Archaebacteria (Archae-ancient; bacteria) are special since, they live in some of the most harsh habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens).
The characteristics of this domain are:-
(i) They are most primitive prokaryotes.
(ii) They are found in stressed environment, such as high salt content (Great salt lake, the dead sea), edge of the ocean, hot sulphur springs, volcanic walls, etc.
(iii) Their cell walls lack peptidoglycan. In most cases, the wall composed of non-cellulosic polysaccharides and some proteins. In some members, there is no cell wall. This feature of having different cell walls is responsible for their survival in extreme condition.
(iv) Most of the archaebacteria are chemoautotrophs.
Types of Archaebacteria
Archaebacteria are of following three types:-
i. Methanogens:-
These are stricdy anaerobes. They live anaerobically in gut of several ruminants such as cows, buffaloes, goat, etc. These bacteria help in fermentation of cellulose. They produce almost 65% of atmospheric methane.
Example Methanobacterium, Methanobacillus, Methanosarcina and Methanococcus.
ii. Halophiles
These are found in extreme saline environments like salt lakes, • salt marshes, salt pans, salt solutions, etc. They are mosdy anaerobes. They contain a chemical called halorhodopsin to pump in chlorides into the cell to prevent cellular dehydration. Halobacterium develops purple membrane having photoreceptor pigment bacteriorhodopsin. In light, it acts as a proton pump and helps in synthesise of ATP. The formation of ATP is a survival mechanism under anaerobic condition. Examples Halobacterium and Halococcus.
iii. Thermoacidophiles:-
These archaebacteria can live in both extreme heat and acidic pH (around 2) environment. Under anaerobic conditions, these organisms oxidise sulphur to sulphuric acid.
2S + 2H2O + 3O2 → 2H2SO4 + Energy
=> Thermoacidophiles can survive in high temperature and low pH conditions because of:-
(a) Special branched chain lipids in cell membranes that reduce cell fluidity.
(b) Enzymes can work at low pH.
(c) Enzymes are resistant to high temperature coagulation. Examples Sulfobolus, Thermoplasma and Thermoproteus.