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.

Automated summary

The experiment investigated the interaction between carbonate alkalinity levels and dietary protein levels on the growth, digestive ability, intestinal immunity, physical barrier function, and microflora of Songpu mirror carp. Results showed that higher carbonate alkalinity levels led to reduced weight gain rate and increased feed conversion ratio, while higher dietary protein levels also had negative effects. The study suggests that maintaining dietary protein levels below 310 g/kg under carbonate alkalinity stress of 15 mmol/L can effectively enhance the growth and intestinal immune response of fish.