Effect of low temperature on the stereoisomer composition of beta-carotene in the halotolerant alga Dunaliella bardawil (Chlorophyta)

Dunaliella bardawil Ben-Amotz & Avron, a beta-carotene-accumulating halotolerant alga, was analyzed for the effect of growth temperatures on its pigment content and on the stereoisomeric composition of beta-carotene by the use of advanced liquid chromatography and photodiode array detection. Decreasing culture temperature from 30 degrees to 10 degrees C increased the beta-carotene content twofold and the ratio of 9-cis to all-trans beta-carotene fourfold, with no significant changes in the other cell pigments. The variation of total beta-carotene content by temperature was correlated with the integral irradiance received by the algal culture during a cell division cycle, whereas the 9-cis stereoisomer increased over the amount expected by that integration. The massive accumulation of 9-cis beta-carotene within the beta-carotene globules is interpreted as indicating that the oily 9-cis stereoisomer protects against the crystallization of all-trans beta-carotene at low temperatures.