The intensification of aquaculture, coupled with climate change, heightens infection risks in fish farms. Probiotics are a promising strategy to mitigate stressors, enhance fish health, and reduce environmental impact. In this study, juvenile seabass supplemented for two months with a synbiotic of marine probiotics bio-encapsulated in algae showed modulation of their intestinal microbiota and improved survival to Vibrio anguillarum (+18%). Gene expression in spleen was analyzed by RNA-Seq to (1) study the disease response in non-supplemented seabass and (2) identify mechanisms potentially involved in survival gain induced by synbiotic supplementation.

After 24 hours of infection, 4,173 Differentially Expressed Genes (DEGs) were identified in non-supplemented fish (control) compared to J0. Gene Ontology (GO) annotation revealed many genes related to innate immune activation and disease biomarkers. However, downregulated genes related to antigen recognition and cytokine activity suggest that the pathogen may reduce the immune response. Fish supplemented with synbiotic also showed innate immune activation but displayed 217 downregulated DEGs compared to non-supplemented fish post-infection. These genes were linked to immune response, ion homeostasis, bacterial defense, blood coagulation or negative regulation of apoptosis, indicating a modulation of disease response. The pathways affected by the synbiotic will be described to understand its protective role against Vibrio anguillarum infection.

These results suggest that marine synbiotic supplementation induces a more controlled and effective innate immune response in seabass during V. anguillarum infection, resulting in significant survival gain. This study highlights the potential of marine synbiotics as a sustainable strategy to improve fish health in aquaculture.