Abstract: The effect of alkalinity on terrestrial plants has been studied extensively,but little is known for aquatic plants,particularly for its effect on freshwater bloom-algae.Therefore,the bloom forming blue-green alga Microcystis aeruginosa FACHB 905 was chosen in the current study and cultured in medium with 2.3 mmol/L HCO₃^-(ALK2.3)or 12.4 mmol/L HCO₃^-(ALK12.4)in order to compare the growth,photosynthetic performance,pigments,malondialdehyde(MDA)and ultrastructure at different alkalinity.Compared with the control,the biomass of ALK2.3 and ALK12.4 treatments decreased by 7% and 55% after 13 days culture,respectively.The contents of chlorophyll a of ALK2.3 and ALK12.4 treatments were also decreased by 22% and 88% during the same period,respectively.At ALK2.3,Chl a was decreased significantly from the 7th day except for the 9th day,while Chlorophyll a/phycocyanin(Chl a/PC)ratio did not change much except on the 7th day.At ALK12.4,Chl a began to increase after the 5th day and reached the maximum on the 7th day.Chl a became lower than that of the control after the 9th day.The Chl a/PC ratio increased on the 3rd day and then declined.At ALK2.3,photosynthetic activity was inhibited at the first 7 days of culture and then recovered to normal level.At ALK12.4,the photosynthetic activity also showed a similar inhibition-recovery mode,but it did not restore the normal level at the end of culture.The MDA contents of both ALK12.4 and ALK2.3 treatments were increased,but the degree was higher under ALK12.4 than under ALK2.3.The number of liposomes in ALK12.4 treatment was more than that of ALK2.3,which in turn was more than that of the control,while number of thylakoids was in a reverse order.In summary,the biomass was decreased by elevated alkalinity,which might be due to the decreased photosynthetic activity and damage of photosynthetic apparatus.