Salinity-induced oxidative stress and regulation of antioxidant defense system in the marine macroalga Ulva prolifera

The physiological and biochemical responses to salinity stresses in Ulva prolifera were investigated, including growth rate, level of oxidative stress and regulation of antioxidant defense system. A six-day exposure to hyposaline (10 parts per thousand) and hypersaline (60 parts per thousand) conditions resulted in a significant decrease in growth rate and maximum photosynthetic quantum efficiency (Fv/Fm) compared with the control (30 parts per thousand). Increases in H2O2 contents correlated to the level of lipid peroxidation, which suggested that oxidative damage occurred in salinity stress and was more severe at 60 parts per thousand than at 10 parts per thousand. The amount of total soluble protein (TSP) significantly increased in a hypersaline condition. The fluctuations of four antioxidant substrates and four antioxidant enzymes were determined after the long-term salinity stress. Compared to growth at 30 parts per thousand, low salinities led to a major increase in activities of catalase (CAT), superoxide dismutase (SOD) and glutathione reductase (GR), coupled with an increase in contents of ascorbate, glutathione and beta-carotenoid.Thalli exposed to hypersaline conditions rapidly accumulated glutathione and did not affect the content of ascorbate, alpha-tocopherol and beta-carotenoid. The activities of CAT, SOD, ascorbate peroxidase (APX) and GR increased in hypersaline conditions, suggesting that reactive oxygen scavenging enzymes played an important role in U. prolifera for adapting to the hypersaline condition. The alterations in antioxidant enzymes and substrates are not consistent between hyposaline and hypersaline stresses in U. prolifera, but the regulation of antioxidant defense system was a vital tolerance mechanism involved in the oxidative stress. (C) 2011 Elsevier B.V. All rights reserved.