Effects of dietary protein levels on growth performance, serum indexes, PI3K/AKT/mTOR/S6K signalling and intestinal microbiota of abalone Haliotis discus hannai

A 120-day feeding trial was conducted to assess the effect of dietary protein levels on growth performance, serum indexes, PI3 K/AKT/mTOR/S6 K signalling pathway and intestinal microbiota of abalone Haliotis discus hannai. Abalones (initial body weight: 15.88-16.54 g; initial shell length 41.70-42.40 mm) were fed nine isoenergetic (similar to 16.08 kJ/g) and isolipidic (similar to 42.0 g/kg) experimental diets with 93.5, 116.9, 151.1, 191.8, 230.3, 275.9, 320.3, 360.5 and 405.9 g/kg protein contents, respectively. Results showed that the optimal dietary protein level for H. discus hannai was determined as 259.4 g/kg through second-order polynomial regression analysis for weight gain rate (WGR). Serum TP and urea nitrogen levels were significantly increased when dietary protein levels were higher than 320.3 g/kg (p < .05). The highest gene expressions of phosphatidylinositol 3 kinase (PI3 K), target of rapamycin (mTOR) and ribosomal protein S6 kinase (S6 K) in muscle were found in the treatment with 275.9 g/kg of dietary protein. The excessive dietary protein level (405.9 g/kg) significantly decreased the gene expression of S6 K. Meanwhile, abalone fed with 275.9 g/kg of dietary protein exhibited the highest microbial diversity in intestine as measured by Chao richness and Shannon diversity index. In conclusion, dietary protein level reduced from 259.4 g/kg to 154.0 g/kg or increased from 259.4 to 323.0 g/kg, the WGR of abalone might be depressed with 5% likelihood. Over high (>360.5 g/kg) or low (<151.1 g/kg) dietary protein levels had negative effects on growth performance, serum indexes, PI3 K/AKT/mTOR/S6 K signalling and microbial diversity in intestine of abalone.