 1. Microalgae are increasingly being investigated as functional feed additives in a variety of fish species, but our knowledge on how microalgae supplementation affects Atlantic salmon remains limited. 2. We hypothesized that microalgae inclusion of 8% in the feed would improve performance, fatty acid and pigment deposition as well as health and immunity of Atlantic salmon reared in recirculating aquaculture systems, RAS. 3. We fed Atlantic salmon smolts with 5 different microalgae-enriched diets containing tetrasilmus chewy, T.C., arthrosperplotensis, A.P., schizokitrium lemassinum, S.L., or chlorella vulgaris either intact, C.V.I., or as broken cell wall derivative, C.V.B., or a control diet, C.D. 4. After 8 weeks of feeding in brackish water, 13 PSU, all groups were transferred to seawater, 32 PSU, for additional 2 weeks. 5. Our results indicate that C.V.B. improved feed conversion and protein retention, but reduced condition factor, P and L to 0.05, compared to fish fed with a control diet. 6. Voluntary feed intake decreased in seawater, but was similar among diet groups. 7. The amount of dacosahexionic acid was particularly high in S.L. fed fish and alphalinolenic acid was enriched in fish fed C.V.I., C.V.B., and T.C., P and L.T., 0.05. 8. Following seawater transfer, fat content and monounsaturated fatty acids decreased in the muscle, while polyunsaturated fatty acids increased. 9. Lutein was present in all muscle samples. This article was authored by Jonas Mueller, Jonas Mueller, Matteo Pauley, and others. We are article.tv. Links in the description below.