Lipid accumulation in microalgae usually occurred when microalgae are cultivated under stress conditions (e. g., nitrogen starvation, nutrient deficiency, pH variations, etc.). Among those stress conditions, nitrogen limitation is the most effective and commonly used strategy for stimulating lipid accumulation in microalgae. Recent reports demonstrated that cultivation under nitrogen starvation conditions leads to a marked increase in the oil/lipid content (Mandal and Mallick, 2009). Hu et al. (2008) collected the data of lipid contents of various microalgae and cyanobacteria species under normal growth and stress conditions in a literature
survey, indicating that under stress conditions, the lipid contents of green microalgae, diatoms, and some other microalgae species are 10-20% higher than under normal conditions. However, the lipid contents of cyanobacteria were usually very low (10%) (Hu et al., 2008).

It is thought that when microalgae are cultivated under nitrogen-starvation conditions, the proteins in microalgae will be decomposed and converted to energy-rich products, such as lipids. Siaut et al. (2011) also concluded that during microalgae growth, starch would first be synthesized to reserve energy, then lipid would be produced as a long-term storage mech­anism in case of prolonged environmental stress (such as nitrogen deficiency). Although a nitrogen-starvation strategy is very effective in increasing lipid content of microalgae, the ni­trogen deficiency conditions often lead to a significant decrease in the microalgae growth rate, thereby causing negative effects on lipid productivity. Therefore, engineering approaches should be conducted to optimize the cultivation time for the microalgae growth period (nitrogen-sufficient condition) and lipid accumulation period (nitrogen-deficient condition) to ensure high overall oil/lipid productivity.