Introduction
Norwegian salmon farming is a vital economic sector but faces significant challenges due to high fish mortality rates, particularly during the seawater (SW) phase . For instance, winter ulcers caused by bacteria such as Moritella viscosa or Tenacibaculum sp. pose a major threat . Functional feeds containing bioactive compounds, such as fucoidan from sugar kelp (Saccharina latissima) offer a p otential solution to improve fish resilience. The RESILIENT SALMON project ( Research Council of Norway – 294821 ) investigates how fucoidan can enhance immune responses and reduce susceptibility to diseases.
Materials and methods
Fucoidan was extracted from sugar kelp using a factorial design to optimize key parameters, such as temperature, acid concentration and time, in a pilot biorefinery process . Bioactivity test in salmon kidney cells (SHK-1) determined the most effective fucoidan fraction for immune stimulation, based on sulfation levels, yield and immunostimulatory effects (Michalak et al 2024). Then, the most promising fraction was upscaled for evaluation in functional diets for Atlantic salmon (Salmo salar).
Experiment 1 (Exp.1) : After an in vitro evaluation in SHK-1 cells, t he immunomodulatory effects of fucoidan in pre-smolt Atlantic salmon were tested in freshwater (FW) phase . Fish were fed experimental diets containing 0% ( negative control), 0.1%, 0.2%, or 0.4% fucoidan, or a positive control diet containing 0.2% MacroGard®. Distal intestine (DI) samples were collected for microbiota characterization (by 16S rRNA-seq) and detection of immune-related biomarkers (using RNAseq, RT-qPCR, immunohistochemistry and proteomics) . Also, p rimary cultures of head kidney leukocytes (HKLs) were obtained from fish fed experimental diets and stimulated with proteins from Aeromonas salmonicida. Then, the protein level of cytokines such as TNF-α and IL-10, as well as effector molecules ( i.e., iNOS and HSP70) were detected by indirect ELISA.
Experiment 2 (Exp. 2): We evaluated wh ether feeding a diet with 0.2% fucoidan for four weeks post-vaccination in FW could enhance the immune response of Atlantic salmon exposed to inactivated Tenacibaculum maritimum ( via intraperitoneal injection) after SW transfer. For this, i mmune biomarkers in the DI and head kidney (HK) were analyzed by RT-qPCR and indirect ELISA at the end of the FW stage, and before and after stimulation in SW.
Experiment 3 (Exp. 3): i nvestigate the impact of feeding a 0.2% fucoidan diet on the resistance of Atlantic salmon to Tenacibaculum dicentrarchi . F ish were fed the fucoidan-supplemented diet for four weeks in FW , followed by four weeks on a control diet in FW. Then, fish were transferred to saltwater and challenged with T. dicentrarchi . The control group, which only received the control diet throughout the experimental period , was also infected with T. dicentrarchi. We assessed mortality rates and immune responses to evaluate the mode of action underlying the observed health effect of the functional diet containing fucoidan.
Results and Discussion
In Exp. 1, fucoidan enhanced immune and antioxidant responses in Atlantic salmon. For instance by upregulating expression of sod-1 and prx in SHK-1 (at 72h) . Also, HKLs from fish fed 0.2% fucoidan or MacroGard® and stimulated with proteins from A. salmonicida had increased production of TNF-α (pro-inflammatory cytokin e) and HSP70 (chaperone) . In contrast, HKLs from fish fed 0.4% fucoidan had an increased level of IL-10 (anti-inflammatory cytokine) . Feeding t he 0.2% fucoidan diet also increased the presence of Bacillus sp in DI and up-regulated pathways related to immune cell development , tissue repair, and cell adhesion . At protein level, fish fed 0.2% fucoidan showed a higher production of MHC II, CD3 , along with the up-regulation of gata3 , suggesting an enhanced adaptive immunity. By proteomic s in DI, we detected higher levels of immune-related proteins (gap junction and NOD-like receptor) and a lower level of proteins associated with amino acid metabolism and fatty acids degradation , indicating a metabolic shift towards immune resilience. Th ese findings suggest that fucoidan , depending on dietary inclusion level, can enhance immune function, gut health, and overall resilience in salmon.
Results from Exp 2, showed that fucoidan supported growth and boosted immune responses when combined with a commercial vaccine. Fucoidan activated key immune-related genes in Atlantic salmon such as pro- and anti-inflammatory cytokines (tnfa , il10) in DI , while in the HK , an upregulation of antimicrobial peptides (i.e., hepcidin , cathelicidin) was detected after stimulation with inactivated T. maritimum. These effects were also confirmed at the protein level, reinforcing fucoidan’s potential as a potent immune-modula ting feed component.
Exp. 3 revealed that fucoidan significantly improved fish survival when exposed to the live T. dicentrarchi . Atlantic s almon fed fucoidan-supplemented diet showed a 51% higher probability of survival compared to the control group. Further, the levels of CD4-1+ T cells and IgM+ B cells in peripheral blood leukocytes (PBL) and the spleen increased a s a result of fucoidan supplementation during the infection. Following the T. dicentrarchi infection, cytokine expression levels were also upregulated in the spleen and PBL of fucoidan- fed fish, suggesting that fucoidan may enhance T cell differentiation and immune activation.
Conclusion
Functional feeds enriched with fucoidan from sugar kelp, combined with nutritional programming, can significantly enhance salmon resilience and health beyond the specific vaccine effects , supporting the sustainable development of the aquaculture industry.
References
Michalak, L.; Morales-Lange, B.; Montero, R.; Horn, S.J.; Mydland, L.T.; Øverland, M. Impact of Biorefinery Processing Conditions on the Bioactive Properties of Fucoidan Extracts from Saccharina l atissima on SHK-1 Cells. Algal Res. 2023, 75, 103221.