Supplementing Sulfate-Based Alginate Polysaccharide Improves Pacific White Shrimp (Litopenaeus vannamei) Fed Fishmeal Replacement with Cottonseed Protein Concentrate: Effects on Growth, Intestinal Health, and Disease Resistance

Sulfate-based alginate polysaccharide (SAP) is a novelty marine prebiotic. To investigate the beneficial effects of SAP in Pacific white shrimp (Litopenaeus vannamei) fed low-fishmeal diets, six diets (whole fishmeal group: FM; fishmeal replacement with cottonseed protein concentrate and SAP supplementary groups: SAP0, SAP1, SAP2, SAP3, and SAP4) were formulated and fed shrimp for 56 days. The results showed that SAP2 and SAP3 groups reached the best weight gain and specific growth rate ( $P<0.05$ ). In serum, the activities of lysozyme and acid phosphatase showed the trend of firstly increased and then decreased ( $P<0.05$ ). In the gut, the highest activities of trypsin, lipase, and amylase were found in SAP2 and SAP3 groups ( $P<0.05$ ); the histological indexes gradually improved with SAP levels increased ( $P<0.05$ ); sequencing of microbiota revealed that the composition and structure of microbiota have been improved, especially in the decreasing abundance of Vibrio, Pseudoalteromonas, and Candidatus Bacilloplasma at genus level. Besides, transcriptomics revealed a degree of similarity in differential gene expression patterns in shrimp; the comparison of RNA-Seq and qRT-PCR verified that SAP improved antioxidant and immunity in shrimp. The challenge revealed that SAP strengthened the resistance against Vibrio parahaemolyticus. Totally, the supplementary SAP to low-fishmeal diets improved growth, intestinal health, and disease resistance in shrimp. Based on the polynomial curve analysis of specific growth rate among SAPs groups, the optimum SAP supplementary level was 1.91%.