Comparative assessment of salinity tolerance based on physiological and biochemical performances in Ulva australis and Pyropia yezoensis

Physiological and biochemical responses of Ulva australis and Pyropia yezoensis were studied under short-term (1-day) and long-term (7-, 14- and 28-day) salinity stresses (5, 30 and 55 practical salinity unit, psu) to explore the salinity tolerance on comparative basis. Reduced growth rate was recorded in both species under the hyposaline (5 psu) condition compared with control (30 psu), and significant (p < 0.05) decrease of growth (-79% and -57% in U. australis and P. yezoensis, respectively) was recorded at 28-day. Chlorophyll a and carotenoids contents were reduced at the hyposaline condition compared with control. However, chlorophyll a and carotenoids levels at the hypersaline condition were higher in P. yezoensis than control, whereas their levels decreased in U. australis. Phycoerythrin and phycocyanin contents in P. yezoensis decreased at the hyposaline condition, whereas their levels increased at the hypersaline condition. Enhanced lipid peroxidation (LPO) level with increasing exposure periods compared with control could be correlated with reactive oxygen species (ROS) level, which suggested oxidative damage under hypo- and hypersaline stresses. Oxidative damage was more severe at 5 than 55 psu. At the hyposaline condition, the activities of superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and glutathione peroxidase (GPx) in P. yezoensis increased at 1- and 7-day but only SOD, CAT and GPx activities increased immediately at 1-day in U. australis. At 55 psu, antioxidant enzyme activities increased marginally in both species at 28-day. Glutathione S-transferase (GST) activity reduced at the hyposaline condition while its activity increased at the hypersaline condition in both species. These results revealed that U. australis and P. yezoensis respond to the salinity stress differently and oxidative stress development is implied by the alterations in antioxidant enzymes. Therefore, the results suggest the potential role of antioxidant enzymes to combat the salinity-induced oxidative stress.