Abstract: A high heavy metal resistant Chlorella ellipsoidea strain was isolated from the electroplating waste water.Former researches had showed the alga had high tolerance to heavy metals and was able to eliminate Cu2+ ,Zn2+ ,Ni2+ and Cd2+ effectively from water.So it could be used for decreasing heavy metal pollution.To understand the physical mechanisms of the alga against the toxicities of heavy metals,we studied the physical responses of the alga to Cd2+ stress.In this research,we focused on the physical reactions to radical injuring in cells caused by Cd2+ . Methods:Photosynthesis O2 release was measured by membrane oxygen electrode;Superoxide anion radical was measured by xanthine oxidase and NBT system;Superoxide dismutase (SOD) activity is determined by hydroxylamine assay-developed from xanthine oxidase assay;Catalase(CAT)was measured by molybdic acid method;Peroxidase(POD)was determined by method of guaiacol oxidation;Proline was measured by the method of Sulosalicylic acid. Results:Chlorophyll content and photosynthetic O2 release were measured as indicators of cadmium toxicities.Results showed that,between 10 to 240μmol/L Cd2+ ,total chlorophyll content and photosynthesis O2 release decreased when the ion concentration increased.The lower concentration of Cd2+ ,10μmol/L induced higher chlorophyll-a,it was 130% of the control.Cadmium caused the ratio of chlorophyll-a/b rising. When Cd2+ was over 30μmol/L,the ratio of chla/b rose from 1.22 to 2.61 These showed Cd2+ in this concentration range could cause toxicities to the C.ellipsoidea. So this concentration range of Cd2+ was used as stress to induce radicals and observe the physical responses of the alga.Results showed Cd2+ produced more O2-. in algal cells.When Cd2+ was 90μmol/L,O2-. was 2.07 times as much as the control in 72hrs after incubated.Meanwhile,POD was induced strongly and reached the highest level.It’s content was 4.84 times concentration of the control.But for 120μmol/L and 240μmol/L Cd2+ ,POD content was 4.10 and 3.63 times respectively;There were four SOD isoenzyme bands in the electrophoresis gel.The least molecular band was Fe-SOD,and the other three were Mn-SOD.Highest SOD content was induced by 120μmol/L Cd2+ ,reached 1.3 times as much as the control.Two types of SOD showed different responses to cadmium stress.Mn-SOD obviously increased.That caused total amount of SOD increasing.But Fe-SOD decreased;Proline content and Cd2+ concentration in algal cells was correlated.When Cd2+ was 10μmol/L,proline content was 1 89 times of the control.Proline reached highest level,2.86 times of the control in 120μmol/L Cd2+ .These results suggested that increasing of free radical elimination enzymes and substances was helpful to the algae to resistant cadmium injuring.However,as one of the most important anti-oxidation enzymes,CAT showed an opposite response to the Cd2+ .Its content went down with Cd2+ going up.When Cd2+ was 90μmol/L,CAT activity was 23.36% of the control.This meant CAT was very sensitive to Cd2+ . Conclusion:Cd2+ could induce radicals which were toxic to algal cell.The high heavy metal resistant C.ellipsoidea strain could produce more SOD,POD and proline to decrease the injuries.