Modified clay(MC),an effective material used for the emergency elimination of algal blooms,can rapidly reduce the biomass of harmful algal blooms(HABs)via flocculation.After that,MC can still control bloom population through indirect effects such as oxidative stress,which was initially proposed to be related to programmed cell death(PCD)at molecular level.To further study the MC induced cell death in residual bloom organisms,especially identify-ing PCD process,we studied the physiological state of the residual Prorocentrum donghaiense.The experimental results showed that flocculation changed the physiological state of the residual cells,as evidenced by growth inhibition and increased reactive oxygen species pro-duction.Moreover,this research provides biochemical and ultrastructural evidence show-ing that MC induces PCD in P.donghaiense.Nuclear changes were observed,and increased caspase-like activity,externalization of phosphatidylserine and DNA fragmentation were detected in MC-treated groups and quantified.And the mitochondrial apoptosis pathway was activated in both MC-treated groups.Besides,the features of MC-induced PCD in a uni-cellular organism were summarized and its concentration dependent manner was proved.All our preliminary results elucidate the mechanism through which MC can further control HABs by inducing PCD and suggest a promising application of PCD in bloom control.