Long-term effects of mineral versus organic fertilizers on activity and structure of the methanotrophic community in agricultural soils

Agricultural practices, such as mineral nitrogen fertilization, have an impact on the soil's ability to oxidize methane, but little is known about the shifts in the methanotrophic community composition associated with these practices. Therefore, the long-term effect of both mineral (NH4NO3) and organic (manure and GFT-compost) fertilizer applications on the soil methanotrophic community activity and structure were investigated. Both high and low affinity methane oxidation rates were lower in the soil treated with mineral fertilizer compared to the other soils. An enhanced nitrate concentration was observed in the mineral fertilized soil but nitrate did not show a direct affect on the high affinity methane oxidation. In contrast, the low affinity methane oxidation was slowed down by increased nitrate concentrations, which suggests a direct effect of nitrate on low affinity methane oxidation. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA gene fragments specific for methanotrophs revealed a distinct community between the mineral and organic fertilized soils as extra Type I methanotrophic bands (phylotypes) became visible in the organic fertilized soils. These phylotypes were not visible in the patterns of the added organic fertilizers suggesting an indirect effect of the organic fertilizers on the methanotrophic community. Additionally, a molecular analysis was performed after the low affinity methane oxidation test. The enhanced methane concentrations used in the test enriched certain low affinity methanotrophs in the organic fertilized soils but not in the mineral fertilized soil. Supporting the molecular and functional observations, fatty acids characteristic for methanotrophs were less abundant in the soil treated with mineral fertilizer compared to the soil treated with compost. In conclusion, the function and molecular and chemical composition of the methanotrophic community are all altered in soil fertilized with mineral fertilizer.     

Effect of Afforestation and Reforestation of Pastures on the Activity and Population Dynamics of Methanotrophic Bacteria

We investigated the effect of afforestation and reforestation of pastures on methane oxidation and the methanotrophic communities in soils from three different New Zealand sites. Methane oxidation was measured in soils from two pine (Pinus radiata) forests and one shrubland (mainly Kunzea ericoides var. ericoides) and three adjacent permanent pastures. The methane oxidation rate was consistently higher in the pine forest or shrubland soils than in the adjacent pasture soils. A combination of phospholipid fatty acid (PLFA) and stable isotope probing (SIP) analyses of these soils revealed that different methanotrophic communities were active in soils under the different vegetations. The C₁₈ PLFAs (signature of type II methanotrophs) predominated under pine and shrublands, and C₁₆ PLFAs (type I methanotrophs) predominated under pastures. Analysis of the methanotrophs by molecular methods revealed further differences in methanotrophic community structure under the different vegetation types. Cloning and sequencing and terminal-restriction fragment length polymorphism analysis of the particulate methane oxygenase gene (pmoA) from different samples confirmed the PLFA-SIP results that methanotrophic bacteria related to type II methanotrophs were dominant in pine forest and shrubland, and type I methanotrophs (related to Methylococcus capsulatus) were dominant in all pasture soils. We report that afforestation and reforestation of pastures caused changes in methane oxidation by altering the community structure of methanotrophic bacteria in these soils.     

Activity, distribution, and abundance of methane-oxidizing bacteria in the near surface soils of onshore oil and gas fields

Methane-oxidizing bacteria (MOB) have long been used as an important biological indicator for oil and gas prospecting, but the ecological characteristics of MOB in hydrocarbon microseep systems are still poorly understood. In this study, the activity, distribution, and abundance of aerobic methanotrophic communities in the surface soils underlying an oil and gas field were investigated using biogeochemical and molecular ecological techniques. Measurements of potential methane oxidation rates and pmoA gene copy numbers showed that soils inside an oil and gas field are hot spots of methane oxidation and MOB abundance. Correspondingly, terminal restriction fragment length polymorphism analyses in combination with cloning and sequencing of pmoA genes also revealed considerable differences in the methanotrophic community composition between oil and gas fields and the surrounding soils. Principal component analysis ordination furthermore indicated a coincidence between elevated CH₄ oxidation activity and the methanotrophic community structure with type I methanotrophic Methylococcus and Methylobacter, in particular, as indicator species of oil and gas fields. Collectively, our results show that trace methane migrated from oil and gas reservoirs can considerably influence not only the quantity but also the structure of the methanotrophic community.     

