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A novel species is proposed for two strains of methanotrophic bacteria (H2 T and Sakb1) isolated from an acidic (pH 4.3) Sphagnum peat bog lake (Teufelssee, Germany) and an acidic (pH 4.2) tropical forest soil (Thailand), respectively. Cells of strains H2 T and Sakb1 were aerobic, Gramnegative, non-motile, straight or curved rods that were covered by large polysaccharide capsules and contained an intracytoplasmic membrane system typical of type II methanotrophs. They possessed both a particulate and a soluble methane monooxygenase and utilized the serine pathway for carbon assimilation. They were moderately acidophilic organisms capable of growth between pH 4.4 and 7.5 (optimum 5.8-6.2). The most unique characteristic of these strains was the phospholipid fatty acid profile. In addition to the signature fatty acid of type II methanotrophs (18 Aerobic methanotrophic bacteria are capable of utilizing methane as the sole source of carbon and energy. They have been divided into two groups, types I and II, belonging to the Gamma-and Alphaproteobacteria, respectively. These types differ in several phenotypic, chemotaxonomic and genotypic features, including the arrangement of intracytoplasmic membranes (ICMs), the predominant phospholipid fatty acids (PLFAs) and the pathway used for carbon assimilation. Methanotrophic bacteria inhabit a wide range of natural environments of diverse temperature, salinity andAbbreviations: DMDS, dimethyldisulfide; FAME, fatty acid methyl ester; ICM, intracytoplasmic membrane; MDH, methanol dehydrogenase; PLFA, phospholipid fatty acid; pMMO, particulate methane monooxygenase; sMMO, soluble methane monooxygenase.The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene sequence and partial sequences of the mxaF, mmoX and pmoA genes of Methylocystis heyeri strain H2 T are AM283543-AM283546, respectively.A supplementary figure showing mass spectra of DMDS adducts of 16 : 1 PLFA of strain H2

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Turnover of glucose and acetate in the presence of active reduction of nitrate, ferric iron and sulfate was investigated in anoxic rice field soil by using [U‐14C]glucose and [2‐14C]acetate. The turnover of glucose was not much affected by addition of ferrihydrite or sulfate, but was partially inhibited (60%) by addition of nitrate. Nitrate addition also strongly reduced acetate production from glucose while ferrihydrite and sulfate addition did not. These results demonstrate that ferric iron and sulfate reducers did not outcompete fermenting bacteria for glucose at endogenous concentrations. Nitrate reducers may have done so, but glucose fermentation may also have been inhibited by accumulation of toxic denitrification intermediates (nitrite, NO, N2O). Addition of nitrate resulted in complete inhibition of CH4 production from [U‐14C]glucose and [2‐14C]acetate. However, addition of ferrihydrite or sulfate decreased the production of 14CH4 from [U‐14C]glucose by only 70 and 65%, respectively. None of the electron acceptors significantly increased the production of 14CO2 from [U‐14C]glucose, but all increased the production of 14CO2 from [2‐14C]acetate. Uptake of acetate was faster in the presence of either nitrate, ferrihydrite or sulfate than in the unamended control. Addition of ferrihydrite and sulfate reduced 14CH4 production from [2‐14C]acetate by 83 and 92%, respectively. Chloroform completely inhibited the methanogenic consumption of acetate. It also inhibited the oxidation of acetate, completely in the presence of sulfate, but not in the presence of nitrate or ferrihydrite. Our results show that, besides the possible toxic effect of products of nitrate reduction (NO, NO2− and N2O) on methanogens, nitrate reducers, ferric iron reducers and sulfate reducers were active enough to outcompete methanogens for acetate and channeling the flow of electrons away from CH4 towards CO2 production.

Three upland soils from Thailand, a natural forest, a 16-year-old reforested site, and an agricultural field, were studied with regard to methane uptake and the community composition of methanotrophic bacteria (MB). The methane uptake rates were similar to rates described previously for forest and farmland soils of the temperate zone. The rates were lower at the agricultural site than at the native forest and reforested sites. The sites also differed in the MB community composition, which was characterized by denaturing gradient gel electrophoresis (DGGE) of pmoA gene fragments (coding for a subunit of particulate methane monooxygenase) that were PCR amplified from total soil DNA extracts. Cluster analysis based on the DGGE banding patterns indicated that the MB communities at the forested and reforested sites were similar to each other but different from that at the farmland site. Sequence analysis of excised DGGE bands indicated that Methylobacter spp. and Methylocystis spp. were present. Sequences of the "forest soil cluster" or "upland soil cluster ␣," which is postulated to represent organisms involved in atmospheric methane consumption in diverse soils, were detected only in samples from the native forest and reforested sites. Additional sequences that may represent uncultivated groups of MB in the Gammaproteobacteria were also detected.The current atmospheric mixing ratio of the greenhouse gas methane (CH 4 ) is 1.75 ppm by volume (8). An estimated 30 Tg of CH 4 year Ϫ1 is consumed via microbiological oxidation in upland soils, accounting for about 6% of the global methane sink (17). Atmospheric methane oxidation has been detected in many different upland soils, including arctic and subarctic tundra soils, grasslands and arable soils of the temperate zone, tropical forest soils, savannah soils, and even arid desert soils (25,31). Most studies have been performed to estimate the methane uptake capacity of upland soils of the temperate zone, and few data are available for tropical and subtropical soils (10,21,22,27,28). Most of these data indicate that the methane uptake rates of tropical and subtropical forest soils are comparable to those of forest soils of the temperate zone, but higher oxidation rates have been reported for some tropical soils (29).Among upland soils, forest soils are much more efficient methane sinks than cultivated soils (1, 3). Changes in land use, especially cultivation of formerly undisturbed soils, reduce the sink strength for atmospheric methane by 60 to 90% (30, 31). Such reductions have been reported for tropical soils as well (10, 21). However, the methane oxidation rate seems to recover much faster after abandonment of agricultural activities (22) compared to systems in the temperate zone.Methane oxidation in upland soils is mediated by methanotrophic bacteria (MB). Seven recognized genera of MB belong to the group containing the type I methanotrophs (Gammaproteobacteria), while the group containing the type II methanotrophs consists of four genera of MB (Alphaproteobacteria)...