THE BIOTECHNOLOGY OF CULTIVATING DUNALIELLA RICH IN BETA CAROTENE: FROM BASIC RESEARCH TO INDUSTRIAL PRODUCTION

Mordhay Avron

Biochemistry Department

Wiezmann Institute of Science

Rehovot, Israel

The unicellular alga Dunaliella exists in several ill-defined species. In hypersaline lakes, the Dunaliella strain which predominates is often red, rather than green, in color, due to massive accumulation of a single pigment, beta-carotene. Of the many strains of the genus Dunaliella described, only two strains, Dunaliella bardawil and Dunaliella salina Teod., have been shown to possess the capacity to produce large amounts of beta-carotene, when cultivated under appropriate conditions[1]. Electron micrographs of Dunaliella bardawil indicate that the massive amounts of beta-carotene are located in a large number of chloroplastic, lipoidal globules located in the interthylakoid space of the chloroplast. Under appropriate cultivation, more than 10% of the dry weight of D. bardawil is accounted for by beta-carotene.

The rate and extent of beta-carotene accumulation in D. bardawil is determined by the conditions under which it is cultivated. Under standard laboratory cultivation conditions, little beta-carotene is synthesized and the algae appear green in color. However, when the light intensity is increased much beyond the intensity required for normal growth, and when the rate of growth is limited, beta-carotene is accumulated to the highest levels.

Beta-Carotene accumulation protects against the deleterious effects of high-intensity irradiation. Thus, strains which do not accumulate beta-carotene, or beta-carotene-poor D. bardawil die when exposed to high irradiation under limiting growth conditions, while beta-carotene-rich D. bardawil survive under the same conditions[2]. This may explain the previously mentioned predominance of D. bardawil or D. salina Teod. over green strains of Dunaliella in some saline lakes in nature, where low concentrations of algae are exposed to high solar irradiation under nutrient-limiting conditions.

The ability of selected strains of Dunaliella to produce beta-carotene by photosynthetic conversion of CO2 stimulated research aimed at probing the possible industrial production of this commercially valuable product[3]. Since these algae grow at high salinity in a complete inorganic medium, few, if any, natural competitors and predators interfere with large-scale cultivation. Furthermore, the lack of a cell wall makes this alga, in contrast with most, easily digestible by most animal species.

Indeed, large-scale cultivation of Dunaliella is carried out today in Israel, in the United States, and in Australia for the commercial production of an algal meal rich in beta-carotene, purified beta-carotene, and other natural products.

Beta-carotene has been used both as a food coloring agent and as a source of vitamin A in animal feed. Adding dry Dunaliella or an extract of the algae high in beta-carotene to a vitamin A deficient chick diet indeed showed that it is an excellent source of the vitamin and, in addition, a yolk-color enhancing agent[4].

Recent epidemiological and oncological studies suggest that normal to high levels of beta-carotene in the body may protect against cancer development in humans. The studies provide evidence that people with above average beta-carotene intakes have a lower incidence of several types of cancer[57].

Natural beta-carotene, as found in D. bardawil and in many fruits and vegetables, contains about 50% of all-trans beta-carotene, with the rest



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