Compartment-Specific Niche Filtering Shapes the Structure and Nutrient-Cycling Potential of Bacterial Communities in Eutrophic Waters with Hydrilla verticillata

Submerged aquatic macrophytes and their microbiomes can help mitigate eutrophication, yet how microbial communities and functions differ across specific plant-associated and surrounding niches remains unclear. Here, we profiled bacterial community composition (16S rRNA gene sequencing) and quantified nitrogen and phosphorus cycling genes (<i>narG</i>, <i>nirK</i>, <i>nirS</i>, <i>nosZ</i>, <i>phoD</i> by qPCR) across eight distinct compartments associated with the submerged macrophyte <i>Hydrilla verticillata</i> in a eutrophic freshwater wetland. The niches spanned open water, bulk sediment, rhizosphere, and plant phyllosphere (leaf/stem surfaces) and endosphere (leaf/stem/root interiors). Alpha diversity differed significantly among niches: sediments (non-rhizosphere and rhizosphere) exhibited the highest Operational Taxonomic Unit (OTU) richness and diversity, whereas leaf-associated niches (phyllosphere and endosphere) had the lowest. Beta diversity showed clear separation by niche, indicating strong habitat filtering. Community composition also varied markedly: the water column was dominated by Bacteroidota (~51% of sequences), plant-associated communities were enriched in Pseudomonadota (43-90%), and sediment niches were dominated by Firmicutes (23~48%). Functional gene abundances showed pronounced niche partitioning. Nitrate/nitrite reduction genes (<i>narG</i>, <i>nirK</i>, <i>nirS</i>) were most enriched on leaf phyllosphere, with <i>narG</i> abundance equally high in the water, whereas the N₂O reductase gene <i>nosZ</i> peaked in sediment niches. The alkaline phosphatase gene <i>phoD</i> had its highest copy numbers in leaf biofilms, with significantly lower levels in internal plant tissues. Overall, neutral processes explained ~61% of community variation, but deterministic assembly was evident in the well-connected water and leaf surface niches. These findings reveal strong niche differentiation in plant-associated microbiomes and suggest that compartmentalized microbial functional capacity within the <i>H. verticillata</i> holobiont enhances nitrogen removal and phosphorus cycling in eutrophic waters.