A Probiotic Fermented Ulva fasciata Biological Extract Feed Supplement Improves Litopenaeus vannamei's Growth Performance, Gut Bacteria, and Immunity-Related Gene Expression.

The current study describes the application of the marine probiotic Enterococcus durans E23 for fermenting and biologically extracting Ulva fasciata. The prepared extract (RE) exhibited promising antibacterial activity against Pseudomonas fluorescens, Enterococcus faecalis, Klebsiella pneumoniae, E. coli, Bacillus sp., and Staphylococcus aureus (inhibition zone diameters 10-21 mm). Total phenolics, flavonoids, and GC-MS analysis revealed that RE contains phenolics, flavonoids, vitamins, and 35 phytochemical compounds. The efficacy of UE23 (RE + E. durans 23) as a feed supplement for Litopenaeus vannamei was assessed. The experiment was conducted for 12 weeks in fiberglass tanks (200 L). A total of 300 post-larval (PL) shrimp, weighing ~ 0.20 ± 0.01 g, were employed. Four experimental groups were allocated to 12 tanks (25 PL/tank). The control group (C) was fed on a commercial diet, while T1, T2, and T3 included adding UE23 at 1, 2, and 3 g kg^-1 concentrations, respectively. Applying UE23 significantly (p < 0.05) enhanced L. vannamei growth performance and immunity gene expression. T2 recorded the highest weight gain and specific growth rate, while the highest survival rate and lowest feed conversion ratio were reported by T3. UE23 successfully elevated the number of lactic acid bacteria in the shrimp gut from 20 in the C group to > 300 in the T3 treatment, and none of the tested pathogens were detected. Further, the expression of the transglutaminase gene was upregulated 14.5-fold compared to the control in the T3 treatment. In conclusion, U. fasciata biological extraction by E. durans E23 is a promising approach in obtaining an innovative shrimp feed additive with substantial bioactivities and nutritive potential.