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

摘要

Changes in carotenoid composition were monitored on a diurnal basis in cells of Dunaliella salina Teed, grown outdoors, under natural irradiance in mass culture. Stressed cells contained approximately four times more beta-carotene than non-stressed cells. The net change in beta-carotene content however, was similar for both treatments and correlated closely with changes in photon flux density during the diurnal cycle. Xanthophyll levels were higher in stressed cells suggesting that excess light was not the only factor involved in stimulating carotenoid accumulation, although xanthophyll content increased in nonstressed cells to a level similar to that of stressed cells, with increasing photon flux density. Light-response curves of zeaxanthin and beta-carotene formation displayed a biphasic nature in stressed cells. In nonstressed cells however, zeaxanthin formation was rapid and the descending light-response curve showed little change in net zeaxanthin levels. High beta-carotene-containing cells had substantial amounts of zeaxanthin and levels of this carotenoid remained higher in stressed cells throughout 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-stressed cells formed more beta-carotene and zeaxanthin in response to increasing irradiance, stressed cells formed beta-carotene and zeaxanthin more rapidly i.e. at lower photon flux density. Zeaxanthin formation in stressed cells occurred as a consequence of violaxanthin de-epoxidation. By comparasion, zeaxanthin production in non-stressed cells occurred by two probable mechanisms: 1, by rapid de-epoxidation of violaxanthin and 2, a slow but continuous de novo synthesis which increased the total zeaxanthin pool.