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The quest for atmospheric methane oxidizers in forest soils


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Aerobic methanotrophs in forest soils are the largest biological sink for atmospheric methane (CH4). Community structures in 53 soils from Europe, Russia, North and South America, Asia and New Zealand located in boreal, temperate and tropical forests were analysed and maximal abundances of 2.1 × 107 methanotrophs g−1 DW were measured. In acidic soils, the most frequently detected pmoA genotypes were Upland Soil Cluster α (USCα) and Methylocystis spp. Phospholipid fatty acids that were labelled by consumption of 14/13CH4 suggested the activity of type II methanotrophs. Cluster 1 (Methylocystaceae), USCγ and Methylocystis spp. were frequently detected genotypes in pH-neutral soils. Genotypes with ambiguous functional affiliation were co-detected (Clusters MR1, RA21, 2) and may represent aerobic methanotrophs, ammonia oxidizers or enzymes with an unknown function. The physiological traits of atmospheric CH4 oxidizers are largely unknown because organisms possessing the key forest soil pmoA genotypes (USCα, USCγ, Cluster 1) have not been cultivated. Some methanotrophic strains belonging to the family Methylocystaceae have been shown to oxidize CH4 at atmospheric mixing ratios. Methylocystis strain SC2 was found to have an alternative particulate CH4 monooxygenase responsible for CH4 oxidation at atmospheric mixing ratios. pH, forest type and temperature might be environmental factors that shape methanotrophic communities in forest soils. However, specific effects on individual species are largely unknown, and only a limited number of studies have addressed environmental controls of methanotrophic diversity, pointing to the need for future research in this area.

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