Harmful algal bloom (HAB) is an ecological disaster to local mariculture.At present,its impact on macrophytes has not been well studied.In this study,we cultivated sexually propagated embryos of Sargassum fusiformis— an edible seaweed—in Prorocentrum donghaiense suspensions at different cell densities (0,0.50× 105,0.75 × 105,1.00× 105,and 1.50× 105 cells/mL) for 10 days,during which growth and photosynthetic activities of the embryos were determined,and a monocultivation was set up for comparison.Results show that the relative growth rate and photosynthetic activities of the embryos co-cultivated with P.donghaiense were inhibited mostly and significantly in the cell densities of 0.75×105,1.00× 105,and 1.50× 105 cells/mL,and the inhibitory effects increased in overall with increased cell densities.The maximum relative electron transport rates (rETRmax) and apparent photosynthetic efficiency (α) of co-cultivated embryos were all significantly lower than monocultivation ones on the 10th day.Furthermore,the photosynthetic activity detected by chlorophyll-a fluorescence transient (i.e.,OJIP),the electron transport among electron transfer accepters of PSⅡ (photosystem Ⅱ) and that from PSⅡ to PSⅠ (photosystem Ⅰ) was restricted,which is probably responsible for the decreases of rETRmax and α in the co-cultivated embryos.In addition,parts of the photosynthetic reaction centers of PSⅡ in the co-cultivated embryos were inactivated.Therefore,P.donghaiense bloom could restrain the development and photosynthetic activities of S.fusiformis embryos,reduce the seedlings stock,and eventually hinder the development of S.fusiformis production industry.