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Kolb, S; Knief, C; Dunfield, P; Conrad, R: Abundance and Activity of Uncultured Methanotrophic Bacteria Involved in the Consumption of Atmospheric Methane in Two Forest Soils, Environmental Microbiology, 7(8), 1150-1161 (2005), doi:10.1111/j.1462-2920.2005.00791.x
The activity and abundance of methanotrophic bacteria were measured in an acidic and a neutral forest soil. The soils exhibited high uptake rates (> 30 mg CH4 m-2 h-1) of atmospheric CH4 at all measurement times throughout the vegetation period. The abundances of various phylogenetic groups of methanotrophs, including some uncultured putative ones, were measured using real-time PCR assays. Each assay specifically targeted the pmoA gene or mmoX gene of a particular group of methanotrophs, or the amoA gene of ammonia-oxidizing bacteria. As-yet uncultured methanotrophs of a group previously dubbed "forest soil cluster" or "USC a" were numerically dominant in the acidic soil, while cultured but taxonomically uncharacterized methanotrophs of a group "Cluster I" were dominant in the neutral soil. Each group was detected in numbers equivalent to about 106 pmoA gene copies (g dry weight soil) –1 and comprised > 90% of the detectable methanotrophic bacteria in the respective soil. Since the numbers of ammonia-oxidizing bacteria were similar but not higher, they could not have accounted for the observed CH4 uptake rates due to their low cell-specific CH4 oxidation activity. Based on CH4 flux and bacterial abundance data, estimated cell-specific CH4 oxidation rates of the detected methanotrophic bacteria were 540-800 ´ 10-18 mol cell-1 h-1, which is high compared to literature values of cultured methanotrophic bacteria. These estimated cell-specific CH4 oxidation rates are sufficiently high to allow not only maintenance but even growth on atmospheric CH4 alone. mRNA transcripts of the pmoA gene were detected in the acidic soil, demonstrating that USC a methanotrophs expressed pmoA under ambient methane mixing ratios. On the other hand, pmoA trancripts of Cluster I or of other methanotrophic groups were not detectable. Our study suggests that USC a and Cluster I methanotrophs are adapted to the low concentration of methane in forest soils by possessing high cell-specific CH4 oxidation activities.
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