|
|
|
|
|
|
 |
|
 |
|
 |
|
|
<tr><th bgcolor=darkgray>Scientific classification
<tr><td>
<tr><td>Kingdom:<td>Archaea
</table>
<tr><th bgcolor=darkgray>Phyla / Classes
<tr><td>
Phylum Crenarchaeota
Phylum Euryarchaeota
    Halobacteria
    Methanobacteria
    Methanococci
    Methanopyri
    Archeoglobi
    Thermoplasmata
    Thermococci
Phylum Korarchaeota
Phylum Nanoarchaeota
</table>
The Archaea are a major group of prokaryotes. They were first identified by Woese in 1977, based on their separation from other prokaryotes on 16S rRNA genetic trees. These two groups were treated as kingdoms, called Archaebacteria and Eubacteria, and he argued that they and the eukaryotes each developed separately from an ancestral progenote with poorly developed genetic mechanisms. In 1990 he promoted these major lines to domains and renamed them the Archaea, Bacteria, and Eucarya.
Although the identity of the Archaea is generally accepted, other biologists have questioned Woese's hypotheses about them. Subsequent studies have confirmed that they are similar to other prokaryotes in most aspects of cell structure and genetics, although there are several notable differences. Further, other trees have found that they are either closely related or ancestral to the eukaryotes. This discredits the progenote hypothesis, and it has been proposed that the archaebacteria and eukaryotes both developed from specialized eubacteria. In this case they should be ranked as a kingdom or lower.
Many archaeans are extremophiles. Some live at extremely high temperatures, often above that of boiling water, as found in geysers and black smokers. They are also found in cold habitats and in very salty, acidic, or alkaline water. Some such as methanogens are found in the digestive tracts of animals like ruminants and termites. Although they were originally thought of as being confined to extreme environments, however, other archaeans are mesophiles and have been found in environments like marshland, sewage, and soil. They vary in morphology and physiology, variously spherical, rod-shaped, spiral, lobed, or even rectangular. They vary from 0.1 to more than 15 μm in diameter, and some form aggregates or filaments up to 200 μm in length. They are usually harmless to other organisms, and none are known to cause disease.
The most notable characteristic distinguishing the archaeans is the composition of their cell membrane. Unlike most (but not all) eubacteria, they have a single cell membrane that lacks peptidoglycan. However, in both eubacteria and eukaryotes the membrane is composed mainly of glycerol-ester lipids. In Archaea, these are replaced by glycerol-ether lipids with branched hydrocarbon groups. They also have flagella notably different in composition and development from the superficially similar flagella of eubacteria.
The Archaea are divided into two main groups, the Euryarchaeota and Crenarchaeota, again based on rRNA trees. A third group, the Korarchaeota, are known only from environmental gene samples and may not really represent a separate group. The single species Nanoarchaeum equitum, discovered in 2002, has been tentatively placed in a fourth group but its affinities are uncertain.
Archaeans have been of interest to those studying the origins and early evolution of eukaryotes. Some trees have suggested they are a paraphyletic group, with the Euryarchaeota closer to the eukaryotes than the Crenarchaeota are. Since the membranes of eukaryotes are similar to those of eubacteria and not those of Archaea, some authors have proposed the nucleus originated from an archaeon symbiotic within a eubacterium, but this is not supported by arguments from cell structure. An alternative hypothesis is that the Archaea are a monophyletic group and their membranes have changed composition, possibly to allow hyperthermophily.
External links
References
- Howland, John L. The Surprising Archaea: Discovering Another Domain of Life Oxford: Oxford University Press. ISBN 0-19-511183-4
- Lake, J.A. (1988). Origin of the eukaryotic nucleus determined by rate-invariant analysis of rRNA sequences. Nature 331 184–186.
da:Archaea
de:Archaeen
et:Arhed
es:Archaea
eo:Arkio
fr:Archéobactérie
la:Archaea
ja:古細菌
pl:Archeowce
fi:Arkkieliöt
sv:Arkéer
zh:古细菌
Related links
Top visited 0 of 0 links
[no links posted yet]
>> place link >>
Discussion
Last posted 0 of 0 messages
[no messages posted yet]
>> post message >>
Watch
You can add this article to your own "watchlist" and receive e-mail notification about all changes in this page.
|
|
 |
|
 |
|
