General characteristics of Archaebacteria ( Ancient bacteria)

Bacteria are the most ubiquitous organism on earth. Prokaryote includes bacteria and blue green algae. Bacteria are of two groups eubacteria and archae bacteria. The eubacteria are commonly found in soil, water and living in or on larger organisms include the gram positive and the gram negative and the cyanobacteria. Archae bacteria are a group of ancient bacteria.They are supposed to be originated just after the origin of life on earth. They can live in extreme environmental conditions. They may be called the 'oldest living fossils'.

The Archae or archaebacteria, form the second subdivision of the prokaryotes and they are similar to eubacteria. On the basis of the evolution of their ribosomal RNA molecules, they appear as different from eubacteria.

• Morphologically they may be spherical rod shaped, spiral lobed, plate shaped irregularly shaped or pleromorphic. They range in diameter from 0-1 to over 15 um in length.
• They multiply either by binary fission, budding, fragmentation.

Most archae bacteria possess cell walls.But their cell wall structure and chemical composition is very different from that of eubacterial peptidoglycan cell wall. Their cell wall is made up of polysaccharides and proteins (glycoproteins). A few genera, such as methano bacterium have walls composed of pseudomurein, a polymer whose structure superficially resembles eubacterial peptidoglycan but differs markedly in chemical compositions. The cell membrane contain branched lipids which help them to tolerate extreme temperature and pH.Polar lipids are present in archae bacterial membranes: phospholipids, sulfolipilds and glycolipids. 7 to 30% of the membrane lipids are non poar lipids.

i) Strict or obligate anaerobes

They live only in the absence of oxygen.They are of two types

1.Methanogens

They produce methane from carbon dioxide or formic acid. In biogas plants these bacteria are responsible for the production of the biogas. Some methanogens live as symbionts in the rumen of the cattle. They help in the digestion of cellulose in their diet.

2.Halophiles

They live in the extreme salt environments. In the presence of light, the cell membrane becomes pigmented. It absorbs the light and captures energy as ATP molecules. They are only organism capable of captures light energy without photosynthesis.

ii) Facultative anaerobes

They are aerobic bacteria but can live in anaerobic conditions. They are mainly known as thermoacidophilies (temperature and acid living archaebacteria). They found on hot sulphur springs. They oxidise sulphur to sulphuric acid under aerobic conditions at about 80⁰C. They are found in marshy places.

Chromosome of archaebacteria is a single closed DNA circle, however their genomes are significantly smaller than the normal eubacetium.