Abstract. In lakes, large amounts of methane are produced in anoxic
sediments. Methane-oxidizing bacteria effectively convert this potent
greenhouse gas into biomass and carbon dioxide. These bacteria are present
throughout the water column, where methane concentrations can range from
nanomolar to millimolar. In this study, we tested the
hypothesis that methanotroph assemblages in a seasonally stratified
freshwater lake are adapted to the contrasting methane concentrations in the epi- and hypolimnion. We further hypothesized that lake overturn would
change the apparent methane oxidation kinetics as more methane becomes
available in the epilimnion. In addition to the change in the methane oxidation kinetics, we investigated changes in the transcription of genes encoding
methane monooxygenase, the enzyme responsible for the first step of methane
oxidation, with metatranscriptomics. Using laboratory incubations of
the natural microbial communities, we show that the half-saturation constant (Km) for
methane – the methane concentration at which half the maximum methane
oxidation rate is reached – was 20 times higher in the hypolimnion than in
the epilimnion during stable stratification. During lake overturn, however,
the kinetic constants in the epi- and hypolimnion converged along with a
change in the transcriptionally active methanotroph assemblage. Conventional
particulate methane monooxygenase appeared to be responsible for methane
oxidation under different methane concentrations. Our results suggest that
methane availability is one important factor for creating niches for
methanotroph assemblages with well-adapted methane oxidation kinetics. This
rapid selection and succession of adapted lacustrine methanotroph
assemblages allowed the previously reported high removal efficiency of
methane transported to the epilimnion to be maintained – even under rapidly
changing conditions during lake overturn. Consequently, only a small
fraction of methane stored in the anoxic hypolimnion is emitted to the
atmosphere.
Supplementary material to "Lake mixing regime selects methane-oxidation kinetics of the methanotroph assemblage"