Selective stimulation of type I methanotrophs in a rice paddy soil by urea fertilization revealed by RNA-based stable isotope probing

Methane-oxidizing bacteria (MOB) in soil are not only controlled by their main substrates, methane and oxygen, but also by nitrogen availability. We compared an unfertilized control with a urea-fertilized treatment and applied RNA-stable-isotope-probing to follow activity changes upon fertilization as closely as possible. Nitrogen fertilization of an Italian rice field soil increased the CH4 oxidation rates sevenfold. In the fertilized treatment, isopycnic separation of 13C-enriched RNA became possible after 7 days when 300 micromol 13CH4 g(dry soil)(-1) had been consumed. Terminal-restriction fragment length polymorphism (T-RFLP) fingerprints and clone libraries documented that the type I methanotrophic genera Methylomicrobium and Methylocaldum assimilated 13CH4 nearly exclusively. Although previous studies had shown that the same soil contains a much larger diversity of MOB, including both type I and type II, nitrogen fertilization apparently activated only a small subset of the overall diversity of MOB, type I MOB in particular.     

Stereospecific effect of hexachlorocyclohexane on activity and structure of soil methanotrophic communities

In the past decades, large amounts of non-insecticidal hexachlorocyclohexane (HCH) isomers (alpha-, beta-, delta- and epsilon-HCH) have been dumped as side-products of the insecticide gamma-HCH (lindane). This study investigates the effect of HCH isomers on methane oxidation, an important soil function performed by methanotrophic bacteria. Both activity and structure of the methanotrophic community were assessed, using methane oxidation assays and PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) respectively. Methane oxidation assays with historically polluted soils revealed that on the long-term methane oxidation was inhibited by HCH pollution. PCR-DGGE and diversity analysis based on Lorenz curves showed that the type I methanotrophic community was less evenly distributed in historically HCH-polluted soils compared with less polluted reference soils. Short-term experiments with methane-enriched consortia further demonstrated that only gamma- and delta-isomers inhibited methane oxidation. Type I methanotrophs of methane-enriched microbial consortia that received gamma- or delta-HCH evolved towards higher species richness. Apparently, for historically HCH-polluted soils, a narrow community remained after long-term exposure while in case of short-term exposures, methane-enriched consortia were converted into less active, but richer communities when they were stressed by the presence of gamma- or delta-HCH. This work demonstrates the importance of incorporating all isomers and possible other side-products in risk assessment studies of persistent organic pollutants and the use of structural analysis of type I methanotrophic communities as evaluating tool.     

Vertical distribution of the methanotrophic community after drainage of rice field soil

Anoxic soils, such as flooded rice fields, are major sources of the greenhouse gas CH(4) while oxic upland soils are major sinks of atmospheric CH(4). Nevertheless, CH(4) is also consumed in rice fields where up to 90% of the produced CH(4) is oxidized in a narrow oxic zone around the rice roots and in the soil surface layer before it escapes into the atmosphere. After 1 day drainage of rice field soil, CH(4) oxidation was detected in the top 2-mm soil layers, but after 8 days drainage the zone of CH(4) oxidation extended to 8 mm depth. Simultaneously, the potential for CH(4) production decreased, but some production was still detectable after 8 days drainage throughout the soil profile. The vertical distribution of the methanotrophic community was also monitored after 1 and 8 days drainage using denaturing gradient gel electrophoresis after PCR amplification with primer sets targeting two regions on the 16S rRNA gene that are relatively specific for methylotrophic alpha- and gamma-Proteobacteria, and targeting two functional genes encoding subunits of key enzymes in all methanotrophs, i.e. the genes for the particulate methane monooxygenase (pmoA) and the methanol dehydrogenase (mxaF). Drainage stimulated the methanotrophic community. Eight days after drainage, new methanotrophic populations appeared and a distinct methanotrophic community developed. The population structure of type I and II methanotrophs was differently affected by drainage. Type II methanotrophs (alpha-Proteobacteria) were present throughout the soil core directly after drainage (1 day), and the community composition remained largely unchanged with depth. Only two new type II populations appeared after 8 days of drainage. Drainage had a more pronounced impact on the type I methanotrophic community (gamma-Proteobacteria). Type I populations were not or only weakly detected 1 day after drainage. However, after 8 days of drainage, a large diversity of type I methanotrophs were detected, altough they were not evenly distributed throughout the soil core but dominated at different depths. A distinct type I community structure had developed within each soil section between 0 and 20 mm soil depth, indicating the widening of suitable habitats for methanotrophs in the rice field soil within 1 week of drainage.     

