Expression of the peanut diacylglycerol acyltransferase 3 increases the neutral lipid content and improves the fatty acid composition of Chlorella vulgaris

Microalgae are a potential source of renewable biofuel with several advantages over conventional crops. Under stress conditions, oleaginous microalgae such as Chlorella vulgaris accumulate high levels of neutral lipids, mainly in the form of triacylglycerol (TAG), which can be converted into biodiesel. However, the growth under stress conditions limits biomass accumulation. DGAT enzymes catalyze the final step in TAG biosynthesis, by transferring a fatty acyl-CoA to diacylglycerol. We describe here the first case in which a higher plants DGAT3-type enzyme has been overexpressed in an oleaginous microalga. Higher plants DGAT3 enzymes differ in their properties from other types of DGAT enzymes and also from the distantly related group of enzymes nominated DGAT3 in algae. We overexpressed in C. vulgaris the DGAT3 of Arachis hypogaea (peanut), since this enzyme utilizes oleoyl-CoA as the preferred acyl donor. Oleic acid is a favorable fatty acid constituent of biofuel due to its low melting point and a relatively low vulnerability to oxidation. The sequence and regulatory regions of AhDGAT3 were optimized for supporting efficient expression. The transformed algal lines showed up to a five-fold increase in the content of neutral lipids. This increase occurred under normal growth conditions, which do not limit biomass accumulation. The transformed algae also showed a four-fold increase in the percentage of oleic acid and a 25% reduction in the percentage of linolenic acid among the lipid-derived fatty acids. Both changes are favorable for biodiesel utilization. This work demonstrates that higher plants DGAT3 enzymes, and particularly the peanut DGAT3, can be utilized for obtaining improved microalgal feedstocks for biofuel production.