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Snow Algae
Snow algae grow on snow.  Freshly fallen snow is white, and in most places it melts before algae can form on it to any obvious extent.  On glaciers, ice caps and other long term snow expanses, many types of algae can grow and often actually change the colour of the snow.
 
Temperature
 
While most algae grow better at higher temperatures, snow algae are adapted to the conditions found on snow.  They prefer temperatures lower than 10 degrees C (50 degrees F).  These algae grow best at close to the freezing point of water; careful measurements have demonstrated some photosynthesis in algae cysts at temperatures as low as minus forty degrees.
 
Types
 

Most snow algae are green algae although there are several from other groups.  Red coloured algae can make what is known as "Water Melon" snow.

 

Life cycles
 
Some of the snow algae have complicated lifecycles.  They often have an active stage that can move through the icy snow, positioning themselves at a good level in the ice.  These can form spores which colour the snow red. 
 
Protection from Cold and Ultraviolet
 

The red pigments give some protection from the sun’s ultraviolet.  Snow algae have got many different chemicals which protect them from either damaging light or from freezing in the cold.

 
Concentration

 
Algae on snow can be present at concentrations as high as a million cells per millilitre.

 
Distribution

Snow algae are found in both the Northern and the Southern Hemispheres although there are some species only found in one hemisphere.

Ice Algae

Snow is not salty when it falls from the sky, and if fresh snow is melted, the water from it will be fresh. There is a quite different group of algae which grow on the ice formed on top of the sea.

By Iwona Erskine-Kellie from Vancouver, Canada (Snow Algae Textures) [CC-BY-2.0 (www.creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
This file is licensed under the Creative Commons Attribution 2.0 Generic license.
 
 
 
 
 
Snow pits in Water Melon Snow
perhaps made by the warming of the red snow more than the white snow.
Will Beback [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (www.creativecommons.
org/licenses/by-sa/3.0/)], via Wikimedia Commons.
This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.