Diversity of the particulate methane monooxygenase gene in methanotrophic samples from different rice field soils in China and the Philippines

Methanotrophic bacteria play a crucial role in regulating the emission of CH4 from rice fields into the atmosphere. We investigated the CH4 oxidation activity together with the diversity of methanotrophic bacteria in ten rice field soils from different geographic locations. Upon incubation of aerated soil slurries under 7% CH4, rates of CH4 oxidation increased after a lag phase of 1-4 days and reached values of 3-10 micromol d(-1) g-dw(-1) soil. The methanotrophic community was assayed by retrieval of the pmoA gene which encodes the a subunit of the particulate methane monooxygenase. After extraction of DNA from actively CH4-oxidizing soil samples and PCR-amplification of the pmoA, the community was analyzed by Denaturant Gradient Gel Electrophoresis (DGGE) and Terminal Restriction Fragment Length Polymorphism (T-RFLP). DGGE bands were excised, the pmoA re-amplified, sequenced and the encoded amino acid sequence comparatively analyzed by phylogenetic treeing. The analyses allowed the detection of pmoA sequences related to the following methanotrophic genera: the type-I methanotrophs Methylobacter, Methylomicrobium, Methylococcus and Methylocaldum, and the type-II methanotrophs Methylocystis and Methylosinus. T-RFLP analysis detected a similar diversity, but type-II pmoA more frequently than DGGE. All soils but one contained type-II in addition to type-I methanotrophs. Type-I Methylomonas was not detected at all. Different combinations of methanotrophic genera were detected in the different soils. However, there was no obvious geographic pattern of the distribution of methanotrophs.     

Effects of O2 and CH4 on presence and activity of the indigenous methanotrophic community in rice field soil

The activity and distribution of methanotrophs in soil depend on the availability of CH4 and O2. Therefore, we investigated the activity and structure of the methanotrophic community in rice field soil under four factorial combinations of high and low CH4 and O2 concentrations. The methanotrophic population structure was resolved by denaturant gradient gel electrophoresis (DGGE) with different PCR primer sets targeting the 16S rRNA gene, and two functional genes coding for key enzymes in methanotrophs, i.e. the particulate methane monooxygenase (pmoA) and the methanol dehydrogenase (mxaF). Changes in the biomass of type I and II methanotrophic bacteria in the rice soil were determined by analysis of phospholipid-ester-linked fatty acid (PLFA) biomarkers. The relative contribution of type I and II methanotrophs to the measured methane oxidation activity was determined by labelling of soil samples with 14CH4 followed by analysis of [14C]-PLFAs. CH4 oxidation was repressed by high O2 (20.5%), and enhanced by low O2 (1%). Depending on the CH4 and O2 mixing ratios, different methanotrophic communities developed with a higher diversity at low than at high CH4 concentration as revealed by PCR-DGGE. However, a prevalence of type I or II populations was not detected. The [14C]-PLFA fingerprints, on the other hand, revealed that CH4 oxidation activity was dominated by type I methanotrophs in incubations with low CH4 mixing ratios (1000 p.p.m.v.) and during initiation of CH4 consumption regardless of O2 or CH4 mixing ratio. At high methane mixing ratios (10 000 p.p.m.v.), type I and II methanotrophs contributed equally to the measured CH4 metabolism. Collectively, type I methanotrophs responded fast and with pronounced shifts in population structure and dominated the activity under all four gas mixtures. Type II methanotrophs, on the other hand, although apparently more abundant, always present and showing a largely stable population structure, became active later and contributed to CH4 oxidation activity mainly under high CH4 mixing ratios.     

