The effects of pristine polystyrene microplastics (pMPs) without any pretreatment at different concentrations (0,10,20,50,and 100 mg/L) on Phaeodactylum tricornutum Bohlin at two initial algae densities (105 and 106 cells/mL) were assessed in this study.Hormesis-like effects were found when microalgae grew with pMPs.The results showed that pMPs inhibited microalgae growth under a high concentration of microplastics tolerated by individual algal cell (low initial algae density) (up to-80.18±9.71％) but promoted growth when the situation was opposite (up to 15.27±3.66％).The contents of photosynthetic pigments including chlorophyll a,chlorophyll c and carotenoids showed resistance to pMPs stress under a low initial algae density and increased with time,but the opposite was true under a high initial algae density.Compared with the low initial algae density group,Qp received less inhibition,and NPQ (heat dissipation) also decreased under the high initial algae density.Under the low initial algae density,OJIP parameters such as Sm,N,Area,Pi Abs,Ψo,ψEo,TRo/RC and ETo/RC were more perturbed initially and returned to the levels of the control group (without pMPs)over time,but they remained stable throughout the experiment at high initial algae density.These results show that microplastics in the marine environment may have different toxic effects on P.tricornutum at different growth stages,which is of great significance for understanding the impact of microplastics on marine microalgae and aquatic ecosystems.