complex connections in natural wetlands than paddy wetlands. Furthermore, the relative importance of stochastic processes was greater than that of deterministic processes, as stochastic processes explained >50% of the variation in communities. These results demonstrated that the co-occurrence relationships and assembly processes of active methanotrophic communities in paddy and natural wetlands were distinct. Overall, the results of this study enhance our understanding of the communities of methane-oxidizing bacteria in paddy and natural wetlands of Northeast China.

Environmental and Microbial Interactions Shape Methane-Oxidizing Bacterial Communities in a Stratified Lake.

In stratified lakes, methane-oxidizing bacteria (MOB) are strongly mitigating methane fluxes to the atmosphere by consuming methane entering the water column from the sediments. MOB communities in lakes are diverse and vertically structured, but their spatio-temporal dynamics along the water column as well as physico-chemical parameters and interactions with other bacterial species that drive the community assembly have so far not been explored in depth. Here, we present a detailed investigation of the MOB and bacterial community composition and a large set of physico-chemical parameters in a shallow, seasonally stratified, and sub-alpine lake. Four highly resolved vertical profiles were sampled in three different years and during various stages of development of the stratified water column. Non-randomly assembled MOB communities were detected in all compartments. We could identify methane and oxygen gradients and physico-chemical parameters like pH, light, available copper and iron, and total dissolved nitrogen as important drivers of the MOB community structure. In addition, MOB were well-integrated into a bacterial-environmental network. Partial redundancy analysis of the relevance network of physico-chemical variables and bacteria explained up to 84% of the MOB abundances. Spatio-temporal MOB community changes were 51% congruent with shifts in the total bacterial community and 22% of variance in MOB abundances could be explained exclusively by the bacterial community composition. Our results show that microbial interactions may play an important role in structuring the MOB community along the depth gradient of stratified lakes.

Atmospheric Methane Oxidizers Are Dominated by Upland Soil Cluster Alpha in 20 Forest Soils of China.

Upland soil clusters alpha and gamma (USCα and USCγ) are considered a major biological sink of atmospheric methane and are often detected in forest and grassland soils. These clusters are phylogenetically classified using the particulate methane monooxygenase gene pmoA because of the difficulty of cultivation. Recent studies have established a direct link of pmoA genes to 16S rRNA genes based on their isolated strain or draft genomes. However, whether the results of pmoA-based assays could be largely represented by 16S rRNA gene sequencing in upland soils remains unclear. In this study, we collected 20 forest soils across China and compared methane-oxidizing bacterial (MOB) communities by high-throughput sequencing of 16S rRNA and pmoA genes using different primer sets. The results showed that 16S rRNA gene sequencing and the semi-nested polymerase chain reaction (PCR) of the pmoA gene (A189/A682r nested with a mixture of mb661 and A650) consistently revealed the dominance of USCα (accounting for more than 50% of the total MOB) in 12 forest soils. A189f/A682r successfully amplified pmoA genes (mainly RA14 of USCα) in only three forest soils. A189f/mb661 could amplify USCα (mainly JR1) in several forest soils but showed a strong preferential amplification of Methylocystis and many other type I MOB groups. A189f/A650 almost exclusively amplified USCα (mainly JR1) and largely discriminated against Methylocystis and most of the other MOB groups. The semi-nested PCR approach weakened the bias of A189f/mb661 and A189f/A650 for JR1 and balanced the coverage of all USCα members. The canonical correspondence analysis indicated that soil NH4+-N and pH were the main environmental factors affecting the MOB community of Chinese forest soils. The RA14 of the USCα group prefers to live in soils with low pH, low temperature, low elevation, high precipitation, and rich in nitrogen. JR1's preferences for temperature and elevation were opposite to RA14. Our study suggests that combining the deep sequencing of 16S rRNA and pmoA genes to characterize MOB in forest soils is the best choice.

Anaerobic methane-oxidizing bacterial communities in sediments of a drinking reservoir, Beijing, China

