Lake Lovojaervi microbiome

Small boreal lakes are known to contribute significantly to global methane emissions. Lake Lovojaervi is a eutrophic lake in Southern Finland with bottom water methane concentrations up to 2 mM. Biogeochemical processes and microbial community structures involved in methane removal were explored through chemical profiling, incubation experiments, and 16S rRNA gene amplicon sequencing. In incubation experiments involving the addition of light and/or oxygen, methane oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobic methane oxidation. In the dark anoxic water column at 7 m depth, nitrite seemed to be the key electron acceptor involved in methane oxidation, yet additions of Fe(III), anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic methane oxidation. Overall, this study shows that photosynthetically fueled methane oxidation can be a key process in methane removal in the water column of humic, turbid lakes, thereby limiting diffusive methane emissions from boreal lakes. Yet, it also highlights the potential importance of a whole suite of alternative electron acceptors, including humics, in these freshwater environments in the absence of light and oxygen.