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(är'kēbăktĭr`ēə), diverse group of bacteriabacteria
[pl. of bacterium], microscopic unicellular prokaryotic organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. Once considered a part of the plant kingdom, bacteria were eventually placed in a separate kingdom, Monera.
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 (prokaryotes), sometimes called the archaea and considered a major group unto themselves. Archaebacteria are contrasted with the Eubacteria, from which they differ biochemically in the arrangement of the bases in their ribosomal RNA and in the composition of their plasma membranes and cell walls. There are three major known groups of Archaebacteria: methanogens, halophiles, and thermophiles. The methanogens are anaerobic bacteria that produce methane. They are found in sewage treatment plants, bogs, and the intestinal tracts of ruminants. Ancient methanogens are the source of natural gas. Halophiles are bacteria that thrive in high salt concentrations such as those found in salt lakes or pools of sea water. Thermophiles are the heat-loving bacteria found near hydrothermal vents and hot springs. Many thermophiles are chemosynthetic (see chemosynthesischemosynthesis,
process in which carbohydrates are manufactured from carbon dioxide and water using chemical nutrients as the energy source, rather than the sunlight used for energy in photosynthesis. Much life on earth is fueled directly or indirectly by sunlight.
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), using dissolved sulfur or other elements as their energy source and iron as a means of respiration. Archaebacteria emerged at least 3.5 billion years ago and live in environments that resemble conditions existing when the earth was young.
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A group of prokaryotic organisms that are more closely related to eukaryotes than bacteria. Based on comparative analyses of small subunit ribosomal ribonucleic acid (rRNA) sequences and selected protein sequences, the three primary lines of descent from the common ancestor are the Archaea (archaebacteria), the Bacteria, and the Eucarya (eukaryotes). Although the Archaea look like Bacteria cytologically (they are both prokaryotes), they are not closely related to them.

The Archaea can be divided into two evolutionary lineages on the basis of rRNA sequence comparisons, the Crenarchaeotae and the Euryarchaeotae. The crenarchaeotes are organisms that grow at high temperatures (thermophiles) and metabolize elemental sulfur. Most are strict anaerobes that reduce sulfur to hydrogen sulfide (sulfidogens), but a few can grow aerobically and oxidize sulfur to sulfuric acid. The euryarchaeotes have a number of different phenotypes. Thermococcus and Pyrococcus are sulfidogens like many crenarchaeotes. Archaeoglobus reduces sulfate to sulfide. Thermoplasma grows under acidic conditions aerobically or anaerobically (as a sulfidogen). Many euryarchaeotes are methane-producing anaerobes (methanogens) and some grow aerobically in the presence of very high concentrations of salt (halophiles). See Methanogenesis (bacteria)

The thermophilic archaea are found in high-temperature environments around the world. They have been isolated from soils and shallow marine sediments heated by nearby volcanoes and from deep-sea hydrothermal vents. Some are used as a source for heat-stable enzymes useful for industrial applications. The methanogenic archaea inhabit the digestive tracts of animals (especially ruminants like cows), sewage sludge digesters, swamps (where they produce marsh gas), and sediments of marine and fresh-water environments. They are of interest commercially because of their ability to produce methane from municipal garbage and some industrial wastes. Halophilic archaea live in the Great Salt Lake, the Dead Sea, alkaline salt lakes of Africa, and salt-preserved fish and animal hides. They are also commonly found in pools used to evaporate seawater to obtain salt.

The discovery of the Archaea caused a major revision in the understanding of evolutionary history. It had previously been thought that all prokaryotes belonged to one evolutionary lineage. Since their cellular organization is simpler, prokaryotes were assumed to be ancestors of eukaryotes. The discovery of the relationship of the Archaea to the Eucarya revealed that prokaryotes do not comprise a monophyletic group since they can be divided into two distinct lineages. Although the three descended from a common ancestor, modern eukaryotes may have arisen from fusions of bacterial and archaeal endosymbionts with ancestral eukaryotes. Chloroplasts and mitochondria arose from free-living bacteria which became endosymbionts. The discovery of the Archaea has also given microbiologists a better picture of the common ancestor. The deepest-branching eukaryotes (like Giardia) are strict anaerobes that lack mitochondria, and they diverged much later than the deepest-branching bacteria and archaea. The earliest archaea and bacteria (Thermotoga and Aquifex) are also anaerobes and are also extreme thermophiles. Therefore the common ancestor of these groups was probably also an extremely thermophilic anaerobe. Therefore, it is possible that life may have arisen in a relatively hot environment, perhaps like that found in deep-sea hydrothermal vents. See Bacteria, Eukaryotae, Prokaryotae

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


A group of unusual prokaryotic organisms that microscopically resemble true bacteria but differ biochemically and genetically, and form a distinct evolutionary group; some occur widely in oxygen-free environments and produce methane, while others are found in extreme salty or acidic conditions or grow at high temperatures.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Identification was made using 16S rDNA, a powerful tool for deducing evolutionary relationships and phylogenetic among eukaryotic organisms, bacteria and archaebacteria (Olmezoglu et al., 2012), was used.
Gamma-glutamyl transferases (GGT) (E.C. are a family of highly conserved enzymes occurring in archaebacteria, eubacteria, fungi, protozoa, nematodes, plants, and mammals [1].
represents a novel genus of marine heterotrophic archaebacteria growing optimally at 100 degC.
In order to simplify the task, we decided to use the classification categories provided in the textbook (Miller & Levine, 2001), excluding archaebacteria and eubacteria.
Unlike bacteria and archaebacteria, the choanoflagellate cell has a clearly defined nucleus.
In today's world, prokaryotes are exemplified by organisms known as bacteria and archaebacteria, which inhabit every conceivable biological niche on the planet.
Phylogenies based on biochemical data indicate that eukaryotic mitochondria are a sister-group of the [alpha]-purple eubacteria, while eukaryotic cells are a sister-taxon to the archaebacteria (Yang et al.
Dyall-Smith, "A plasmid vector with a selectable marker for halophilic archaebacteria," Journal of Bacteriology, vol.
As Case noted, attempted reconciliation of five Kingdoms (Bacteria, Protoctists, Animals, Plants, Fungi) with three Domains (Bacteria, Archaea, Eucarya) resulted in a "compromise" of six kingdoms (Archaebacteria, Eubacteria, Protista, Fungi, Plantae, Animalia)--an outcome that does not fully represent the intent of either the five-kingdom or three-domain system.
What's more, these origin-of-life researchers suspect that the two major groups of bacteria, known as archaebacteria and eubacteria, originated on two separate occasions about 3.8 billion years ago.
As it happens, the majority of the archaebacterial sequences are not consistent with currently accepted views of the Tree of Life which cluster the archaebacteria (Archaea) with eukaryotes (Eukarya).
In the late 1970s, Carl Woese and his colleagues at the University of Illinois at Urbana-Champaign noted that the prokaryotic kingdom itself consists of two distinct groups: Eubacteria ("true" bacteria), comprising most known bacteria, and Archaebacteria, consisting chiefly of methane-producing microbes and microbes that live in high-salt aquatic environments.