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The living organisms are classified into bacteria, Archaebacteria and Eukaryota. Previously Archaebacteria were considered bacteria because of their small size and absence of a well-defined nucleus. Later they are classified under a separate group from bacteria because of their distinct characteristics.

Archaebacteria are prokaryotic single-celled microorganisms. The term prokaryotic casts light on its cellular organisation. They lack a well-defined nucleus and the genetic material is a simple circular chromosome. They have a special ability to survive in envharshironments like boiling waters like hot water springs, extremely saline conditions, or even acidic environments. Archaebacteria are both aerobic and anaerobic organisms.

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What are Archaebacteria?

The term “Archae” is derived from the Greek word “archaios” which means ancient or primitive. They are primitive organisms and are believed to be evolved first through evolution. The organisms under the domain Archaebacteria are simple unicellular prokaryotes. They are microscopic and invisible to the naked eye.

Archaebacteria were once considered bacteria because of their close similarity with bacterial cells under the microscope. However, they are considered a separate domain because of the dissimilarities shown with bacteria. Archaebacteria lack the peptidoglycan cell wall and share some similarities with eukaryotes. Some researchers consider Archaebacteria as the link between bacteria and organisms of Eukaryota

Examples of Archaebacteria

Some examples of Archaebacteria are Thermophyllus aquaticus, Sulfolobus tokodaii, Methanobrevibacter smithii, and Pyrolobus fumarii.

Characteristics of Archaebacteria

  • Archaebacteria are enclosed by a rigid cell wall composed of polysaccharides and glycoconjugates. The cell wall is devoid of peptidoglycan which makes them differ from bacteria. The rigid cell wall protects them from extreme external conditions by protecting the internal osmotic pressure

  • Cell membranes in Archaebacteria are unique to the domain with respect to the composition. It is made up of ether-linked phospholipids

  • The membrane-bound organelles are absent in Archaebacteria since they are prokaryotes. They lack a well-defined nucleus and other organelles like mitochondria, lysosomes, Golgi apparatus, and endoplasmic reticulum. The genetic material is a single, tightly wound, circular chromosome similar to that present in bacteria

  • Plasmids may be present floating in the cytoplasm. They generally code for antibiotic resistance enzymes

  • The cells may be spiral, round or rod-shaped

  • Archaeal RNA polymerase is unique and resembles that of eukaryotes in some manner. Hence the gene transcription is also unique to the domain

  • Archaebacteria are both aerobic, and anaerobic and some Archaebacteria are extremophiles. They live in extreme and harsh environments of temperature, acidity and salinity. Some species live in sulphur-rich environments also

  • Sexual reproduction is absent in archaebacteria. They reproduce asexually by binary fission. However, the exchange of genetic material is by transformation, conjugation and transduction. The genetic exchange results in the recombination of genetic material which is essential for evolution

Types of Archaebacteria

Archaebacteria are of three types, Methanogens, Halophiles, and Thermophiles.


  • Methanogens are strict anaerobes. They obtain energy from decomposition products and produce methane

  • They live in marshy habitats like sewage treatment plants and also as endosymbionts in the intestinal tracts of ruminant animals like cows and buffaloes

  • The natural gas existing on earth present-day is a product of methanogens. This ability is utilised for commercially producing methane through gobar gas plants. Organisms like Methanococcus and Methanobacterium fall under this category


  • Halo means “salt” and philic means “loving”. They are organisms living in extremely saline environments. Hence their name is halophiles.

  • They are aerobic chemoheterotrophs.

  • The presence of high cellular salt content and special membrane lipids help them survive in saline environments like salt lakes and salt marshes. Examples are Halobacterium and Halococcus


  • Thermophiles as the name suggests, they are heat (Thermo) loving (philic). They are chemosynthetic organisms living in extremely high temperatures. They utilise sulphur to oxidise it and produce energy

  • They can be thermophiles, hyperthermophiles (tolerance to extremely hot temperatures 80℃) and thermoacidophiles (tolerate high temperatures as well as highly acidic conditions with pH 2 also).

  • The presence of special membranous branched lipids and enzymes for survival in acidic environments allows these organisms to survive in such harsh conditions. They inhabit boiling hot water springs and thermal vents

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Importance of Archaebacteria

  • The methanogenic Archaebacteria are capable of producing methane by utilising organic compounds and this ability is employed in biogas production from the organic wastes

  • The thermophilic Archaebacteria are tolerant to extreme temperature conditions. Thermus aquaticus is a source of Taq DNA polymerase which is heat resistant enzyme with wide applications in molecular biology techniques

  • They are a source of heat-resistant enzymes which have major applications in detergent preparation.

  • The soil-inhabiting Archaebacteria are efficient in nutrient upcycling and contribute to plant growth

  • Since they are primitive life forms, intense studies can help scientists understand the processes of evolution and those conditions that prevailed on ancient earth

  • Archaebacteria are producers and consumers of ecosystems belonging to extreme conditions

  • Because of their ability to utilise the organic substances, Archaebacteria are employed for environmental-related processes like bioremediation and biodegradation

Difference between bacteria and Archaea

  • Bacteria contain cell walls composed of peptidoglycan layers while Archaebacteria have cell walls made of polysaccharides and glycoconjugates. Archaebacteria are devoid of peptidoglycan cell walls.

  • Cell membranes of Archaebacteria have ether-linked phospholipids which are a unique characteristic of this domain. Unlike archaebacteria, the bacteria contain ester-linked phospholipid cell membranes

  • The mode of reproduction in Arcahebacteria is asexual and is by binary fission. Archaebacteria unlike bacteria are unable to produce spores for reproduction.


Archaebacteria are the primitive living organisms of the earth. They are single-celled prokaryotes which were once considered bacteria due to their microscopic nature. However, with the improvements in advanced bio techniques, the archaebacteria were learnt to have certain distinguishing features from bacteria and hence are considered a separate domain. They are aerobic as well as anaerobic organisms. Archaebacteria are noted for their ability to survive in extreme environments. They are of different types named methanogens, halophiles, thermophiles, and thermoacidophiles. Although their primitive characteristics are essential for understanding the evolution of earth and living organisms, archaebacteria have some commercial important values also. Their methanogenic ability is used for the commercial production of methane in gobar gas plants. The thermophilic bacteria are commercial producers of thermally stable enzymes.


Q1. Which type of Archaebacteria are present in ruminants?

Ans. Methanogens are present in the intestines of ruminants for digesting the organic substances and producing methane. Cows, buffaloes, Reinders, and antelopes come under the category of ruminants, they have a special organ called rumen in their bodies. The rumen is noted for the presence of a high number of microorganisms especially for digesting fibre content in their diet.

Q2. How do Archaebacteria perform locomotion?

Ans. Archaebacteria perform locomotion with the help of flagella. The flagellum is a long thin appendage attached to the outer membrane. Flagella may be single, multiple or absent which is unique to the species.

Q3. Do Archaebacteria have pigments like chlorophyll?

Ans. Archaebacteria are devoid of membrane-bound cell organelles like chloroplasts and pigments like chlorophyll. The Halobacterium is the only Archaebacterian having pigments that is rhodopsin. However, the Halobacterium is not a photoautotroph.

Q4. What is the response of Archaebacteria to antibiotics?

Ans. Certain species of Archaebacteria show high resistance to antibiotics which is coded by the special genes present on plasmids

Q5. Do Archaebacteria reproduce sexually?

Ans. Sexual reproduction is absent in Archaebacteria and reproduction is by asexual binary fission

Updated on 13-Oct-2022 11:19:47
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