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Archaebacteria

Archaebacteria Sub-kingdom of the kingdom Prokaryote, which, on the basis of both RNA and DNA composition and biochemistry, differs significantly from other bacteria. They are thought to resemble ancient bacteria that first arose in extreme environments such as sulphur-rich, deep-sea vents. Archaebacteria have unique protein-like cell walls and cell membrane chemistry, and distinctive ribosomes. They include methane-producing bacteria, which use simple organic compounds such as methanol and acetate as food, combining them with carbon dioxide and hydrogen gas from the air, and releasing methane as a by-product. The bacteria of hot springs and saline areas have a variety of ways of obtaining food and energy, including the use of minerals instead of organic compounds. They include both aerobic and anaerobic bacteria. Some hot springs bacteria can tolerate temperatures up to 88°C (190°F) and acidities as low as pH 0.9. One species, Thermoplasma, may be related to the ancestor of the nucleus and cytoplasm of the more advanced eukaryote cells. Some taxonomists consider archaebacteria to be so different from other living organisms that they constitute a higher grouping called a domain. See also taxonomy

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Archaebacteria

Archaebacteria (är´kēbăktĬr´ēə), diverse group of bacteria (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 chemosynthesis), 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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archaebacteria

archaebacteria (domain Archaea) Organisms belonging to the kingdoms Crenarchaeota and Euryarchaeota; formerly these were grouped together as the kingdom Archaebacteria.

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archaebacteria

archaebacteria (domain Archaea) Organisms belonging to the kingdoms Crenarchaeota and Euryarchaeota; formerly these were grouped together as the kingdom Archaebacteria.

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archaebacteria

archaebacteria (domain Archaea) Organisms belonging to the kingdoms Crenarchaeota and Euryarchaeota; formerly these were grouped together as the kingdom Archaebacteria.

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archaebacteria

archaebacteria(domain Archaea) Organisms belonging to the kingdoms Crenarchaeota and Euryarchaeota; formerly these were grouped together as the kingdom Archaebacteria.

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archaebacteria

archaebacteria See Archaea.

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Archaebacteria

Archaebacteria

General characteristics

Groups of Archaebacteria

Resources

Life on Earth can be divided into three large collections, or domains. These are the Eubacteria (or true bacteria), Eukaryota (the domain that humans belong to), and Archae. The members of this last domain are the archaebacteria.

Most archaebacteria (also called archae) look bacteria-like when viewed under the microscope. They have features that are quite different, however, from both bacteria and eukaryotic organisms. These differences led American microbiologist Carl Woese to propose in the 1970s that archaebacteria be classified in a separate domain of life. Indeed, because the organisms are truly separate from bacteria, Woese proposed that the designation archaebacteria be replaced by archae.

Archae are similar to eukaryotic organisms in that they lack a part of the cell wall called the peptidoglycan. Also, archae and eukaryotes share similarities in the way that they make new copies of their genetic material. However, archae are similar to bacteria in that their genetic material is not confined within a membrane, but instead is spread throughout the cell. Thus, archae represent a blend of bacteria and eukaryotes (some scientists call them the missing link), although generally they are more like eukaryotes than bacteria.

General characteristics

Archaebacteria are described as being obligate anaerobes; that is, they can only live in areas without oxygen. Their oxygen-free environments, and the observations that habitats of Archaebacteria can frequently be harsh (so harsh that bacteria and eukaryotic organisms such as humans cannot survive), supports the view that Archaebacteria were one of the first life forms to evolve on Earth.

Archaebacteria are microscopic organisms with diameters ranging from 0.00020.0004 in (0.51.0 micrometer). The volume of their cells is only around one-thousandth that of a typical eukaryotic cell. They come in a variety of shapes, which can be characterized into three common forms. Spherical cells are called cocci, rod-shaped cells are called bacilli, and spiral cells can either be vibrio (a short helix), spirillum (a long helix), or spirochete (a long, flexible helix). Archaebacteria, like all prokaryotes, have no membrane-bound organelles. This means that the archaebacteria are without nuclei, mitochondria, endoplasmic reticula, lysosomes, Golgi complexes, or chloroplasts. The cells contain a thick cytoplasm that contains all of the molecules and compounds of metabolism and nutrition. Archaebacteria have a cell wall that contains no peptidoglycan. This rigid cell wall supports the cell, allowing an archaebacterium to maintain its shape and protecting the cell from bursting when in a hypo-tonic environment. Because these organisms have no nucleus, the genetic material floats freely in the cytoplasm. The DNA consists of a single circular molecule. This molecule is tightly wound and compact, and if stretched out would be more than 1,000 times longer than the actual cell. Little or no protein is associated with the DNA. Plasmids may be present in the arch-aebacterial cell. These are small, circular pieces of DNA that can duplicate independent of the larger, genomic DNA circle. Plasmids often code for particular enzymes or for antibiotic resistance.

Groups of Archaebacteria

Archaebacteria can be divided into three groups. The first group is comprised of the methane producers (or methanogens). These archaebacteria live in environments without oxygen. Methanogens are widely distributed in nature. Habitats include swamps, deep-sea waters, sewage treatment facilities, and even in the stomachs of cows. Methanogens obtain their energy from the use of carbon dioxide and hydrogen gas.

