Haematococcus pluvialis - The red colored green alga

Haematococcus pluvialis is a single celled green freshwater alga found all over the world. Each cell is encased in a transparent capsule and varies in size from 10-50 microns depending on its life cycle stage, but most are around 20 microns. H. pluvialis can be found in two major stages, active and inactive. In the active stage the algae are motile and move with two long whip-like projections called flagella. In the inactive stage the cells become non-motile encased cysts which are able to tolerate periods without water and huge temperature fluctuations. One of the protective strategies the algae use during the non-motile stage is the production of a molecule called astaxanthin.

Astaxanthin is a red colored molecule responsible for H. pluvialis red color. But other than turning the cells partially or totally red, astaxanthin also protects the cells from UV radiation as it is a very powerful antioxidant. The high concentration of astaxanthin in these algae is also responsible for their macro appearance when gathered in large numbers. If you have ever had a small unchlorinated pool or a birdbath which acquired a red layer on the bottom, these might be the reason.
So despite being a green alga capable of photosynthesis, the presence of astaxanthin is thus able to give a red color to the surfaces they cover in contrast to the more common green color associated with algae growth.

However, the astaxanthin is not only able to protect the algae. The strong antioxidant capabilities of this molecule is also good for humans and should/could be beneficial for a number of health conditions such as arthritis, skin conditions and even cancer to name a few. Another place you can find this powerful antioxidant is in fish. But animals don't produce it themselves, so the only way the fish can acquire it is by eating things like H. pluvialis. In fact the pink color in both salmon, some crustaceans and even flamingos are due to their ingestion of H. pluvialis and their content of astaxanthin.