Abstract: Sargassum fusiforme seedlings were used to determine their photosynthetic responses to different seawater salinities (15‰, 20‰, 29‰, 45‰). The results showed that photosynthesis of S. fusiforme was significantly affected by excessive low-and high-salinity stress. Specifically, at salinity of 20‰, photosynthesis of S. fusiforme seedling was slightly affected, which indicated low salinity tolerance in S. fusiforme seedlings. However, excessive low salinity level (15‰) significantly depressed algal photosynthesis. At high salinity of 45‰, S. fusiforme seedlings were damaged by high salt stress, its photosynthetic activity and utilization of inorganic carbon were significantly inhibited. In a short time treatment(1-12 h), seedling photosynthesis was slightly affected, and it's pigment contents, maximum relative election transport rate (rETR_max), and maximal photochemical efficiency (F_v/F_m) exhibited changes of decreasing followed by increase. In prolonged culture(240 h), excessive low-(15‰) or high-salinity (45‰) significantly inhibited algal photosynthesis, meaning notable decreasing of its rETR_max, light use efficiency(α), maximum net photosynthesis rate (P_nmax) and photosynthetic carbon fixation (v_max). Moreover, excessive high salinity stressed the photosynthesis process of S. fusiforme seedling by hindering the synthesis of its photosynthetic pigments. It indicated that the strategies for preventing low-and high-salinity stress on S. fusiforme seedlings should be practically performed when the salinity level extremely changed, to avoid the severe stress of prolonged salinity change on the seedlings.

Key words: Sargassum fusiforme, photosynthesis, salt stress, seaweed aquaculture, chlorophyll fluorescence