PurposeNitrate/nitrite-dependent anaerobic methane oxidation (N-DAMO) connects the global cycle of carbon and nitrogen in aquatic ecosystems. The aims of this study were to investigate the spatiotemporal variation of N-DAMO bacteria and its driving factors in a drinking reservoir which are strongly affected by human consumption.MethodsCloning analyses were used to study the pmoA and 16S rRNA genes of N-DAMO bacteria. Sequencing and phylogenetic analyses were used to investigate the bacterial composition and structure. Redundancy analyses (RDA) and spearman’s correlations analyses were applied to analyze the relationships between bacterial distribution and environmental factors.ResultThere were remarkable spatial variations of N-DAMO bacteria in winter. Shannon biodiversity of 16S rRNA genes was higher in winter than that in summer. Compared with other freshwater ecosystems, there was higher diversity of N-DAMO bacteria in Miyun Reservoir. The N-DAMO bacterial communities (16S rRNA and pmoA genes) in sampling sites near to dam were different from the bacterial communities in other sampling sites. The N-DAMO bacterial community structure in dam areas may be associated with the water column depth in front of the dam. Moreover, Spearman’s correlations revealed that DO, NO3− -N, NO2−-N, and NH4+-N in interstitial water and sediment were potential determinant factors influencing the diversity of N-DAMO bacteria (16S rRNA and pmoA genes).ConclusionThere were distinct seasonal variations in 16S rRNA and spatial variations in pmoA genes. N-DAMO bacterial communities contained novel Methylomirabilis oxyfera-like pmoA genes in Miyun Reservoir. Nitrogen (NO2−-N, NO3−-N, and NH4+-N) were the dominant factor influencing the N-DAMO bacterial community structure in the drinking reservoir. N-DAMO bacterial community structure in dam areas indicates that water depth and DO might be the dominant factor influencing the N-DAMO bacterial communities in the reservoir.

Interspecies Chemical Signaling in a Methane-Oxidizing Bacterial Community.

Multiple species of bacteria oxidize methane in the environment after it is produced by anaerobic ecosystems. These organisms provide reduced carbon substrates for species that cannot oxidize methane themselves, thereby serving a key role in these niches while also sequestering this potent greenhouse gas before it enters the atmosphere. Deciphering the molecular details of how methane-oxidizing bacteria interact in the environment enables us to understand an important aspect that shapes the structures and functions of these communities. Here we show that many members of the Methylomonas genus possess a LuxR-type acyl-homoserine lactone (acyl-HSL) receptor/transcription factor that is highly homologous to MbaR from the quorum-sensing (QS) system of Methylobacter tundripaludum, another methane oxidizer that has been isolated from the same environment. We reconstitute this detection system in Escherichia coli and use mutant and transcriptomic analysis to show that the receptor/transcription factor from Methylomonas sp. strain LW13 is active and alters LW13 gene expression in response to the acyl-HSL produced by M. tundripaludum These findings provide a molecular mechanism for how two species of bacteria that may compete for resources in the environment can interact in a specific manner through a chemical signal.IMPORTANCE Methanotrophs are bacteria that sequester methane, a significant greenhouse gas, and thereby perform an important ecosystem function. Understanding the mechanisms by which these organisms interact in the environment may ultimately allow us to manipulate and to optimize this activity. Here we show that members of a genus of methane-oxidizing bacteria can be influenced by a chemical signal produced by a possibly competing species. This provides insight into how gene expression can be controlled in these bacterial communities via an exogenous chemical signal.

Analysis of the Temporal and Spatial Distribution of the Diversity of the Denitrifying Anaerobic Methane-Oxidizing Bacterial Community in the Sediments of the Hunhe River and Its Relationship with Environmental Factors

16S rRNA and pmoA functional genes were used as biomarker genes to investigate the temporal and spatial distribution of community diversity of Candidatus Methylomirabilis oxyfera (M. oxyfera) in the sediments of the Hunhe River by clone library technology. The dependence relationship between the physicochemical property of water and sediment samples and the diversity characteristics of the M. oxyfera community were analyzed by multivariate direct gradient analysis. Among the examples collected in September 2014, the OTU number of the 16S rRNA gene of M. oxyfera was 2-5, the Shannon-Wiener diversity index was 0.21-1.4, and the distribution characteristics in the middle reaches > upstream > downstream were presented. The OTU number and Shannon-Wiener diversity index of the pmoA functional gene in upstream samples are significantly higher than those in the middle and lower reach samples, and the Shannon-Wiener diversity indices in the upstream samples are 3.5 times and 2.3 times higher than that of the middle and the downstream samples, respectively. The community diversity of M. oxyfera showed a distinct regional distribution. Samples were collected at 3 sampling points in March 2015. The OTU values of 16S rRNA and pmoA for M. oxyfera are 6 and 5 respectively, which were obviously higher than those in September 2014. The Shannon-Wiener index is also higher than that in September 2014 (1.4>0.68; 57>0.00). The community diversity of M. oxyfera showed obvious seasonal distribution characteristics. Multivariate direct gradient analysis results showed that the concentration of DOC in water, sediment conductivity, TOC concentration, and nitrite concentration in the sediment are the main environmental factors affecting the community diversity of M. oxyfera.

Quorum Sensing in a Methane-Oxidizing Bacterium.

