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

Abstract

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‰) and hypersaline (60‰) conditions resulted in a significant decrease in growth rate and maximum photosynthetic quantum efficiency (Fv/Fm) compared with the control (30‰). Increases in H 2O 2 contents correlated to the level of lipid peroxidation, which suggested that oxidative damage occurred in salinity stress and was more severe at 60‰ than at 10‰. 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‰, 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 β-carotenoid. Thalli exposed to hypersaline conditions rapidly accumulated glutathione and did not affect the content of ascorbate, α-tocopherol and β-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.