Regulation Mechanism of Gibberellic Acid-3 for Astaxanthin Biosynthesis in Heterotrophic Growing Chromochloris zofingiensis

Chromochloris zofingiensis has been proven as a potential resource for large-scale astaxanthin production, but little information on phytohormones for its growth and astaxanthin accumulation could be obtained. This study explored the impact of gibberellic acid-3 (GA3) on growth and astaxanthin biosynthesis in heterotrophic C. zofingiensis. After 6 days of cultivation with GA3, biomass and astaxanthin yields in 7.5 L fermenters reached 268.5 g·L^–1 and 0.34 g·L^–1, respectively, which were 6% and 89% higher than those in the control. GA3 changed transcription levels of genes linked to carbon metabolism, lipid metabolism, astaxanthin production, and ABC transporters. Genes related to astaxanthin biosynthesis, such as phytoene synthase (PSY), phytoene desaturase (PDS), beta-carotenoid hydroxylase (CHYb), and beta-carotenoid ketolase (BKT), were up-regulated under GA3 induction. The enhancement of carbon metabolism and lipid metabolism led to elevated consumption of substrates and generation of reducing power, thus facilitating astaxanthin biosynthesis. By using GA3 and arginine together, the astaxanthin yield increased to 0.39 g·L^–1, which was 18% higher than that obtained under GA3 induction. It could be concluded that GA3 showed significant effects on astaxanthin biosynthesis. This research proposed novel feasible approaches to enhance astaxanthin production in heterotrophic C. zofingiensis.