Differential transcriptomic response of Atlantic salmon to fermented sunflower meal under commercial seawater farming conditions

Dietary inclusion of plant-based ingredients is hampered by gastrointestinal inflammation in salmonids, as well as feed/food competition. Fermented sunflower meal (FSFM), a by-product from oil production under biorefinery processing, can support fish health without compromising growth or competing with food production. Thus, this study assessed the physiological modulation of dietary FSFM in immune organs of post-smolts Atlantic salmon (Salmo salar) farmed under commercial conditions. Results showed that while the antioxidant properties of the diets (control diet with 5% non-FSFM, 5% FSFM, and 10% FSFM) did not change significantly, fish fed 5% FSFM exhibited a distinct transcriptional modulation (compared to all other groups) in the head kidney (HK) and spleen (SP) that was linked to metabolic and immune pathways. For instance, upregulation of oxidative phosphorylation, phagosome activity, and amino acid metabolism was detected, whereas melanogenesis and signaling cascades associated with VEGF, GnRH, and Wnt were downregulated. Regarding fish fed 10% FSFM, phagosome and oxidative phosphorylation pathways were also upregulated in the HK compared to the control. This was complemented by the upregulation of antibody production pathway in both HK and SP, and regulation of the actin cytoskeleton in SP. Conversely, the spliceosome and efferocytosis pathways were downregulated in the HK. Interestingly, a network analysis showed metabolic and immune clusters connected by cell death (i.e., necroptosis and ferroptosis) and lipid metabolism, highlighting the relation between nutrition and immunity. Overall, these findings demonstrate that FSFM is a suitable feed ingredient that modulates a balanced immune response in Atlantic salmon during the seawater stage.