In this article we will discuss about:- 1. Definition of Archaebacteria 2. Types of Archaebacteria 3. Uses.
Definition of Archaebacteria:
They are a group of most primitive prokaryotes which are believed to have evolved immediately after the evolution of the first life. They have been placed in a separate subkingdom or domain of Archaea by a number of workers.
Archaebacteria are characterised by absence of peptidoglycan in their wall. Instead the wall contains protein and non-cellulosic polysaccharides. It has pseudomurein in some methanogens. The cell membranes are characterised by the presence of a monolayer of branched chain lipids. Their 16S rRNA nucleotides are quite different from those of other organisms.
Many archaebacteria even now live under extremely hostile conditions where very few other organisms can dare subsist, e.g., salt pans, salt marshes, hot sulphur springs. The archaebacteria are of two broad categories, obligate anaerobes and facultative anaerobes.
Obligate anaerobes can live under anaerobic conditions only. They get killed in the presence of oxygen, e.g., methanogens. Facultative anaerobes are actually aerobic archaebacteria which can bear anaerobic conditions comfortably. They are represented by thermoacidophiles and halophiles.
Types of Archaebacteria:
Archaebacteria are of three major types— methanogens, halophilic and thermoacedophilic, Methanogens and halophiles are placed in division euryarchaeota while thermoacidophiles are placed in division creuarchaeota.
1. Methanogens:
The archaebacteria are strict anaerobes. Nutritionally they are “autotrophs” which obtain both energy and carbon from decomposition products. They occur in marshy areas where they convert formic acid and carbon dioxide into methane with the help of hydrogen.
This capability is commercially exploited in the production of methane and fuel gas inside gobar gas plants e.g., Methanobacterium, Methcinococcus.
Some of the methanogen archaebacteria live as symbionts (e.g., Methanobacterium) inside rumen or first chamber in the stomach of herbivorous animals that chew their cud (ruminants, e.g., cow, buffalo). These archaebacteria are helpful to the ruminants in fermentation of cellulose.
2. Halophiles (Halophils):
Halophiles are named so because they usually occur in salt rich substrata (2.5-5.0 M) like salt pans, salt beds and salt marshes e.g., Halobacterium, Halococcus. They are aerobic chemo-heterotrophs.
Their cell membranes have red carotenoid pigment for protection against harmful solar radiations. Under anaerobic conditions, halophiles cannot use external materials. At this time they subsist on ATP synthesised by membrane pigment system from solar radiations.
Halophiles are able to live under high salt conditions due to four reasons:
(1) Presence of special lipids in the cell membranes.
(2) Occurrence of mucilage covering.
(3) Absence of sap vacuoles and hence plasmolysis.
(4) High internal salt content.
Halophiles growing in salt pans and salt beds give offensive smell and undesirable pigmentation to the salt.
3. Thermoacidophiles (Thermoacidophils):
These archaebacteria have dual ability to tolerate high temperature as well as high acidity. They often live in hot sulphur springs where the temperature may be as high as 80°C and pW as low as 2, e.g., Thermo plasma, Thermoproteus.
Basically these archaebacteria are chemosynthetic, i.e., they obtain energy for synthesis of food from oxidising sulphur. Under aerobic conditions they usually oxidise sulphur to sulphuric acid.
2S + 2H2O + 3O2 → 2H2SO4
If the conditions are anaerobic, the thermoacidophiles may reduce sulphur to H2S. Bicarbonates are also precipitated into the carbonate form by their activity.
Thermoacidophiles are able to tolerate high temperature as well as high acidity due to two reasons:
(1) Branched chain lipids in the cell membranes.
(2) Presence of special resistant enzymes capable of operating under acidic conditions.
Archaebacteria are also known as living fossils because they represent one of the earliest forms of life which experimented on the absorption of solar radiations for the first time, lived comfortably under anaerobic conditions and developed techniques to oxidize the chemicals present in the substratum on the availability of oxygen.
Uses of Archaebacteria:
(i) Archaebacteria are employed in the production of gobar gas from dung and sewage,
(ii) In ruminants, they cause fermentation of cellulose.