Oil from algae
For microalgae that are able to survive heterotrophically, exogenous carbon sources offer prefabricated chemical energy, which the cells often store as lipid droplets [31]. Heterotrophically cultivated Chlorella protothecoides has been shown to accumulate as much as 55% of its dry weight as oil, compared to only 14% in cells grown photoautotrophically [ 22••]. Another natural mechanism through which microalgae can alter lipid metabolism is the stress response owing to a lack of bioavailable nitrogen [ 32]. Although nitrogen deficiency appears to inhibit the cell cycle and the production of almost all cellular components, the rate of lipid synthesis remains higher, which leads to the accumulation of oil in starved cells [ 30••]. Interestingly, nitrogen deprivation also promotes the accumulation of the antioxidant pigment astaxanthin in the green alga Haematococcus pluvialis [ 33]. Both of these adaptive responses help to ensure the cells’ survival during times of stress — while lipids serve as energy stores, astaxanthin seems to play a role in the protection against reactive oxygen species.