Archaea
Crenarchaeota
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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:
- 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...)
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. They are also found in the digestive systems of
ruminants and
termites. 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, or even rectangular. 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.
From a
nutritional 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). No archaea capable of photosynthesis have yet been found.
Archaea 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. A few authors consider the Archaea and eukaryotes highly derived Bacteria (making that kingdom paraphyletic).
Comparisons of rRNA indicate the archea are more closely related to eukaryotes than eubacteria are, but the membranes of archaea differ greatly from the other two. To reconcile this inconsistency, some authors propose that the nucleus of eukaryotes arose from symbiosis between an archaeal cell inside a eubacterium.
See also
