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:
- thermophiles, who live at high temperatures,
- hyperthermophiles, who live at really high temperatures (present record is 113°C!)
- psychrophiles, who like it cold (one in the Antarctic grows best at 4°C)
- halophiles, who live in very saline environments (like the Dead Sea)
- acidophiles, who live at low pH (as low as pH 1 and who die at pH 7!)
- alkaliphiles, who thrive at a high pH.
There are three main groups:
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.
- the muck of swamps and marshes
- the rumen of cattle (where they live on the hydrogen and CO2 produced by other microbes living along with them)
- sewage sludge
- the gut of termites.
4H2 + CO2 -> CH4 + 2H2O
Two methanogens have had their complete genomes sequenced:
[View the data]
- Methanococcus jannaschii and
- Methanobacterium thermoautotrophicum
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.
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.
The archaea have a curious mix of traits characteristic of
- the other prokaryotes - the bacteria as well as traits found in
The table summarizes some of them.
- single, circular chromosome
- no introns
- bacterial-type membrane transport channels
- Many metabolic processes
- energy production
- polysaccharide synthesis
- Many traits found in the bacteria first appeared in the ancestors of all the present-day groups.
- The split leading to the archaea and the eukaryotes occurred after the bacteria had gone their own way.
- However, the acquisition by eukaryotes of
occurred after their line had diverged from the archaea.
have suggested that the archaea may be the little-changed descendants of the first forms of life on earth.
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
- ability to live in extreme environments
- autotrophism; that is, their ability to make food using materials (H2, S, CO2) in the earth's crust
Archaea may also be enlisted to aid in cleaning up contaminated sites, e.g., petroleum spills.
- enzymes to be added to detergents (maintain their activity at high temperatures and pH)
- an enzyme to covert corn starch into dextrins
- Taq polymerase: a DNA polymerase isolated from Thermus aquaticus, a denizen of a Yellowstone hot spring. Used for PCR - the polymerase chain reaction.
5 March 2001