Once upon a time, man thought that all living organisms could be divided into two kingdoms: animal and vegetable. By the 19th century, with the advent of the microscope and greater understanding of the diversity of life, came another scheme of two main divisions: the Prokaryotae and the Eukaryotae. The Eukarya (eukaryotes) embody all macroscopic organisms, including humans, animals, and plants as well as some microscopic unicellular organisms such as yeast. The Prokaryotae (prokaryotes) include unicellular organisms such as bacteria. The distinguishing characteristic between the divisions is the presence of a well-defined nucleus (membrane-bound organelle housing the chromosomes and DNA-related functions) in individual cells, found in eukaryotes and lacking in prokaryotes.
After 1977, based on Carl Woese and his lab’s analysis of bacteria-like organisms with biochemical characteristics distinct from true bacteria, the prokaryotes were reclassified into two domains: Bacteria (formerly Eubacteria) and Archaea (formerly Archaebacteria). Archaea, the third domain of life, the other domains being Bacteria and Eukarya, are similar to bacteria in appearance but are separate life forms biochemically and genetically. Comparative studies with archaea have enabled greater elucidation of evolutionary history, and with multiple enzymes that are stable at high temperatures, the microbes have practical application in medicine, food science, and other industries.
Archaea were designated a third domain of life by comparing the sequences of nucleotide bases in RNA from their ribosomes (cell manufacturing sites of proteins) and subsequent verification by comparative genomics. One of the first archaea described by Woese was a methanogen, a microbe that produced the swamp gas methane from hydrogen and carbon dioxide in the environment. These microbes had a ribosomal RNA (rRNA) pattern different from that shown by bacteria, prompting the case for another domain of life.
Many more members of the domain Archaea have since been identified. Initially, it appeared that Archaea were only found in extreme environments, such as hot springs and deep-sea hydrothermal vents, or in highly alkaline or acid waters, or very salty lakes. Because many species live in extreme conditions, they remained unrecognized for so long and were difficult to culture in the laboratory. However, researchers now know archaea can be found in normal habitats as well, including soils, rice paddies, freshwater, and throughout the oceans. Some species of Archaea have been found in the rumens of animals, including the digestive tract of humans.
The Archaea have been separated into two major phyla, the Euryarchaeota and Crenarchaeota.
Archaea are chimeras with characteristics shared with Eukarya, other features shared with Bacteria, and yet have their own unique characteristics such as the ability of some species to produce methane gas. Like bacteria, they are single-celled organisms, lacking a membrane-enclosed nucleus, and having circular DNA. Archaea resemble Eukarya in that their DNA is associated with histone proteins and in the similarity of their cell machinery for DNA replication, gene transcription, and DNA repair. Additionally, the cell membranes and cell walls of Archaea are different in composition from that of Eukarya or Bacteria.
The Archaea are thought to be the closest representative of an ancestral progenitor for eukaryotes. The third domain includes many different and fascinating microbes of increasing value to industry and research.
References and Read-more-about-it:
1. Howland, J.L. (2000). The Surprising Archaea. Oxford: Oxford University Press.
2. Bolt EL, Delmas S. Archaea: a microbial cockatrice. May 2009. Microbiology Today, 89-90.
3. The Earthlife Web, The Archaea. Available at: http://www.earthlife.net/prokaryotes/archaea.html
Accessed July 5, 2010.
4. Rother M, Metcalf WW. Genetic Technologies for Archaea. 2005. Curr Opin Microbiol, 8(6):745-51.
5. Introduction to Archaea, Life’s extremists…. Available at: http://www.ucmp.berkeley.edu/archaea/archaea.html Accessed July 5, 2010.