Bioflocs attenuate Mn-induced bioaccumulation, immunotoxic and oxidative stress via inhibiting GR-NF-κB signalling pathway in Channa asiatica.

Manganese (Mn) is a relatively common element in aquatic ecosystems and can be bio-concentration, but the mechanism of manganese poisoning on fish health is unclear. Here, this study's objective was to evaluate the potential mechanisms of bioflocs in ameliorating Mn-induced toxicity in Channa asiatica. Three hundred sixty juveniles were randomly divided into 12 tanks. Four C:N ratios in triplicate tanks were tried: C/N = 7.6:1 with a commercial diet (control), C/N 10:1, C/N 15:1 and C/N 20:1, and the bio-accumulation, immunotoxic, oxidative stress, GR-NF-κB related genes expression and intestinal histomorphology were assessed in three different periods after Mn exposure (0 h, 48 h and 96 h). The results showed that bioflocs had a significant protective effect on Mn poisoning by preventing alterations in bio-accumulation levels, LSZ, AKP, C3, C4 and IgM, of which the C/N 15:1 group had the best relief effect. Furthermore, bioflocs also assisted in the recovery of liver T-SOD, CAT, GPX and T-AOC levels while decreasing the content of MDA. Moreover, C/N 15:1 group significantly down-regulated the expression level of NF-κB, TNF-α, IL-1β and IL-8 and up-regulated significantly IκBα, GR, HSP70 and HSP90 expression levels considerably (P < 0.05). From the intestinal section, the C/N 15:1 group resistance was the best one, and there was no difference between C/N 20:1 group and control group. These results revealed that administration of bioflocs (C/N 15:1) has the potential to combat Mn toxicity in C. asiatica, and the specific pathway may be GR-NF-κB.