Antioxidant response of the microalga Dunaliella salina under salt stress

Hydrogen peroxide content in Dunaliella salina cells increases concomitantly with salinity, and maximum content occurred at 5.0 M NaCl concentration. Activity of the antioxidant enzyme superoxide dismutase (SOD, EC 1.15.1.1) increased significantly by two- and three-fold in 1.0 and 2.0 M salt concentrations, respectively, thereafter declining significantly. Unlike SOD, catalase (EC 1.11.1.6) activity decreased in tandem with salinity; however, there was no significant decrease in catalase activity up to 2.0 M salt concentration (compared to 0.5 M salinity). There was no significant decrease in catalase activity from 2.0 M to 5.0 M salinity, whereas a significant decrease was observed in 4.0 and 5.0 M salinity (compared to 1.0 M salinity). Ascorbic acid peroxidase (EC 1.11.1.11) activity decreased concurrently with salt stress up to 2.0 M, thereafter no significant change was observed in cells grown in varying salt concentrations (0.5-5.0 M NaCl). Detailed study of hydrogen peroxide content and antioxidant enzymes in response to varying salinity (0.5-5.0 M NaCl) has not been reported previously in Dunaliella salina. Antioxidant enzymes and H2O2 scavenging systems have short-term adaptation mechanisms for protection against salt stress and are not important in imparting salt stress tolerance to Dunaliella salina at high salinity because of production of secondary antioxidant metabolites, like beta-carotene, which protect the alga against salt induced oxidative damages.