Lipid Accumulation Mechanisms in Auto- and Heterotrophic Microalgae.

Microalgae lipids have attracted great attention in the world as a result of their potential use for biodiesel productions. Microalgae are cultivated in photoautotrophic conditions in most cases, but several species are able to grow under heterotrophic conditions, in which microalgae are cultivated in the dark where the cell growth and reproduction are supported by organic carbons. This perspective is covering the related studies concerning the difference between hetero- and autotrophic cultivation of microalgae. The auto- and heterotrophic central carbon metabolic pathways in microalgae are described, and the catalyzing reactions of several key metabolic enzymes and their corresponding changes in the protein level are summarized. Under adverse environmental conditions, such as nutrient deprivation, microalgae have the ability to highly store energy by forming triacylglycerol (TAG), the reason for which is analyzed. In addition, the biosynthesis of fatty acids and TAGs and their difference between auto- and heterotrophic conditions are compared at the molecular level. The positive regulatory enzymes, such as glucose transporter protein, fructose-1,6-bisphosphate aldolase, and glycerol-3-phosphate dehydrogenase, and the negative regulation enzymes, such as triose phosphate isomerase, played a crucial role in the lipid accumulation auto- and heterotrophic conditions.