In the presence of oxygen, Sulfolobus spp. In anaerobic environments, they oxidize sulfur to produce sulfuric acid, which is stored in granules. Sulfolobus spp. They have flagella and, therefore, are motile. Thermoproteus has a cellular membrane in which lipids form a monolayer rather than a bilayer, which is typical for archaea. Its metabolism is autotrophic. To synthesize ATP, Thermoproteus spp. The phylum Euryarchaeota includes several distinct classes.
Species in the classes Methanobacteria, Methanococci, and Methanomicrobia represent Archaea that can be generally described as methanogens. Methanogens are unique in that they can reduce carbon dioxide in the presence of hydrogen, producing methane. They can live in the most extreme environments and can reproduce at temperatures varying from below freezing to boiling. Methanogens have been found in hot springs as well as deep under ice in Greenland.
Some scientists have even hypothesized that methanogens may inhabit the planet Mars because the mixture of gases produced by methanogens resembles the makeup of the Martian atmosphere. Some genera of methanogens, notably Methanosarcina , can grow and produce methane in the presence of oxygen, although the vast majority are strict anaerobes. Halobacteria require a very high concentrations of sodium chloride in their aquatic environment.
One remarkable feature of these organisms is that they perform photosynthesis using the protein bacteriorhodopsin , which gives them, and the bodies of water they inhabit, a beautiful purple color Figure 2. Figure 2. Halobacteria growing in these salt ponds gives them a distinct purple color. Notable species of Halobacteria include Halobacterium salinarum , which may be the oldest living organism on earth; scientists have isolated its DNA from fossils that are million years old.
Archaea are not known to cause any disease in humans, animals, plants, bacteria, or in other archaea. Although this makes sense for the extremophiles, not all archaea live in extreme environments. Many genera and species of Archaea are mesophiles, so they can live in human and animal microbiomes, although they rarely do. As we have learned, some methanogens exist in the human gastrointestinal tract.
Yet we have no reliable evidence pointing to any archaean as the causative agent of any human disease. Still, scientists have attempted to find links between human disease and archaea.
The archaeal S-layer can be made of either protein or glycoprotein, often anchored into the plasma membrane of the cell. The proteins form a two-dimensional crystalline array with a smooth outer surface. A few S-layers are composed of two different S-layer proteins. While archaea lack peptidoglycan, a few contain a substance with a similar chemical structure, known as pseudomurein. Methanochondroitin is a cell wall polymer found in some archaeal cells, similar in composition to the connective tissue component chondroitin, found in vertebrates.
Some archaea have a protein sheath composed of a lattice structure similar to an S-layer. These cells are often found in filamentous chains, however, and the protein sheath encloses the entire chain, as opposed to individual cells. While archaea have ribosomes that are 70S in size, the same as bacteria, it was the rRNA nucleotide differences that provided scientists with the conclusive evidence to argue that archaea deserved a domain separate from the bacteria.
In addition, archaeal ribosomes have a different shape than bacterial ribosomes, with proteins that are unique to archaea. This provides them with resistance to antibiotics that inhibit ribosomal function in bacteria. Many of the structures found in bacteria have been discovered in archaea as well, although sometimes it is obvious that each structure was evolved independently, based on differences in substance and construction. Cannulae , a structure unique to archaea, have been discovered in some marine archaeal strains.
These hollow tube-like structures appear to connect cells after division, eventually leading to a dense network composed of numerous cells and tubes. This could serve as a means of anchoring a community of cells to a surface.
Another structure unique to archaea is the hamus , a long helical tube with three hooks at the far end. Hami appear to allow cells to attach both to one another and to surfaces, encouraging the formation of a community.
Pili have been observed in archaea, composed of proteins most likely modified from the bacterial pilin. The resulting tube-like structures have been shown to be used for attachment to surfaces. What is similar between the bacterial flagellum and the archaeal flagellum? Both are used for movement, where the cell is propelled by rotation of a rigid filament extending from the cell. After that the similarities end. What are the differences? Methods such as metagenomics allow for the study of genetic material without the need to grow cultures of a particular species in a lab, allowing researchers to study the genetic blueprints of more microbes than ever before.
Archaea are generally pretty friendly. A lot of archaea live in mutualistic relationships with other living things, meaning they provide some kind of benefit to another species and get something good in return.
For example, the vast numbers of methanogens archaea that produce methane as a by-product that live in the human digestive system help to get rid of excess hydrogen by utilising it to produce energy.
This hydrogen is a waste product produced by the bacteria that help break down the food we eat, so getting rid of the excess means bacteria can do their job more effectively and efficiently. Many forms of archaea can utilise totally inorganic forms of matter—hydrogen, carbon dioxide or ammonia for example—to generate organic matter themselves.
Most other living things require at least some kind of organic material to generate energy, so archaea occupy a unique place in the global food web in this regard. Archaea may also give us a glimpse into how to look for life beyond Earth.
We now know that there are so many environmental conditions—regardless of how extreme they may appear to be—that are capable of supporting life, so we can widen the boundaries of our search for life on other planets like Mars, perhaps. Haloarchaea , for example, are known for surviving in super-salty conditions with very little water and are capable of surviving in a state of near-starvation for a very long time—as in, potentially millions of years at a time.
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