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Updated on February 3, 2015


THE ARCHAEBACTERIA (Ancient bacteria)

They are spherical/ rod shaped/ pleomorphic; single/ multiple/ filamentous. They reproduce by binary fission/ budding/ fragmentation. Nutrition is aerobic/ facultative anaerobic/ chemolithotrophic to organotrophic.they are found in extreme aquatic and terrestrial habitats; known as extremophiles (are mesophilic or hyperthermophilic; grows in freshwater or concentrated brines or symbionts in digestive system). Their cell wall is diverse; no peptidoglycan and no muramic acid; has pseudomurein, protein sulfur layer or polyssaccharide. Their membrane lipids have ether linkages (tetraethers and pentacyclic rings). They contain single closed DNA circle; may have plasmids like bacteria. Some genomes have histone proteins like eukaryotes. DNA replication is bidirectional. Their carbohydrate metabolism is by modified Embden Meyerhof Pathway or Entner Deodroff Pathway. In halophiles and thermophiles, complete Tricarboxylic Acid Pathway is present; methanogens does not have complete TCA pathway; carbondioxide fixation takes place by reductive TCA cycle and reductive acetyl CoA cycle. Thermostability is provided by rigid proteins that increases supercoiling in DNA.

  1. Taxonomy: 2 phyla

    Euryarchaeota (3 orders)

    1. Methanogens
    2. Halophiles
    3. Thermophiles

    Crenarcheota (2 orders)

    1. Thermoproteales
    2. Sulfolobales
    3. Desulfurococcales
    4. Caldisphaerales



Methanogens are a type of microorganism that produces methane as a by-product of metabolism in conditions of very low oxygen. They can live in environments without oxygen. They are widely distributed in swamps, deep-sea waters, sewage treatment facilities, and even in the stomachs of cows: They are often present in bogs, swamps, and other wetlands, where the methane they produce is known as "marsh gas." Methanogens also exist in the guts of some animals, including cows and humans, where they contribute to the methane content of flatulence.

  1. Some types of methanogen, including those of the Methanopyrus genus, are extremophiles, organisms that thrive in conditions most living things could not survive in, such as hot springs, hydrothermal vents, hot desert soil, and deep subterranean environments. Methanocaldococcus genus, are mesophiles, meaning they thrive best in moderate temperatures. Methanobrevibacter smithii is the prominent methanogen in the human gut, where it helps digest polysaccharides, or complex sugars. They are strict anaerobes; obtain their energy from the use of carbon dioxide, hydrogen gas, formate, methanol, acetate etc.

    They use a source of carbon, such as carbon dioxide or acetate, to drive their metabolism, called methanogenesis, along with hydrogen as a reducing agent. Therefore, they have the ecological benefit of removing excess hydrogen and carbon from anaerobic environments. A methanogen that metabolizes carbon dioxide is classified as hydrogenotrophic, while those that metabolize acetate are called acetotrophic or aceticlastic.

    Methanogens are used to dissolve components of sewage. The methane they give off can be harnessed as a source of power and as a clean burning fuel and an excellent energy source. Methane is a greenhouse gas which contributes to global warming. They have 5 orders and 26 genera.

Bacilli Methanogens
Bacilli Methanogens


Halophiles (salt loving):

  1. Halophiles (Greek = salt lovers) are “Organisms which require a minimum concentration of sodium chloride in their environment.” Halophilic microorganisms live in areas with high salt concentration, such as the Dead Sea or the Great Salt Lake in Utah. They are unique because they require high levels of salt that would be lethal to most organisms.

    These are aerobic chemoheterotrophs with respiratory metabolism; motile/non motile; different cell shapes (cubes/pyramids).They require 1.5 M NaCl; optimum 3-4 M NaCl. Cells accumulate organic or inorganic solutes to balance osmolarity. They produce a purple pigment called bacteriorhodopsin, which allows them to use sunlight as a source of photosynthetic energy, similar to plants. Halobacterium salinarum uses 4 different light utilizing rhodopsin molecules.

    Halophilic microorganisms are normally easy to cultivate and they are harmless, since they cease growth and lyse at the low salinity of our usual environment. They are involved in the production and spoilage of salted foods. It has 17 genera.




  1. They have 3 genera with thermoacidopiles that lack cell wall. They live and reproduce in a sizzling temperature range that could be anywhere between 45 and 80 degrees Celsius. The record breaker is an even more extreme kind of extremophile and it's called Strain 121 - so named because it flourishes in temperatures of 121 degrees Celsius.

    The genus name is derived from the Greek thermē and plasma, meaning “warmth” (or “heat”) and “formative substance,” respectively, which describe the thermophilic (heat-loving) nature of these organisms. They are present as irregular filaments or cocci in hot acidic coal mines. Optimum growth temperature: 55- 59%; pH <2. Cell membrane strengthenes by diglycerol tetraethers, lipopolysaccharides and glycoproteins.

    They are capable of both aerobic and anaerobic metabolism. Their survival in anaerobic habitats is dependent on sulfur respiration, a form of chemolithotrophic metabolism in which carbon and energy are obtained from the reaction of sulfur with organic compounds. Extremely thermophilic sulfur metabolizers reduce sulfur to sulfide; grows at 88-1000 C.They contain Histone like proteins stabilize DNA forming particles. Thermus aquaticus has revolutionized molecular biology and the biotechnology industry. The cells contain an enzyme that both operates at a high temperature and is a key to making genetic material. This enzyme has been harnessed as the basis for a technique called the polymerase chain reaction (PCR). Two species of Thermoplasma have been described: T. acidophilum, discovered in coal refuse and first reported in 1970, and T. volcanium, initially discovered in solfataric fields on Vulcano Island, Italy, and reported in 1988

Thermoplasma volcanium
Thermoplasma volcanium


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