Dietary sulfate-based alginate polysaccharide supplementation boosts growth, gut health, immune response and disease resistance in juvenile hybrid grouper (? Epinephelus fuscoguttatus x ? E. lanceolatus) fed cottonseed protein concentrate-based diets

To study the dietary prebiotic role of Sulfate-Based alginate polysaccharide (SAP) in cottonseed protein concentrate-based diets, six iso-nitrogenous and iso-lipid feeds were prepared to feed grouper, including a fishmeal control group (FM) was set at 50% fishmeal level, on this basis 5 groups replacing 50% fishmeal protein with cottonseed protein concentrate and supplementing sulfate-based alginate polysaccharide 0%, 1%, 2%, 3% and 4% (SAP0, SAP1, SAP2, SAP3 and SAP4 groups). The healthy and vigorous grouper (initial weight = 8.63 +/- 0.01 g) were selected and randomly divided into 6 treatment groups in triplicates (30 fish in each repetition). The feeding experiment was conducted for 8 weeks, followed by a one-week Vibrio harveyi challenge. The experi-mental results showed that the weight gain and specific growth rate reached the significant maximums in SAP2 and SAP3 groups (P < 0.05). The malondialdehyde and triglyceride levels in serum decreased, while the anti-oxidant indicators and non-specific immune enzymes activity (lysozyme, acid phosphatase, alkaline phospha-tase) in serum and intestinal tissue got increased (P < 0.05). The intestinal digestive enzymes activity and histomorphology indices (plica height, plica width and muscle layer thickness) were increased (P < 0.05). Mi-crobial sequencing revealed that the ACE and Chao1 indices increase firstly and then decreased, while the Shannon and Simpson indices showed a decreasing trend (P < 0.05). The expression level of pro-inflammatory factors (IL-1 beta and TNF-alpha) was down-regulated, yet the expression of anti-inflammatory factors (IL-10) was up-regulated in intestinal tissue (P < 0.05). The cumulative mortality of Vibrio harveyi challenge was significantly reduced (P < 0.05). In one word, the adverse effects on growth, immunity and intestinal health of grouper feeding cottonseed protein concentrate-based diets were repaired by the supplementing sulfate-based alginate polysaccharide. According to the binary regression analysis of specific growth rate among SAPs treatments, the optimum level of supplementing sulfate-based alginate polysaccharide of grouper was 2.25%.