Seasonal Composition and Structure of Methane-Cycling Communities in Alpine Lake Sediments of the Rila Mountains

The global methane budget is largely driven by biogenic sources, many of which remain insufficiently characterized. Here, we investigated the community composition and seasonal dynamics of methanogenic and methanotrophic assemblages to elucidate the key contributors to methane cycling and the environmental factors shaping these processes in lake sediments of the Rila Mountains (Bulgaria). Methanogenic communities are primarily composed of Methanothrix, Methanosarcina, Methanobacterium and Methanoregula with summer peaks in Methanothrix and Methanoregula, and cold-season proliferation of Methanobacterium. Methanotrophic communities are dominated by representatives of the Pseudomonadota, including Crenothrix, Methylobacter, and Methylocystis with summer maxima observed for Crenothrix and Methylobacter. Methanosarcina and Methylocystis showed relatively stable abundances throughout the ice-free season. Ordination and correlation analyses revealed that temperature, pH, and carbon (organic and inorganic) concentration and lability emerged as the environmental drivers influencing on microbial communities, with seasonally variable effects on methane-cycling microorganisms. These findings provide a foundation for future research on methane cycling in alpine lake ecosystems of the Rila Mountains and contribute to improving predictions of methane emissions under changing climatic conditions.