Archaebacteria belong to the three-domain system of biological organism classification. Archaea, the more appropriate term for these organisms, visually appear similar to bacteria under the microscope, but their metabolic differences separate them from prokaryotes. Many Archaea are classified as extremophiles, meaning that they require physically or geochemical extreme conditions that are otherwise harmful and inhabitable to most other organisms on Earth. Biologists separate Archaea into three main groups-Euryarchaeota, Crenarchaeota and Korarchaeota.
- Extreme halophiles A type of Euryarchaeota, Halophilic Archaea require extremely salty environments in which to live. The minimum necessary salt concentration is nine percent, while many of these organisms can survive in solutions with up to 32% salt. This group includes ten genera, in which there are twenty species. Not many natural environments with the required salt concentration exist, and they mostly occur where sea water has been trapped; subsequent water evaporation decreases the water concentration while increasing the salt concentration. One well-known natural habitat for halophiles is the Great Salt Lake in Utah.
- Hyperthermophiles Another example of Euryarchaeota, hyperthermophiles prefer extremely hot environments, such as hydrothermal vents in natural hot springs. These organisms can endure water temperatures of up to 100 degrees Celsius, the temperature of boiling water.
- Archeoglobus Archeoglobus is the only type of Archaea that fixes sulfur. It reduces sulfate to hydrogen sulfide. Like hyperthermophiles, Archeoglobus prefers hot marine sediment near hydrothermal vents.
- Ferroglobus Ferroglobus Archara require environments similar to that of hyperthermophiles. In addition to high temperatures, Ferroglobus needs an environment free of oxygen, making it an obligate anaerobe. Its metabolic processes fix both iron and nitrogen.
- Picrophilus Picrophilus is part of the Thermoplasmatales, another type of Euryarchaeota. These Archaea are extreme acidophiles, meaning that they prefer and require very acidic living conditions. Picrophilus has the most stringent pH requirement. It grows optimally at a pH of 0.7 and begin to die at a pH of 4.0.
- Thermoplasma Thermoplasmas is another acidophile. Their optimal pH begins at 2.0. Unlike other Archaea, thermoplasma do not posses a cell wall.
- Methanogens Another form of Euryarchaeota, methanogens produce methane as a by products of their biochemical processes. They are the most common of the Archaea that require oxygen-free conditions and reside in swamps, marshes, landfills, and lakes and in the gastrointestinal tracts of many animals and insects, including camels, sheep, cows, humans, dogs, horses, and termites.
- Sulfabolus Sulfabolus is a type of Crenarchaeota, which live in extreme cold or hot environments. Sulfabolus requires hot conditions and is found in volcanoes. It also needs oxygen to live. Its biochemical processes fix both iron and sulfur.
- Acidianus This Archaea is another Crenarchaeota also found in volcanoes. It can live either with or without. As its name suggests, it prefers an acidic environment as well.
- Korarchaeota Scientists know the least about this third main group of Archaea. They have identified only one species in a single place-a hydrothermal pool at Yellowstone National Park.