CO2浓度增加对长白山森林土壤甲烷氧化影响

大气CO2浓度升高可能对森林土壤的甲烷(CH4)氧化速率产生影响.本文采用开顶箱技术,对连续6年高浓度CO2(500 μmol·mol-1)处理的长白山森林典型树种蒙古栎树下土壤CH4氧化速率进行研究,并利用CH4氧化菌的16S rRNA特异性引物以及CH4单加氧酶功能基因引物分析了土壤中CH4氧化菌的群落结构与数量.结果表明:CO2浓度增高后,生长季土壤甲烷氧化量与对照和裸地相比分别降低了4％和22％;基于16S rRNA特异性引物的DGGE分析表明,CO2浓度增高导致两类甲烷氧化菌的多样性指数降低;CO2浓度增高对土壤中Ⅰ类甲烷氧化菌数量无显著影响,而使土壤中Ⅱ类甲烷氧化菌数量显著减少,功能基因pmoA拷贝数与对照和裸地相比分别降低了15％和46％.CO2浓度增高导致森林土壤甲烷氧化菌数量与活性降低,土壤含水量的增加可能是导致这一现象的主要原因.     

Effect of afforestation and reforestation of pastures on the activity and population dynamics of methanotrophic bacteria

We investigated the effect of afforestation and reforestation of pastures on methane oxidation and the methanotrophic communities in soils from three different New Zealand sites. Methane oxidation was measured in soils from two pine (Pinus radiata) forests and one shrubland (mainly Kunzea ericoides var. ericoides) and three adjacent permanent pastures. The methane oxidation rate was consistently higher in the pine forest or shrubland soils than in the adjacent pasture soils. A combination of phospholipid fatty acid (PLFA) and stable isotope probing (SIP) analyses of these soils revealed that different methanotrophic communities were active in soils under the different vegetations. The C18 PLFAs (signature of type II methanotrophs) predominated under pine and shrublands, and C16 PLFAs (type I methanotrophs) predominated under pastures. Analysis of the methanotrophs by molecular methods revealed further differences in methanotrophic community structure under the different vegetation types. Cloning and sequencing and terminal-restriction fragment length polymorphism analysis of the particulate methane oxygenase gene (pmoA) from different samples confirmed the PLFA-SIP results that methanotrophic bacteria related to type II methanotrophs were dominant in pine forest and shrubland, and type I methanotrophs (related to Methylococcus capsulatus) were dominant in all pasture soils. We report that afforestation and reforestation of pastures caused changes in methane oxidation by altering the community structure of methanotrophic bacteria in these soils.     

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.     

Methanotrophic communities in Australian woodland soils of varying salinity

Despite their large areas and potential importance as methane sinks, the role of methane-oxidizing bacteria (MOB) in native woodland soils is poorly understood. These environments are increasingly being altered by anthropogenic disturbances, which potentially alter ecosystem service provision. Dryland salinity is one such disturbance and is becoming increasingly prevalent in Australian soils. We used microarrays and analysis of soil physicochemical variables to investigate the methane-oxidizing communities of several Australian natural woodland soils affected to varying degrees by dryland salinity. Soils varied in terms of salinity, gravitational water content, NO(3)-N, SO(4)-S and Mg, all of which explained to a significant degree MOB community composition. Analysis of the relative abundance and diversity of the MOB communities also revealed significant differences between soils of different salinities. Type II and type Ib methanotrophs dominated the soils and differences in methanotroph communities existed between salinity groups. The low salinity soils possessed less diverse MOB communities, including most conspicuously, the low numbers or absence of type II Methylocystis phylotypes. The differences in MOB communities suggest niche separation of MOB across varying salinities, as has been observed in the closely related ammonia-oxidizing bacteria, and that anthropogenic disturbance, such as dryland salinity, has the potential to alter MOB community and therefore the methane uptake rates in soils in which disturbance occurs.     

CO2浓度增加对长白山森林土壤甲烷氧化影响

大气CO2浓度升高可能对森林土壤的甲烷(CH4)氧化速率产生影响.本文采用开顶箱技术,对连续6年高浓度CO2（500 μmol·mol-1）处理的长白山森林典型树种蒙古栎树下土壤CH4氧化速率进行研究,并利用CH4氧化菌的16S rRNA特异性引物以及CH4单加氧酶功能基因引物分析了土壤中CH4氧化菌的群落结构与数量.结果表明： CO2浓度增高后,生长季土壤甲烷氧化量与对照和裸地相比分别降低了4%和22%;基于16S rRNA特异性引物的DGGE分析表明,CO2浓度增高导致两类甲烷氧化菌的多样性指数降低;CO2浓度增高对土壤中Ⅰ类甲烷氧化菌数量无显著影响,而使土壤中Ⅱ类甲烷氧化菌数量显著减少,功能基因pmoA拷贝数与对照和裸地相比分别降低了15%和46%.CO2浓度增高导致森林土壤甲烷氧化菌数量与活性降低,土壤含水量的增加可能是导致这一现象的主要原因.     

