Astaxanthin, a highly esteemed carotenoid, is widely utilized in the food and health industries as a safe coloring agent, a feed supplement for poultry and aquaculture, and a nutraceutical. The freshwater green alga Chromochloris zofingiensis has been identified as the natural source of this ketocarotenoid. 

C. zofingiensis has attracted significant attention for its ability to grow heterotrophically, achieving a high biomass yield (>200 g·L⁻¹) compared to photoautotrophic growth (0.3 g·L⁻¹). While heterotrophic conditions enhance astaxanthin production due to higher biomass, inducing astaxanthin often reduces biomass dramatically. This trade-off highlights the need for optimized culture methods to improve both biomass and astaxanthin content for efficient and economically viable production. 

Recently, a research group led by Prof. BI Yonghong from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences elucidated the regulation mechanism of Gibberellic acid-3 (GA3) in enhancing astaxanthin biosynthesis in heterotrophic Chromochloris zofingiensis. This study was published in the Journal of Agricultural and Food Chemistry. 

In this study, the researchers explored the effect of phytohormone and discovered that GA3 could significantly enhance biomass and astaxanthin accumulation in heterotrophic growing C. zofingiensis. After 6 days of cultivation with GA3, the biomass and astaxanthin yields in 7.5 L fermenters reached 268.5 g·L^-1 and 0.34 g·L^-1, respectively, representing a 6% and 89% increase compared to the control group. 

Using transcriptomic and metabolomic analyses, the researchers revealed that GA3 changed the transcription of carbon metabolism, carotenoid synthesis, lipid metabolism, and ATP-binding cassette (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 upregulated under GA3 induction. The enhancement of carbon metabolism and lipid metabolism led to elevated consumption of substrates and generation of reducing power, which facilitated astaxanthin biosynthesis. 

Furthermore, the study demonstrated that C. zofingiensis could accumulate biomass and astaxanthin concurrently by combined utilization of GA3 and Arginine. 

This study not only revealed the regulation mechanism of GA3 for astaxanthin biosynthesis in heterotrophic microalga C. zofingiensis, but also proposed novel feasible approaches to enhance astaxanthin production in large-scale cultivation of C. zofingiensis. 

Journal Cover：Gibberellic acid-3 (GA3) played a significant role in biomass and astaxanthin accumulation in heterotrophic Chromochloris zofingiensis. (Image by IHB)

(Editor: MA Yun)