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To gain an understanding of the dynamics of photosynthesis in the target algae during the algicidal activity of a marine bacterium, we focused on the study of the behavior of the dinoflagellate Alexandrium tamarense against the algicidal bacterium Vibrio sp.DHQ25.Algal cells were found to be susceptible to the algicidal bacterium DHQ25 using algieidal activity assay.Loss of photosynthetic pigments, accompanied by a decline in PSII photochemical efficiency (Fv/Fm), in A.tamarense was detected under bacterial filtrate stress.Transmission electron microscope analysis showed obvious morphological modifications of chloroplast dismantling as a part of the algicidal process.The PSII electron transport chain was seriously blocked, with its reaction center damaged.This damage was detected in a relative transcriptional level ofpsbA and psbD genes, which encode the D1 and D2 proteins in the PSII reaction center.This study indicated that marine bacteria with indirect algicidal activity played an important role in the changes of photosynthetic performance in an Harmful algal bloom species.To our knowledge, this is the first report concerning a bacteria-induced algicidal process towards bloom-forming A.tamarense by addressing the photosynthetic dynamic response to the stress.