Introduction
Farmed salmon will play a significant role in bridging the demand gap for high-quality food. However, salmon farming is faced with several challenges including access to high quality feed resources, and multi-stressor conditions such as suboptimal environmental conditions and diseases leading to mortalities and economic losses. Therefore , the need for sustainable novel feed resources to replace the conventional feed resources currently used in salmon feeds has never been more urgent. N ovel aquafeed ingredients should be sustainable and address both nutritional and health related challenges in salmon farming. Microbial ingredients (MIs ) such as yeast and fungi provide a viable alternative to address both challenges due to their high nutritional value
and bioactive components with proven health benefits
. Paecilomyces variotii (PEKILO®) is a filamentous fungus with high crude protein content (60-70%) and it contains bioactive components such as β -glucans, mannans and nucleic acids w hich induce health benefits for fish. Considering this, our study under the NordForsk -funded NordicFeed project was designed to characterize the immuno modulating effects of P . variotii in Atlantic salmon ( Salmo salar ) using in vitro and in vivo techniques. We hypothesized that the functional bioactive components in P . variotii will improve overall health and welfare in Atlantic salmon .
Materials and methods
The immunomodulatory effect of P . variotii fermented on sulphite stillage as substrate was characterized using adherent mononuclear leucocytes isolated from head kidney (HK) and spleen of Atlantic salmon (ca. 2 kg) and incubated for 6, 24 and 48 hours with or without the presence of heat inactivated Moritella viscosa (Figure 1, left panel). For the in vivo assessment, vaccinated Atlantic salmon pre- smolts were fed diets with P . variotii replacing 0, 5, 10, or 20% of crude protein from fishmeal and soy protein concentrates for 4 weeks in freshwater . G rowth and health parameters were analyzed (Figure 1, right panel).
Results
Results from the in vitro studies (Figure 1, left panel) indicated that P . variotii induces a strong immune response in the HK of Atlantic salmon at all time-points evaluated , but most strongly after 6 h . Co-stimulation of the s pleen and HK leucocytes with bo th P . variotii and M . viscosa induces an even stronger immune response, particularly in the HK. Upregulation of regulatory cytokine in the HK and downregulation in the spleen was observed when the bacteria alone or a combination with P . variotii was added to cell cultures. In addition, P . variotii enhanced the antimicrobial activity of leucocytes with higher transcript level s of β -glucan receptors, demonstrating recognition of this bioactive compound in the feed ingredient . Our results from the f eeding trial (Figure 1, right panel) indicates that all inclusion levels of P . variotii resulted in feed intake, specific growth rate like the control diet and a dose-dependent linear decrease in the feed conversion ratio with increasing inclusion of the mycoprotein . The i mmune responses induced by P . variotii were systemic. Low and high doses of P . variotii modulated a strong T cell response, innate responses, and enhanced antimicrobial activity. Medium and high dosage also modulated a Th1 response as well as receptors and signaling molecules characteristic of β-glucan recognition (Fig ure 1). Further , inclusion of P . variotii in diets for Atlantic salmon increased specific antibody titres against V . anguillarum.
Conclusion
P . variotii can replace up to 20% of crude protein in the diet of Atlantic salmon without compromising growth performance of the fish. In addition, P . variotii has significant health benefits in Atlantic salmon beyond its nutritional value including enhancing the natural killer cells, the complement system, antimicrobial activity of leucocytes, modulated type 1 and type 2 responses and improved humoral responses.
References
1. Agboola, J.O., M. Øverland, A. Skrede, and J.Ø. Hansen, Yeast as major protein‐rich ingredient in aquafeeds: a review of the implications for aquaculture production. Reviews in Aquaculture, 2021. 13(2): p. 949-970. https://doi.org/10.1111/raq.12507
2. Morales-Lange, B., B. Djordjevic, A. Gaudhaman, C.M. Press, J. Olson, L.T. Mydland, L. Mercado, M. Imarai, M. Castex, and M. Øverland, Dietary Inclusion of Hydrolyzed Debaryomyces hansenii Yeasts Modulates Physiological Responses in Plasma and Immune Organs of Atlantic Salmon (Salmo salar) Parr Exposed to Acute Hypoxia Stress. Frontiers in Physiology, 2022. 13. doi.org/10.3389/fphys.2022.836810