What is Archaebacteria?
Archaebacteria are a unique type of single-celled organism that has challenged the traditional way scientists classify life. Until the advent of advanced genetic and molecular biology studies, archaebacteria were considered to be part of the same kingdom of single-celled organisms as bacteria, known as Monera.
However, sophisticated genetic and biochemical studies have revealed that archaebacteria are so different from other modern life forms that they constitute a distinct domain in the new “phylogenetic tree of life,” called “Archaea.”
Archaebacteria are modern descendants of a very ancient lineage of bacteria that evolved around sulfur-rich deep-sea vents. They are different from “modern” bacteria and all other modern life forms in many ways, including their unique biochemistry, which gives scientists insight into the workings of ancient life.
In fact, some scientists propose that the archaebacteria Thermoplasma may be ancestors of the nuclei of our own eukaryotic cells, which are believed to have developed through the process of endosymbiosis.
Another remarkable trait of archaebacteria is their ability to survive in extreme environments, including very salty, very acidic, and very hot surroundings. Archaebacteria have been recorded surviving temperatures as high as 190° Fahrenheit and acidities as high as 0.9 pH.
Their ability to survive in such harsh conditions is due to their unique biochemical characteristics, such as their cell membrane structure and metabolism, which are different from those of other organisms.
Archaebacteria have also challenged scientists’ ideas about how to define a species because they practice a lot of horizontal gene transfer, where genes are transferred from one individual to another during their lifetimes.
This makes it difficult to determine how closely different cells are related or if archaebacteria cells have the stable combinations of traits that scientists typically use to define a species.
Archaebacteria Characteristics
Here are some of the important characteristics of archaebacteria:
- Obligate or Facultative Anaerobes: Archaebacteria are able to thrive in the absence of oxygen, making them obligate or facultative anaerobes. They are the only organisms that are able to undergo methanogenesis, a process in which they produce methane as a byproduct of their metabolism.
- Unique Cell Membranes: The cell membranes of archaebacteria are composed of lipids, which are different from the lipids found in the cell membranes of other organisms. The lipids in archaebacterial cell membranes have branched chains and ether linkages, which make them more stable in extreme environments.
- Rigid Cell Walls: The cell walls of archaebacteria are rigid and provide shape and support to the cell. They also protect the cell from bursting under hypotonic conditions. The cell wall of archaebacteria is composed of pseudo-murein, which is similar to the peptidoglycan found in bacterial cell walls but is resistant to the effects of lysozyme.
- No Membrane-Bound Organelles: Archaebacteria do not have membrane-bound organelles such as nuclei, endoplasmic reticulum, mitochondria, lysosomes, or chloroplasts. Instead, all the compounds required for nutrition and metabolism are found in the thick cytoplasm of the cell.
- Extremophiles: Archaebacteria can live in a variety of environments and are often referred to as extremophiles. They can survive in acidic and alkaline aquatic regions, as well as in temperatures above boiling point. They can also withstand a very high pressure of more than 200 atmospheres.
- Antibiotic Resistance: Archaebacteria are indifferent to major antibiotics because they contain plasmids that have antibiotic-resistance enzymes.
- Asexual Reproduction: The mode of reproduction in archaebacteria is asexual and is known as binary fission. The cell divides into two equal parts, each of which grows and divides again.
- Unique Gene Transcription: Archaebacteria perform unique gene transcription, which is different from both prokaryotes and eukaryotes. They use a system of transcription factors and RNA polymerases that are distinct from those found in other organisms.
- Divergent Evolution: The differences in the ribosomal RNA of archaebacteria suggest that they diverged from both prokaryotes and eukaryotes. This has led to the suggestion that archaebacteria represent a separate domain of life, distinct from both bacteria and eukaryotes.
Types of Archaebacteria
There are three main types of archaebacteria – Crenarchaeota, Euryarchaeota, and Korarchaeota, and each has unique characteristics and adaptations that allow them to thrive in a variety of extreme environments.
1. Crenarchaeota
Crenarchaeota are known for their extreme heat tolerance and ability to survive in acidic environments. They have specialized proteins and biochemistry that allow them to function at temperatures as high as 230°F.
These microorganisms can be found in hot springs and deep-sea vents, where they have been hypothesized to have originated during the early stages of life on Earth. Crenarchaeota are also involved in the nitrogen cycle, converting nitrogen into forms that can be used by other organisms.
2. Euryarchaeota
Euryarchaeota, on the other hand, is known for its ability to produce methane, which is an important step in the carbon cycle. They are the only known organisms that can perform cellular respiration using carbon as an electron acceptor.
Methanogens, a type of Euryarchaeota, can be found in environments like marshes, wetlands, and the stomachs of ruminants like cows, where they break down complex carbon compounds into methane. Without methanogens, the Earth’s carbon cycle would be disrupted.
3. Korarchaeota
Korarchaeota are the least understood of the three main types of archaebacteria and are thought to be the oldest lineage of archaebacteria, possibly the oldest surviving organisms on Earth.
They can be found in hydrothermal environments similar to Crenarchaeota, and have genes that are found in both Crenarchaeota and Euryarchaeota, as well as unique genes.
Scientists hypothesize that the other two types of archaebacteria may have descended from a common ancestor similar to Korarchaeota.
Examples of Archaebacteria
1. Halophiles
Halophiles are a type of archaebacteria that are adapted to living in extremely salty environments such as salt flats, salt lakes, and salt pans. These microorganisms have evolved unique cellular mechanisms to maintain their cellular structure and function in high salt concentrations.
Some halophiles are able to survive in salt concentrations up to ten times that of seawater. Some examples of halophiles include Haloquadratum walsbyi, Halobacterium salinarum, and Halorubrum chaoviator.
2. Methanogens
Methanogens are a type of archaebacteria that are able to produce methane gas as part of their metabolic process. They are found in environments such as the gastrointestinal tract of animals, wetlands, and deep-sea hydrothermal vents.
Methanogens are important in the cycling of carbon and play a role in producing biogas from waste materials. Some examples of methanogens include Methanobrevibacter smithii, Methanococcus maripaludis, and Methanosarcina acetivorans.
3. Thermophiles
Thermophiles are a type of archaebacteria that are adapted to living in extremely hot environments such as hot springs and deep-sea hydrothermal vents. These microorganisms have evolved unique cellular mechanisms to maintain their cellular structure and function at high temperatures.
Some thermophiles are able to survive in temperatures up to 122 degrees Celsius. Some examples of thermophiles include Pyrococcus furiosus, Thermus aquaticus, and Sulfolobus acidocaldarius.