hermophilic and hyperthermophilic a

hermophilic and hyperthermophilic archaea, specifically crenarcheotes in the class Thermoprotei, are known to inhabit environments such as hot springs, ocean vents, and geysers which are inhospitable to many other forms of life.(Figure 2)


Figure 1: Scanning electron micrograph of Pyrodictium archaea. By R. Rachel, 1997 http://tolweb.org/Crenarchaeota
Such habitats not only have extremely high temperatures--at some ocean vents, temperature might even reach 400 degrees Celsius--but have high concentrations of dissolved minerals. At the same time, they have low concentrations of oxygen, if they are not completely anaerobic, and they are often low in pH as well. Though various genera of these archaea flourish at different temperatures, the most striking examples of hyperthermophiles include archaea in the genera Pyrodictium, (Figure 1) which are adapted to live around thermal vents at the ocean floor, at temperatures ranging from 100-110 degrees Celsius. Acidophiles are also prevalent in this class, especially those belonging to the order Sulfolobales: Sulfolobus solfatericus flourishes at a pH ranging from only 2 to 4. The acidophiles are usually found in environments rich in sulfur, and they obtain energy using H2, H2S, and elemental sulfur as electron donors. Yet there are many archaea which grow at neutral or high pH: rod-shaped species in the genera Thermoproteus, Thermofilum, and Pyrobaculum, as well as coccoid species in the genus Desulfurococcus. 11



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.