Among the many known species of Dunaliella, only a few subspecies of D. salina Teod. have been shown to produce and accumulate large amounts of 0-carotene. In hypersaline lakes, which are generally low in available nitrogen and exposed to high solar irradiation, these p-carotene producing strains of Dunaliella predominate to a seasonal bloom of about 0.1 mg 0-carotene per litre. Under such stressful environmental conditions more than 12 per cent of the algal dry weight is 0-carotene, usually associated with a sharp decline in the thylakoids' chlorophyll. The p-carotene in
Dunaliella accumulates within distinctive oily globules in the interthylakoid spaces of the chloroplast periphery. Analysis of the globules showed that the 0-carotene of Dunaliella is composed mainly of two stereoisomers - all-trans and 9-cis - with the rest a few other mono-cis and di-cis stereoisomers of the p-carotene. Both the amount of the accumulated p-carotene and the 9-cis to all-trans ratio depend on light intensity and on the algal division time as affected by the growth conditions. Thus, any growth stress which will slow down the rate of cell division under light will in turn increase @-carotene production in Dunaliella. In fact, high light and many environmental stress conditions such as high salt, low temperature, extreme pH, nutrient deficiency, and others affect the content of p-carotene in Dunaliella. It was previously suggested that the equation of the amount of light absorbed by the cell during one division cycle integrates the effect of all growth variables on the content and isomeric ratio of p-carotene in Dunaliella (Ben-Amotz and Avron, 1990). The exceptions to this integration are nitrogen deficiency and low growth temperatures; both induce exceptionally extreme intracellular accumulation of pcarotene under any light intensity. Taking into account that nitrogen starvation inhibits chlorophyll production in algae as part of its inhibitory effect on protein biosynthesis, the prolonged nitrogen independent carotenoid biosynthesis will then protect the chlorophyll-low cells against the lethal damage of light. The effect of low temperatures on Dunaliella is analyzed by specific stimulation effect of chilling on the biosynthesis of 9-cis p-carotene. The physicochemical properties of 9-cis p-carotene differ from those of all-trans 0-carotene; all-trans p-carotene is practically insoluble in oil and is easily crystallized at low temperatures, while 9-cis p-carotene is much more soluble in hydrophobic-lipophilic solvents, very difficult to crystallize and generally oily in its concentrated form. To avoid cellular crystallization of all-trans P-carotene and to survive on growth at low temperatures, Dunaliella produces a higher ratio of 9-cis to all-trans p-carotene, where the 9-cis stereoisomer functions in vivo as an oily matrix to all-trans p-carotene (Ben-Amotz, 1996).