Salinity stress and hydrogen peroxide regulation of antioxidant defense system in Ulva fasciata

The regulation of antioxidant defense system in macroalgae exposed to salinity stress was examined in Ulva fasciata Delile. As compared to the 30 parts per thousand control, a long-term (4 days) exposure to hyposaline (5, 15 parts per thousand) and hypersaline (60, 90, 120, 150 parts per thousand) conditions inhibited growth rate and TTC reduction ability. A decrease in maximum quantum efficiency (F-v/F-m ratio) and the maintenance of superoxide dismutase activity under salinity stress indicate the potential generation of reactive oxygen species in chloroplasts. An exposure to 15, 60, and 90 parts per thousand decreased seawater H2O2 contents but increased thallus H2O2 contents that are positively correlated with TBARS and peroxide contents. Alleviation of oxidative damage and H2O2 accumulation at 15 and 90 parts per thousand by a H2O2 scavenger, dimethylthiourea, suggests that oxidative damage occurring under moderate hyposaline and hypersaline conditions is ascribed to accumulated H2O2. Increased glutathione reductase activity and glutathione content and decreased ascorbate content are responsible for accumulated H2O2 at 15, 60, and 90 parts per thousand, while ascorbate peroxidase activity increased only at salinity >= 90 parts per thousand. Catalase and peroxidase activities also increased at 60 and 90 parts per thousand for H2O2 removal, but only catalase showed activity increase at 15 parts per thousand. For the regeneration of ascorbate, the activities of both dehydroascorbate reductase and monodehydroascorbate reductase were increased at 5 and 15 parts per thousand while only monodehydroascorbate reductase activity increased at 60 and 90 parts per thousand. It is hypothesized that the availability of antioxidants and the activities of antioxidant enzymes are increased in U. fasciata to cope with the oxidative stress occurring in hyposaline and hypersaline conditions.