Abstract：Abstract Nannochloropsis is a kind of economic microalgae with great development potential and its large-scale farming mainly depends on outdoor raceway pond. Low temperature becomes one of main limiting factors which affect the yield and quality of Nannochloropsis. Salicylic acid (SA) can enhance the low temperature resistance of many plants. This study transferred N. oceanica cells (25 ℃, mid-logarithmic phase) to low temperature (15 ℃) stress for 96 h. According to the difference of final concentrations of salicylic acid and cultivation temperature, the experiment was divided into 5 groups ("25 ℃+SA0", "15 ℃+SA0", "15 ℃+SA5", "15 ℃+SA15" and "15 ℃+SA25"). The cell density, soluble sugar, soluble protein, malondialdehyde (MDA) content, fatty acid composition and transcript levels of 3 fatty acid desaturase genes (Δ5 desaturase, Δ6 desaturase and Δ12 desaturase) were determined of 5 time points (0, 24, 48, 72 and 96 h). The results showed that growth of N. oceanica was inhibited under low temperature stress, while exogenous salicylic acid could promote its growth even though not exceed the group under normal cultivation temperature (25 ℃). Soluble sugar, soluble protein and MDA contents were compared between groups "15 ℃+SA0", "15 ℃+SA5" and "15 ℃+SA15". The results showed that SA could significantly increase its soluble sugar and soluble protein content, decline its MDA content, greatly augment the percentage of unsaturated fatty acids. The best effect emerged under the condition of 15 mg/L salicylic acid. The transcript levels of 3 fatty acid desaturase gene were also analyzed of groups "15 ℃+SA0" and "15 ℃+SA15". The results indicated that the transcript levels of Δ5 desaturase and Δ6 desaturase were decreased in N. oceanica under low temperature stress, while exogenous SA could ease the effect of low temperature stress and enhance the transcript levels of Δ5 desaturase and Δ6 desaturase which corresponds to an increase in the percentage of unsaturated fatty acids. The transcript level of Δ12 desaturase was decreased, while exogenous SA did not significantly promote its transcript. Thus it could be seen that SA could increase intracellular osmotic adjusting materials, reduce peroxidation of membrane lipid, increase the content of unsaturated fatty acid in order to maintain membrane fluidity, eventually relieve the damage of low temperature stress to N. oceanica and improve its low temperature resistance. It is the first time to explore the effect of salicylic acid on low temperature resistance of N. oceanica. The study, with better theoretical significance and economic value, will provide a theoretical basis for optimizing microalgae cultivation and stress resistance.
