Stage-Dependent Dynamics and Assembly Processes of PhoD-Harboring Bacterial Communities Driven by Ulva prolifera Green Tides

The phoD gene encodes alkaline phosphatase, which hydrolyzes organic phosphorus and releases bioavailable phosphorus for direct utilization by marine organisms. phoD-harboring bacteria are reported to be sensitive to environmental changes. As a common ecological disturbance, annual Ulva prolifera green tides in the southern Yellow Sea pose significant ecological challenges, yet the responses and assembly processes of phoD-harboring bacterial communities remain poorly understood. In this study, high-throughput sequencing was used to characterize these communities across the pre-bloom, bloom and post-bloom stages. The results revealed significant stage-specific shifts in community structure, with the bloom and post-bloom stages exhibiting higher similarity to each other than the pre-bloom stage. Abundant taxa were more sensitive to environmental fluctuations across all stages and were characterized by broader niche breadths but reduced phylogenetic diversity during the bloom. In contrast, rare taxa maintained relatively stable diversity but showed marked niche contraction. Neutral community model and βNTI analyses demonstrated that stochastic processes dominated community assembly overall. Green tide drove rare taxa toward heterogeneous selection and drift, while abundant taxa shifted toward homogeneous selection during the post-bloom stage. Co-occurrence network analysis showed increased microbial correlations during the bloom, implying a trend toward greater network stability of phoD-harboring bacterial communities under green tide disturbance. The lagged responses, functional redundancy and divergent ecological strategies of abundant and rare taxa may explain how green tides drive variations in microbes involved in the phosphorus cycle. These findings provide new insights into the microbial regulatory mechanisms of the nutrient cycle in coastal ecosystems affected by large-scale U. prolifera green tides.