Archaea

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Members of the Archaea comprise one of the three principal domains of living organisms in the universal phylogenetic tree of life. The other two principal domains are the Bacteria and the Eukarya. The phylogenetic tree is a theoretical representation of all living things, constructed on the basis of comparative ribosomal RNA sequencing and reflecting evolutionary relationships rather than structural similarities.

Many scientists hypothesize that the Archaea are the closest modern relatives of Earth's first living cells. Called "universal ancestors," these are the cells from which all other life is believed to have evolved. This hypothesis is based on two types of evidence. Genetic analyses indicate that the Archaea domain branches off of the phylogenetic tree at a point that is closest to the tree's root. Furthermore, it has been observed that many of the Archaea prefer to live in extremes of temperature, salt concentration, and pH—environmental conditions thought to be similar to those found on Earth over 3.5 billion years ago, when life first originated.

The Archaea share certain characteristics with Bacteria, others with Eukarya, and have some characteristics that are unique. For example, cells of the Archaea are structurally more similar to Bacteria, live predominantly as single cells, and have cell walls, although the walls do not contain the complex material called peptidoglycan that is a signature molecule of the Bacteria. While some Eukarya have cell walls, it is not a universal characteristic of that domain, and the Eukarya walls are composed of chitin or cellulose, neither of which occurs in cell walls of Archaea or Bacteria. Like the Bacteria, the Archaea lack a membrane-enclosed nucleus and their DNA exists in a circular form. On the other hand, their DNA is associated with histones, a characteristic of Eukarya, and their cell machinery (such as proteinsynthesizingenzymes and RNA polymerases) more closely resembles that found in the Eukarya. The lipids that comprise their membranes are unique, resembling neither the Bacteria nor the Eukarya.

Certain members of the Archaea are able to produce methane gas, another unique characteristic. Methane is one of the most important greenhouse gases. An Italian scientist named Alessandro Volta first discovered it as a type of "combustible air" over two hundred years ago. He trapped gas from marsh sediments and showed that it was flammable long before we knew that it was produced by members of the Archaea that lived in salt marsh sediment. Other important habitats for Archaea with this unique ability include the digestive tracts of animals and sewage sludge digesters.

The ability of many members of the Archaea to thrive in environmental conditions that we would find extreme is perhaps one of their most fascinating characteristics. There are genus like Halobacterium, which inhabit extremely salty environments, such as the Great Salt Lake in Utah and the Dead Sea in Israel. The salt concentration in these lakes is at least ten times that of seawater. Still other lakes, like Lake Magadi in Kenya, are not only extremely salty, but are also extremely alkaline, with pH values as high as 10 or 12. Archaea can be found even here, and their names reflect their habitat: Natronobacterium, Natronosomonas, and Natronococcus ("natro" means "salt"). The reddish-purple color sometimes seen in seawater-evaporating ponds, where solar salt is prepared, is the result of the growth of red-pigmented Archaea.

Extremes of temperature offer no challenge to certain members of the Archaea. A number of species, in fact, require temperatures over 80 °C in order to grow. Some live quite happily in the superheated outflow of geothermal power plants. Others thrive in the conditions of extreme acidity and temperature found in sulfur-rich, acidic hot springs like those in Yellowstone National Park, in the United States.

Archaea also populate the areas surrounding deep-sea vents, underwater volcanoes that form when the earth's crust opens along the ocean floor's spreading centers. The deep-sea vents have the hottest temperatures at which any living organism has been found. As of 2001, the current record for heat tolerance belonged to Pyrolobus fumarii, which can grow in water at a maximum temperature of 113 °C, well above boiling. At the opposite extreme, Archaea are among the few organisms found in the frigid waters of the Antarctic.

As a consequence of their ability to thrive in extreme conditions, the Archaea have become increasingly valuable. For example, the DNA polymerase of Thermus aquaticus, an Archaea found in the Yellowstone hot springs, is a heat tolerant enzyme that is crucially important in modern molecular biology laboratories, because of its use in the polymerase chain reaction. Archaea have also become important for commercial purposes. Their enzymes, sometimes called extremozymes, have made their way intolaundry detergent, for example, where they digest proteins and lipids in hot water or cold, and in extremely alkaline conditions, thus helping to remove life's little messes.

The high-salt density of the Dead Sea makes it difficult for humans to swim in its waters. Human bodies are much more buoyant in the Dead Sea waters than in fresh water. It is possible to lay back in the water—floating as if on an air mattress.

Cell, Eukaryotic; Eubacteria; Polymerase Chain Reaction; Ribosome.

Madigan, Michael T., John M. Martinko, and Jack Parker. Brock Biology of Microorganisms, 9th ed., Upper Saddle River, NJ: Prentice Hall, 2000.

Campbell, Neil A. Biology, 4th ed. Menlo Park, CA: Benjamin Cummings, 1996.

Madigan, Michael T., and Barry Marrs. "Extremophiles." Scientific American (April, 1997): 82-87.

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