Usually microalgae only synthesize small amounts of triacylglycerols (TAGs) under normal nutrient conditions, but can synthesize a large num­ber of TAGs with a significant change in the fatty acid composition un­der stress conditions. Limiting nutrient availability such as nitrogen and phosphorus starvation during microalgae cultivation is a common meth­od to induce lipid synthesis [30,31]. When the nitrogen is exhausted and becomes the limiting factor, microalgae will continue to absorb organic carbons, which are to be converted to lipids. The nutrient limitation also results in a gradual change in lipid composition, i. e., from free fatty acids to TAGs which are more suitable for biodiesel production [32]. Phospho­rus is another important nutrient that influences algae growth and lipid accumulation. Khozin-Goldberg et al. [33] found that phosphate limitation could cause significant changes in the fatty acid and lipid composition of Monodus subterraneus. Some studies found that phosphorus deficiency led to reduced lipid content of Nannochloris atomus and Tetraselmis sp.

[34] . Silicon is a necessary element for the growth of diatom. Roessler

[35] found that silicon deficiency could induce lipid accumulation in Cy — clotella cryptica by two distinct processes: (1) An increase in the propor­tion of newly assimilated carbons which are converted to lipids; (2) A slow conversion of previously assimilated carbon from non-lipid compounds to lipids. Unfortunately, higher lipid content achieved through nutrient limitation is usually at the expense of lower biomass productivity because nutrient deficiency limits cell growth. As mentioned above, lipid produc­tivity, representing the combination of lipid content and biomass yield, is a more meaningful performance index to indicate the ability of lipid production of microalgae. Therefore, it is necessary to develop a nutrient management strategy which will first facilitate rapid biomass accumula­tion and then induce lipid accumulation in order to achieve maximum lipid productivity.