Effects of supplemental fulvic acid on survival, growth performance, digestive ability and immunity of large yellow croaker (Larimichthys crocea) larvae

Abstract

A 30-day feeding trial was designed to evaluate the effect of supplemental fulvic acid (FA) on survival, growth performance, digestive ability and immunity of large yellow croaker (Larimichthys crocea) larvae (initial body weight 11.33 ± 0.57 mg). Four isonitrogenous and isolipids diets containing 0.00%, 0.01%, 0.02% and 0.04% FA were formulated, respectively. Results showed that the supplementation of 0.04% FA significantly improved survival rate of large yellow croaker larvae. Meanwhile, supplemental FA significantly increased final body weight and specific growth rate. Based on the specific growth rate, the optimal supplementation was 0.0135% FA. Larvae fed the diet with 0.01% FA had significantly higher villus height than the control. The supplementation of 0.01%-0.02% FA significantly increased the muscular thickness of intestine. Moreover, supplementation of FA significantly increased mRNA expression of intestinal epithelial proliferation and barrier genes (pcna, zo-1 and zo-2). Diets supplemented with 0.02%-0.04% FA significantly increased the activity of trypsin in the intestinal segment, while 0.01%-0.02% FA significantly increased the activity of trypsin in the pancreatic segment. Compared with the control, supplementation of FA remarkably increased activities of alkaline phosphatase and leucine aminopeptidase in the brush border membrane of intestine. Larvae fed the diet with 0.01% FA significantly increased activities of lysozyme and total nitric oxide synthase. Furthermore, the supplementation of 0.01% to 0.02% FA significantly decreased the mRNA expression of pro-inflammatory cytokines (tnf-α and il-6). Concurrently, supplemental FA significantly increased anti-inflammatory cytokine (il-10) mRNA expression level. In conclusion, this study indicated that the supplementation of FA could improve the survival rate and growth performance of larvae by promoting intestinal development, digestive enzymes activities and innate immunity.

Keywords: fulvic acid; immune response; intestinal morphology; large yellow croaker; larval nutrition.

FIGURE 1

Relationship of specific growth rate (SGR, %/d) with supplemental FA levels of large yellow croaker larvae.

FIGURE 2

Effects of supplemental fulvic acid on intestinal morphology of large yellow croaker larvae. Control group (0.00%) (A); 0.01% FA group (B); 0.02% FA group (C); 0.04% FA group (D). MT, muscular thickness; VH, villus height; EH, enterocyte height (HE staining; Scale bar = 50 μm).

FIGURE 3

Effects of supplemental fulvic acid on intestinal cell proliferation-related and barrier-related genes mRNA expression in intestine of large yellow croaker larvae (mean ± S.E.M, n = 3). Pcna, proliferating cell nuclear antigen; odc, ornithine decarboxylase; zo-1, zonula occludens-1; zo-2, zonula occludens-2. Bars with different letters showed a significant difference (p < 0.05, Tukey’s test).

FIGURE 4

Effects of supplemental fulvic acid on LZM (A), TNOS (B) and iNOS (C) activities in the visceral mass of large yellow croaker larvae (mean ± S.E.M, n = 3). LZM, lysozyme; TNOS, total nitric oxide synthase; iNOS, inducible nitric oxide synthase. Bars with different letters showed a significant difference (p < 0.05, Tukey’s test).

FIGURE 5

Effects of supplemental fulvic acid on genes related to inflammation mRNA expression in liver of large yellow croaker larvae (mean ± S.E.M, n = 3). Ifn-γ, interferon γ; tnf-α, tumor necrosis factor α; il-1β, interleukin-1β; il-6, interleukin-6; il-10, interleukin-10. Bars with different letters showed a significant difference (p < 0.05, Tukey’s test).