The second group of Archaebacteria are known as the extreme halophiles. Halophile means salt loving. Members of this second group live in areas with high salt concentrations, such as the Dead Sea or the Great Salt Lake in Utah. In fact, some of the archaebacteria cannot tolerate a relatively unsalty environment such as seawater. Halophilic microbes produce a purple pigment called bacteriorhodopsin, which allows them to use sunlight as a source of photosynthetic energy, similar to plants.

The last group of archaebacteria lives in hot, acidic waters such as those found in sulfur springs or deep-sea thermal vents. These organisms are called the extreme thermophiles. Thermophilic means heat

KEY TERMS

Chloroplast Green organelle in higher plants and algae in which photosynthesis occurs.

Domain One of the three primary divisions of all living systems: Archae, Bacteria, or Eukaryota.

Enzyme Biological molecule, usually a protein, which promotes a biochemical reaction but is not consumed by the reaction.

Eukaryote A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Golgi complex Organelle in which newly synthesized polypeptide chains and lipids are modified and packaged.

Lysosome The main organelle of digestion, with enzymes that can break down food into nutrients.

Mitochondria An organelle that specializes in ATP formation, the powerhouse of the cell.

Nucleus A membrane-bound organelle in a eukaryote that isolates and organizes the DNA.

Organelle An internal, membrane-bound sac or compartment that has a specific, specialized metabolic function.

loving. They thrive at temperatures of 160°F (70°C) or higher and at pH levels of pH = 1or pH = 2 (the same pH as concentrated sulfuric acid).

Archaebacteria reproduce asexually by a process called binary fission. In binary fission, the bacterial DNA replicates and the cell wall pinches off in the center of the cell. This divides the organism into two new cells, each with a copy of the circular DNA. This is a quick process, with some species dividing once every twenty minutes. Sexual reproduction is absent in the archaebacteria, although genetic material can be exchanged between cells by three different processes. In transformation, DNA fragments that have been released by one bacterium are taken up by another bacterium. In transduction, a bacterial phage (a virus that infects bacterial cells) transfers genetic material from one organism to another. In conjugation, two bacteria come together and exchange genetic material. These mechanisms give rise to genetic recombination, allowing for the continued evolution of the archaebacteria.

Archaebacteria are fundamentally important to the study of evolution and how life first appeared on Earth. The organisms are also proving to be useful and commercially important. For example, methanogens are used to dissolve components of sewage. The methane they give off can be harnessed as a source of power and fuel. Archaebacteria are also used to clean up environmental spills, particularly in harsher environments where most bacteria will fail to survive.

A thermophilic archaebacterium called Thermus aquaticus has revolutionized molecular biology and the biotechnology industry. This is because the cells contain an enzyme that both operates at a high temperature and is key to making genetic material. This enzyme has been harnessed as the basis for a technique called the polymerase chain reaction (PCR). PCR is now one of the bedrocks of molecular biology.

Another increasingly popular reason to study archaebacteria is that they may represent one of the earliest forms of life that existed on earth. This has prompted the suggestion the development of life on other planets may involve similar microbes.

See also Evolution, divergent.

Resources

BOOKS

Alberts, Bruce, et al. Molecular Biology of the Cell. 4th ed. New York: Garland, 2002.

Oren, A. Halophilic Microorganisms and their Environments. New York: Springer, 2002.

Ventosa, Antonio. Halophilic Microorganisms. New York: Springer, 2003.

Brian Hoyle

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Archaebacteria

Archaebacteria

Life on Earth can divided into three large collections, or domains. These are the Eubacteria (or "true" bacteria ), Eukaryota (the domain that humans belong to), and Archae. The members of this last domain are the archaebacteria.

Most archaebacteria (also called archae) look bacteria-like when viewed under the microscope . They have features that are quite different, however, from both bacteria and eukaryotic organisms. These differences led American microbiologist Carl Woese to propose in the 1970s that archaebacteria be classified in a separate domain of life. Indeed, because the organisms are truly separate from bacteria, Woese proposed that the designation archaebacteria be replaced by archae.

Archae are similar to eukaryotic organisms in that they lack a part of the cell wall called the peptidoglycan. Also, archae and eukaryotes share similarities in the way that they make a new copy of their genetic material. However, archae are similar to bacteria in that their genetic material is not confined within a membrane , but instead is spread throughout the cell. Thus, archae represent a blend of bacteria and eukaryotes (some scientists call them the "missing link"), although generally they are more like eukaryotes than bacteria.

General characteristics

Archaebacteria are described as being obligate anaerobes; that is, they can only live in areas without oxygen . Their oxygen-free environments, and the observations that habitats of Archaebacteria can frequently be harsh (so harsh that bacteria and eukaryotic organisms such as humans cannot survive), supports the view that Archaebacteria were ones of the first life forms to evolve on Earth.

