Detection and classification of atmospheric methane oxidizing bacteria in soil

Well-drained non-agricultural soils mediate the oxidation of methane directly from the atmosphere, contributing 5 to 10% towards the global methane sink(1,2). Studies of methane oxidation kinetics in soil infer the activity of two methanotrophic populations: one that is only active at high methane concentrations (low affinity) and another that tolerates atmospheric levels of methane (high affinity). The activity of the latter has not been demonstrated by cultured laboratory strains of methanotrophs, leaving the microbiology of methane oxidation at atmospheric concentrations unclear(3,4). Here we describe a new pulse-chase experiment using long-term enrichment with (CH4)-C-12 followed by shortterm exposure to (CH4)-C-13 to isotopically label methanotrophs in a soil from a temperate forest. Analysis of labelled phospholipid fatty acids (PLFAs) provided unambiguous evidence of methane assimilation at true atmospheric concentrations (1.8-3.6 p.p.m.v.). High proportions of C-13-labelled C-18 fatty acids and the co-occurrence of a labelled, branched C-17 fatty acid indicated that a new methanotroph, similar at the PLFA level to known type II methanotrophs, was the predominant soil micro-organism responsible for atmospheric methane oxidation.