DNA- and RNA-level analyses reveal active microbial nitrogen transformation processes and microecological response mechanisms in the sediments of Lake Erhai.

Lake eutrophication is driven by both external nutrient inputs and internal nutrient cycling. In semi-enclosed faulted plateau lakes, limited hydraulic exchange reduces the nutrient-buffering capacity. While external inputs are controlled, internal release of nitrogen and NH4+ resulting from microbial activity becomes the predominant source of pollution. In this study, surface sediment and water samples were collected from seven representative monitoring sites at Lake Erhai, Yunnan Province, during summer and spring. By integrating DNA- and RNA-level analyses, we investigated active microbial nitrogen transformation processes in terms of total and active community structures and the abundance of nitrogen-cycling and related functional genes. Co-occurrence networks were constructed to identify key taxa and their functional roles. Notably, Candidatus_Accumulibacter emerged as a central taxon within the active microbial community during summer, participating in multiple metabolic pathways, including nitrogen fixation, nitrogen assimilation, and sulfur oxidation. Although DNA-level analysis revealed no significant seasonal differences in the abundance of nitrogen-cycling functional genes, RNA-level analysis indicated higher expression of the ammonification-associated ureC gene and the assimilatory gdhA gene in summer than in spring. Moreover, the relative metabolic activity of organic nitrogen transformation peaked in spring, indicating that organic nitrogen mineralization outpaces inorganic nitrogen assimilation during this season. Limited nitrification activity may contribute to the sustained accumulation of NH4+, offering a potential explanation for the long-term ammonium enrichment. This study provides new insights into the microbial mechanisms underlying nitrogen cycling in inland plateau lake ecosystems.