Effects of diluted seawater on the growth, antioxidation capacity and accumulation of bioactive compounds of the brown alga, Sargassum muticum

Environmental condition is a significant factor that directly affects the growth, metabolism and productivity of marine algae. This study examined the influences of diluted seawater on the growth, antioxidation capacity and accumulation of bioactive compounds of the marine macroalga, Sargassum muticum. Compared to the natural seawater (30 ppt, parts per thousand) control, the specific growth rate was clearly elevated at diluted seawater with 15 and 20 ppt during the initial 3 days, and was not statistically different at diluted seawater from 15 to 30 ppt at 15 days, while it decreased significantly at a low seawater concentration of 10 ppt. Chlorophyll a, carotenoids and total protein content fluctuated slightly under the different seawater solutions, suggesting that the algal cells underwent metabolic regulation during the period of adaptation to culture media changes. Changes in seawater concentration altered antioxidant enzyme activity, for instance, total superoxide dismutase, catalase and peroxidase, especially at low seawater solutions with 10 and 15 ppt. The malondialdehyde content also increased substantially, indicating that oxidative damage occurred in diluted seawater below 15 ppt. Moreover, the alginate content increased with decreasing seawater concentration, the alginate productivity was significantly increased by 22.32%-54.79% in all dilutions at 3 days and this promotion continued only at 15 and 20 ppt salinities after 7 days. The fucoxanthin content reduced significantly in the 10 and 15 ppt dilutions at 15 days, however, its productivity increased slightly at 10 and 15 ppt salinities in the first 3 days. The phlorotannin content increased slightly in the seawater dilutions with 20 ppt at 3 and 7 days, meanwhile, its productivity was significantly elevated during the initial 3 days at all seawater dilutions and the highest value was found at 20 ppt treatment. These results provide valuable data for the development and management of the aquaculture of S. muticum.