Metagenomic insights into the trophic gradient influence on nitrogen cycling microbiomes in plateau lakes

The increasing prevalence of nitrogen (Nr) pollution in lake ecosystems is a growing global concern. Understanding the dynamics of Nr-cycling microbial communities in these environments is crucial for assessing how ecosystem processes and functions respond to trophic gradients. This study investigates the microbial Nr-metabolism in plateau lakes with varying trophic states across a broad geographical range. A detailed metagenomic study revealed that increasing trophic status index (TSI) reduced the α-diversity of Nr-cycling microbial communities, while TSI and altitude jointly shaped the β-diversity patterns. The Nr-cycling microorganisms predominantly belonged to the phylum Proteobacteria, with the most abundant functional genes associated with organic Nr degradation and synthesis, dissimilatory/assimilatory nitrate reduction to ammonium (DNRA and ANRA), and denitrification processes (DNiF). Key Nr functional genes exhibited differential enrichment across lakes, indicating changes in Nr-metabolism strategies along the trophic gradient. A total of 126 metagenome-assembled genomes (MAGs) contributed to Nr-cycling, with the majority assigned to Proteobacteria (36) and Planctomycetes (25). Among these, MAG110 was enriched in eutrophic lakes and possessed near-complete DNiF and ANRA pathways, while MAG115, predominant in oligotrophic lakes, relied solely on ANRA. This functional divergence reflects trophic-specific ecological adaptations, that denitrification is favored in nutrient-rich, low-oxygen conditions and Nr- retention is prioritized under Nr-limited environments. Moreover, enzymes like nitronate monooxygenase (encoded by both genomes) and nitroalkane oxidase highlight a novel metabolic interaction between Nr-transformations and organic C1 compound oxidation in freshwater ecosystems. Overall, this study highlights the complex relationship among trophic status, microbial diversity, and Nr-metabolism in lake ecosystems.