Snow algae grow in semi-permanent to permanent snow or ice in the alpine or polar regions of the world. Their optimum growth temperatures are generally below 10° C. These algae have successfully adapted to their harsh environment through the development of a number of features which include pigments, polyols (sugar alcohols, e.g. glycerine), sugars and lipids (oils), mucilage sheaths, motile stages and spore formation. Large blooms in the summer months can reach cell concentrations of 105 to 106 cells per mL. and colour whole snowbanks red, orange, green or grey depending on the species and habitat conditions.
The snow algal flora is thought to be dominated by chlamydomonads, a group of green algae characterised by single cells with two flagella at their anterior ends. However, species from other algal groups are also important and the dominant alga in many of the glaciers around the world is a saccoderm desmid Mesotaenium berggrenii, an alga that colours the snow grey.Many of the snow algal species go through a complicated life cycle involving vegetative and or motile cells which are usually green in colour and immotile spores or cysts which may be red, orange or yellow green in colour. The green vegetative cells give rise to green snow and the red and orange snow are generally caused by the spore stages of the snow algae although some snow algae may be red-pigmented in their vegetative state. These pigments protect the cells from high light and UV radiation damage during the summer months. The pigments may take the form of iron tannin compounds, as in M. berggrenii, or orange to red-pigmented lipids as in the majority of the snow algae.
The spores usually have thick walls and large amounts of lipid reserves, polyols and sugars. These spores are able to withstand sub-zero temperatures in winter and also high soil temperatures and desiccation in summer which would kill normal vegetative cells.
The motile stages enable them to re-colonise the snow from germinating spores left behind on the soil as well as to position themselves at the optimum depth for photosynthesis in the snow/ice column.The cells of some species also secrete copious amounts of mucilage which enable them to adhere to one another and to snow crystals and prevent the cells from being washed away by meltwater. The mucilage also forms a protective coat and delays desiccation. It may have an additional function as an UV shield.
Snow algae were probably derived from species of soil or aquatic algae. Their distribution is governed by factors such as nutrients, pH, salinity, aspect and sunlight.
Snow algae, in adapting to an extreme environment, may have developed compounds (such as sunscreens and low temperature enzymes) and genes of commercial value.
A few species are common worldwide, but others are restricted to either the Northern or Southern Hemispheres.