PIGMENT AND STRUCTURAL-CHANGES IN CHLORELLA-ZOFINGIENSIS UPON LIGHT AND NITROGEN STRESS

The green alga Chlorella zofingiensis was found to respond very rapidly to exposure to combined conditions of high light intensity and nitrogen deficiency by accumulation of secondary carotenoids. Accumulation of secondary carotenoids was detected as early as 60 min after induction to light stress (300 mu mol . m(-2). s(-1)) in a nitrogen-free medium. Canthaxanthin and astaxanthin (free and monoesters) were detected 2-3 h later, and after an additional 12 h an astaxanthin diester also appeared. The accumulation of total secondary carotenoids was linear in relation to time. After 24 h the main secondary carotenoids were the monoester of astaxanthin (about 50% of total secondary carotenoids) and canthaxanthin (20-25%). During the first 8 h of stress the content of the primary carotenoids beta-carotene and lutein increased but subsequently the content of both chlorophyll and the primary carotenoids was reduced. The reduction in the content of the photosynthetic pigments was followed by a degradation of the thylakoids and a reduction of the potential rate of photosynthesis (alpha), but not of P-max. After 3 days under light stress the chloroplast was modified to a chromoplast-like organelle, full of secondary carotenoids and free of thylakoid membranes. Twelve hours after the induction of stress, lipid bodies containing secondary carotenoids appeared around the chloroplast and accumulated at the periphery of the cell. The profile of the secondary carotenoids in the lipid bodies was similar to that in the chromoplast, both containing double the amount of astaxanthin diester and half the amount of free astaxanthin as compared with total cell carotenoids. After 3 days under stress, a hydrophobic layer rich in secondary carotenoids formed inside the cell wall. Our results suggested that the lipid layer functions as a light filter to reduce irradiation of the cell components, to prevent photooxidative damage and to reduce water losses.