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To investigate the effect of cerium(Ce)on Scendesmus obliquus,the growth,chlorophyll a content and chlorophyll fluorescence were studied by lab cultured experiments.Results from the present investigation noted that 0.50-5.00 mg/L Ce3+ resulted in a considerable stimulation to the growth and the chlorophyll a content of Scendesmus obliquus,but concentrations above 50.0 mg/L appeared an inhibitory effect.Both the growth and the chlorophyll a content were also at a maximum at a concentration of 5 mg/L.Both the donor and acceptor side of PSII reaction centers are sensitive to external stress.In our study,the OEC of PSII in the low concentration of Ce3+ was not sensitive to Cerium,but sensitivity was intensely increased when treated with high concentration.Ψo and φEo were significantly lower in the treatment than in the control.Ce3+ led to decrease in rate of electron transport(ET0/ABS and ET0/TR0).An increase of VJ suggests that Ce3+ inhibits electron transfer from QA- To QB and subsequently results in accumulation of QA-.Our results provide not only new insights into cerium stress responses in Scendesmus obliquus but also a good foundation for realizing the cytological mechanism for the toxic effects of Ce3+ on Scendesmus obliquus.