The Archaea reference article from the English Wikipedia on 24-Jul-2004
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Archaea

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Archaea
Scientific classification
Domain (biology)>Domain*: Archaea
Phyla* & classes
Crenarchaeota
Euryarchaeota
  • Halobacteria
  • Methanobacteria
  • Methanococci
  • Methanopyri
  • Archeoglobi
  • Thermoplasmata
  • Thermococci
Korarchaeota
Nanoarchaeota
* Or kingdom (see text)
The Archaea are one of the three major groups of living organisms, together with bacteria and eukaryota. They are prokaryotes, like bacteria, and were originally included among them. Their separate identity was discovered in the late 1970s by Dr. Carl Woese at the University of Illinois by genetic comparison. Originally they were termed the Archaebacteria, and the other prokaryotes the Eubacteria, but now there is a growing tendency to restrict the term bacteria to the latter and the names have adjusted accordingly. The Archaea may be treated as a single kingdom or as a domain, in which case the subgroups may be ranked as kingdoms.

Archaea differ from the true bacteria in many important respects, as well as from the eukaryotes. These differences include:

Originally Archaea were thought to only live in extreme environments, including water whose temperature exceeds that of boiling water, like geysers, very salty, acid or alkaline water or black smokers. Some are also known to prefer cold habitats. However they are now known to be very common in marshland, sewage treatment plants -- indeed most soil contains at least one species of Archae. A few have been found in animal digestive systems. The environmental conditions archaea prefer and their unusual biochemistry make them usually harmless to organisms belonging to the other two domains. No case of infection of a human with archaea has been reported so far.

They are very diverse, both in morphology and physiology. Some are single-celled, while others form filaments or aggregates. They may be spherical, rod-shaped, spiral, lobed. Their size varies in diameter from 0.1 to more than 15 µm (filaments up to 200 µm).

They show a great diversity in multiplication modes, which may be by binary fission, budding or fragmentation.

For a nutrional point of view, they range from being chemolithoautotrophic to organotrophic. Physiologically, they can be aerobic, facultatively anaerobic, or strictly anaerobic. Some are mesophiles, others hyperthermophiles (may live over 100°C).

Arachaea also include the methanogens in their number, the only organisms known to produce methane, which are one of the several possible explanations for the recently discovered presence of methane on Mars.

Table of contents
1 Groupings
2 Evolution
3 See also
4 External links

Groupings

There are two main groups of Archaea, the Crenarchaeota and Euryarchaeota. The Korarchaeota have been described from RNA samples, but the actual organisms remain unknown, and the Nanoarchaeota are known from a single species discovered in 2002, Nanoarchaeum equitum.

Evolution

The Archaea appear to be close relatives of the eukaryotes, and some work has suggested that the Euryarchaeota may be closer to them than the Crenarchaeota, though more recent studies support their monophyly. Woese argued that the Bacteria, Archaea, and eukaryotes all diverged separately from an ancestral progenote, but this has little support, and a few authors consider the Archaea and eukaryotes highly derived Bacteria (making that kingdom paraphyletic).

See also

External links