Prorocentrum donghaiense blooms frequently occur in the Yangtze River Estuary and the adjacent East China Sea and have destroyed marine ecosystem and caused enormous economic losses over the past two decades.Thus,highly efficient,low-cost,ecofriendly approaches must be developed to control P.donghaiense blooms.In this study,a bacterium strain (Y42),which was isolated from the coastal surface sediment,was identified as Paracoccus sp.and used to inhibit P.donghaiense growth.The culture supematant of the Y42 bacteria was able to lyse P.donghaiense and the algicidal activity of the Y42 supernatant was stable under different temperatures,light intensities and wide pH range (pH 4-10).To clarify the algicidal mechanism of strain Y42,we observed the alga-lytic process and detected the photosynthetic activity of P.donghaiense after exposure to the Y42 supematant.Scanning-and transmission electron microscopy analysis showed that the Y42 supematant destroyed the membrane integrity and caused algal cell breakage at the megacytic zone.Photosynthetic pigment loss and a significant decline in both the photosynthetic efficiency (Fv/Fm) and electron transport rate (rETR) indicated that the Y42 supematant destroyed the photosynthetic system of P.donghaiense.The present study reveals the potential of strain Y42 for the control of P.donghaiense blooms.