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

The green alga Chlorella zofingiensis was found to respond very rapidl y to exposure to combined conditions of high light intensity and nitro gen deficiency by accumulation of secondary carotenoids. Accumulation of secondary carotenoids was detected as early as 60 min after inducti on to light stress (300 mu mol . m(-2). s(-1)) in a nitrogen-free medi um. 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 i n relation to time. After 24 h the main secondary carotenoids were the monoester of astaxanthin (about 50% of total secondary carotenoids) a nd canthaxanthin (20-25%). During the first 8 h of stress the content of the primary carotenoids beta-carotene and lutein increased but subs equently the content of both chlorophyll and the primary carotenoids w as reduced. The reduction in the content of the photosynthetic pigment s was followed by a degradation of the thylakoids and a reduction of t he 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 m embranes. Twelve hours after the induction of stress, lipid bodies con taining secondary carotenoids appeared around the chloroplast and accu mulated at the periphery of the cell. The profile of the secondary car otenoids in the lipid bodies was similar to that in the chromoplast, b oth containing double the amount of astaxanthin diester and half the a mount of free astaxanthin as compared with total cell carotenoids. Aft er 3 days under stress, a hydrophobic layer rich in secondary caroteno ids formed inside the cell wall. Our results suggested that the lipid layer functions as a light filter to reduce irradiation of the cell co mponents, to prevent photooxidative damage and to reduce water losses.