Aerobic methanotrophic bacteria use methane as their sole source of carbon and energy and serve as a major sink for the potent greenhouse gas methane in freshwater ecosystems. Dissecting the molecular details of how these organisms interact in the environment may increase our understanding of how they perform this important ecological role. Many bacterial species use quorum sensing (QS) systems to regulate gene expression in a cell density-dependent manner. We have identified a QS system in the genome of Methylobacter tundripaludum, a dominant methane oxidizer in methane enrichments of sediment from Lake Washington (Seattle, WA). We determined that M. tundripaludum produces primarily N-3-hydroxydecanoyl-l-homoserine lactone (3-OH-C10-HSL) and that its production is governed by a positive feedback loop. We then further characterized this system by determining which genes are regulated by QS in this methane oxidizer using transcriptome sequencing (RNA-seq) and discovered that this system regulates the expression of a putative nonribosomal peptide synthetase biosynthetic gene cluster. Finally, we detected an extracellular factor that is produced by M. tundripaludum in a QS-dependent manner. These results identify and characterize a mode of cellular communication in an aerobic methane-oxidizing bacterium.IMPORTANCE Aerobic methanotrophs are critical for sequestering carbon from the potent greenhouse gas methane in the environment, yet the mechanistic details of chemical interactions in methane-oxidizing bacterial communities are not well understood. Understanding these interactions is important in order to maintain, and potentially optimize, the functional potential of the bacteria that perform this vital ecosystem function. In this work, we identify a quorum sensing system in the aerobic methanotroph Methylobacter tundripaludum and use both chemical and genetic methods to characterize this system at the molecular level.

Responses of methanogenic and methanotrophic communities to warming in varying moisture regimes of two boreal fens

Peatlands are one of the major sources of the powerful greenhouse gas methane (CH4). Our aim was to detect responses of methanogenic archaeal and methane-oxidizing bacterial (MOB) communities that control the methane (CH4) cycle to climatic warming. This study took place in two boreal fens three years after experimental warming in un-manipulated wet and drier regimes, thus simulating future climate scenarios. We determined active methanogen and MOB communities as transcripts of mcrA and pmoA genes, along with the abundance of these genes, CH4 production and oxidation potentials, and in situ CH4 fluxes. Methanogenic community remained similar, although methanogen abundance decreased after warming. In the wet regime, this decrease resulted in a small but significant reduction on the potential CH4 production in such peat layers where the average production potential was high. Drying alone, however, reduced the potential CH4 production more than warming, and this impact was strong enough to mask the small warming impact in the drier regime. Warming did not affect the MOB community or the potential CH4 oxidation in the wet regime; however, type Ib MOB abundance decreased and MOB related to genus Methylocapsa became typical after warming in the drier regime of the southern fen. The in situ measured CH4 fluxes indicated similar patterns as potential measurements; both warming and drying reduced methane emissions, drying more than warming. These results indicate that methanogens and MOB may have different controlling patterns on CH4 fluxes when facing global warming. These patterns may further differ not only between moisture regimes, but inside the same habitat type, here boreal fen. Irrespective of this variation, the in situ CH4 fluxes still seem to respond similarly across sites.

Effect of Elevated CO2 Concentration, Elevated Temperature and No Nitrogen Fertilization on Methanogenic Archaeal and Methane-Oxidizing Bacterial Community Structures in Paddy Soil.

Elevated concentrations of atmospheric CO2 ([CO2]) enhance the production and emission of methane in paddy fields. In the present study, the effects of elevated [CO2], elevated temperature (ET), and no nitrogen fertilization (LN) on methanogenic archaeal and methane-oxidizing bacterial community structures in a free-air CO2 enrichment (FACE) experimental paddy field were investigated by PCR-DGGE and real-time quantitative PCR. Soil samples were collected from the upper and lower soil layers at the rice panicle initiation (PI) and mid-ripening (MR) stages. The composition of the methanogenic archaeal community in the upper and lower soil layers was not markedly affected by the elevated [CO2], ET, or LN condition. The abundance of the methanogenic archaeal community in the upper and lower soil layers was also not affected by elevated [CO2] or ET, but was significantly increased at the rice PI stage and significantly decreased by LN in the lower soil layer. In contrast, the composition of the methane-oxidizing bacterial community was affected by rice-growing stages in the upper soil layer. The abundance of methane-oxidizing bacteria was significantly decreased by elevated [CO2] and LN in both soil layers at the rice MR stage and by ET in the upper soil layer. The ratio of mcrA/pmoA genes correlated with methane emission from ambient and FACE paddy plots at the PI stage. These results indicate that the decrease observed in the abundance of methane-oxidizing bacteria was related to increased methane emission from the paddy field under the elevated [CO2], ET, and LN conditions.

