Bacterial communities play vital roles in coastal biogeochemical cycling and ecological stability. Despite their importance, a significant knowledge gap exists regarding their spatiotemporal dynamics and assembly mechanisms in the tropical coastal waters of the Leizhou Peninsula, China. To investigate the bacterial community structure, co-occurrence networks, and assembly processes, we conducted 16S rRNA gene amplicon sequencing on water samples collected seasonally from August 2022 to June 2023. The bacterial communities were dominated by Proteobacteria and Cyanobacteria, and were characterized by a distinct warm-season peak in the relative of Cyanobium. Alpha diversity indices exhibited significant seasonal fluctuations, reaching a minimum in August (autumn) and a maximum in December (winter). These variations were strongly regulated by water temperature and phosphate availability. Redundancy analysis (RDA) identified salinity as the primary deterministic factor shaping community composition. Seasonal environmental heterogeneity, rather than spatial variation, primarily governed bacterial community dynamics. We also observed a seasonal succession in community assembly mechanisms with deterministic filtering dominated in autumn, whereas stochastic processes prevailed in other seasons. Predicted functional profiles indicated a stable core metabolism, although local anthropogenic inputs stimulated specific metabolic adaptations in industrial and aquaculture zones. Our findings reveal that seasonal environmental filtering (especially temperature and salinity) and a shifting balance between stochastic and deterministic assembly processes govern bacterial dynamics in this tropical coastal ecosystem, with anthropogenic inputs modulating local metabolic functions. This study provides fundamental insights into the mechanisms maintaining microbial diversity and stability in tropical coastal waters facing seasonal and human pressures.

Harmful blooms of the haptophyte Phaeocystis globosa can substantially reshape surrounding microbial communities, with potential consequences for bloom dynamics and biogeochemical cycling. However, distributional breadth patterns across archaeal, bacterial, viral and eukaryotic groups, as well as their cross-domain co-occurrence patterns, remain poorly characterized. In this study, surface waters were collected from a P. globosa bloom-affected site (Tiaoshun Bridge, TS) and a non-bloom reference site (Jinshawan, JSW) in Zhanjiang Bay, China, and analyzed via shotgun metagenomic sequencing, sample-associated distributional breadth estimation, and FDR-corrected Spearman correlation network analysis. Chlorophyll a was markedly higher at TS than at JSW (49.53 vs. 9.93 μg·L⁻¹), consistent with greater P. globosa colony abundance (36 vs. 8 colonies·L⁻¹), confirming strong spatial heterogeneity in bloom intensity. The eukaryotic community at TS was dominated by Haptophyta and Phaeocystaceae, whereas Ascomycota, Bacillariophyta, Bacteroidota, and Nucleocytoviricota were enriched at JSW. Generalist taxa predominated among archaea and bacteria, whereas microbial eukaryotes exhibited stronger distributional specialization. The highly modular network was bacteria-dominated (87.3% of all nodes). Notably, under the specified thresholds, P. globosa showed only two significant positive associations, exclusively with two Pseudoalteromonas taxa. These findings indicate that bloom-associated microbial networks are structured primarily by bacterial covariation rather than by direct algal-centered linkages, and identify Pseudoalteromonas as candidate bloom-associated bacteria for future hypothesis testing.