Spatial distribution of prokaryotes in a harmful dinoflagellate bloom near the Huangqi Peninsula, Fujian, China.

In recent decades, increasingly frequent harmful algal blooms (HABs) have severely impacted the coastal aquaculture across China, but the mechanisms underlying their initiation and termination remain unclear. Emerging evidence suggests that planktonic prokaryotes potentially play a critical role in HAB dynamics. Using 16S rDNA high-throughput sequencing, this study investigated the spatial variations in planktonic prokaryotic communities during a mixed Prorocentrum and Karenia bloom in Lianjiang coastal waters in late May 2024. The results revealed that the bloom significantly reduced bacterial diversity, and the community composition was highly associated with the bloom stages: the relative abundance of the primary phylum Proteobacteria was significantly higher in the blooming region; and the relative abundances of dominant species, such as Lentibacter algarum, Phaeobacter italicus, Pelagimonas phthalicica, and Caulobacter mirabilis, significantly differed between bloom and non-bloom areas. Correlation analysis showed weak associations between environmental factors and major prokaryotic taxa, but complex interspecific interactions within the bacterial community. In addition, significant relations were observed between key prokaryotes and primary eukaryotic species. Functional prediction showed that pathways related to quorum sensing, nutrient cycling, and environmental stress resistance were significantly enriched in this area. Collectively, these findings suggested that the main bloom-forming species were the primary factor influencing the planktonic bacterial communities, and that algal-bacterial interactions, especially quorum sensing mechanisms, might contribute to bloom decline. Additionally, the intensive aquaculture in this region synergistically influenced the structure of planktonic prokaryotic communities.