Aerobic methanotrophic communities at the Red Sea brine-seawater interface

The central rift of the Red Sea contains 25 brine pools with different physicochemical conditions, dictating the diversity and abundance of the microbial community. Three of these pools, the Atlantis II, Kebrit and Discovery Deeps, are uniquely characterized by a high concentration of hydrocarbons. The brine-seawater interface, described as an anoxic-oxic (brine-seawater) boundary, is characterized by a high methane concentration, thus favoring aerobic methane oxidation. The current study analyzed the aerobic free–living methane-oxidizing bacterial communities that potentially contribute to methane oxidation at the brine-seawater interfaces of the three aforementioned brine pools, using metagenomic pyrosequencing, 16S rRNA pyrotags and pmoA library constructs. The sequencing of 16S rRNA pyrotags revealed that these interfaces are characterized by high microbial community diversity. Signatures of aerobic methane-oxidizing bacteria were detected in the Atlantis II Interface (ATII-I) and the Kebrit Deep Upper (KB-U) and Lower (KB-L) brine-seawater interfaces. Through phylogenetic analysis of pmoA, we further demonstrated that the ATII-I aerobic methanotroph community is highly diverse. We propose four ATII-I pmoA clusters. Most importantly, cluster 2 groups with marine methane seep methanotrophs, and cluster 4 represent a unique lineage of an uncultured bacterium with divergent alkane monooxygenases. Moreover, non-metric multidimensional scaling (NMDS) based on the ordination of putative enzymes involved in methane metabolism showed that the Kebrit interface layers were distinct from the ATII-I and DD-I brine-seawater interfaces.

Composition of Methane-Oxidizing Bacterial Communities as a Function of Nutrient Loading in the Florida Everglades

Agricultural runoff of phosphorus (P) in the northern Florida Everglades has resulted in several ecosystem level changes, including shifts in the microbial ecology of carbon cycling, with significantly higher methane being produced in the nutrient-enriched soils. Little is, however, known of the structure and activities of methane-oxidizing bacteria (MOB) in these environments. To address this, 0 to 10 cm plant-associated soil cores were collected from nutrient-impacted (F1), transition (F4), and unimpacted (U3) areas, sectioned in 2-cm increments, and methane oxidation rates were measured. F1 soils consumed approximately two-fold higher methane than U3 soils; additionally, most probable numbers of methanotrophs were 4-log higher in F1 than U3 soils. Metabolically active MOB containing pmoA sequences were characterized by stable-isotope probing using 10 % (v/v) (13)CH(4). pmoA sequences, encoding the alpha subunit of methane monooxygenase and related to type I methanotrophs, were identified from both impacted and unimpacted soils. Additionally, impacted soils also harbored type II methanotrophs, which have been shown to exhibit preferences for high methane concentrations. Additionally, across all soils, novel pmoA-type sequences were also detected, indicating presence of MOB specific to the Everglades. Multivariate statistical analyses confirmed that eutrophic soils consisted of metabolically distinct MOB community that is likely driven by nutrient enrichment. This study enhances our understanding on the biological fate of methane being produced in productive wetland soils of the Florida Everglades and how nutrient-enrichment affects the composition of methanotroph bacterial communities.

Spatial patterns of iron- and methane-oxidizing bacterial communities in an irregularly flooded, riparian wetland.

Iron- and methane-cycling are important processes in wetlands with one connected to plant growth and the other to greenhouse gas emission, respectively. In contrast to acidic habitats, there is scarce information on the ecology of microbes oxidizing ferrous iron at circumneutral pH. The latter is mainly due to the lack of isolated representatives and molecular detection techniques. Recently, we developed PCR–DGGE and qPCR assays to detect and enumerate Gallionella-related neutrophilic iron-oxidizers (Ga-FeOB) enabling the assessment of controlling physical as well as biological factors in various ecosystems. In this study, we investigated the spatial distribution of Ga-FeOB in co-occurrence with methane-oxidizing bacteria (MOB) in a riparian wetland. Soil samples were collected at different spatial scales (ranging from meters to centimeters) representing a hydrological gradient. The diversity of Ga-FeOB was assessed using PCR–DGGE and the abundance of both Ga-FeOB and MOB by qPCR. Geostatistical methods were applied to visualize the spatial distribution of both groups. Spatial distribution as well as abundance of Ga-FeOB and MOB was clearly correlated to the hydrological gradient as expressed in moisture content of the soil. Ga-FeOB outnumbered the MOB subgroups suggesting their competitiveness or the prevalence of Fe2+ over CH4 oxidation in this floodplain.

