Enhanced Carotenoid Biosynthesis by Oxidative Stress in Acetate-Induced Cyst Cells of a Green Unicellular Alga, Haematococcus pluvialis

In a green alga, Haematococcus pluvialis, a morphological change of vegetative cells into cyst cells was rapidly induced by the addition of acetate or acetate plus Fe²⁺ to the vegetative growth phase. Accompanied by cyst formation, algal astaxanthin formation was more enhanced by the addition of acetate plus Fe²⁺ than by the addition of acetate alone. Encystment and enhanced carotenoid biosynthesis were inhibited by either actinomycin D or cycloheximide. However, after cyst formation was induced by the addition of acetate alone, carotenoid formation could be enhanced with the subsequent addition of Fe²⁺ even in the presence of the inhibitors. The Fe²⁺ -enhanced carotenogenesis was inhibited by potassium iodide, a scavenger for hydroxyl radical, suggesting that hydroxyl radical formed by an iron-catalyzed Fenton reaction may be required for enhanced carotenoid biosynthesis. Moreover, it was demonstrated that four active oxygen species, singlet oxygen, superoxide anion radical, hydrogen peroxide, and peroxy radical, were capable of replacing Fe²⁺ in its role in the enhanced carotenoid formation in the acetate-induced cyst. From these results, it was concluded that oxidative stress is involved in the posttranslational activation of carotenoid biosynthesis in acetate-induced cyst cells.