Dinoflagellate blooms reshape bacterioplankton assembly and alter free-living and particle-attached community dynamics with implications for harmful algal bloom processes.

Although shifts in bacterial communities during harmful algal blooms (HABs) have been studied extensively, the assembly processes of bacterioplankton communities across complete bloom cycles remain insufficiently resolved. In this study, based on 16S rRNA gene high-throughput sequencing, we characterized the succession and assembly of bacterioplankton communities across four stages of a coastal bloom caused by Scrippsiella acuminata. We found that the alpha diversity of free-living (FL) bacterioplankton decreased with bloom progression, whereas the alpha diversity of particle-attached (PA) bacterioplankton increased during bloom development. Among taxa that responded strongly to the bloom, Rhodobacteraceae, Flavobacteriaceae, and SAR86_clade showed marked changes in abundance. Null-model analysis suggests that stochastic processes, mainly drift and dispersal limitation, dominated the assembly of both FL and PA bacterial communities, but that the bloom increased the influence of deterministic processes, especially selection. Rhodobacteraceae showed the closest association with the bloom-forming dinoflagellate and may participate in infochemical exchange through compounds such as indole-3-acetic acid (IAA). Together, these results show that bloom progression restructures FL and PA bacterioplankton differently while shifting community assembly toward stronger environmental filtering.IMPORTANCEHarmful algal blooms (HABs) reshape microbial food webs and biogeochemical cycling in coastal waters. This study shows that free-living and particle-attached bacterioplankton respond differently to bloom progression, with contrasting diversity trajectories and distinct taxonomic shifts. It also demonstrates that stochastic assembly remains dominant throughout the bloom, whereas deterministic selection becomes more important during bloom stages. These findings provide a mechanistic framework for understanding how HABs restructure marine bacterioplankton and identify bacterial groups potentially involved in bloom-associated interactions.