Source-dependent effects of dietary astaxanthins on health and mucosal immunity in Atlantic salmon (Salmo salar)

This study examined the effects of using synthetic vs a natural source of astaxanthin (AXT) on the histomorphometry of mucosal organs (distal mid-intestine, skin, and gills), erythrocyte morphology, liver and trunk kidney health, and the expression of mucosal immune-related mucin genes (muc2, muc5ac1, muc5ac2, muc5b) and antimicrobial peptide (AMP) genes (def1, def2, def3, cath1) in Atlantic salmon (Salmo salar). Three experimental diets were formulated based on AXT sources: synthetic astaxanthin referred to as the control diet (CO), and two additional diets containing an emulsion of wall-broken whole-cell algal astaxanthin (AW) and extracted algal astaxanthin (AE), both obtained from the microalga Haematococcus pluvialis. Each dietary group had six replicates, and the feeding trial lasted for 105 days. Afterward, a subset of fish from each group underwent a 14-day challenge test to assess stress tolerance. The results showed that the histoarchitecture of the distal mid-intestine, skin, and gills was affected in both the CO and AW groups during the feeding and challenge phases. Mucosal mapping revealed increases in volumetric density, mucous cell (MC) area, and mucosal barrier integrity in these groups. The frequency of erythrocyte abnormalities was lower in the CO and AW groups compared to the AE group. Conversely, the AE diet was associated with significantly larger vacuole area (%) and mean vacuole size (μm) in hepatocytes. Similarly, the AE group showed a higher percentage (%) and larger neutral lipid droplets (μm) in liver tissue. Structural anomalies in the trunk kidneys, assessed via semi-quantitative scoring, were more severe in the AE group than in the CO and AW groups. The expression of mucosal immune-related mucins and AMP genes was upregulated in both the CO and AW groups compared to the AE group. Notably, muc2, muc5b, and def3 in the distal mid-intestine, and muc2 in the skin showed significantly higher expression in the CO group than in the AE group. Overall, the findings suggest that supplementing the diet with synthetic and emulsified wall-broken whole-algal cell astaxanthin (57.0 and 43.0 mg/kg, respectively) influenced mucosal mapping and histomorphometry, which are linked to a supportive mucosal immune status and improved health in Atlantic salmon through a combination of structural and physiological changes.