부산대학교 도서관

Long-term acclimation to irradiance stress HL of the green alga Dunaliella salinaTeod. UTEX 1644 entails substantial accumulation of zeaxanthin along with a lowering in the relative amount of other pigments, including chlorophylls and several carotenoids. This phenomenon was investigated with wild type and the zea1mutant of D. salina, grown under conditions of low irradiance LL, or upon acclimation to irradiance stress HL. In the wild type, the zeaxanthin to chlorophyll ZeaChl mol : mol ratio was as low as 0.009 : 1 under LL and as high as 0.8 : 1 under HL conditions. In the zea1mutant, which constitutively accumulates zeaxanthin and lacks antheraxanthin, violaxanthin and neoxanthin, the ZeaChl ratio was 0.15 : 1 in LL and 0.57 : 1 in HL. The divergent ZeaChl ratios were reflected in the coloration of the cells, which were green under LL and yellow under HL. In LL-grown cells, all carotenoids occurred in structural association with the Chl-protein complexes. This was clearly not the case in the HL-acclimated cells. A β-carotene-rich fraction occurred as loosely bound to the thylakoid membrane and was readily isolated by flotation following mechanical disruption of D. salina. A zeaxanthin-rich fraction was specifically isolated, upon mild surfactant treatment and differential centrifugation, from the thylakoid membrane of either HL wild type or HL-zea1mutant. Such differential extraction of β-carotene and Zea, and their separation from the Chl-proteins, could not be obtained from the LL-grown wild type, although small amounts of Zea could still be differentially extracted from the LL-grown zea1strain. It is concluded that, in LL-grown D. salina, xanthophylls including most of Zea in the zea1strain are structurally associated with and stabilized by the Chl-proteins in the thylakoid membrane. Under HL-growth conditions, however, zeaxanthin appears to be embedded in the lipid bilayer, or in a domain of the chloroplast thylakoids that can easily be separated from the Chl-proteins upon mild surfactant treatment. In conclusion, this work provides biochemical evidence for the domain localization of accumulated zeaxanthin under irradiance-stress conditions in green algae, and establishes protocols for the differential extraction of this high-value pigment from the green alga D. salina.