Modelling perspectives on microbial methane oxidation in stratified lakes

Freshwater lakes and impoundments are an important source of methane into the atmosphere. Methane is the second most important greenhouse gas after carbon dioxide and accounts for about 75% of the climate impact of lakes. The only process that reduces methane emissions from lakes is the oxidation by methanotrophic bacteria. This diverse group of bacteria has the unique ability to use methane and oxygen to gain energy and produce biomass. Aim of this dissertation was to improve our understanding of the ecology of these bacteria and their importance to prevent methane emissions from freshwater lakes. From the outset, the interdisciplinary project was designed to complement field studies with modelling. The focus was in particular on the methane dynamics during the autumnal overturn of stratified lakes. During this critical phase, large amounts of methane that accumulate in the anoxic hypolimnion during stratification are transported to the surface. In the studied lake, methanotrophic bacteria were able to oxidise about 98% of the methane accumulated in the hypolimnion before it reached the atmosphere. Using model simulations, we showed that this high percentage was a result of the convectively driven deepening of the thermocline that kept methane fluxes on average at about 27% of the microbial methane oxidation capacity. A scenario analysis with modified meteorological data suggested that microbial methane oxidation is fairly robust even at higher mixing velocities. In a next step, the thesis addressed the observed diversity and seasonal succession of methanotrophic bacteria. Using a radioactive tracer and metatranscriptomics, we showed that the different communities that dominate methane oxidation in the period of autumn overturn differ significantly in their methane oxidation kinetics. In a final step, we have developed a model for niche differentiation of methanotrophic bacteria in stratified lakes and showed that differences in methane oxidation kinetics can explain at least some of the observed diversity. However, a systematic analysis of the trait combinations that allowed coexistence revealed that these combinations were confined to very narrow regions in the parameter space. We argue that convergence to physiologically limited trait combinations may be a mechanism that ensures that these combinations of traits arise