Archaebacteria : Definition,Characteristics,Types and Their Uses

In this article we will discuss about:- 1. Definition of Archaebacteria 2. General Characteristics of Archaebacteria 3.Types of Archaebacteria 4. Uses of Archaebacteria.

1. 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.

2. General Characteristics of Archaebacteria

Archaebacteria may be Gram-positive or Gram-negative. Cells are generally invested with a cell- wall, except those of Thermo plasma, a wall-less mycoplasma-like genus. Archaebacterial cells may be spherical, rod-shaped, spiral, irregularly lobed as in Sulfolobus, or filamentous. Cell diameter ranges between 0.1 μm and 1.5 μm. Cells multiply by several means, like binary fission, budding, fragmentation etc.

The organisms may be aerobic, anaerobic, chemolithotrophic or chemoorganotrophic. They mostly occur in extreme environments, though some are mesophilic. Gram- positive archaebacteria have a thick homogeneous cell wall mainly containing complex polysaccharide. The Gram-negative forms have a thinner wall consisting mainly of protein or glycoprotein.

An outer membrane characteristically present in the Gram-negative true bacteria is absent in archaebacteria. Murein is absent in the cell wall of both Gram-positive and Gram-negative archaebacteria. In some, like Methanobacterium, a peptidoglycan-like polymer, called pseudomurein is present. Due to the absence of murein, the archaebacteria are insensitive to β-lactam antibiotics like penicillins, cephalosporin’s, etc.

A unique feature of archaebacteria is the presence of ether-linked isopranyl lipids in their membranes. The composition of the archaebacterial lipids may change in response to the environment. Under more extreme environmental conditions, like high salinity and high temperature, the composition changes to provide a more rigid and stable membrane to withstand the environmental stress.

Like all other prokaryotes, archaebacteria have a covalently linked closed circular genome, but its size is generally smaller than that of other prokaryotes. The genome size ranges between 0.8 to 1.1 x 109 Daltons. G + C content of DNA varies widely, from 21 to 68 moles%. Ribosomes are of 70S type, but their shape may vary from those of other prokaryotes.

So far as biochemical characteristics are concerned, the archaebacteria do not appear to use the EMP for glucose dissimilation, because of the absence of 6-phosphofructokinase. Tricarboxylic acid cycle has been found to be operative either fully or partly in many halophilic and thermophilic archaebacteria. Mode of metabolism may range from typical organotrophy to strict chemolithotrophy.

Halo bacterium, an extreme halophile, can utilize light energy to produce ATP with the help of a special pigment, called bacteriorhodopsin. The chemolithotrophic members do not use the Calvin cycle for C02-fixation, but several other pathways like reductive TCA cycle or reductive acetyl-CoA pathway. At least three members of the group have been reported to be able to fix atmospheric nitrogen.

3. 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.

4. 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