Evolution of Thermophilic Archaea
Crenarcheota and Hyperthermophiles: An Overview
Thermophilic 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. Such habitats are high in concentrations of reduced minerals and temperature, and low in concentration of oxygen and often in pH. Though various genera of these archaea flourish at different temperatures, the most striking examples of hyperthermophiles include archaea in the genera Pyrodictium, 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.
Though certain species of Thermoprotei use oxygen in respiration, most of the metabolisms of these archaea are anaerobic. 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. 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.
Possible Microfossils at Kangaroo Caves
In 2000, strange filaments were first reported in volcanic hosted massive sulfide deposits (VHMS) at the Kangaroo Caves Formation, East Pilbara, Western Australia. It was hypothesized that these filaments were microfossils, specifically of ancient thermophilic chemotrophs. Though non-biological filamentous mineral growths can be easily found in sulfide deposits, the filaments at Kangaroo Caves differed in that they were unbranched, with constant diameters. Moreover, they were entangled and clustered in specific ways which resemble those of certain modern thermophilic microbes.
The rocks and the structures in them are about 3.2 billion years old, meaning that if these filaments really are the remains of primitive microbes, they were flourishing in the Archaean Eon, at the same time as many ancient cyanobacteria. Neither cell structure nor metabolism can be determined by looking at these filaments; even if we could be sure that they were microfossils, we could still not tell whether they were bacteria or archaea. However, it is exciting evidence that thermophilic life did exist on early Earth and perhaps evolved even earlier. It is possible that these thermophiles are the ancestors of modern thermophilic archaea.
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Section 3
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Conclusion
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References
Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2009, Kenyon College.
