Accumulation of Secondary Carotenoids in Chlorella zofingiensis

We have previously found that in Chlorella emersonii, grown under synergistic conditions of high irradiance and low nitrogen, chlorophyll and primary carotenoids were degraded, whereas secondary carotenoids were produced. In this study Chlorella zofingiensis was cultivated under similar conditions. Maximal growth was achieved in a culture growing under light irradiance of 150 μmol quanta m^-2 s^-1 and a nitrogen concentration of 0.5 mg/mL of KNO₃. Higher nitrogen concentrations or higher irradiance inhibited cell division. The secondary carotenoids produced were identified as canthaxanthin (about 30 %) and astaxanthin (about 70 %) in the form of mono- and diesters. Maximal accumulation occurred in cells starved of nitrogen and grown at a light irradiance of 300 μLmol m^-2 s^-1. Exposure of a thin layer of algal cells to sunlight resulted in de-esterification of the astaxanthin esters. It is suggested that secondary carotenoids have a photo-protective role, that is, under high light conditions they protect chlorophyll and other photosynthetic pigments against damage.