PHYSIOLOGICAL RESPONSES OF DUNALIELLA TERTIOLECTA TO Hg2+-INDUCED OXIDATIVE STRESS

Some species of the genus Dunaliella are the major primary producers in Maharlu salt lake, which is polluted with different heavy metals including Hg2+. Short-term (48 h) and prolonged exposure (28 d) of D. tertiolecta to 5, 10, 20, and 50 ?M HgCl2 decreased Chlorophyll "a" content, but caused enhanced carotenoid production and increased ascorbate peroxidase (APX) activity, both of which function as antioxidants, buffering oxidative stress by scavenging reactive oxygen species (ROS). Increase in total carotenoids in terms of pictogram per cell (pg cell-1) was higher in prolonged exposure, whereas APX activity was greater in shortterm exposure to HgCl2. In both experiments, despite the increase in antioxidant capacity, algal growth in terms of cell number per ml of culture decreased relative to the control. It seems that generation of ROS in the presence of HgCl2 exceeds the total antioxidant capacity of the cells, which is evident from the increase in cell malondialdehyde (MDA) content and, as a result, reduced cell number. The SDS-PAGE profile of the extracted proteins showed enhanced synthesis of two protein bands of about 29 and 38 kD.