Archaebacteria are known to be the oldest living organisms on earth. They belong to the kingdom Archaea and are classified as bacteria because they resemble bacteria when observed under a microscope. Apart from this, they are completely distinct from prokaryotes. However, they share slightly common characteristics with the eukaryotes.
These can easily survive under very harsh conditions such as the bottom of the sea and the volcanic vents and are thus known as extremophiles.
Archaebacteria
Characteristics of Archaebacteria
Following are the important characteristics of archaebacteria:
- Archaebacteria are obligate anaerobes, i.e., they flourish in the strict absence of oxygen., and that is why only they can undergo methanogenesis.
- The cell membranes of the Archaebacteria are composed of lipids.
- The rigid cell wall provides shape and support to the Archaebacteria. It also protects the cell from bursting under hypotonic conditions.
- The cell wall is composed of Pseudomurein, which prevents archaebacteria from the effects of Lysozyme. Lysozyme is an enzyme released by the immune system of the host, which dissolves the cell wall of pathogenic bacteria.
- These do not possess membrane-bound organelles such as nuclei, endoplasmic reticulum, mitochondria, lysosomes or chloroplast. Its thick cytoplasm contains all the compounds required for nutrition and metabolism.
- They can live in a variety of environments and are hence called extremophiles. They can survive in acidic and alkaline aquatic regions, and also in temperature above boiling point.
- They can withstand a very high pressure of more than 200 atmospheres.
- Archaebacteria are indifferent towards major antibiotics because they contain plasmids which have antibiotic resistance enzymes.
- The mode of reproduction is asexual, known as binary fission.
- They perform unique gene transcription.
- The differences in their ribosomal RNA suggest that they diverged from both prokaryotes and eukaryotes.
Types of Archaebacteria
Archaebacteria are classified on the basis of their phylogenetic relationship. The major types of Archaebacteria are discussed below:
Crenarchaeota
The Crenarchaeota are Archaea, which exist in a broad range of habitats. They are tolerant to extreme heat or high temperatures. They have special proteins that help them to function at temperatures as high as 230 degrees Celsius. They can be found in deep sea vents and hot springs, regions with superheated water. These include thermophiles, hyperthermophiles, and thermoacidophiles.
Euryarchaeota
These can survive under extremely alkaline conditions and have the ability to produce methane, unlike any other living being on earth. These include methanogens and halophiles.
Korarchaeota
They possess the genes common with Crenarchaeota and Euryarchaeota. All three are believed to have descended from a common ancestor. These are supposed to be the oldest surviving organism on earth. These include hyperthermophiles.
Thaumarchaeota
These include archaea that oxidize ammonia.
Nanoarchaeota
This is an obligate symbiont of archaea belonging to the genus Ignicoccus.
Importance of Archaebacteria
The importance of archaebacteria can be understood from the following points:
- Archaebacteria have compelled the scientists to reconsider the common definition of species. Species are a group with gene flow within its members. The archaebacteria exhibit gene flow across its species.
- The Archaebacteria are methanogens, i.e., they are capable of producing methane. They act on the organic matter and decompose it to release methane which is then used for cooking and lighting. Thus they play the role of primary producers.
Examples of Archaebacteria
Following are the important examples of archaebacteria:
Lokiarcheota
It is a thermophilic archaebacterium found in deep-sea vents known as the Loki’s castle. It has a unique genome. Some of the genes of the genome are involved in phagocytosis. They also possess the eukaryotic genes that are used by the eukaryotes to control their shapes. It is believed that Lokiarcheota and eukaryotes shared a common ancestor several billion years ago.
Methanobrevibacter smithii
It is a methane-producing bacteria found in the human gut. It helps in the breakdown of complex plant sugars and extracts energy from the food consumed by us. Some help to protect against colon cancer. People suffering from colon cancer and obesity have very high levels of this bacteria in their gut.
The Archaebacteria cannot perform photosynthesis and show high levels of gene transfer between lineages. The discovery of Archaebacteria has made scientists believe that life can exist even in extreme environmental conditions.
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