Methanogens

• Methanogens produce methane as a metabolic waste product of carbon dioxide and hydrogen. These microorganisms live in anaerobic (oxygen-free) environments as diverse as marshlands, deep-sea hydrothermal vents, oil wells and animals' digestive tracts. Foul-smelling marsh gas and sewer gas can occur where methanogens thrive. While extremes of salinity and temperature have little affect on these archaeans, oxygen is toxic to most methanogen species.
  
  

Halophiles

• Halophiles thrive in extremely saline environments. Seawater has an average salinity of 3.5 percent; halophilic organisms live in concentrations at least 10 times that of seawater. Despite their name, halobacteria belong to domain Archaea, not to domain Bacteria. Large colonies appear purplish or reddish; evaporating lakes and wastewater from salt mines may appear dramatically red due to halophiles in the water. Unlike other species of archaea, most modern halophile species have aerobic metabolisms.
  
  

Thermophiles

• Thermophile archaeans form visible greenish colonies in a Yellowstone National Park hot spring.Karl Weatherly/Photodisc/Getty Images
  
  As their name suggests, thermophiles live in extremely hot conditions. Phylum Thermobacteria includes archaean species that thrive at temperatures above 180 degrees Fahrenheit (80 degrees Celsius). Some species within the genus tolerate lower temperatures, while others can sustain life only in near-boiling water. Fumaroles, geysers and hot springs provide habitats to thermophile colonies. The organisms' varied colors form zones around some springs as each species finds its own favored temperature in which to live.
  
  

Thermoacidophiles

• Morning Glory Pool in Yellowstone National Park is rich with thermoacidophiles.Jupiterimages/Photos.com/Getty Images
  
  These archaeans require both high temperatures and low pH to survive. Members of the genus Sulfolobus produce their own acidic environment as they metabolize sulfur and excrete sulfuric acid as a metabolic byproduct. Sulfur and sulfur-containing compounds frequently accompany volcanic activity, and where sulfur appears, thermoacidophiles typically flourish. Ponds in mine tailings provide a hospitable environment for these archaeans as well. Like other archaeans, thermoacidophiles contain pigments that tint their aqueous environment; sulfolobus organisms, for example, produce a brilliant swimming-pool aqua hue.