Detection and classification of atmospheric methane oxidizing bacteria in soil

Well-drained non-agricultural soils mediate the oxidation of methane direct ly from the atmosphere, contributing 5 to 10% towards the global methane si nk(1,2). Studies of methane oxidation kinetics in soil infer the activity o f two methanotrophic populations: one that is only active at high methane c oncentrations (low affinity) and another that tolerates atmospheric levels of methane (high affinity). The activity of the latter has not been demonst rated by cultured laboratory strains of methanotrophs, leaving the microbio logy of methane oxidation at atmospheric concentrations unclear(3,4). Here we describe a new pulse-chase experiment using long-term enrichment with (C H4)-C-12 followed by shortterm exposure to (CH4)-C-13 to isotopically label methanotrophs in a soil from a temperate forest. Analysis of labelled phos pholipid fatty acids (PLFAs) provided unambiguous evidence of methane assim ilation at true atmospheric concentrations (1.8-3.6 p.p.m.v.). High proport ions 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.