A novel Scenedesmus sp. strain performs simultaneous nitrogen and phosphate removal from mariculture tail water at different salinities

Microalgae represent a highly valuable resource for the removal of nitrogen and phosphorus from aquaculture effluents. However, their biomass production and nutrient removal efficiency are significantly influenced by the limited salinity tolerance. This study successfully isolated a novel strain of Scenedesmus sp. with remarkable salinity adaptability. Growth analysis showed that salinity-acclimated Scenedesmus sp. had cell densities of 1.98 × 10⁶, 0.92 × 10⁶, and 0.65 × 10⁶ cells/mL at 0 ‰, 5 ‰, and 10 ‰ salinity after 12 days of culture, and high salinity significantly increased the contents of protein (84.58 % at 15 ‰), soluble sugar (192.20 % at 20 ‰), and total lipid (7.97-fold increase at 15 ‰) compared to freshwater condition. Moreover, photosynthetic efficiency exhibited salinity-dependent responses, with chlorophyll a content peaking at 274.81 % (10 ‰), while key photosynthetic parameters (YII and Fv/fm) showed 23.20–62.27 % reduction at high salinities. The microalgae could remove 72.42 % NH₄⁺-N and 94.30 % PO₄³⁻-P at 0 ‰ salinity, and 69.32 % NH₄⁺-N and 76.35 % PO₄³⁻-P at 10 ‰ salinity. Transcriptomic analysis revealed that salinity-induced differential gene expression at 10 ‰ salinity primarily affected photosynthesis-related pathways (light capture and reactions) and energy metabolism, which explains the observed decline in nutrient removal efficiency at higher salinities. The combined physiological and molecular findings provide valuable insights for applying functional microalgae in pollutant treatment of euryhaline aquaculture systems.