Radioactive Fingerprinting of Microorganisms That Oxidize Atmospheric Methane in Different Soils

Microorganisms that oxidize atmospheric methane in soils were characterized by radioactive labelling with ¹⁴CH₄ followed by analysis of radiolabelled phospholipid ester-linked fatty acids (¹⁴C-PLFAs). The radioactive fingerprinting technique was used to compare active methanotrophs in soil samples from Greenland, Denmark, the United States, and Brazil. The ¹⁴C-PLFA fingerprints indicated that closely related methanotrophic bacteria were responsible for the oxidation of atmospheric methane in the soils. Significant amounts of labelled PLFAs produced by the unknown soil methanotrophs coeluted with a group of fatty acids that included i17:0, a17:0, and 17:1ω8c (up to 9.0% of the total ¹⁴C-PLFAs). These PLFAs are not known to be significant constituents of methanotrophic bacteria. The major PLFAs of the soil methanotrophs (73.5 to 89.0% of the total PLFAs) coeluted with 18:1 and 18:0 fatty acids (e.g., 18:1ω9, 18:1ω7, and 18:0). The ¹⁴C-PLFAs fingerprints of the soil methanotrophs that oxidized atmospheric methane did not change after long-term methane enrichment at 170 ppm CH₄. The ¹⁴C-PLFA fingerprints of the soil methanotrophs were different from the PLFA profiles of type I and type II methanotrophic bacteria described previously. Some similarity at the PLFA level was observed between the unknown soil methanotrophs and the PLFA phenotype of the type II methanotrophs. Methanotrophs in Arctic, temperate, and tropical regions assimilated between 20 and 54% of the atmospheric methane that was metabolized. The lowest relative assimilation (percent) was observed for methanotrophs in agricultural soil, whereas the highest assimilation was observed for methanotrophs in rain forest soil. The results suggest that methanotrophs with relatively high carbon conversion efficiencies and very similar PLFA compositions dominate atmospheric methane metabolism in different soils. The characteristics of the methane metabolism and the ¹⁴C-PLFA fingerprints excluded any significant role of autotrophic ammonia oxidizers in the metabolism of atmospheric methane.     

The active methanotrophic community in hydromorphic soils changes in response to changing methane concentration

Methanotrophic communities were studied in several periodically water-saturated gleyic soils. When sampled, each soil had an oxic upper layer and consumed methane from the atmosphere (at 1.75 ppmv). In most gleyic soils the K(m(app)) values for methane were between 70 and 800 ppmv. These are higher than most values observed in dry upland soils, but lower than those measured in wetlands. Based on cultivation-independent retrieval of the pmoA-gene and quantification of partial pmoA gene sequences, type II (Alphaproteobacteria) methanotrophs of the genus Methylocystis spp. were abundant (> 10(7) pmoA target molecules per gram of dry soil). Type I (Gammaproteobacteria) methanotrophs related to the genera Methylobacter and Methylocaldum/Methylococcus were detected in some soils. Six pmoA sequence types not closely related to sequences from cultivated methanotrophs were detected as well, indicating that diverse uncultivated methanotrophs were present. Three Gleysols were incubated under different mixing ratios of (13)C-labelled methane to examine (13)C incorporation into phospholipid fatty acids (PLFAs). Phospholipid fatty acids typical of type II methanotrophs, 16:0 and 18:1omega7c, were labelled with (13)C in all soils after incubation under an atmosphere containing 30 ppmv of methane. Incubation under 500 ppmv of methane resulted in labelling of additional PLFAs besides 16:0 and 18:1omega7c, suggesting that the composition of the active methanotrophic community changed in response to increased methane supply. In two soils, 16:1 PLFAs typical of type I methanotrophs were strongly labelled after incubation under the high methane mixing ratio only. Type II methanotrophs are most likely responsible for atmospheric methane uptake in these soils, while type I methanotrophs become active when methane is produced in the soil.     

