Characterization and manipulation of a DGAT2 from the diatom Thalassiosira pseudonana: Improved TAG accumulation without detriment to growth, and implications for chloroplast TAG accumulation

Improving the ability of microalgae cells to accumulate triacylglycerol (TAG) without a negative effect on growth or biomass accumulation is desirable for economically-viable biofuels production. Metabolic engineering is one method to increase TAG yields, provided that appropriate gene target(s) are identified. Microalgal lipid and TAG synthesis is incompletely characterized, but it is clear that it differs in fundamental ways from plants and other eukaryotes. We characterize a DGAT2 homolog from the diatom Thalassiosira pseudonana, and show that it shifts intracellular location from the chloroplast during exponential growth, to the endoplasmic reticulum in stationary phase when large cytoplasmic lipid droplets accumulate. This is the first direct localization of a chloroplast DGAT2, which confirms the ability of these organelles to accumulate TAG in evolutionarily-diverse classes of microalgae. The enzyme is also not directly localized to cytoplasmic lipid droplets, suggesting that TAG synthesis and lipid droplet formation are separate processes. Overexpression of the enzyme resulted in significantly increased TAG accumulation without a negative effect on growth or biomass accumulation. Overexpression lines had a distinct shift in fatty acid composition relative to wild-type. The results shed light on fundamental processes of algal lipid metabolism and a practical benefit of improved productivity in transgenic lines.