Physiological Responses and Functional Performance of Eucheumatoid Seaweeds Under Prolonged Low-Salinity Stress

Low-salinity events caused by heavy rainfall and freshwater runoff pose significant environmental constraints on eucheumatoid seaweed mariculture in coastal areas. In this study, five eucheumatoid seaweeds, including three strains of Kappaphycus alvarezii (Hn-R, Hn-G, and Ma-R), Kappaphycus striatum, and Eucheuma denticulatum, were exposed to 22, 26, and 30 psu for 36 days. Growth performance, photosynthetic pigments, chlorophyll fluorescence parameters, oxidative damage, and antioxidant responses were determined. To compare the actual physiological performance of different seaweeds under each salinity condition, a functional performance score was calculated using membership function analysis based on relative growth rate, chlorophyll a, carotenoids, photosynthesis, and malondialdehyde. Low salinity generally reduced growth, photosynthetic pigment content, and photosystem II performance, while increasing malondialdehyde content and antioxidant-related parameters, indicating that prolonged low-salinity exposure induced both physiological inhibition and oxidative stress responses. At 22 psu, K. alvarezii Ma-R showed the strongest functional performance, followed by K. striatum and E. denticulatum. At 26 psu, E. denticulatum showed the strongest functional performance, followed by K. alvarezii Ma-R and K. striatum. These results indicate that eucheumatoid seaweeds differ in their functional performance under mild and more severe low-salinity tolerance, providing physiological evidence for selecting candidate eucheumatoid seaweeds for salinity-fluctuating coastal mariculture.