In Atlantic salmon, the specific red-pink flesh color is formed by the accumulation of astaxanthin from food. While wild salmon get their pigment from their prey, farmed Atlantic salmon needs supplemented formulated diets. Nowadays, the share between synthetic- and natural sources is approximately 90:10, with an increasing interest in new natural sources. The cost of carotenoid supplements accounts for 6-10% of the total feed cost, whereas the deposition for flesh pigmentation in salmonids is less than 22%. Natural astaxanthin from the algae Haematococcus pluvialis contains only 3S,3’S stereoisomer, the predominant pigment in wild salmon, suggesting an improved biochemical effect compared to the synthetic form. In addition, some experimental diets containing algae-extracted astaxanthin have shown improved immune responses and stress tolerance in many aquatic species. Therefore, our study was performed to understand whether astaxanthin from H. pluvialis could enhance astaxanthin assimilation and potentially boost growth and immune responses.
A 108-day feeding experiment was conducted in a flow-through system with seawater taken from a depth of 250 m of eighteen 800 L-tanks, including three diets: AS (synthetic astaxanthin), AW (whole algae astaxanthin), and AX (algae-extracted astaxanthin) (3 x 6 replicates). Twenty fish (approximately 652 g) were stocked in each tank and fed twice daily. Environmental conditions were maintained in the optimal range of Atlantic salmon. Three fish were sampled from each tank on days 30, 60, and 108 to assess growth performance, visual flesh color, and astaxanthin content in Norwegian Quality Cut. At the end of the experiment, liver and pyloric caeca in each treatment (n = 4) were collected for RNA extraction and sequencing for differential expression analysis.
No significant difference in either color assessment or growth performance was observed after 108 days. The average fish weight in AW and AX was similar and lower than in AS, but the difference was not significant (P > 0.05). At the end of the experiment, there was no significant difference in astaxanthin content between the three dietary treatments (P > 0.05) (Fig. 1).
Regarding transcriptomics, differential gene expression in both tissues was more pronounced when comparing the timelines than the comparisons between experimental diets (Fig. 2). Gene Ontology (GO) annotation was mainly distributed in the biological process category. At Day 108, enriched GOs and KEGG pathways were involved in the biosynthesis and metabolism of steroids, lipids, and fatty acids compared to Day 0. Noticeably, GO terms of fatty acid degradation and biosynthesis of unsaturated fatty acids were enriched in AS only. On the other hand, in AW and AX, H-2 class I histocompatibility antigen, Q8 α-chain gene in the liver, and pyloric caeca involved in immune responses were upregulated.