Divergent Responses of Abundant and Rare Bacterial Communities to Environmental Variables in Highly Urbanized Coastal Regions: N-NO Mediates the Community Assembly and Co-occurrence Networks

Understanding how rare and abundant bacterial taxa respond to environmental shifts is essential for predicting microbial community dynamics in highly urbanized estuarine ecosystems, where anthropogenic disturbances and nutrient fluctuations significantly reshape ecological processes. Here, we investigated the responses of rare and abundant bacterial communities to the environments in densely urbanized coastal areas (i.e., EO: eight outlets; NMR: nearshore marine region) within Pearl River Estuary, subsequently exploring the relationships of assembly processes and co-occurrence networks with N-NO. Both taxonomic and phylogenetic divergences among abundant and rare taxa exhibited positive associations with various environmental parameters. Broader environmental thresholds in rare taxa than abundant taxa suggested greater environmental adaptability in rare bacterial communities. Null model analyses revealed disparate stochastic processes governed community assembly for rare and abundant taxa. Correlations of environmental parameters with beta-nearest taxon index (βNTI) identified temperature (T), dissolved oxygen (DO), salinity (Sal), and nitrite nitrogen (N-NO) as pivotal factors shaping community assembly in urbanized coastal areas. Furthermore, significant positive associations between βNTI and N-NO suggest that the fluctuations in N-NO concentrations are associated with notable transitions in abundant and rare community assembly dynamics, shifting from deterministic to stochastic processes. In the co-occurrence networks, significant correlations with N-NO were observed in different modules of rare and abundant taxa. In NMR, module 2 showed a significantly positive correlation, while modules 1 and 3 exhibited significantly negative correlations. In contrast, only module 1 of abundant taxa displayed a significantly positive correlation in EO. It contributes fresh perspectives on environmental adaptability, advancing the comprehension of ecological processes alongside co-occurrence patterns amid fluctuating N-NO levels within highly urbanized coastal regions.