Archaea is a domain of unicellular prokaryotic organisms. They were earlier referred to as archaebacteria. They differ from bacteria in having different cell wall components. They are found in extreme environmental conditions such as hot springs, marshy areas and extreme salty areas. According to their habitat, they are known as thermoacidophiles, methanogens and halophiles.
They are found in diverse habitats. They form the microbiota in many organisms. In humans, they are found in the gut, skin and mouth. They are an important part of the ecosystem and perform diverse functions such as carbon fixation, nutrient recycling, decomposition, etc.
Classification and Characteristics
Carl Woese et al. proposed the three-domain system. Archaea are classified as a separate domain. It is the third domain along with Bacteria and Eukarya. Archaea are morphologically similar to bacteria, but molecular studies have shown that they are more closely related to eukaryotes and possess genes similar to eukaryotes.
Woese and Fox proposed that archaebacteria have a separate line of descent on the basis of differences in 16S rRNA genes. Archaebacteria do not possess a peptidoglycan layer in their cell wall, unlike bacteria.
Archaea means “ancient”. Methanogens were the first representatives of this domain. They live in anaerobic conditions. Later, thermoacidophiles and halophiles were also included in Archaea. Now we know that they are not only present in extreme environments, but are widely distributed in nature.
Archaea are classified based on molecular phylogenetics by comparing the sequence of rRNA genes. The two main phyla of Archaea are:
- Euryarchaeota – It includes methanogens and halobacteria. They live in extreme temperatures or high salt environments and marshy areas. E.g. Halobacteria, Thermoplasma, Methanococcus.
- Crenarchaeota – They are mostly present in the marine environment. Many are sulphur dependent extremophiles. They are thermophilic or hyperthermophilic organisms. Many species lack histones. E.g. Sulfolobus, Pyrobaculum.
Korarchaeota and Nanoarchaeota are the newly proposed phylum.
Some of the salient features of Archaea are the following:
- They are found in various shapes and sizes. The size ranges from 0.1 μm to over 15 μm. They occur in various forms such as spiral, sphere, and rod. Even flat square cells have been found in Haloquadratum walsbyi.
- Most archaea possess a cell wall. The cell walls of archaea do not contain peptidoglycan. They have pseudopeptidoglycan or S-layer of surface proteins.
- The cell membranes of archaea have ether-linked lipids instead of ester-linked in bacteria and eukarya. Fatty acids are linked to glycerol by an ether linkage.
- They do not have a membrane-bound nucleus or cell organelles.
- They have a single circular chromosome and also have plasmids like bacteria.
- The RNA polymerase of archaea is more complex than bacteria and similar to the eukaryotes. It is made up of multiple polypeptides.
- Protein synthesis or translation is also similar to eukaryotic translation. The initiator tRNA carries unmodified methionine.
- They are autotrophic or symbiotic.
- Archaea show various types of metabolism and utilise different sources of energy. Chemolithotrophs derive energy from the oxidation of inorganic compounds and chemoorganotrophs derive energy from the organic molecules.
- Some of the archaea derive energy from sunlight (phototrophs) but they do not show photosynthesis using chlorophyll.
- Methanogenesis is the characteristic property of methanogens. They belong to the phylum Euryarchaeota. They are responsible for the synthesis of methane or biogas.
- Some archaea perform CO2 fixation by a modified Calvin cycle or other metabolic pathways, e.g. reverse Krebs cycle, reductive acetyl-CoA pathway, etc. They use energy from the oxidation of inorganic compounds such as ammonia, hydrogen sulphide, sulphur, etc.
- Archaea reproduce by binary fission or multiple fission. They show horizontal gene transfer, wherein a gene is transferred from one species to another.
Types of Archaea
The three main types of archaebacteria are halophiles, thermoacidophiles and methanogens.
Methanogens
They are obligate anaerobes. They are present in marshy areas and in the gut of many ruminating animals. They produce methane. They are used to prepare biogas (methane) from the dung of these animals commercially. E.g. Methanobacterium, Methanococcus, etc. They are also present in the gut of humans and are responsible for flatulence. They play a key role in anaerobic wastewater treatment. They are spherical or rod-shaped. They usually cannot survive in the presence of oxygen. They may contain a cell wall made up of pseudopeptidoglycan or an S-layer made up of surface proteins. Methanogens produce methane from various substrates such as CO2, formate, methanol and methylamines. The process is known as methanogenesis.
Thermoacidophiles
These can thrive in extreme temperatures as well as highly acidic conditions. They live in deep-sea vents and hot sulphur springs. They can tolerate high temperatures as well as low pH. They have branched-chain lipids in their membranes. E.g. Thermoplasma, are facultative anaerobes and derive energy from sulphur or organic carbon. Thermoproteus is anaerobe, they are hydrogen-sulphur autotrophs. They reduce sulphur.
Halophiles
They thrive in high salt concentrations. E.g. Halobacterium, Halococcus. They belong to the family Halobacteriaceae. They are protected in a highly acidic environment by the presence of special kinds of lipids in the membrane and mucilage covering. They show high internal salt content and the absence of plasmolysis. They are protected from harmful solar radiation by the red carotenoid pigment present in their cell membrane.
Some of the important genera of Archaea are the following:
- Sulfolobus – They grow in volcanic springs at pH 2-3 and temperatures of 75-80 °C, making them thermoacidophiles. Sulfolobus are irregularly shaped and have flagella.
- Pyrodictium – They are submarine hyperthermophilic and can adapt to the hot-cold habitat. They show optimal growth in the temperature range of 80°C to 105°C. They have flat, disk-shaped cells. They are found in deep-sea vents, where the inside temperature becomes as high as 400°C and the outside temperature typically remains as 3°C.
- Nanoarchaeum – They are the smallest known archaea. The cells are around 400 nm in diameter. Nanoarchaeum was found to have the smallest known genome in 2003. They are found in hydrothermal vents.
- Cenarchaeum – They form symbiotic relationships with sponges. They are found living in sponges, e.g. as an endosymbiont in a sponge Axinella mexicana.
- Halobacterium – They are aerobic. They are dependent on amino acids in aerobic conditions. They have an S-layer made up of surface glycoproteins that surrounds the lipid bilayer. Many species have gas vesicles in them. The cell has a high potassium concentration and maintains equilibrium with the hypersaline environment.
- Halococcus – They also derive energy from amino acids and organic acids. They use chlorine pumps to maintain osmotic balance with the hypersaline environment.
- Methanobacterium – They are obligate anaerobic organisms. They are non-motile. They are widely distributed in low oxygen environments such as hot springs, wastewater, etc. They produce methane from formate or by reducing carbon dioxide with hydrogen.
- Thermoplasma – They are thermoacidophiles. They are facultative anaerobes. The lipoglycan membrane is responsible for its stability in extreme temperatures and highly acidic environments.
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