Harley, John P. Department of Biological Sciences, Eastern Kentucky University, Richmond, Kentucky.
Last reviewed:January 2019
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A group of diverse microscopic prokaryotic organisms that constitute one of the three domains of life. Taxonomists recognize three domains of cellular life: Archaea, Bacteria, and Eukaryota (also termed Eukarya or Eucarya) [Fig. 1]. The members of the Archaea and Bacteria comprise the prokaryotes and look identical under a microscope; however, they differ in their chemical composition and are unrelated. Archaea microorganisms are often found in some of the most extreme environments on Earth, including anaerobic, hypersaline, extremely cold, and extremely high temperature environments. For example, they comprise about one-third of the prokaryotic biomass in coastal Antarctic waters, providing food for many marine animals. In addition, some are symbionts in the digestive system of animals. Notably, the members of the Archaea are not known to cause disease in humans or animals. See also: Bacteria; Eukaryota; Microbiology; Prokaryote
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