Implications of functional zoning on river sediment bacteria in a transitional medium-sized city: Insights from...

Functional zoning reshapes sediment microbiomes and ecosystem functions, yet it remains unclear how environmental drivers and cross-area migration of keystone species jointly influence developed and developing areas and their interactions. To fill this gap, we integrated environmental surveys with 16S rRNA amplicon sequencing in Tongxiang, a transitional city, across UA (Urbanized area), AA (Agricultural area), DA1 (Developing area 1), and DA2 (Developing area 2) to quantify bacterial communities, co-occurrence networks, assembly processes, and cross-area ecological links. Although environmental conditions exhibited spatial homogenization among areas, differences in heavy metal accumulation and nutrient enrichment driven by area-specific development significantly influenced microbial community structures. Urbanization simplified microbial networks, with fewer edges and nodes and lower modularity, and reduced the number of keystone species to 2, compromising ecosystem resilience, whereas agricultural development enhanced diversity and showed the highest values of these network indicators and 18 keystone species, indicating stronger network stability. Deterministic processes dominated microbial assembly in urbanized areas (NST >0.5), while stochastic processes prevailed in agriculturally influenced areas (NST <0.5). Driven by the expansion of microbial species from functional areas, SourceTracker-based tracking of keystone species indicated potentially directional tendencies in ecological transitions, with DA1 leaning toward urbanization and DA2 leaning toward agriculturalization. Heavy metals were key drivers regulating keystone species migration into DA1, whereas nutrient enrichment coupled with heavy metal accumulation promoted keystone species expansion into DA2. Urbanization-associated ecological pressures in DA1 reduced β-diversity and network complexity, such as relative modularity, whereas agricultural influence in DA2 enhanced β-diversity and was characterized by greater network stability and lower vulnerability. These findings unveil that functional development across both developed and developing areas collectively fundamentally reshapes microbial community dynamics, alters community assembly processes, drives keystone species migration, and regulates ecosystem stability and adaptability, offering novel insights for understanding and managing urban ecosystem evolution.