Carbonate alkalinity and dietary protein levels affected growth performance, intestinal immune responses and intestinal microflora in Songpu mirror carp (Cyprinus carpio Songpu)

An eight week 3 x 2 two-factorial experiment was conducted to investigate the interaction between carbonate alkalinity levels at 0 and 15 mmol/L, and dietary protein levels at 310, 340 and 370 g/kg on the growth rate, digestive ability and the intestinal immunity, physical barrier function and microflora of Songpu mirror carp (Cyprinus carpio songpu). Fish at an initial average weight of 17.86 +/- 0.19 g were randomly allotted to 18 aquariums each with 30 fish. Results revealed that weight gain rate (WGR) significantly decreased as carbonate alkalinity levels increased (p < 0.05), whereas the feed conversion ratio (FCR) of fish significantly increased (p < 0.05). Amylase activity in the intestine significantly decreased with increasing carbonate alkalinity levels (p < 0.05). A carbonate alkalinity of 15 mmol/L significantly reduced the superoxide dismutase (SOD) and total antioxidant capacity (T-AOC) activities, elevated the malondialdehyde (MDA) contents in the intestine and hepatopancreas and decreased the contents of Complement 3 (C3), Complement (C4) and Ig M (p < 0.05). The relative expressions of occludin, claudin3, claudin7, myosin light chain kinase (MLCK) and interleukin 10 (IL-10) in the intestine were significantly down-regulated with increasing carbonate alkalinity levels (p < 0.05), whereas the relative expressions of interleukin 6 (IL-6), interleukin 8 (IL-8) and tumor necrosis factor (TNF-alpha) in the intestine were significantly up-regulated (p < 0.05). The Proteobacteria abundance significantly increased with increasing carbonate alkalinity level (p < 0.05). The WGR significantly decreased as dietary protein levels increased (p < 0.05). The MDA contents in intestine and hepatopancreas also showed a significant rise with increasing dietary protein levels (p < 0.05). Increments in dietary protein levels significantly reduced the C3 and C4 contents of fish intestine (p < 0.05). Fish in the 370 g/kg diet protein level group showed a significantly lower relative expression of occludin and TNF-alpha (p < 0.05). There was an effect on the C3 content, the relative expression of occludin, the abundance of Firmicutes and Firmicutes: Bacteroidetes ratio between carbonate alkalinity and dietary proteins over time (p < 0.05). In summary, under the long-term carbonate alkalinity stress of 15 mmol/L, dietary protein level for Songpu mirror carp should be controlled below 310 g/kg, which could effectively weaken the oxidative damage produced by carbonate alkalinity stress and further strengthen the growth and intestinal immune response of fish.