Thermophiles, meaning heat-loving organisms, are organisms with an optimum growth temperature of 50 °C or more, a maximum of up to 70 °C or more, and a minimum of about 40 degrees C, but these are only approximate. Some extreme thermophiles (hyperthermophiles) require a very high temperature (80 °C to 105 °C) for growth. Their membranes and proteins are unusually stable at these extremely high temperatures. Thus many important biotechnological processes utilize thermophilic enzymes because of their ability to withstand intense heat.
Many of the hyperthermophiles Archea require elemental sulfur for growth. Some are anaerobes that use the sulfur instead of oxygen as an electron acceptor during cellular respiration. Some are lithotrophs that oxidize sulfur to sulfuric acid as an energy source, thus requiring the microorganism to be adapted to very low pH (i.e., it is an acidophile as well as thermophile). These organisms are inhabitants of hot, sulfur-rich environments usually associated with volcanism, such as hot springs, geysers, and fumaroles. In these places, especially in Yellowstone National Park, we find a zonation of microorganisms according to their temperature optima. Often these organisms are coloured, due to the presence of photosynthetic pigments.
Figure 2: Obsidian Pool, in the Mud Volcano area of Yellowstone National Park. Many species of thermophilic archaea live here. By Norm Pace, 1997 http://tolweb.org/Crenarchaeota
Though certain species of Thermoprotei use oxygen in respiration, most of the metabolisms of these archaea are anaerobic. Moreover, most of these archaea are also chemolithotrophs, meaning that they must take their energy from inorganic sources. Hydrogen sulfide, hydrogen, methane, sulfur, and nitrogen are all potential energy sources for chemolithotrophs. A particularly high number of species reduce sulfur and various sulfates for energy, particularly those which inhabit solfataric fields. Solfataric fields, composed of soils heated up by volcanic emissions from magma chambers, are known for their high elemental sulfur content. 11 However, sulfur use is not limited to archaea which thrive on land. Ignicoccus islandicus oxidizes hydrogen with sulfur, forming hydrogen sulfide. Pyrodictium might oxidize hydrogen with sulfur or else use anaerobic fermentation.
The hostile habitats in which crenarcheotes live resemble, in many cases, the conditions on early Earth. The planet was extremely hot and radioactive, much more so than today; though it is thought that a crust developed relatively soon after Earth's formation, this crust was thin and made entirely of igneous rock. The early atmosphere consisted of water vapor, CO2, nitrogen, hydrogen, methane, NH3, and CO. 12 As the early atmosphere is believed to have had no oxygen in it, the first life forms to develop must clearly have been anaerobic. If life forms evolved before Earth cooled--and it is quite possible that they did, as they could have flourished beneath Earth's forming crust, away from crashing meteorites--these organisms might have been the ancestors of today's thermophilic prokaryotes, including archaea. Though it is relatively certain among microbiologists that the first microbes, ancestors of all life on Earth today, evolved nearly 4 billion years ago, it is still quite uncertain whether these early microbes were thermophiles.