Using an analogous approach to that of Holmes et al. (1999), but on a much larger set of soils, Knief et al. (2003) demonstrated that different methanotrophs were present and active in different soils that oxidized atmospheric methane. In acidic soils, the USCa group was the predominant pmoA sequence detected. However, a novel group of pmoA sequences distantly related (<75% derived amino acid identity) to sequences of type I methanotrophs (Gammaproteobacteria) was predominantly detected in soils with pH values above 6.0. This group was named 'upland soil cluster g' (USCg). Soils were incubated under low mixing ratios of 13CH4, and incorporation of 13C into PLFAs characteristic of methanotrophic Gammaproteobacteria was observed in soils where sequences of the USCg were detected. The results indicate that the atmospheric methane oxidizers in neutral to alkaline soils are novel type I species. The prevalence of USCa sequences in acid (pH < 6) soils and the close phylogenetic relationship to the acidophile Methylocapsa suggest that this group is acidophilic, whereas the USCg group is neutrophilic (Knief et al., 2003).
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