Prorocentrum donghaiense has become one of the major algae that cause phytoplankton blooms. A marine bacterium, Halomonas sp. DH-e with high algicidal activity to P. donghaiense was isolated in previous studies. However, the mechanism underlying the algicidal activity of DH-e was unknown. In the present study, the post-culture medium of DH-e was supplemented into the culture of P. donghaiense. Samples of algae cells and phycosphere bacteria were collected at different times. Illumina Miseq sequencing was used to evaluate bacterial community dynamics in the phycosphere of microalgae. Taxonomic analysis identified 3 phyla, including Proteobacteria, Bacteroidetes and Verrucomicrobia, with Proteobacteria dominating and accounting for more than 95% of all bacterial species. Alteromonadaceae increased gradually, while Rhodobacteraceae decreased during the culture with the post-culture medium. Principal component analysis suggested that bacterial community composition was similar during 0-12 h, but changed significantly at 24 h, 36 h and 48 h. Redundancy analysis revealed a close correlation between phycosphere bacterial community composition and pH, chlorophyll a and algae cell mass. In conclusion, the results suggested that algicidal activity of DH-e was mediated via the secretion of unidentified components in the post-culture medium, which induced variation of phycosphere bacterial community and subsequently contributed to the lysis of algae cells. Fig. 5. Redundancy analysis (RDA) ordination diagram showing the relationship among samples, bacteria and environmental factors. TN, total nitrogen; TP, total phosphorus; N/P, ratio of nitrogen to phosphorus; Chl a, chlorophyll a; DAC, the density of algae cells.