Stereoisomers of β-Carotene and Phytoene in the Alga Dunaliella bardawil

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Abstract
Dunaliella bardawil, a halotolerant green alga, was previously shown to accumulate high concentrations of .beta.-carotene when grown outdoors under defined conditions. The .beta.-carotene of alga cultivated under high light intensity in media containing a high salt concentration is composed of approximately 50% all-trans .beta.-carotene and 40% 9-cis .beta.-carotene. We show here that the 9-cis to all-trans ration is proportional to the integral light intensity to which the alga are exposed during a division cycle. In cells grown under a continuous white light of 2000 micoreinsteins per square meter per second, the ratio reached a value of around 1.5, while in cells grown under a light intensity of 50 microeinsteins per square meter per second, the ratio was around 0.2. As previously shown, algae treated with the herbicide norflurazon accumulate phytoene in place of .beta.-carotene. Electron micrographs showed that the phytoene is accumulated in many distinct gloubules located in the interthylakoid spaces of the chloroplasts. Here too, two isomers are present, apparently all-trans and 9-cis phytoene, and their ratio is dependent upon the integral light intensity to which the aglae are exposed during a division cycle. In the presence of norflurazon, Dunaliella bardawil grown under a light intensity of 2000 microeinsteins per square meter per second contained about 8% phytoene with a 9-cis to all-trans ratio of about 1.0. This ratio decreased to about 0.1 when the light intensity was reduced to 50 microeinsteins per square meter per second. These data suggest that the isomerizaiton reaction which leads to the production of the 9-cis isomer occurs early in the path of carotene biosynthesis, at or before formation of all-trans phytoene. The presence of the 9-cis isomer of .beta.-carotene and the dependence of its preponderance on light intensity seem to be a common feature of many plant parts. Thus carrots which are exposed to minimal light contain no 9-cis isomer while sun-exposed leaves, fruits, and lowers contain 20 to 50% of the 9-cis isomer.