Citrate, a possible precursor of astaxanthin in Phaffia rhodozyma: influence of varying levels of ammonium, phosphate and citrate in a chemically defined medium.

The influence of ammonium, phosphate and citrate on astaxanthin production by the yeast Phaffia rhodozyma was investigated. The astaxanthin content in cells and the final astaxanthin concentration increased upon reduction of ammonium from 61 mM to 12.9 mM (from 140 mug/g to 230 mug/g and 1.2 mug/ml to 2.3 mug/ml, respectively). Similarly, both the astaxanthin content and a staxanthin concentration increased by reducing phosphate from 4.8 mM to 0.6 5 mM (160 mug/g to 215 mug/g and 1.7 mug/ml to 2.4 mug/ml, respectively). L ow concentrations of ammonium or phosphate also increased the fatty acid co ntent in cells. By analogy with lipid synthesis in other oleaginous yeasts, an examination of the data for varying nitrogen and phosphate levels sugge sted that citrate could be the source of carbon for fatty acids and caroten oid synthesis. Supporting this possibility was the fact that supplementatio n of citrate in the medium at levels of 28 mM or higher notably increased t he final pigment concentration and pigment content in cells. Increased caro tenoid synthesis at low ammonium or phosphate levels, and stimulation by ci trate were both paralleled by decreased protein synthesis. This suggested t hat restriction of protein synthesis could play an important role in carote noid synthesis by P. rhodozyma.