| Summary: | Upland soils of tundra function as a constant sink for atmospheric CH<sub>4</sub> but the identity of methane oxidizers in these soils remains poorly understood. Methane uptake rates of −0.4 to −0.6 mg CH<sub>4</sub>-C m<sup>−2</sup> day<sup>−1</sup> were determined by the static chamber method in a mildly acidic upland soil of the lichen-dominated forested tundra, North Siberia, Russia. The maximal CH<sub>4</sub> oxidation activity was localized in an organic surface soil layer underlying the lichen cover. Molecular identification of methanotrophic bacteria based on retrieval of the <i>pmoA</i> gene revealed Upland Soil Cluster Alpha (USCα) as the only detectable methanotroph group. Quantification of these <i>pmoA</i> gene fragments by means of specific qPCR assay detected ~10<sup>7</sup><i>pmoA</i> gene copies g<sup>−1</sup> dry soil. The <i>pmoA</i> diversity was represented by seven closely related phylotypes; the most abundant phylotype displayed 97.5% identity to <i>pmoA</i> of <i>Candidatus</i> Methyloaffinis lahnbergensis. Further analysis of prokaryote diversity in this soil did not reveal 16S rRNA gene fragments from well-studied methanotrophs of the order <i>Methylococcales</i> and the family <i>Methylocystaceae</i>. The largest group of reads (~4% of all bacterial 16S rRNA gene fragments) that could potentially belong to methanotrophs was classified as uncultivated <i>Beijerinckiaceae</i> bacteria. These reads displayed 96–100 and 95–98% sequence similarity to 16S rRNA gene of <i>Candidatus</i> Methyloaffinis lahnbergensis and “<i>Methylocapsa gorgona”</i> MG08, respectively, and were represented by eight species-level operational taxonomic units (OTUs), two of which were highly abundant. These identification results characterize subarctic upland soils, which are exposed to atmospheric methane concentrations only, as a unique habitat colonized mostly by USCα methanotrophs.
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