Compositional and metabolic responses of European seabass (Dicentrarchus labrax) to a dietary gradient of long-chain (≥ C20) polyunsaturated fatty acids

Decrease in the inclusion of fish oil (FO) in current aquafeed formulations reduces the availability of omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA) to marine fish like the European seabass Dicentrarchus labrax. Here we investigated the compositional and physiological responses of European seabass to a dietary gradient of n-3 LC-PUFA. Six isoenergetic and isonitrogenous diets (D1-D6) were fed to triplicate groups of juvenile seabass (23 g), containing blends of FO and vegetable oil (VO) that resulted in a gradient of n-3 LC-PUFA varying from 0.23 to 3.40% of diet as fed. Sampling was carried out after four months feeding and biochemical composition, as well as lipid metabolic gene expression in liver and mid-intestine analysed. The only effect of diet on proximate composition was a decrease in hepatic lipid content as dietary n-3 LC-PUFA increase whereas, in contrast, fatty acid (FA) compositions of both tissues were substantially modified by diet. Concentrations of LC-PUFA biosynthesis intermediates decreased in both tissues as dietary n-3 LC-PUFA increased with some preference detected towards n-6 FA. Two genes, namely fads2 and igf1, were moderately influenced by dietary VO in liver while genes involved in lipogenesis (srebp1, srebp2 and fas) and LC-PUFA biosynthesis (fads2 and elovl5) were markedly down-regulated, in mid-intestine of seabass in response to increasing dietary n-3 LC-PUFA. The expression of genes of β-oxidation (pparα, cpt1α and fabp1) did not vary among treatment. Collectively, our data suggest that the nutritional regulation of key lipid metabolic genes in European seabass was more pronounced in mid-intestine than in liver. Overall, when compared to a previous study of similar design in gilthead seabream (Sparus aurata), the European seabass seems less sensitive to a dietary gradient of vegetable oil.