Hot spring. Blue-green algae (not always blue- green in colour) in a geothermal pool or hot spring in a hydrothermal area. The pool edges are encrusted with mineral deposits. Water superheated by underground geothermal activity dissolves many of the minerals from surrounding rocks. When the water escapes to the surface its power as a solvent is reduced and minerals are precipitated. Such chemical nutrients and the hot, anaerobic, aquatic environment allow many colourful primitive algae and bacteria to flourish. Some blue-green algae are able to tolerate temperatures of up to 165 Celsius. Photographed at Yellowstone National Park, USA.