Introduction
Vitamin D is an essential fat-soluble micronutrient in animals, and the two most important derivates are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). The D2 originates from vegetable and fungal origins and the D3 from animal origins (Cardwell et al., 2018). Mammals can additionally synthesize pre-vitamin D3 in the skin from 7-dehydrocholesterol (7-DHC). This is done in humans by UV-B radiation of 290-315 nm which causes the epidermal 7-DHC to rearrange and form pre-vitamin D3. (Holick et al., 1977). This is also common among land vertebrates such as lizards and birds and also among some insects (Oonincx et al., 2018).
The main origin of D3 in fish such as Atlantic salmon, has been thought to be via the food chain (Lock et al., 2009). It has been stated that fish is fully dependent on dietary D3 (Lock et al., 2009; Soto-Dávila et al., 2020).
We wanted to test whether UV-B light stimulated the production of vitamin D3 in Atlantic salmon.
Material and methods
For the trials, 4 indoor holding tanks (3 x 3 m, 8 angular) with a water volume of ~ 10.8 m3 (depth 1.2 m) were used. The indoor area was enlightened with ordinary white light from tube lights in the roof, at a continuous 24/7 light regime for all tanks. In addition, two UV-B-light sources (LumenIZE Pro T5 Forest 6% 54W, Arcadia reptile, Great Britain) were installed 30 cm above the water surface in two of the tanks (UV-B exposed fish) at a 24/7 light regime. For comparison fish from two random tanks were used (control/non-exposed fish). Atlantic salmon were reared from roe (Benchmark genetics, Norway) at Salmon Evolutions smolt facility at Dale in Norway following the normal smolt production. The average weight in start of the trial was 6g. Two separate trials were done.
Fish were sampled from the 4 indoor tanks at 3 separate occasions in both trials, at day 0 (start of the trial), day 31, and day 77 in trial 1 and day 0, day 27, and day 68 in trial 2. Fish were tawed, and filets with pin bones extracted was analysed for Cholecalciferol (D3) by Eurofins Food & Feed Testing Norway AS (EN 12821:2009 mod.). The method consists of saponification and extraction followed by HPLC analysis and detection.
Results
The level of D3 in muscle was markedly affected by the presence of a UV-B light source. Filet D3 levels started out at ≤ 1 µg/100 g in all tanks during both trials. D3 levels in fish exposed to UV-B increased during both trials, whereas in the control groups D3 levels remained at low levels throughout. Fish from UV-B treated tanks had significantly higher D3 levels than fish from the control tanks.
Conclusion
The groups exposed to UV-B light had 2,5 to 6 times higher Vitamin D3 in the muscle after 4 weeks and more than 5 times as much after about 10 weeks than the control groups. The study suggests that Atlantic salmon in the freshwater phase synthesize pre-vitamin D3 from 7-dehydrocholesterol using UV-B light.
In today’s salmon farming, where more of the farming occurs in closed and submerged facilities with no or limited access to UV-B from natural sunlight, this knowledge is important.
Acknowledgements
The authors would like to acknowledge the thorough work conducted by the staff at Salmon Evolution Dale. We are grateful to the Møre og Romsdal County Council for the financial support through MobPro, project 356883.
References
Cardwell, G., Bornman, J.F., James, A.P., Black, L.J., 2018. A Review of Mushrooms as a Potential Source of Dietary Vitamin D. Nutrients 10(10), 1498.
Holick, M.F., Frommer, J.E., McNeill, S.C., Richtand , N.M., Henley, J.W., Potts, J.T., 1977. Photometabolism of 7-dehydrocholesterol to previtamin D3 in skin. Biochemical and Biophysical Research Communications 76(1), 107–114. https://doi.org/https://doi.org/10.1016/0006-291X(77)91674-6
Lock, E.J., Waagbø, R., Wendelaar Bonga, S., Flik, G., 2009. The significance of vitamin D for fish: a review. Aquaculture Nutrition 16(1), 100–116. https://doi.org/https://doi.org/10.1111/j.1365-2095.2009.00722.x
Oonincx , D.G.A.B., van Keulen, P., Finke, M.D., Baines, F.M., Vermeulen, M., Bosch, G., 2018. Evidence of vitamin D synthesis in insects exposed to UVb light. Scientific Reports 8(1), 10807. https://doi.org/10.1038/s41598-018-29232-w
Soto-Dávila, M., Valderrama, K., Inkpen, S.M., Hall, J.R., Rise, M.L., Santander, J., 2020. Effects of Vitamin D2 (Ergocalciferol) and D3 (Cholecalciferol) on Atlantic Salmon (Salmo salar) Primary Macrophage Immune Response to Aeromonas salmonicida subsp. salmonicida Infection. Frontiers in Immunology 10. https://doi.org/10.3389/fimmu.2019.03011