Co-Expression of Lipid Transporters Simultaneously Enhances Oil and Starch Accumulation in the Green Microalga Chlamydomonas reinhardtii under Nitrogen Starvation

Lipid transporters synergistically contribute to oil accumulation under normal conditions in microalgae; however, their effects on lipid metabolism under stress conditions are unknown. Here, we examined the effect of the co-expression of lipid transporters, fatty acid transporters, (FAX1 and FAX2) and ABC transporter (ABCA2) on lipid metabolism and physiological changes in the green microalga Chlamydomonas under nitrogen (N) starvation. The results showed that the TAG content in FAX1-FAX2-ABCA2 over-expressor (OE) was 2.4-fold greater than in the parental line. Notably, in FAX1-FAX2-ABCA2-OE, the major membrane lipids and the starch and cellular biomass content also significantly increased compared with the control lines. Moreover, the expression levels of genes directly involved in TAG, fatty acid, and starch biosynthesis were upregulated. FAX1-FAX2-ABCA2-OE showed altered photosynthesis activity and increased ROS levels during nitrogen (N) deprivation. Our results indicated that FAX1-FAX2-ABCA2 overexpression not only enhanced cellular lipids but also improved starch and biomass contents under N starvation through modulation of lipid and starch metabolism and changes in photosynthesis activity. The strategy developed here could also be applied to other microalgae to produce FA-derived energy-rich and value-added compounds.

Automated summary

Lipid transporters play a synergistic role in oil accumulation in microalgae under normal conditions, but their effects under stress conditions are unclear. In this study, the co-expression of fatty acid transporters (FAX1 and FAX2) and ABC transporter (ABCA2) in the green microalga Chlamydomonas was found to significantly increase the TAG content, as well as membrane lipids, starch, and biomass. The overexpression also upregulated genes involved in TAG, fatty acid, and starch biosynthesis, and caused changes in photosynthesis activity and ROS levels during nitrogen deprivation. This strategy could be applied to other microalgae for the production of FA-derived energy-rich compounds.