They are believed to have evolved immediately after the origin of life on earth, as even now these are living under extremely adverse condition such as extreme salty areas (halophiles), hot springs (thermosacidophiles) and marshy area (methanogens). Very few other organism can survive under such environmental conditions. So these are termed as “living fossils”.
These possess introns in DNA, their ribosomal proteins are highly acidic, these prokaryotes possess histone proteins different from that of eukaryotes. These being the most primitive and ancient most bacteria.
Archaebactgeria differ from other bacteria in having different cell wall structure and this feature is responsible for their survival in extreme conditions. The cell wall in archaebacteria contains proteins and non-cellulosic polysaccharides. It lacks peptidoglycan. The characteristic cell wall material in bacteria and cyanobacteria. It consists of glycoprotein , pseudomurein and non cellutosic polysaccharide. Pseudomurein is like bacterial peptidoglycan, but contain N- acetyltalosaminuronic acid instead of NAM and lacks D- Amino acid. The cell membrane contains branched chain lipids (phytanyl side chains) which decreases membrane fluidity. This chemical composition of the cell membrane enables these organisms to withstand extremes of temperature and pH.
Archaebacteria re divided into three groups – methanogens, halophiles and thermoacidophiles
a) Methanogens : They are obligate anaerobes occurring in marshy habitats. They are capable of converting CO2. Methanol and formic acid (HCOOH) into methane and hence the name methanogens. This property is exploited commercially in the production of fuel gas and methane in gobar gas plants (biogas fermenters ). Some of the methanogens live in rumen of herbivorous animals like buffalo, cow etc. (ruminants). These microorganisms assist in fermentation of cellulose in such animals, e.g., Methanococcus, Methanobacterium , Methanosarcina, Methanospirillum.
b) Halophiles : They are aerobic chemoheterotrophic coccoid forms and are Gram negative. They occur in high salt concentration medium like sea, salt lake, brines, marshes, salted fish etc.
in high light intensity, a reddish pigment bacteriorhodopsin develops in their membrane to trap sun light to produce ATP, but they cannot use this ATP in food synthesis. Sap vacuoles are absent in halophiloes. Hence, they cannot get plasmolysed in high salt concentration. They maintain a high osmotic concentration of KCI in their cells. These bacteria get lysed if NaCI level falls below 10% , e.g., Halococcus, Halobacterium. These can grow well in a medium containing 25-30% of NaCI
c) Thermoacidophiles : They are capable of tolerating high temperature as well as high acidity and hence, the name thermoacidophiles. They often live in hot- water springs where the temperature is as high as 80 degree C and the pH as low as 2. They oxidize sulphur to suphuric acid under aerobic conditions and the energy obtained in this reaction is utilized for the synthesis of organic food. The medium becomes highly acidic due to the production of sulphuric aicd. Under anaerobic conditions sulphur is reduced to H2S , e.g., Thermoplasma, Themoproteus. Hence, these are chemosynthetic in nature.
Did You Know?
Thermoacidophiles are capable of withstanding extremely low pH and high temperature due to :
a) The cell membrane containing branched chain lipids
b) The presence of resistant enzymes which can operate under acidic conditions.
It seems probable that archaebactgeria, living in extreme environmental conditions got separated from the main line of bacterial evolution quite early. They survived through long period of geological changes successfully, existing at present and are perhaps the oldest “living fossil”