Does astaxanthin protect Haematococcus against light damage?

The photoprotective function of the ketocarotenoid astaxanthin in Haematococcus was questioned. When exposed to high irradiance and/or nutritional stress, green Haematococcus cells turned red due to accumulation of an immense quantity of the red pigment astaxanthin. Our results demonstrate that: 1) The addition of diphenylamine, an inhibitor of astaxanthin biosynthesis, causes cell death under high light intensity: 2) Red cells are susceptible to high light stress to the same extent or even higher then green ones upon exposure to a very high light intensity (4000 μmol photon m^-2 s^-1); 3) Addition of ¹O₂ generators (methylene blue, rose bengal) under noninductive conditions (low light of 100 μmol photon m^-2 s^-1) induced astaxanthin accumulation. This can be reversed by an exogenous ¹O₂ quencher (histidine): 4) Histidine can prevent the accumulation of astaxanthin induced by phosphate starvation. We suggest that: 1) Astaxanthin is the result of the photoprotection process rather than the protective agent: 2) ¹O₂ is involved indirectly in astaxanthin accumulation process.