Quantitative profiling of bacterial community restructuring and functional potential variation across upstream and downstream sediments of the Three Gorges Dam

Although numerous studies have shown that dam construction alters microbial communities in aquatic ecosystems, most of these findings are based on relative microbiome profiling, which yields only compositional rather than absolute information. To address this limitation, we employed quantitative microbiome profiling on sediments collected from upstream (GDK, XXH, MH) and downstream (HLM, NJG) sites of the Three Gorges Dam to evaluate bacterial community and functional responses 21 years after dam operation commenced. Downstream sediments exhibited significantly lower levels of moisture, total nitrogen (TN), dissolved organic carbon (DOC), cation exchange capacity (CEC), and available potassium (AK) compared with upstream sediments ( p < 0.001). These environmental changes were associated with a 56.49-64.45% reduction in bacterial abundance ( p < 0.001). α-diversity declined significantly, along with 88.16% of ASVs, predominantly within Pseudomonadota (formerly Proteobacteria). Furthermore, overall bacterial community composition differed markedly between upstream and downstream sediments. Pseudomonadota dominated upstream communities (~2.32×10 11 copies), whereas Desulfobacterota was most abundant downstream (~7.00×10 10 copies). Our results indicated marked declines in the abundances of functional genes associated with inferred functional potential for carbon, nitrogen, sulfur, and phosphorus cycling in downstream sediments. Specifically, genes involved in carbon (e.g., carbohydrate degradation), nitrogen (e.g., nitrification, denitrification), sulfur (e.g., dissimilatory sulfate reduction), and phosphorus (e.g., purine metabolism, which involves organic phosphorus in nucleotides) cycling decreased significantly ( p < 0.001). These findings suggest that dam construction may trigger cascading effects, in which reduced moisture and nutrient availability are associated with decreased microbial abundance, simplified community structure, and diminished biogeochemical functional potential.