Unexpected stimulation of soil methane uptake as emergent property of agricultural soils following bio‐based residue application

Intensification of agriculture to meet the global food, feed, and bioenergy demand entail increasing re‐investment of carbon compounds (residues) into agro‐systems to prevent decline of soil quality and fertility. However, agricultural intensification decreases soil methane uptake, reducing, and even causing the loss of the methane sink function. In contrast to wetland agricultural soils (rice paddies), the methanotrophic potential in well‐aerated agricultural soils have received little attention, presumably due to the anticipated low or negligible methane uptake capacity in these soils. Consequently, a detailed study verifying or refuting this assumption is still lacking. Exemplifying a typical agricultural practice, we determined the impact of bio‐based residue application on soil methane flux, and determined the methanotrophic potential, including a qualitative (diagnostic microarray) and quantitative (group‐specific qPCR assays) analysis of the methanotrophic community after residue amendments over 2 months. Unexpectedly, after amendments with specific residues, we detected a significant transient stimulation of methane uptake confirmed by both the methane flux measurements and methane oxidation assay. This stimulation was apparently a result of induced cell‐specific activity, rather than growth of the methanotroph population. Although transient, the heightened methane uptake offsets up to 16% of total gaseous CO₂ emitted during the incubation. The methanotrophic community, predominantly comprised of Methylosinus may facilitate methane oxidation in the agricultural soils. While agricultural soils are generally regarded as a net methane source or a relatively weak methane sink, our results show that methane oxidation rate can be stimulated, leading to higher soil methane uptake. Hence, even if agriculture exerts an adverse impact on soil methane uptake, implementing carefully designed management strategies (e.g. repeated application of specific residues) may compensate for the loss of the methane sink function following land‐use change.     

Revealing the community and metabolic potential of active methanotrophs by targeted metagenomics in the Zoige wetland of the Tibetan Plateau

The Zoige wetland of the Tibetan Plateau is one of the largest alpine wetlands in the world and a major emission source of methane. Methane oxidation by methanotrophs can counteract the global warming effect of methane released in the wetlands. Understanding methanotroph activity, diversity and metabolism at the molecular level can guide the isolation of the uncultured microorganisms and inform strategy-making decisions and policies to counteract global warming in this unique ecosystem. Here we applied DNA stable isotope probing using ¹³C-labelled methane to label the genomes of active methanotrophs, examine the methane oxidation potential and recover metagenome-assembled genomes (MAGs) of active methanotrophs. We found that gammaproteobacteria of type I methanotrophs are responsible for methane oxidation in the wetland. We recovered two phylogenetically novel methanotroph MAGs distantly related to extant Methylobacter and Methylovulum. They belong to type I methanotrophs of gammaproteobacteria, contain both mxaF and xoxF types of methanol dehydrogenase coding genes, and participate in methane oxidation via H₄MPT and RuMP pathways. Overall, the community structure of active methanotrophs and their methanotrophic pathways revealed by DNA-SIP metagenomics and retrieved methanotroph MAGs highlight the importance of methanotrophs in suppressing methane emission in the wetland under the scenario of global warming.     

Response of methanotrophic activity and community structure to temperature changes in a diffusive CH4/O2 counter gradient in an unsaturated porous medium

Microbial methane oxidation is a key process in the global methane cycle. In the context of global warming, it is important to understand the responses of the methane-oxidizing microbial community to temperature changes in terms of community structure and activity. We studied microbial methane oxidation in a laboratory-column system in which a diffusive CH₄/O₂ counter gradient was maintained in an unsaturated porous medium at temperatures between 4 and 20 °C. Methane oxidation was highly efficient at all temperatures, as on average 99 ± 0.5% of methane supplied to the system was oxidized. The methanotrophic community that established in the model system after initial inoculation appeared to be able to adapt quickly to different temperatures, as methane emissions remained low even after the system was subjected to abrupt temperature changes. FISH showed that Type I as well as Type II methanotrophs were probably responsible for the observed activity in the column system, with a dominance of Type I methanotrophs. Cloning and sequencing suggested that Type I methanotrophs were represented by the genus Methylobacter while Type II were represented by Methylocystis . The results suggest that in an unsaturated system with diffusive substrate supply, direct effects of temperature on apparent methanotrophic activity and community may be of minor importance. However, this remains to be verified in the field.