Ozonation pretreatment is typically implemented to improve algal cell coagulation.However,knowledge on the effect of ozonation on the characteristics and coagulation of associated algal organic matter,particularly cellular organic matter (COM),which is extensively released during algal bloom decay,is limited.Hence,this study aimed to elucidate the impact of ozonation applied before the coagulation of dissolved COM from the cyanobacteria Microcystis aeruginosa.Additionally,the degradation of microcystins (MCs) naturally present in the COM matrix was investigated.A range of ozone doses (0.1-1.0 mg O3/mg of dissolved organic carbon-DOC) and ozonation pH values (pH 5,7 and 9) were tested,while aluminium and ferric sulphate coagulants were used for subsequent coagulation.Despite negligible COM removal,ozonation itself eliminated MCs,and a lower ozone dose was required when performing ozonation at acidic or neutral pH (0.4 mg O3/mg DOC at pH 5 and 7 compared to 0.8 mg O3/mg DOC at pH 9).Enhanced MC degradation and a similar pattern of pH dependence were observed after preozonation-coagulation,whereas coagulation alone did not sufficiently remove MCs.In contrast to the benefits of MC depletion,preozonation using ≥0.4 mg O3/mg DOC decreased the coagulation efficiency (from 42％/48％ to 28％-38％/41％-44％ using Al-Fe-based coagulants),which was more severe with increasing ozone dosage.Coagulation was also influenced by the preozonation pH,where pH 9 caused the lowest reduction in COM removal.The results indicate that ozonation efficiently removes MCs,but its employment before COM coagulation is disputable due to the deterioration of coagulation.