Light Elicits Astaxanthin Biosynthesis and Accumulation in the Fermented Ultrahigh-Density Chlorella zofinginesis

The growth and astaxanthin production of Chlorella zofingiensis were examined under both heterotrophic and photoautotrophic conditions, and it was found that, in comparison to the photoautotrophic mode, the heterotrophic mode led to high algal densities but attenuated intracellular astaxanthin accumulation. Following the heterotrophy-photoautotrophy transition, a considerable increase in the astaxanthin content was observed, accompanied by the upregulation of key carotenogenic genes, including phytoene synthase (PSY), β-carotenoid hydroxylase (CHYb), β-carotenoid ketolase 1 (BKT1), and β-carotenoid ketolase 2 (BKT2). In contrast, the astaxanthin content and carotenogenic genes underwent an opposite change following the photoautotrophy-heterotrophy transition, suggesting the key role of light in stimulating astaxanthin biosynthesis. To improve the astaxanthin production by C. zofingiensis, a novel heterotrophy-photoinduction culture strategy without dilution was developed and evaluated. The astaxanthin content and productivity reached 2.7 mg g^-1 of dry weight and 9.9 mg L^-1 day^-1, respectively, which were 4.0- and 2.5-fold higher than that obtained under the heterotrophic condition.