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

## TL;DR

This study explores how different parts of the aquatic plant Hydrilla verticillata host distinct bacterial communities that help manage nutrients in eutrophic waters.

## Contribution

The study reveals compartment-specific niche filtering and microbial functional specialization in plant-associated and surrounding environments.

## Key findings

- Sediment niches showed highest bacterial diversity, while leaf-associated niches had lowest.
- Plant-associated communities were enriched in Pseudomonadota, while sediments were dominated by Firmicutes.
- Leaf phyllosphere had high narG and phoD gene abundance, suggesting roles in nitrogen and phosphorus cycling.

## Abstract

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 (narG, nirK, nirS, nosZ, phoD by qPCR) across eight distinct compartments associated with the submerged macrophyte Hydrilla verticillata 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 (narG, nirK, nirS) were most enriched on leaf phyllosphere, with narG abundance equally high in the water, whereas the N2O reductase gene nosZ peaked in sediment niches. The alkaline phosphatase gene phoD 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 H. verticillata holobiont enhances nitrogen removal and phosphorus cycling in eutrophic waters.