In freshwater lakes, large amounts of methane are formed 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 concentrations. In this study, we tested the hypothesis that methanotroph assemblages in a seasonally stratified lake exhibit contrasting methane oxidation kinetics in the methane-rich hypolimnion compared to the epilimnion with low methane concentrations. We further examined the change of methane oxidation kinetics during autumn overturn as more methane becomes available in the epilimnion. Together with the change of methane oxidation kinetics, we investigated changes in the transcription of genes encoding the methane monooxygenase (MMO), which is the enzyme responsible for the first step of methane oxidation. We show that the half-saturation constant (Km) obtained from laboratory experiments with the natural microbial community differed by two orders of magnitude between epi- and hypolimnion during stable stratification. During lake overturn, however, the kinetic constants at the lake surface and in the deep-water converged along with a change of the methanotroph assemblage. Conventional particulate MMO seemed responsible for the methane-oxidation under different methane concentrations. Our results suggest that changing methane availability creates niches for methanotroph assemblages with well-adapted methane-oxidation kinetics. This rapid selection and succession of adapted lacustrine methanotroph assemblages seem to support that the reported high removal efficiency of more than 90% is 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.
