Sterol biosynthesis and phytosterol bioconversion in Crassostrea gigas larvae: new evidence from mass-balance feeding studies.

Dietary sterols are important for bivalve larval growth and survival. The aim of the study was to determine quantitatively sterol incorporation and synthesis in Pacific oyster Crassostrea gigas larvae by means of a mass-balance approach. The flow-through larval rearing technique allowed accurate quantification of microalgal ingestion and consequently of sterol ingestion. Sterol incorporation was calculated using the sterol composition of the larvae between 2 sample points. Two sets of experiments were done using two cultured microalgae: Tisochrysis lutea (T) and Chaetoceros neogracile (Cg) as mono- and bi-specific diets (TCg). Accumulation of tissue sterols in oyster larvae, in addition to those present in the diet, indicate that C. gigas larvae appear to have the ability to synthetize sterols de novo under low dietary sterol supply, e.g., when fed the T diet. Sterol synthesis was dependent upon sterol dietary supply; larvae fed T exhibited greater sterol incorporation at the pediveliger stage than larvae fed TCg. Larval sterol compositions under the different dietary regimes indicate likely bioconversion pathways modifying dietary sterols. Larvae fed T bioconverted dietary brassicasterol mainly to cholesterol via a 22-dehydrocholesterol intermediate. Brassicasterol was also actively synthetized in larvae fed T and TCg, suggesting a possible metabolic role of this sterol in C. gigas larvae. Apparent desmosterol synthesis under all experimental conditions suggests a role as a membrane component or as an intermediate in cholesterol synthesis. Our data also indicate that C. gigas larvae require approximately 13 ng cholesterol larvae^-1 to achieve competence for metamorphosis. This mass-balance approach will allow the determination of other biochemical requirements in larval nutrition.