Table of Contents:
Methane (CH4), a major greenhouse gas, is present in the atmosphere at a mixing ratio of about 1.7 ppmv. Aerobic upland soils are a net sink for atmospheric methane. At the beginning of this work only little was known about the methanotrophic bacteria (MB) responsible for atmospheric methane oxidation, although they were thought to be oligotrophs adapted to the trace level of CH4 in the atmosphere. Cultivation-independent molecular ecology techniques were combined with studies on pure cultures of MB to investigate this.
For the cultivation-independent studies 58 soils of different chemical properties, plant cover, and land use were sampled. By combining diverse molecular techniques, and by multivariate analysis of this large dataset, two groups of as-yet uncultivated MB were found to be primarily responsible for atmospheric CH4 uptake. MB were identified by retrieval and sequence analysis of the pmoA gene, and by analysis of the phospholipid fatty acids that were labelled with 13C after incubation of soils with 13CH4. In this way a group of MB (upland soil cluster g) only distantly related to cultivated Gammaproteobacteria, which was present and active in most pH-neutral soils, was identified for the first time. An as-yet uncultivated group of Alphaproteobacteria (upland soil cluster a) was detectable and active in most acidic soils. MB of the known genera Methylocaldum, Methylobacter, Methylosinus and Methylocystis were also present in upland soils and were successfully isolated. Growth studies using these known MB have provided insight into the process of atmospheric CH4 oxidation. Large differences were observed in the abilities of different MB to grow and remain active under low CH4 mixing ratios, although this was not related to substrate affinity as is usually postulated. It was evident from these growth studies that CH4 supply could control which MB are active in soils, and this was confirmed by cultivation-independent examination of a Gleysol after incubation under different mixing ratios of CH4. Some Methylocystis strains were the most oligotrophic of the cultivated MB. These oxidised atmospheric CH4 (1.7 ppmv) with little loss of activity for prolonged periods (>6 weeks). In addition to the two groups of uncultivated MB found in the soil studies, Methylocystis strains may therefore be active at consuming atmospheric methane in certain cases, for example in periodically flooded soils where population growth can be supported by occasional CH4 production.