Ice algae and snow algae are algae and cyanobacteria which grow on long-lasting snow and ice fields like glaciers. When liquid water is available between the snow and ice crystals, they may color the surface green, yellow or red during the summer months. The red pigment of some species is an intracellular protection against excessive visual light and ultraviolet radiation of the sun, which otherwise can cause photoinhibition of photosynthesis or mutations. Without it, the algae at the surface would suffer chromosome breaks and DNA mutations.

There are also ice algal communities on sea ice. These algae (mainly diatoms) are important in polar ecosystems (especially Antarctica) because they provide food for krill. Krill scrape off the algae from the underside of the ice, which is colored brown by the algae. The algae may be found between ice crystals or attached to them, in the water or saltwater channels between ice crystals.

Chlamydomonas nivalis 
Chlamydomonas nivalis is a green microalga which causes, besides other closely relative species, Watermelon snow.

Watermelon snow is snow that is reddish or pink in color, and that can have a smell similar to a fresh watermelon. This type of snow is common during the summer in alpine and coastal polar regions, such as the Sierra Nevada of California. Here, at altitudes of 10,000 to 12,000 feet (3,000–3,600 m), the temperature is cold throughout the year, and so the snow lingers from winter storms. When someone steps on the snow with algae, the footprints look red. 

Chlamydomonas  nivalis is a green alga which owes its red color to a bright red carotenoid pigment (Astaxanthin). This protects the chloroplast and cell nucleus from strong visible and ultraviolet radiation. The green and red pigments absorb light and heat, which gives the alga liquid water as the snow melts around it. Algal blooms may go 25 cm (10 inches) deep. Since each cell measures 20 to 30 micrometers in diameter, a teaspoon of melted snow contains a million or more cells. The algae build up in 'sun cups', which are shallow depressions in the snow. The carotenoid pigment absorbs heat, which deepens the sun cups, and makes the glaciers and snowbanks melt faster.

During the winter months, when white snow covers them, the algae become dormant. In spring, nutrients, increased levels of light and meltwater, stimulate germination. Once they germinate, the resting cells release smaller green flagellate cells which travel towards the surface of the snow. Once the flagellated come close to the surface, they may lose their flagellae and form thick-walled resting cells, or they may function as gametes, fusing in pairs to form zygotes.

Some specialised species feed on C. nivalis, including protozoans such as ciliates, rotifers, nematodes, ice worms and springtails.

History 

The first accounts of watermelon snow are in the writings of Aristotle. Watermelon snow has puzzled mountain climbers, explorers, and naturalists for thousands of years.

In May 1818, four ships sailed from England to search for the Northwest Passage and chart the Arctic coastline of North America. Bad weather made them finally turn the ships back, but the expedition made important contributions to science. Captain John Ross noticed crimson snow that streaked the white cliffs like streams of blood as they were rounding Cape York on the northwest coast of Greenland. A landing party stopped and brought back samples to England. The Times wrote about this discovery on December 4, 1818:

When Ross published his story of the voyage in 1818, the story had an appendix of plants by Robert Brown. In it, Brown compared the red snow to an alga.

Other websites 
Armstrong W.P. 1999. "Watermelon Snow: a strange phenomenon caused by algal cells of the Chlorophyta"  Wayne's Word Noteworthy Plants: Aug 1998. (24 April 2006).

References 

Snow
Algae