Effects of low-protein diets with varying carbohydrate-to-lipid ratios on growth performance, antioxidant activity of Litopenaeus vannamei, and microbiome in biofloc systems

A 53-day experiment investigated the impact of varying dietary carbohydrate-to-lipid (C/L) ratios in biofloc technology (BFT) systems on the growth, feed efficiency, enzyme activity, and metabolism of Pacific white shrimp Litopenaeus vannamei, while also analyzing the microbial community structure and function in the biofloc. Five isonitrogenous and isoenergetic diets (C/L 1.0, 4.1, 6.2, 10.5 and 14.2, 29 % crude protein) were formulated, along with a control diet (35 % crude protein, 7 % lipid). In the groups with dietary C/L ratio of 4.1 and 6.2 group, shrimp showed improved nitrogen retention (NR), energy retention (ER), and higher trypsin activities, compared to the control group. As the C/L ratio increased from 1.0 to 10.5, haemolymph glucose levels increased significantly, while triglyceride and total cholesterol levels were highest at a C/L ratio of 4.1, subsequently decreasing significantly with further increases in the C/L ratio. Antioxidant enzyme activities (CAT and SOD) were elevated in the BFT groups, with the lowest malondialdehyde (MDA) levels observed at C/L ratios of 4.1 and 6.2. Pyruvate kinase (PK) and phosphofructokinase (PFK) activities increased with increasing C/L ratio, while phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) activities decreased. There were no significant differences in the microbial composition, function, or nutrient content of the bioflocs across BFT groups, all of which maintained water quality and provided supplemental nutrition for shrimp. Based on quadratic fitting of weight gain (WG) and specific growth rate (SGR), the optimal dietary C/L ratio for L. vannamei in biofloc systems ranged from 6.44 to 6.52. This study provides theoretical guidance for formulating low-protein and low-cost feeds for farming Litopenaeus vannamei in biofloc systems.