High-altitude lakes are sensitive sentinels of climate change, and yet the microbial processes driving biogeochemical cycles under warming remain poorly understood. Seasonal dynamics of sediment microbial communities in four glacial lakes within the cirques of the Seven Rila Lakes (Rila Mountains, Bulgaria) were investigated during June, August, and October 2024. Environmental monitoring showed pronounced seasonal variability with summer peaks in temperature, dissolved organic carbon and nitrogen, and increased inputs of labile organic matter. Multivariate analyses revealed strong seasonal structuring of microbial assemblages, with the largest shifts occurring between the early ice-free season and summer. Archaeal communities were dominated by Halobacteriota over time, whereas lake warming corresponded to a shift from early-season dominance of Actinomycetota and Bacillota to the summer prevalence of Pseudomonadota and Cyanobacteriota, as well as a proliferation of minor phyla Gemmatimonadota. At the genus level, summer enrichment of archaeal methanogen Methanosaeta and bacterial genera Paucibacter, Ca. Accumulibacter, Methylibium, Crenothrix, and Pseudanabaena was observed. Canonical correspondence analysis identified temperature and nutrient availability as the primary drivers of microbial community restructuring. Our results provide empirical evidence that warming and associated changes in organic matter inputs drive shifts in sediment microbial communities, with direct implications for carbon cycling and methane production. Additionally, the results highlight the sensitivity of high-altitude lakes to global warming and emphasize the critical role of microbial communities in modulating ecosystem responses to climate change.
Angelova et al. (Sat,) studied this question.