Effects of fulvic acid on zebrafish (Danio rerio) growth, immunity and antioxidant status

This experiment aimed to determine the efficacy of fulvic acid (FLA) on growth performance, innate immune system, antioxidant parameters, and expression of immune and antioxidant-related genes in zebrafish (Danio rerio). To this end, 12 tanks (3 per group), each containing 50 zebrafish (with an average weight of 85.7 ± 10.05 mg) in 72 L, were assigned to diets containing FLA at four levels: 0 (control), 0.25 (FLA1), 0.5 (FLA2), and 1 (FLA3) g/kg diet. Following an eight-week culture period, no significant differences in growth performance were observed among the treatment groups (P > 0.05). However, lysozyme activity, total immunoglobulin (Ig), and total protein concentrations in whole-body extracts were significantly enhanced in the 0.5-1 g FLA/kg diet groups compared to the other treatments (P < 0.05). No significant differences were observed among the groups in catalase (CAT), glutathione peroxidase (GPx), or superoxide dismutase (SOD) activities (P > 0.05). The supplementation of FLA significantly upregulated the gene expression of interferon-α (IFN-α) and tumor necrosis factor-alpha (TNF-α), with the highest expression observed in the 0.5 g FLA/kg diet group (P < 0.05). Additionally, interleukin 1 (IL-1) expression was markedly elevated in this group in comparison to the other treatments (P < 0.05). While there was a significant increase in GPx gene expression with dietary FLA (P < 0.05), no notable differences were observed among FLA treatments (P > 0.05). CAT gene expression remained consistent across all groups (P > 0.05). In contrast, SOD gene expression significantly increased in response to all FLA-supplemented diets, with the highest level observed in the 0.5 g FLA/kg group (P < 0.05). These findings suggest that FLA may serve as an effective dietary supplement to enhance the immune response and antioxidant capacity in zebrafish.