Evaluation of protein requirement of spotted seabass (Lateolabrax maculatus) under two temperatures, and the liver transcriptome response to thermal stress

This study investigated the interacting impacts of dietary protein level and water temperature on growth of spotted seabass (Lateolabrax maculatus). Six diets were formulated to contain 35, 38, 41, 44, 47 or 50% crude protein, and each diet was fed to two different groups of fish (2.0 +/- 0.01 g) kept at a low temperature (27 degrees C; LT) or high temperature (33 degrees C; HT). Quadruplicate groups of fish were fed the test diets to apparent satiation three times daily for 8 weeks. The results demonstrated that growth and feed utilization were significantly (P < 0.05) influenced by both dietary protein level and temperature but there was no interaction effect (P > 0.05). The highest weight gain was recorded in the P47LT group which was significantly higher than those of P35LT, P38LT, P41LT and all HT groups. A second-order polynomial regression analysis based on weight gain showed that the protein requirements of spotted seabass reared at 27 degrees C and 33 degrees C are 46.8 and 44.9%, respectively. Moreover, feed conversion ratio decreased by increasing the protein level up to 47% at both temperatures. Intestinal protease activity was enhanced by increasing the dietary protein level at both temperatures, and HT groups exhibited higher activity than their respective LT groups. Hepatic citrate synthase and succinate dehydrogenase activities were significantly influenced by protein level and water temperature and their highest value was detected in the P47LT group. Higher water temperature led to a significant enhancement of serum triglyceride concentration and alkaline phosphatase activity. Also, we assessed the hepatic transcriptome and compared the genes associated with metabolism of fish reared at LT or HT. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that most of the DEGs were involved in the metabolism processes of amino acids, fatty acids and glucose and some signaling pathways. The transcriptomic results indicated that high temperature could induce changes in major nutrient metabolism and heat stress in spotted seabass. In summary, the minimum dietary protein requirements of spotted seabass are different when reared at two temperatures and this may be attributed to alterations of liver metabolism.