Contrasting Assembly and Network Roles of Abundant and Rare Bacteria in Reservoir and Soil Habitats

Reservoir water and the adjacent soil are ecologically interconnected yet distinct microhabitats in saline coastal wetland ecosystems, but direct comparisons of their bacterial community composition and assembly remain limited. Here, we integrated high-throughput 16S rRNA gene sequencing with statistical, null model, and network analyses to compare diversity patterns, assembly mechanisms, and interactions of abundant and rare bacterial taxa in both habitats. Soil communities exhibited greater taxonomic diversity but a lower overall abundance, while reservoir communities displayed a pronounced vertical stratification, in contrast to the more spatially uniform soil communities at the sampled scale. Key environmental drivers differed: salinity (reflecting the harsh saline context) and nutrient levels structured reservoir communities, whereas the nutrient availability and cation exchange capacity predominated in soils. Stochastic processes mainly governed the assembly of abundant taxa in both habitats, whereas deterministic selection more strongly structured rare taxa, especially in soils subject to harsh saline conditions. The co-occurrence network analysis revealed higher connectivity and modularity in soils, with moderate taxa acting as critical connectors between modules. In contrast, rare taxa played a pivotal role in sustaining network stability in the reservoir. Together, these findings demonstrate distinct, habitat-dependent assembly mechanisms and ecological roles of abundant and rare bacterial taxa in saline coastal wetland microhabitats, providing insights that can inform wetland conservation and ecosystem management.