Plant growth regulators promote lipid and carotenoid accumulation in Chlorella vulgaris

Microalgae are photosynthetic organisms with the ability to produce a variety of high-value compounds such as polyunsaturated fatty acids (PUFAs), proteins, pigments, and lipids. The high cost of microalgae production is one of the biggest obstacles for their commercialization. Plant growth regulators might be an ideal choice since they could potentially induce microalgae to produce lipids and other high-value secondary metabolites thereby reducing production cost. This study investigated the effects of eight plant growth regulators (PGRs), namely, salicylic acid (SA); 1-naphthaleneacetic acid (NAA); gibberellin (GA(3)); 6-benzylaminopurine (6-BA); 2,4-epibrassinolide (EBR); abscisic acid (ABA); ethephon (ETH); and spermidine (SPD) on the induction of lipids, proteins, carotenoids, and unsaturated fatty acids (UFAs) in Chlorella vulgaris. Moreover, the expression profiles of seven fatty acid biosynthethis genes were studied in the PGR-treated biomass. All PGRs used in the study caused significant increases in total lipid contents in non-dose-dependent manners when compared to control. However, lipid productivities were increased due to four of the eight PGRs (ABA, 6-BA, NAA, and ETH). Similar to lipids, total carotenoid contents were significantly higher in all of the PGR-treated microalgal biomass except ABA. However, soluble protein contents were not affected by the PGR treatments except SA at 10 mg L-1. Furthermore, 6-BA, NAA, ABA, and ETH treatments resulted in significant increases in UFAs especially DHA, linolenic acid, arachidonic acid, and EPA which were confirmed by the upregulation of fatty acid biosynthesis genes including stearoyl-ACP-desaturase, omega-3 fatty acid desaturase, biotin carboxylase, and acyl-acyl carrier protein. Our findings, therefore, indicate that the treatment with PGR used in this study could be a useful tool to produce biodiesel and other high-value metabolites from microalgal biomass.