OPERATION OF THE XANTHOPHYLL CYCLE IN NONSTRESSED AND STRESSED CELLS OF DUNALIELLA-SALINA TEOD IN RESPONSE TO DIURNAL CHANGES IN INCIDENT IRRADIATION - A CORRELATION WITH INTRACELLULAR BETA-CAROTENE CONTENT

Changes in carotenoid composition were monitored on a diurnal basis in cells of Dunaliella salina Teed, grown outdoors, under natural irradi ance in mass culture. Stressed cells contained approximately four time s more beta-carotene than non-stressed cells. The net change in beta-c arotene content however, was similar for both treatments and correlate d closely with changes in photon flux density during the diurnal cycle . Xanthophyll levels were higher in stressed cells suggesting that exc ess light was not the only factor involved in stimulating carotenoid a ccumulation, although xanthophyll content increased in nonstressed cel ls to a level similar to that of stressed cells, with increasing photo n flux density. Light-response curves of zeaxanthin and beta-carotene formation displayed a biphasic nature in stressed cells. In nonstresse d cells however, zeaxanthin formation was rapid and the descending lig ht-response curve showed little change in net zeaxanthin levels. High beta-carotene-containing cells had substantial amounts of zeaxanthin a nd levels of this carotenoid remained higher in stressed cells through out the diurnal cycle. Changes in intracellular beta-carotene occurred coincident with operation of the xanthophyll cycle during the diurnal period. Non-stressed and stressed cells showed similar de-epoxidation of the xanthophyll cycle at peak photon flux denisty. Although non-st ressed cells formed more beta-carotene and zeaxanthin in response to i ncreasing irradiance, stressed cells formed beta-carotene and zeaxanth in more rapidly i.e. at lower photon flux density. Zeaxanthin formatio n in stressed cells occurred as a consequence of violaxanthin de-epoxi dation. By comparasion, zeaxanthin production in non-stressed cells oc curred by two probable mechanisms: 1, by rapid de-epoxidation of viola xanthin and 2, a slow but continuous de novo synthesis which increased the total zeaxanthin pool.