Plant Physiol, February 2000, Vol. 122, pp. 609-618

Exposure to Low Irradiances Favors the Synthesis of 9-cis ,-Carotene in Dunaliella salina (Teod.)¹

Carotenoid Research Group, School of Biological and Earth Sciences, Liverpool John Moores University, Byrom Street, Liverpool L3 3AF, United Kingdom.

We examined the effect of irradiance on the synthesis of -carotene and its isomers by Dunaliella salina. Growth irradiance had a marked effect both on growth of the alga (which was suppressed at both low and high irradiances) and on the accumulation of -carotene. The accumulation of -carotene but not -carotene was closely linked to an increase in irradiance. Growth at low irradiances (20-50 µmol m² s¹) promoted a high ratio of 9-cis to all-trans -carotene (>2:1), while exposure to high irradiances (200-1,250 µmol m² s¹) resulted in a large reduction in this ratio (to <0.45:1). A similar pattern was seen for the geometric isomers of -carotene, with exposure to low irradiance favoring the accumulation of the 9-cis form. The carotenoid biosynthesis inhibitors 4-chloro-5(methylamino)-2-(---trifluoro-m-tolyl)-3-(sH)-pyridazinone and 2-(4-chlorophenylthio)triethylamine caused the accumulation of the precursors phytoene and lycopene, respectively, in D. salina. High-performance liquid chromatography and infrared analysis showed that phytoene adopted the 15-cis and all-trans forms (as in higher plants), and that lycopene primarily adopted the all-trans form. This indicates that isomerization of -carotene takes place during or after cyclization.