It is a group of very primitive procaryotes, also called living fossils. They were discovered by Woese.
Archaebacteria live under most extreme of environments like salt marshes and hot sulphur springs high temperature.
The cell wall does not contain cellulose or peptidoglycan (= mucopeptide = murein). It contains proteins and noncellulosic polysaccharides.
Wall contains pseudomurein in some methanogens.
Cell membrane possesses a single layer of branched chain lipids (glycero-hydrocarbon-glycerol) instead of phospholipid bilayer. Phospholipids possess ether bonds (– O – ) instead of ester bonds (O. CO).
Branched chain lipids decrease membrane fluidity and increase tolerance to high temperature.
Ribosomal ribonucleotides of 16 S rRNA are different from other organisms.
The archaebacteria show character of eukaryotes (presence of introns, glycerol esters in membrane lipid, no D-amino acid in cell wall) and prokaryotes (Incipient nucleoid, 70S type of ribosome, no meiosis and membrane bound organelle).
Archaebacteria are of three types :
Methanogens : They are obligate anaerobes, gram negative.
Cell wall of these bacteria contains protein (e.g. Methanogenium, Methanospirillum, Methanococcus) or noncellulosic polysaccharides (e.g. Methanosarcina) or Pseusomurein (e.g., Methanobacterium, Methanobrevibacter) in which instead of NAM, N-acetyl talosaminuronic acid is present and the D-amino acids are replaced by L-amino acids.
These bacteria produce methane from formic acid and carbondioxide used in the production of biogas in gobar gas plant. E.g., Methanobacterium, Methanococcus. Some of them live in rumen of cattle (e.g., Methanobacterium) and cause fermentation of cellulose.
Halophilies (Red) : facultative anaerobic chemo organotrophs, gram negative, require 17-23% (~ 20 % ) osmotic molarity 2.5 to 5m. NaCl for better growth.
Capable tolerating high acidity and high temperature. They live in salt rich media e.g., Halobacterium, Halococcus. Halophiles.
Some inhabit deep sea volcanic hydrothermal vents where water is boiling and flourish there well at temperature 104°C.
In light, they develop reddish pigment bacteriorrhodopism in their membrane to trap sun light to produce ATP but they can not use this ATP to produce food.
They can tolerate salt concentration due to -
Unique composition of cell membrane containing branched chain lipids.
Mucilage covering around the cell wall.
Absence of sap vacuoles due to which they can not be plasmolysed in high salt concentration.
They maintain a high osmotic concentration of KCl in their cells.
Accumulation of halogens.
Bacteriorhodopsin that provides ATP at low oxygen levels. These bacteia get lysed if NaCl level falls below 10%.
Thermoacidophiles : They are facultative anaerobe, chemoautotrophs, Gram negative, capable of tolerating high acidity and high temperature.
They live in hot sulphur springs (80°C, 2 pH) e.g., Thermoproteus.
Thermoacidophiles can also perform chemosynthesis .
Thermoplasma (it resembles Mycoplasma by lacking cell wall and undergo lysis at neutral pH).
- Sulfolobus (found in hot acidic springs at temperature upto 85°C). They tolerate high temperature due to homopolar bonds in their proteins. These thermoacidophiles oxide sulphur to under aerobic conditions and , and the energy so obtained is utilized for the synthesis of food. This makes medium acidic. Under anaerobic conditions, S is reduced to .