CH4 production and oxidation processes in a boreal fen ecosystem after long-term water table drawdown

Fens, which extend over vast areas in the Northern hemisphere, are sources of the greenhouse gas CH4. Climate change scenarios predict a lowering water table (WT) in mires. To study the effect of WT drawdown on CH4 dynamics in a fen ecosystem, we took advantage of a WT drawdown gradient near a ground water extraction plant. Methane fluxes and CH4 production and oxidation potentials were related to microbial communities responsible for the processes in four mire locations (wet, semiwet, semidry, and dry). Principal component analyses performed on the vegetation, pH, CH4, and WT results clearly separated the four sampling locations in the gradient. Long-term lowering of WT was associated with decreased coverage of Sphagnum and aerenchymatic plants, decreased CH4 field emissions and CH4 production potential. Based on mcrA terminal restriction fragment length polymorphism the methanogen community structure correlated best with the methane production and coverage of aerenchymatic plants along the gradient. Methanosarcinaceae and Methanocellales were found at the pristine wet end of the gradient, whereas the Fen cluster characterized the dry end. The methane-oxidizing bacterial community consisted exclusively of Methylocystis bacteria, but interestingly of five different alleles (T, S, R, M, and O) of the particulate methane monooxygenase marker gene pmoA. The M allele was dominant in the wet locations, and the occurrence of alleles O, S, and T increased with drainage. The occurrence of the R allele that characterized the upper peat layer correlated with CH4 oxidation potential. These results advance our understanding of mire dynamics after long-term WT drawdown and of the microbiological bases of methane emissions from mires.

Effects of grazing by sheep on the structure of methane-oxidizing bacterial community of steppe soil

Soils of three sites were studied in the Inner Mongolia steppe; one site non-grazed for 26 yr (NG26), another site non-grazed for 6 yr (NG6) and a third site freely grazed all along (FG). The composition of methantrophic communities was characterized by pmoA gene fragments (coding for a subunit of particulate methane monooxygenase) that were PCR amplified from total soil DNA extracts, using denaturing gradient gel electrophoresis (DGGE) method. Cluster analysis based on the DGGE band patterns indicated that the methanotrophic communities structure of NG6 and FG soils were similar to each other but different from that of NG26 soil. Sequence analysis showed that most bands belonged to the cluster of USC γ. This is the first report that USC γ cluster is dominated in the grassland soil.

Methanotrophic bacteria in boreal forest soil after fire

Methane-oxidizing bacteria are the only terrestrial sink for atmospheric methane. Little is known, however, about the methane-oxidizing bacteria that are responsible for the consumption of atmospheric methane, or about the factors that influence their activity and diversity in soil. Effects of fire and its end-product, wood ash, on the activity and community of methane oxidizing bacteria were studied in boreal forest 3 months and 12 years after the treatments. Fire significantly increased the atmospheric CH 4 oxidation rate. Both fire and wood ash treatments resulted in increased soil pH, but there was no correlation with methane oxidation rates. Changes in the methane-oxidizing bacterial community due to treatments were not detected by cultivation-independent recovery and comparative sequence analysis of pmoA gene products from soil. Phylogenetic analysis showed that a majority of the pmoA sequences obtained belonged to the “upland soil cluster α”, which has previously been detected in diverse forest environments.

Stable isotope pulse-chasing and compound specific stable carbon isotope analysis of phospholipid fatty acids to assess methane oxidizing bacterial populations in landfill cover soils.

The oxidation of methane by bacteria residing in soils constitutes an important terrestrial methane sink. These bacteria are particularly abundant in the covering soils of landfill caps due to the supply of high concentrations of methane from the landfill below. Only about 0.1% of soil bacteria are amenable to available methods of culturing, resulting in the need for a method of in situ analysis. A combination of phospholipid fatty acid (PLFA) analysis and stable isotopic labeling has been employed in this investigation as a means of cultivation-independent bacterial analysis. Soil samples taken from the profiles of two landfill caps, one of clay and one of sand, were incubated with 13C-labeled methane. PLFAs were analyzed by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) in order to determine their 13C content, from which the PLFA distribution of the methane-oxidizing bacteria was calculated. Neither landfill cap supported communities of bacteria capable of oxidizing ambient levels of methane but only those elevated levels that are usually attributable to landfills. The clay-capped landfill profile exhibited a change in the methane-oxidizing bacterial community with depth, whereas the sand-capped landfill site displayed a mixture of both type I and II methanotrophs throughout the profile. Two additional samples, taken from sites where methane production was evident, were particularly dominated by type II methanotrophic bacteria.