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These organisms are microscopic prokaryotes. When the first ones were discovered (in 1977), they were considered bacteria. However, when their ribosomal RNA was sequenced, it became obvious that they bore no close relationship to the bacteria and were, in fact, more closely related to the eukaryotes (including ourselves!) For a time they were referred to as archaebacteria, but now to emphasize their distinctness, we call them Archaea.

They have also been called Extremophiles in recognition of the extreme environments in which they have been found: The 200-odd species discovered so far have been placed in two groups:


There are three main groups:

1. Methanogens

These are found living in such anaerobic environments as They are autotrophic; using hydrogen as a source of electrons for reducing carbon dioxide to food and giving off methane ("marsh gas", CH4) as a byproduct.

4H2 + CO2 -> CH4 + 2H2O

Two methanogens have had their complete genomes sequenced: [View the data]

2. Halophiles

These are found in extremely saline environments such as the Great Salt Lake in the U.S. and the Dead Sea. They maintain osmotic balance with their surroundings by building up the solute concentration within their cells.

3. Thermoacidophiles

As their name suggests, these like it hot and acid (but not as hot some of the Crenarchaeota!). They are found in such places as acidic sulfur springs (e.g., in Yellowstone National Park) and undersea vents ("smokers").


The first members of this group to be discovered like it really hot and so are called hyperthermophiles. One, Pyrolobus fumaris, lives at 113°C (the boiling point of water at sea level is 100°C).

Many like it acid as well as hot and live in acidic sulfur springs at a pH as low as 1 (the equivalent of dilute sulfuric acid). These use hydrogen as a source of electrons to reduce sulfur in order to get the energy they need to synthesize their food (from CO2).

One member of the group, Aeropyrum pernix, has had its genome completely sequenced.

Other members of this group seem to make up a large portion of the plankton in cool, marine waters. As yet, none of these has been isolated and cultivated in the laboratory.

Evolutionary Position of the Archaea

The archaea have a curious mix of traits characteristic of

The table summarizes some of them.

Eukaryotic Traits Bacterial Traits
  • single, circular chromosome
  • operons
  • no introns
  • bacterial-type membrane transport channels
  • Many metabolic processes
    • energy production
    • nitrogen-fixation
    • polysaccharide synthesis

What can we conclude from this collection of traits?

The Origin of Life?

Their have suggested that the archaea may be the little-changed descendants of the first forms of life on earth.

Economic Importance of the Archaea

Because they have enzymes that can function at high temperatures, considerable effort is being made to exploit the archaea for commercial processes such as providing Archaea may also be enlisted to aid in cleaning up contaminated sites, e.g., petroleum spills.
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5 March 2001