Growth and rapid succession of methanotrophs effectively limit methane release during lake overturn

Mayr et al. show that the methanotroph assemblage grows fast enough to limit methane outgassing to the atmosphere during lake overturn when methane from the bottom water is transferred to the surface layer in lake Rotsee. The study suggests that an expanding and changing methanotroph assemblage is responsible for the increasing methane oxidation capacity during autumn overturn. Lakes and reservoirs contribute substantially to atmospheric concentrations of the potent greenhouse gas methane. Lake sediments produce large amounts of methane, which accumulate in the oxygen-depleted bottom waters of stratified lakes. Climate change and eutrophication may increase the number of lakes with methane storage in the future. Whether stored methane escapes to the atmosphere during annual lake overturn is a matter of controversy and depends critically on the response of the methanotroph assemblage. Here we show, by combining 16S rRNA gene and pmoA mRNA amplicon sequencing, qPCR, CARD-FISH and potential methane-oxidation rate measurements, that the methanotroph assemblage in a mixing lake underwent both a substantial bloom and ecological succession. As a result, methane oxidation kept pace with the methane supplied from methane-rich bottom water and most methane was oxidized. This aspect of freshwater methanotroph ecology represents an effective mechanism limiting methane transfer from lakes to the atmosphere.