The lipid content of algal cells is closely related to the metabolic pathways that the algae undergo. Photoautotrophic growth is usually characterized by lower rates and lower lipid content compared with heterotrophic growth in which algae use organic matter as carbon and energy source [36,37]. Ef­forts have been made to control metabolic activities during cultivation in order to maximize lipid accumulation. Wu et al. [38] added organic carbon (glucose) to and reduced inorganic nitrogen in the cultivation of Chlorella protothecoides, and found that the induced heterotrophic growth resulted in 55.2% lipid content, which was about four times that in photoautotro­phic growth. Xiong et al. [39] developed a photosynthesis-fermentation model with double CO2 fixation in both photosynthesis and fermenta­tion stages which provided an efficient approach for the production of algal lipid. In this model, cultivation of C. protothecoides could realize 69% higher lipid yield on glucose achieved at the fermentation stage, and 61.5% less CO2 released compared with typical heterotrophic metabolism.

1.4.1 CULTURE CONDITIONS

Lipid synthesis may also be induced under other stress culture conditions, such as extreme light [40], temperature [41], salinity [42], pH [43], and CO2 conditions [44]. Therefore, during cultivation, process conditions must be closely monitored and/or appropriately adjusted for optimal lipid productivity.

1.4 PROCESSING OF MICROALGAE LIPID