Dietary Sheep Blood Zymolysate Modulates Gut Barrier Integrity, Microbiota Composition, and Nutritional Metabolism in Common Carp (Cyprinus carpio).

Sustainable protein sources are pivotal to the green development of aquaculture. As an underutilized abattoir byproduct, sheep blood holds great potential for valorization via enzymatic hydrolysis. This study evaluated the impacts of dietary sheep blood zymolysate (SBZ) supplementation (0.3%, 0.6%, and 1.0% dosages, denoted as TL, TM, and TH groups, respectively) on growth performance, serum biochemistry, intestinal barrier function, and gut microbiota of common carp (Cyprinus carpio). A total of 240 carp (initial weight: 48.0 ± 0.2 g) were randomly assigned to four groups (three replicates/group and 20 fish/replicate) with a basal diet as control (CON) for an 8-week feeding trial. Results demonstrated that TL and TM groups exhibited significantly higher average daily gain (ADG), weight gain rate (WGR), and specific growth rate (SGR) than the CON group (p  < 0.01). The TM group (0.6% SBZ) showed the most prominent benefits: It had elevated whole-body crude protein content (p  = 0.037), serum albumin (ALB) and total protein (TP) levels (p  < 0.0001 and p = 0.028, respectively), and intestinal villus height (VH) (p  < 0.0001), along with upregulated expression of intestinal tight junction genes (ZO-1, Occludin, and Claudin-1) and nutrient transporter genes (excitatory amino acid transporter [EAAT], cationic amino acid transporter [CAT], and peptide transporter 1 [PepT1]) (p  < 0.01). Additionally, TH group (1.0% SBZ) increased muscle histidine (His) content, while TM and TH groups enhanced muscle arginine (Arg) levels. At the microbial level, SBZ supplementation significantly enriched the relative abundance of phylum Pseudomonadota and genus Gemmobacter and reshaped the gut microbial community structure. Overall, dietary SBZ supplementation improves carp growth, body protein deposition, intestinal morphology, and barrier integrity and modulates gut microbiota, with 0.6% SBZ being the optimal dosage for integrated performance. This study provides empirical support for the high-value utilization of sheep blood byproducts in sustainable aquafeed formulation.