Alginate Oligosaccharide: A Promising Functional Additive for Growth, Intestine Function, Immunity, Antioxidation and Apoptosis Modulation in Largemouth Bass (Micropterus salmoides)

A 56-day feeding trial was designed to investigate the effects of alginate oligosaccharide (AOS) on the growth, immune response, antioxidant activity and apoptosis pathways of largemouth bass (Micropterus salmoides). We formulated six isonitrogenous and isoenergetic diets with different concentrations of AOS (0% (control), 0.05%, 0.1%, 0.15%, 0.2% and 0.25%). The results showed that the WGR of the AOS0.15–0.2 groups were markedly increased, and the FBW and SGR of the AOS0.1–0.2 groups were also markedly boosted. In addition, no significant differences were observed in FCR, SR and FI in the treatment groups. According to SGR and WG second-degree polynomial regression analysis, the optimum AOS addition level for juvenile largemouth bass was 0.14–0.15%. On the other hand, no notable differences were observed in crude protein, moisture, crude lipid or crude ash content between groups, and no notable differences were also observed in the levels of AST and ALT in the plasma between all groups. Additionally, ALP activities were considerably higher in the AOS0.15–0.25 groups. In terms of intestinal digestion and absorption function, AOS0.1 group and AOS0.15 group significantly increased the intestinal amylase and lipase activities, and A0S0.1–0.2 groups significantly increased the intestinal trypsin activities, while proper dietary supplementation with AOS significantly improved villus muscular thickness, villus height, and villus width. Furthermore, proper dietary supplementation with AOS significantly up-regulated the mRNA levels of occ, clau, C6A6, C7A5, C7A8B, C6A14 and pept1 in the intestine. No significant differences were observed in the mRNA levels of C7A6, C7A1A and C7A10A between all groups. With respect to the antioxidant and immune functions of the intestine, the analysis revealed no remarkable differences between the groups concerning SOD, GPX activity or T-AOC content in the intestine. However, a significant increase in CAT activity of the intestine was observed in the AOS0.05–0.15 groups, and MDA levels were lower in all AOS-added groups. Apart from the above, AOS0.15–0.25 groups significantly reduced intestinal TNF-α concentration. No notable differences were observed in the intestinal contents of TGF-β, IL-10 and IL-6 between all groups. Additionally, proper dietary supplementation with AOS could improve antioxidant effects and inhibit inflammation by regulating the gene expressions of the related-Nrf2 and NF-κB signaling pathway, including nrf2, keap1, Mn-sod, gpx, nf-κb, il-10 and tgf-β. There was no significant difference in the mRNA levels of cat, fox, il-8 and tnf-α. With respect to cell apoptosis in the intestine, TUNEL assay results showed that green positive cells were significantly lower in the AOS0.05–0.2 groups than the AOS0 group. Additionally, proper dietary supplementation with AOS could inhibit cell apoptosis by regulating the mRNA levels of bxl-xl, caspase 3, caspase 8, caspase 9 and bcl-2. However, there was no significant effect on the level of bax mRNA in any of the treatment groups. In summary, proper dietary supplementation with AOS exerted positive effects on growth, intestinal digestion and absorption function, immune antioxidant responses, and apoptosis pathways to a certain extent.