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
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.
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
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.
Archaebacteria can be divided into three groups.
The first group is comprised of the methane producers
(or methanogens). These archaebacteria live in environarc ments 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.
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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