Category : 11th Class
They are present in rumen (first part of stomach) of cattles. This is simplest and most primitive group of bacteria. The cell wall of these bacteria is made of polysaccharides and proteins (peptidoglycans and muramic acid are absent in cell wall). Further branched chain lipids are present in plasma membrane of archaebacteria, due to which these can face extremes of conditions of temperature and pH. Archaebacteria are considered to be 'oldest of living fossils'. Three main groups of archaebacteria are following.
(1) Methanogens : These are strict anaerobic bacteria and mainly occur in muddy areas and also in stomach of cattle, where cellulose is fermented by microbes. These are responsible for methane gas \[(C{{H}_{4}})\] formation in bio-gas plants, because they have capacity to produce CH4 from CO2 or formic acid (HCOOH). e.g., Methanobacterium, Methanobacillus, Methanosarcina and Methanococcus.
(2) Salt lovers archaebacteria or Halophiles : These are also anaerobic bacteria, which occur in extreme saline or salty conditions (upto 35% of salt or NaCl in culture medium). A purple pigmented membrane containing bacteriorhodopsin is developed in sun-light in these bacteria, which utilizes light energy for metabolic activities (different from photosynthesis). e.g., Holobacterium and Halococcus.
(3) Thermoacidophiles : These are the bacteria which are found in hot sulphur springs (upto 80oC). As against first two groups of archaebacteria, these are aerobic bacteria. These have the capacity to oxidize sulphur to \[{{H}_{2}}S{{O}_{4}}\] at high temperature and high acidity (i.e., pH 2.0), hence given the name Thermoacidophiles, i.e., temperature and acid loving.
\[2S+2{{H}_{2}}O+3{{O}_{2}}\to 2{{H}_{2}}S{{O}_{4}}\]+ energy.
e.g., Sulfobolus, Thermoplasma, Thermoproteus.
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