Archaebacteria are microscopic organisms with diameters ranging from 0.0002–0.0004 in (0.5–1.0 micrometer). The volume of their cells is only around one-thousandth that of a typical eukaryotic cell. They come in a variety of shapes, which can be characterized into three common forms. Spherical cells are called cocci, rod shaped cells are called bacilli, and spiral cells can either be vibrio (a short helix), spirillum (a long helix), or spirochete (a long, flexible helix). Archaebacteria, like all prokaryotes, have no membrane bound organelles. This means that the archaebacteria are without nuclei, mitochondria, endoplasmic reticula, lysosomes, Golgi complexes, or chloroplasts. The cells contain a thick cytoplasm that contains all of the molecules and compounds of metabolism and nutrition . Archaebacteria have a cell wall that contains no peptidoglycan. This rigid cell wall supports the cell, allowing an archaebacterium to maintain its shape, and protecting the cell from bursting when in a hypotonic environment. Because these organisms have no nucleus, the genetic material floats freely in the cytoplasm. The DNA consists of a single circular molecule . This molecule is tightly wound and compact, and if stretched out would be more than 1,000 times longer than the actual cell. Little or no protein is associated with the DNA. Plasmids may be present in the archaebacterial cell. These are small, circular pieces of DNA that can duplicate independent of the larger, genomic DNA circle. Plasmids often code for particular enzymes or for antibiotic resistance.


Groups of Archaebacteria

Archaebacteria can be divided into three groups. The first group is comprised of the methane producers (or methanogens). These archaebacteria live in environments without oxygen. Methanogens are widely distributed in nature. Habitats include swamps, deep-sea waters, sewage treatment facilities, and even in the stomachs of cows. Methanogens obtain their energy from the use of carbon dioxide and hydrogen gas.

The second group of Archaebacteria are known as the extreme halophiles. Halophile means "salt loving." Members of this second group live in areas with high salt concentration , such as the Dead Sea or the Great Salt Lake in Utah. In fact, some of the archaebacteria cannot tolerate a relatively unsalty environment such as seawater. Halophilic microbes produce a purple pigment called bacteriorhodopsin, which allows them to use sunlight as a source of photosynthetic energy, similar to plants.

The last group of archaebacteria lives in hot, acidic waters such as those found in sulfur springs or deep-sea thermal vents. These organisms are called the extreme thermophiles. Thermophilic means heat loving. They thrive at temperatures of 160°F (70°C) or higher and at pH levels of pH=1 or pH=2 (the same pH as concentrated sulfuric acid ).

Archaebacteria reproduce asexually by a process called binary fission. In binary fission, the bacterial DNA replicates and the cell wall pinches off in the center of the cell. This divides the organism into two new cells, each with a copy of the circular DNA. This is a quick process, with some species dividing once every twenty minutes. Sexual reproduction is absent in the archaebacteria, although genetic material can be exchanged between cells by three different processes. In transformation, DNA fragments that have been released by one bacterium are taken up by another bacterium. In transduction, a bacterial phage (a virus that infects bacterial cells) transfers genetic material from one organism to another. In conjugation, two bacteria come together and exchange genetic material. These mechanisms give rise to genetic recombination, allowing for the continued evolution of the archaebacteria.

Archaebacteria are fundamentally important to the study of evolution and how life first appeared on Earth. The organisms are also proving to be useful and commercially important. For example, methanogens are used to dissolve components of sewage. The methane they give off can be harnessed as a source of power and fuel. Archaebacteria are also used to clean up environmental spills, particularly in harsher environments where most bacteria will fail to survive.

A thermophilic archaebacterium called Thermus aquaticus has revolutionized molecular biology and the biotechnology industry. This is because the cells contain an enzyme that both operates at a high temperature and is key to making genetic material. This enzyme has been harnessed as the basis for a technique called the polymerase chain reaction (PCR ). PCR is now one of the bedrocks of molecular biology .

See also Evolution, divergent.


Resources

books

Howland, J.L. The Surprising Archaea. New York: Oxford University Press, 2000.


periodicals

Doolittle, W.F. "What are the Archaebacteria and Why are They Important?" Biochemical Society Symposium 58 (1992): 1–6.

Woese, C.R. "Bacterial Evolution." Microbiological Reviews 51 (1987): 221–271.

Woese, C.R., O. Kandler, and M.L. Wheelis. "Towards a Natural System of Organisms: Proposal for the Domains Archae, Bacteria, and Eucaya." Proceedings of the National Academy of Sciences USA 87 (1990): 4576–4579.


Brian Hoyle

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chloroplast

—Green organelle in higher plants and algae in which photosynthesis occurs.

Domain

—One of the three primary divisions, Archae, Bacteria, or Eukaryota, of all living systems.

Enzyme

—Biological molecule, usually a protein, which promotes a biochemical reaction but is not consumed by the reaction.

Eukaryote

—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Golgi complex

—Organelle in which newly synthesized polypeptide chains and lipids are modified and packaged.

Lysosome

—The main organelle of digestion, with enzymes that can break down food into nutrients.

Mitochondria

—An organelle that specializes in ATP formation, the "powerhouse" of the cell.

Nucleus

—A membrane-bound organelle in a eukaryote that isolates and organizes the DNA.

Organelle

—An internal, membrane-bound sac or compartment that has a specific, specialized metabolic function.

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