General Characteristics of Archaebacteria (Greek – archae – ancient):
- These are the most primitive form of life.
- These are the most ancient bacteria. Some fossils found with these bacteria are 3,5 billion years old. As they were from the time of harshest conditions on the earth, they adapted themselves to live in any harshest condition. These bacteria are special since they live in some of the harshest habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens).
- They have unique cell membrane chemistry. Archaebacteria have cell membranes made of ether-linked phospholipids, while in case of bacteria and eukaryotes both make their cell membranes out of ester-linked phospholipids. The presence of this ether containing linkages in Archaea adds to their ability to withstand extreme temperature and highly acidic conditions.
- Their cell membrane has no peptidoglycans. Archaebacteria use a sugar that is similar to, but not the same as, the peptidoglycan sugar used in bacteria cell membranes.
- They are not influenced by antibiotics that destroy bacteria.
- Their rRNA is unique and is much different from rRNA of bacteria. Their t-RNA and r-RNA possess unique nucleotide sequences found nowhere else.
- Most of the archaebacteria are autotrophs. They use pigment bacteriorhodopsin for photosynthesis.
- Examples: Extreme halophiles – i.e. organisms which thrive in the highly salty environment, and hyperthermophiles – i.e. the organisms which thrive in the extremely hot environment, are best examples of Archaea.
Classification of Archaebacteria on the Basis of Habitat and metabolic activities:
Methanogens or Methanogenic Archaebacteria:
- As they are anaerobic autotrophs, they produce methane as the result of their metabolic activities. They produce methane gas from carbon dioxide and acetic acid from sewage in marshy condition.
CO2 + 4H2 → CH4 + 2H2O
CH3COOH → CH4 + CO2
- Methanogens are present in the gut of several ruminant animals such as cows and buffaloes and they are responsible for the production of methane (biogas) from the dung of these animals. Methane is greenhouse gas that leads to global warming.
- Methanogens die in presence of oxygen. Hence they can be found in swamp and marshes in which all oxygen is consumed. The typical smell in this areas is due to the production of methane.
- Methanogens help in the fermentation of cellulose. They do not decompose the organic matter but utilize the end products of decomposition.
- e.g. Methanobacillus, Thiobacillus etc.
Thermoacidophiles or Thermoacidophilic Archaebacteria:
- They are aerobic or facultative anaerobic chemoautotrophs.
- They are adapted to live in extremely hot (about 80 °C) and extremely low temperature (below freezing point) and acidic conditions (pH up to 2).
- They are found in hot springs (Sulfolobus). in refuse piles of coal mines (Thermoplasma) or geothermal area of Iceland (Thermoproteus)
- Most of the thermoacidophiles use hydrogen sulphide as their energy source. They are chemotrophs
2S + 2H2O + 3O2 → 2H2SO4 + Energy (aerobic condition)
- Under anaerobic condition, sulphur is reduced to hydrogen sulphide
- They precipitate bicarbonate into carbonate due to their activities.
- e.g. Thermoplasma, Picrophilus, Thermococci, Pyrococcus, Sulfolobus, etc.
Halophiles or Halophilic Archaebacteria:
- They aerobic or facultative anaerobic heterotrophs.
- They live in salty environments such as a great salt lake or dead sea, marshes, brine, salt-rich soil where the salt concentration is in range of 2.5 M to 5 M.
- They have high intracellular concentrations. Their enzymes and ribosomes function efficiently at higher salt concentration.
- They contain special photoreceptor pigment called bacteriorhodopsin. Due to which they acquire purple colour.
- Bacteriorhodopsin protects halophiles from strong solar radiations. It helps in the synthesis of ATP.
- It shows chemotrophic nature of nutrition.
- e.g. Halobacteria, halococcus, etc.