Aerobic Methane Oxidation in the Water Column of the Shallow Eutrophic Temperate Lake Kasumigaura

Lake Kasumigaura, the second-largest lake in Japan, is shallow, well-oxygenated, and eutrophic. Since shallow lakes are hotspots for methane（CH₄）emissions, understanding CH₄ oxidation - the process that reduces CH₄ emissions - is essential. This study examines aerobic CH₄ oxidation in Lake Kasumigaura to clarify seasonal fluctuations over six years and the factors that influence it. There was a seasonal dynamic in aerobic CH₄ oxidation in Lake Kasumigaura, with the highest rates in autumn and the lowest in spring. The peak values for both the specific CH₄ oxidation rate (0.0764 ± 0.04 h^-1) and the CH₄ consumption rate (14.9 ± 13.3 nM CH₄ h^-1) occurred in autumn. Moderate activity was recorded in both summer and winter. Using quantitative PCR and Next-Generation Sequencing, it was found that, unlike in other freshwater lakes, Lake Kasumigaura's methanotrophic community is mainly composed of Methylocystis, a Type II methanotroph. Redundancy analysis revealed niche differentiation: Methylocystis (Type II) correlates strongly with TN and NO₃^-, while Type I methanotrophs are less abundant and more sensitive to dissolved CH₄ and water temperature (W.T.). Variations in the dominant methanotrophs (revealed by pmoA gene analyses) in lake water, along with the complex relationships between environmental factors and these communities, highlight the unique biogeochemical properties of Lake Kasumigaura.