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The Archaea

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					                                               The Archaea
       Archaea are microscopic, single-celled organisms. Being prokaryotes they
have no membrane-bound organelles within their cells as you and I do, this
means no nucleus, no mitochondria, no chloroplasts, etc. Their DNA is normally
a single molecule circular in shape whereas our DNA comes in linear form in
several or many molecules. Their ribosomes are of the 70S type, ours are of the
80S type (except in mitochondria) and chloroplasts and plasmids are relatively
common. Lastly, they have no microtubule cytoskeleton.
       Biochemically they are nearly as different from Bacteria as they are from
Eukaryotes which is why they are in a Kingdom of their own. Scientists believe
that all three groups of living things, Bacteria, Archaea and Eukaryota all arose
separately from some unknown ancestor. Of 27 distinguishing characteristics
listed in Brock and Madigan 2000, Bacteria and Archaea share 15, Eukaryotes
and Archaea share 8, whilst Bacteria and Eukaryotes share only 3. One of these
is the possession of Plasmids which is common in both Bacteria and Archaea but
very rare in Eukaryotes.
                                       Technical Info
Archaea differ from bacteria in that they have histone proteins associated with their DNA
as we do. Like us, they have no muramic acid in their cell walls and they use methionine
as their initiator tRNA whereas bacteria use Formylmethionine. Also like us, their
ribosomes are sensitive to diphtheria toxin, those in bacteria are not. They are insensitive
to chloramphenicol, streptomycin and kanamycin, whereas most bacteria are sensitive to
these substances. Like bacteria, some of them are capable of denitrification and nitrogen
fixation but unlike bacteria none of them are capable of nitrification. Also like bacteria
some of them are capable of growth at temperatures above 80(C. No eukaryotes (us,
plants, etc.) are capable of this. They differ from both eukaryotes and bacteria in that
their membrane lipids are ether-linked not ester-linked and in that they are capable of
methanogenesis.
       Archaea though ancient creatures are a relatively recent discovery. Modern
science only learned about them in 1977 when they were discovered by Carl
Woese and George Fox.




http://www.earthlife.net/prokaryotes/archaea.html
       Far fewer Archaea are known to science than Bacteria, in fact only 209
species were listed in 1999, but now that more people are aware of, and looking
for them many more species are sure to be found. Most Archaea are anaerobic
(living in the absence of Oxygen) and many live in uncommon and extreme
environments, i.e. hot springs, Arctic ice floes, highly saline waters and highly
acidic or alkaline soils. Many of the world record holders for extreme
environments are Archaea. Their tolerances range from 4(C to 110(C, and from
pH -0.06 to 9.5, examples are given in the group synopses. Nearly half of the
known Archaea are Methanogenic, meaning that they give off methane as a by-
product of their metabolic activity. Though many Archaea (singular Archaeon)
live in environments which are hazardous to most other organisms, some live
much closer to us and species of Archaea have been found in animal, including
in the human, digestive tract.
       The Archaea have a diverse variety of shapes and exist not only as rods and
dots (cocci) like bacteria but also as triangles, discs, plates and cup-shapes.
       The Archaea are a diverse and fascinating group of micro-organisms and
the Korarchaeota (one of the group's main divisions) may contain the most
ancient of living things and therefore are the closest thing we have to our original
'ancestral organism'.
       The 209 species of Archaea are divided into 63 genera of which 24 are
monotypic (meaning that there is only one species in the genus). The Archaea
are divided into 3 main groups called Euryarchaeota, Crenarchaeota and
Korarchaeota. (Note: The higher classification of the Archaea - as in many other
taxa - is in dispute and the above groups may be called Kingdoms, Sections or
Phyla depending on the source used.) Here I have included some descriptions of
the ecological variety of the main types of Archaea.




http://www.earthlife.net/prokaryotes/archaea.html
                                              Korarchaeota
       Korarchaeota are a recently discovered group of Archaea. Their 16S rRNA
analysis places them as being distantly related to other Archaea and probably
ancestral to them in an evolutionary sense. They have been found in the
Yellowstone hot spring, Obsidian Pool, and though they are now living in culture
little more is known about them than that they have an optimum growth
temperature of 85(C.




http://www.earthlife.net/prokaryotes/archaea.html

				
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