Perkinsus marinus, a protozoan parasite of the eastern oyster, has a requirement for dietary sterols

Perkinsus marinus, a protozoan parasite of the eastern oyster, Crassostrea virginica, causes high mortality in its host along the Atlantic and Gulf coasts of North America. P. marinus meronts cultured in vitro in medium containing complete lipid supplement (cod liver oil, cholesterol and α tocopherol acetate in detergent) are able to synthesize a wide variety of lipids, yet cultures cannot be maintained in lipid-free medium. To determine P. marinus lipid requirements meronts were inoculated into media containing different combinations of lipid components in detergent. Treatments included complete lipid supplement (positive control), detergent only (negative control), cholesterol in detergent, α tocopherol acetate in detergent and cholesterol + α tocopherol acetate in detergent. Meronts proliferated in the positive control medium and media containing cholesterol or cholesterol + α tocopherol acetate, but failed to proliferate in the negative control medium and the medium containing just α tocopherol acetate. Gas chromatography analysis of P. marinus meronts grown in medium with added 13C sodium acetate (0.5 mg mL− 1) revealed the presence of fatty acids containing 13C, but the only sterol present was cholesterol containing no 13C. These results suggest that P. marinus cannot synthesize sterols and must sequester them from its host.