An investigation of the mechanisms for sterol synthesis and dietary sterol bioconversion in the heterotrophic protists Oxyrrhis marina and Gyrodinium dominans

The ability of the marine heterotrophic protists Oxyrrhis marina and Gyrodinium dominans to synthesize sterols de novo and modify dietary sterols was investigated using 13C-labeled substrates. De novo sterol synthesis of O. marina was determined by incorporation of 13C acetate into the culture medium. For G. dominans which has low tolerance of acetate, a protozoan prey Perkinsus marinus that cannot synthesize sterols, was cultured with 13C acetate then fed to G. dominans. Both heterotrophs utilized dietary 13C to synthesize fatty acids de novo, but not sterols. The ability of O. marina and G. dominans to alkylate, saturate, and desaturate dietary sterols was tested using P. marinus incorporated with 13C-labeled cholesterol as prey. O. marina did not modify the dietary 13C-cholesterol, but G. dominans produced 5 labeled sterols (brassicasterol, C28:1, and unknown C28, C29 and C30 sterols) indicating that G. dominans has the ability to desaturate and alkylate dietary cholesterol. The ability of O. marina and G. dominans to dealkylate dietary sterols was tested by feeding them gelatin acacia microspheres (GAMs) containing 13C-labeled brassicasterol. Neither heterotroph dealkylated brassicasterol to make cholesterol, but G. dominans alkylated and saturated brassicasterol to make 2 sterols (C29:1 and C30:0). The lack of dealkylation of brassicasterol by both protist species suggests problems with the substrate and/or delivery system since previous studies suggest that dealkylation of brassicasterol occurs when either species is fed algae containing this sterol.