Evaluation of the Optimum Dietary Arachidonic Acid Level and Its Essentiality for Black Seabream (Acanthopagrus schlegelii): Based on Growth and Lipid Metabolism

The aim of this study was to investigate how dietary arachidonic acid (ARA) level affects growth performance and lipid metabolism in juvenile black seabream (Acanthopagrus schlegelii). A feeding trial was conducted for 8 weeks, during which the fish (0.99 ± 0.10 g) were fed six isonitrogenous and isolipidic diets with varying ARA levels of 0.1%, 0.59%, 1.04%, 1.42%, 1.94%, and 2.42%. Fish fed the diet with 1.42% ARA had significantly higher weight gain (WG) and specific growth rate (SGR) than the other groups (p < 0.05), except for the ARA1.04. As the ARA level increased, the liver and muscle effectively accumulated n−6 polyunsaturated fatty acids (n−6 PUFAs; p < 0.05). However, eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and n−3 PUFA contents of liver and muscle significantly decreased by increasing dietary ARA level (p < 0.05). Results of liver histology showed dramatically increased vacuolar fat droplets leading to hepatic fat pathological changes in fish fed diets with ARA levels of 1.94% and 2.42% (p < 0.05). Serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities increased with increasing dietary ARA level which was accompanied with elevated liver lipid content (p < 0.05). Consistently, triglyceride (TG) and nonesterified fatty acid (NEFA) concentrations of serum and liver, and serum cholesterol (CHO) concentration increased (p < 0.05). As the level of dietary ARA increased, the indicators of lipid metabolism such as sirtuin 1 (sirt1) and peroxisome proliferator-activated receptor α (pparα) also increased (p < 0.05). However, after reaching their peak in ARA1.04 group, the level of these indicators declined (p < 0.05). The same trend was observed for the expression of genes related to the downstream pathways. While the mRNA levels of sterol regulatory element–binding protein-1 (srebp-1) and its downstream genes were markedly increased with the increase of dietary ARA level (p < 0.05). In conclusion, these data suggested that the optimum dietary ARA requirement of A. schlegelii is 1.03% of diet based on the WG. The study revealed that a diet containing 1.04% ARA can activate the expression levels of sirt1 and pparα leading to promoted lipolysis. However, dietary ARA levels of ≥1.42% induced lipid accumulation in the liver, as they suppressed the mRNA levels of sirt1 and pparα, while elevating the expression level of genes related to lipogenesis.