Stochastic and deterministic processes jointly govern bacterial community assembly along a heterogeneous mangrove...

Sediment microbial communities drive biogeochemical cycling in mangroves, but how they assemble under heterogeneous degradation remains unclear. Along a mangrove degradation gradient across five sites (MS1∼MS5), bacterial communities were analyzed using multiple approaches. The normalized stochasticity ratio (NST) yielded an overall value of 0.541 for the metacommunity. Site-specific NST values varied considerably (0.325-0.655), with stochastic dominance observed at two sites (MS2 and MS3) and deterministic dominance at the other sites. Null model analysis (βNTI/RC) further revealed that stochastic processes (dispersal limitation and undominated) accounted for 46-55% of pairwise comparisons. These results indicate that deterministic and stochastic processes jointly contributed to bacterial community assembly, with their balance varying among sites. In addition to these assembly patterns, we observed systematic shifts in community properties: Alpha-diversity increased from healthy to dead state at MS2 and MS4 but peaked at the marginal state at MS5. Beta-diversity shifts were largest at MS4 and MS5. Key taxa were systematically replaced along the gradients, accompanied by rewiring of co-occurrence networks. Predicted functional potential showed a shift toward heterotrophic pathways (e.g., alkane degradation). Collectively, our findings demonstrate that the balance between deterministic and stochastic processes is not universal but shifts with local environmental stress, underscoring the context-dependent nature of community assembly in degrading mangroves.