ASTAXANTHIN FROM MICROBIAL SOURCES

Salmon farming increased substantially in the 1980s, which created a large market for astaxanthin, the principal pigment of salmon. Carotenoids are the most widely distributed class of pigments in nature and have essential biological functions in animals. They also impart attractive pigmentation to many farmed animals. Since animals lack the ability to synthesize carotenoids, the pigments must be supplemented to feeds, usually at considerable expense to the farmer (10 to 15% of total feed costs). Astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione) is the principal carotenoid pigment of salmonids and gives attractive pigmentation in the eggs, flesh, and skin. Worldwide production of farm-raised salmon increased rapidly in the past decade, and more than 200,000 T were raised in 1990. Currently, chemically synthesized astaxanthin and canthaxanthin (beta,beta-carotene-4,4'-dione) are added to salmonid feeds as pigmenters, but there is considerable interest within the aquaculture industry in using natural sources of astaxanthin. The principal biological sources being considered are crustacea and crustacean extracts, the green microalga Haematococcus, and the yeast Phaffia rhodozyma. Each natural pigment source has its limitations and they currently cannot compete economically with the synthetic additive. Crustacean meals have relatively low contents of astaxanthin and high levels of moisture, ash, and chitin. The alga Haematococcus has a high concentration of astaxanthin (0.2 to 2%), but industrial application is limited by the lengthy autotrophic cultivation in open freshwater ponds and the requirement for disruption of the cell wall to liberate the carotenoid. The yeast P. rhodozyma has desirable properties as a biological source of pigment, including rapid heterotrophic metabolism and production of high cell densities in fermentors, but its content of astaxanthin in wild strains is only 200 to 300-mu-g/g yeast (0.02 to 0.03%). Mutants have been isolated that produce > 3000-mu-g total carotenoid per gram of yeast (> 0.30%) in shake flasks after 5 d growth, and measurement of carotenoid fluorescence in individual cells indicates that levels of 10,000 to 15,000-mu-g/g can be obtained. High-producers, however, are often unstable and further strain development is required. In this article, biological sources of astaxanthin are critically evaluated and compared with the synthetic compound presently being used in animal feeds.