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The Archées or Archaea (of the Greek archaios , “old” and backterion , “stick”) are a major group of Procaryote S. They constitute a Taxon the alive one characterized by cells without core and being distinguished from the Eubactéries by certain biochemical characters, like the constitution of the cellular membrane or the mechanism of Réplication of the DNA.

Before work of Phylogénie, Archaea, which was still called archeobacteries, belonged to the reign of the monères in classification with five Règnes of the alive one. The more detailed analyzes showed that archées were as different from the Bacteria as those are to it Eucaryote S: this work led to classification in three fields of alive the.

The Archaea are extremely diversified. Some are known for their capacity with living under extreme conditions and occupy of the ecological niches which they are often alone to occupy (pH close to 0, higher temperature with 100°C, high salinity for example), but there exists much of alive Archées in Biotope S more current and very varied like the ground, the lakes, the sea or the intestine of the animals. They would contribute up to 20% of the total of the Biomasse.

History of discovered of Archaea

The Archaea were identified in 1977 thanks to work of Molecular biology of Carl Woese (professor at the University of the Illinois to Urbana, the United States) and George Fox. By studying the sequences of ribosomal ARN 16S, they separated Archaea from the other Procaryotes. These two groups were called in the beginning Archaéobacteria and Eubacteria. These groups then were famous Archaea and Bacteria to insist on the fact that they compose with the eucaryotes three fields quite distinct from the alive one.

Diversity of Archaea, Habitat

Archaea are very diverse, as well from a morphological point of view as physiological. They are unicellular beings with a size varying between 0,1 and 15 µm, but some develop to form filaments or aggregates (filaments up to 200 µm). They can be spherical (hulls), spirals, in the shape of stick, rectangular…

They show a great diversity of modes of reproduction, by binary fission, budding or fragmentation.

From a nutritional point of view, they are divided into very many groups, since the chimiolithoautotrophes (drawing their energy from chemical gradients of nonbiological origin) with the organotrophe S.

From a physiological point of view, they can be Aérobie S, Anaérobie S optional or strictly anaerobes.

Some prefer average temperatures (Mésophile S), much prefer very high temperatures (hyperthermophiles).
In anaerobic medium, hypersalin, some also develop in cold mediums (watery and terrestrial).

The archeobacteries often develop in extreme niches, where the living conditions are very difficult or impossible for the majority of the other organizations. Certain species are hyperthermophiles (found in extremely hot environments), others are found in cold, saline habitats (strong salt concentration up to 30%), alkaline or acides.
Pyrococcus , Methanopyrus , Thermococcus , Sulfolobus , Pyrodictium of Archées hyperthermophiles are isolated from hot sources, geysers, ebullient ponds or hydrothermal fluids on the level of smoker at sea deep (the hydrothermal fluids there emergent of the magmatic rooms at temperatures going from 200 to 350 °C). Pyrolobus fumarii is able to multiply up to 113 °C. Pyrobaculum comes from deep hot oil tanks. The Sulfolobus are acidophiles.

Members about the Halobacteria them ( Haloferax , Halobacterium, Halococcus, Halorubrum, Natrinema, Natronococus …) live in mediums with strong salt concentration (littoral sailor, salt-water marshes, Dead Sea, Big lake Salted of Utah). They often have a red pigmentation with yellow because of the Caroténoïde S and are responsible for the coloring of certain lakes (Lac Magadi in Kenya for example).

There exists of alive Archées mésophiles in marshes, the waste water, the ground, the sediments ( Methanobacterium, Methanosarcina, Methanogenium …). The archeobacteries methanogenes (producing of methane) of the marshes are responsible for marsh gases (Poitevin for example). Many Archées methanogenes are met in the digestive tract the ruminants ( Methanomicrobium, Methanosarcina ), the Termite S or the human ones.

Until today, there is no clear demonstration that there exists archées Pathogène S, although relations were proposed between the presence of archées methanogenes and diseases parodontales.

Although a great number of Archaea are not cultivable today in laboratory, of many species can be cultivated by using adapted culture media, and by reproducing the environmental conditions as well as possible their natural habitats.

Comparison between the cells of Archaea and those of the Bacteria and Eucaryotes

The Archaea are similar to the bacteria per many aspects of their cellular structure and their Métabolisme. However, the mechanisms and the Protein S implied in the processes of Réplication, transcription and translation present features similar to those met at the Eucaryote S. the characteristics of Archées compared to the two other fields of alive (Bacteria and Eucaryotes) are the following ones:

the structure and the chemistry of the cellular, atypical walls (traditional absence of Peptidoglycane in the bacteria)
the lipidic structure of their membrane: the Lipides of the archeobacteries consist of long isoprenic alcohol chains attached to the Glycérol by bonds ether - whereas the other organizations manufacture the lipids of their membranes by assembling two chains of fatty-acids with a glycerol molecule via a bond ester
the presence of ARN polymerase S unusual, much more complex than the ARN-polymerases of the bacteria, and surprisingly close to those of the eucaryotes.
a circular Chromosome of bacterial type but comprising Genes in mosaic similar to those of the Eucaryotes.
the proteins intervening in the processes of Réplication and repair of DNA resemble those met at the eucaryotes.
them Metabolism (methanogenes…)

The position of Archaea in the tree of the alive one

Although one does not know of them fossils, the traditional Phylogénie lets think that Archées exist since 3,5 billion years. But of other phylogenetic analyzes reveal them much later, towards ~850 million years, at the same time as the Eucaryotes.

Of small size and simple form, the procaryotes evolved/moved over very long periods and set up a group extremely varied. Their interactions with the environment are so varied that they play a crucial role in all the cycles of the terrestrial Biosphère.

These organizations a long time were gathered under the generic term of procaryotic , with the Bactérie S. to differentiate them, the microbiologists had worked out a system of comparison and classification based on small differences visible with the microscope, like on physiological differences (capacity to be developed on a certain medium for example).

As soon as it was a question of elucidating the genealogical relations between the various procaryotes, the biologists had to go obviously: the nutritional and phenotypical differences would not make it possible to classify the various organizations correctly. During years 1970, the biologists became aware of the irreplaceable existence of information, in the middle same of the cells of the living beings, allowing to determine the Phylogénie, DNA. The Gène identified in a cell is varying it from a gene which existed very many years ago. The comparison gene with gene between two organizations thus makes it possible to measure the time passed since the divergence starting from the common ancestor.

Carl Woese realized that ribosomal ARN (or ARNr, one of the molecules contained in the cell) of the organizations which he studied made it possible to highlight the existence of two clearly separate groups: bacteria and archeobacteries. Actually, Woese also realized that ARNr of archées were in fact as different from ARNr of the bacteria as of that of the eucaryotes. He concluded from it that one solely any more did not have to separate in two great groups the world of the alive one, according to the presence or from the absence from a core, but rather in three primitive fields: bacteria, archées and eucaryotes.

Today, of many studies confirmed the character Monophylétique of this group. These micro-organisms resemble by their form the bacteria, but from a molecular point of view, so some their features bring them closer to the bacteria, others rather bring closer them to the eucaryotes. It is thus not possible to see archées as being ancestors of the bacteria.

Great divisions at Archaea

On the basis of only metabolic criterion, archées were divided into three great groups:

the archeobacteries methanogenes
the halophilous Archéobactéries
the thermophilous Archéobactéries

The phylogeny, based on the genetic comparisons, mainly of the ARNr 16S (a component of ribosomes), made it possible to classify Archaea in two phyla:

the position of two others is not elucidated:

References

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