Antioxidant role of astaxanthin in the green alga Haematococcus pluvialis

The green unicellular alga, Haematococcus pluvialis has two antioxidative mechanisms against environmental oxidative stress: antioxidative enzymes in vegetative cells and the antioxidative ketocarotenoid, astaxanthin, in cyst cells. We added a reagent that generates superoxide anion radicals (O2 ), methyl viologen, to mature and immature cysts of H. pluvialis. Tolerance to methyl viologen was higher in mature than in immature cysts. Mature (astaxanthin-rich) cysts showed high antioxidant activity against O2 in permeabilized cells, but not in astaxanthin-free cell extracts, while immature (astaxanthin-poor) cysts had very low antioxidant activities against O2 in both. The results suggested that astaxanthin accumulated in the cyst cells functions as an antioxidant against excessive oxidative stress. The same levels of antioxidant activities against O2 in both permeabilized cells and cell extracts from vegetative cells suggested the presence of antioxidative enzymes (superoxide dismutase).