These algae can grow in snow and ice. They live in very cold locations such as the Alps, which gives them an advantage over "cold-sensitive" algae. They are responsible for the well-known "blood snow". The snow is red because the algae have red pigments which can be easily seen in the snow. The pigment protects them from the harmful UV radiation.
Explanation for children:
Snow and ice algae live high up in the mountains and color snow red and ice gray to brown. High up in the mountains you can get sunburnt very quickly. However, since these algae cannot be protected with sunscreen, they have to protect themselves from UV radiation in a different way. Therefore they have protective colors that appear red or grey to brown to us.