- Phytoplankton: Microalgae live at the surface of the water and vitamin D2 is probably formed via sun exposure of provitamin D (ergosterol). This is supported by the fact that microalgae tested in August have a higher vitamin D content than in October and December.
- Phytoplankton: Microalgae have been reported by some to also contain (in addition to ergosterol and vitamin D2) both vitamin D3 and provitamin D3. There is evidence for microalgae (phytoplankton) as the basis of the food chain is the origin of vitamin D3 in fish. The occurrence of vitamin D3 in microalgae suggests that vitamin D3 may exist in the plant kingdom. And vitamin D3 has been identified in several plant species.
- Zooplankton: Zooplankton principally contains 7-dehydrocholesterol as their provitamin D content. Codfish also consume zooplankton (i.e. Copepoda). Levels of vitamin D3 may vary depending on the time of year and depth in the sea that the zooplankton are found.
- Fish skin: Although there is some evidence for the synthesis of vitamin D3 in the skin of fish, vitamin D3 synthesis is likely to be negligible or minimal because of the limited UVB-light in their natural habitats combined with low amounts of 7-dehydrocholesterol in fish skin.