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The Archaea are one of the three major groups of living organisms, together with the bacteria and eukaryotes. 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:
- wall structures and chemistry (lack of peptidoglycan and Gram
staining)
- lipidic membrane structure (their lipid bilayers consist of branched chain hydrocarbons linked by ether linkages to
glycerol)
- metabolism (methanogens, sulfate reducers...)
Many Archaea 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 and in the
marine environment generally. 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 stricly 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.
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
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