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I am curious as to the species or "types" of archaea that have been found to reside within animals symbiotically. One of the only ones I can think of off the top of my head are methanogens, which live in the gut of ruminants for example.
A pioneering study using 16S RNA primers found only Methanobrevibacter smithii in the human intestine.
Eckburg PB et al. (2005) Diversity of the human intestinal microbial flora. Science 308 1635-1638
In a recent review there is a comprehensive Table of archaea found in humans (this includes non-intestinal flora):
Dridi B et al. (2011) Archea as emerging organisms in complex human microbiomes. Anaerob2 17: 56-63
Much of the work that is catalogued by Dridi et al. is repeated identifications of Methanobacteriales (Methanobrevibacter smithii, Methanosphaera stadtmanae and Methanobrevibacter oralis), but there are others from the Thermoplasmateles, the Halobacteriales and the Sulfolobales, plus some that are thought to be members of unknown archaeal orders.
What types of archaea have been found in animals? - Biology
Description of the Kingdom
Archaeans are “unicellular organisms lacking a nucleus and lacking peptidoglycan in the cell wall. Once grouped with the bacteria, archaeans possess distinctive membrane lipids.” Since they do lack a nucleus, they are considered to be prokaryotes. They were first described as methane producing microbes that can be called methanogens. These methanogens are killed by oxygen, produce unusual enzymes, and have cell walls different from all known bacteria. However, we now know that the archaea includes some rather strange organisms. Members of Archaea are known as “life’s extremists” because they can live through and even thrive under some extremely intense conditions. These living conditions range from oil deposits deep beneath the earth’s surface, the guts of animals, extremely salty water, and temperatures that are close to boiling. The differences in their RNA structures is what separates them from the bacteria kingdom (as well as the other four groups), although it had initially been thought that they were a type of bacteria.
How organisms are grouped into the kingdom
Archaea strongly resemble bacteria cells, but there are several key areas in which they are much more similar to eukaryotes. As modern science moves forward, we are finding that archaeans may be more closely related to eukaryotes than bacteria, despite the initial assumption not so long ago that archaeans were just another type of bacteria.
There are three main types of archaea: crenarchaeota, euryarchaeota, korachaeota. Creharchaeota are those archaeans with the ability to withstand extreme temperatures and intense acidities euryarchaeota are those that produce methane as well as the salt lovers as for the korachaeota, this is simply a group for all other archaea for which little is known. There are several sub-sects to these three main groups: the methanogens, or those that produce methane gas as a waste product of their energy-making processes halophiles are the archaeans that live in salty environments thermophiles are those archaeans that live in hot temperatures psychrophiles, or the archaeans that exist in extremely cold temperatures.
Archaeans are often very small – less than one micron (or one one-thousandth of a millimeter) long. Archeans have been found to have a large variety of shapes these shapes range from lobed and lumpy spheres to perfectly smooth spheres, from short, thick rods to long, hair-like structures, and from triangles to square-like figures. When it comes to shape, the sphere mentioned previously is referred to as a “coccus.” It is also important to remember that archeans are single-celled organisms when considering what structures they will contain. Examples of some of the previously listed shaped cells include:
Another variance in the outward appearance of archaeans is that some possess just one flagellum some possess a multiple flagella while still others have none attached to their cell membrane. Archaeans do have a cell membrane, but do not have internal membranes. The DNA of archaeans, therefore, exists in a single loop known as a plasmid. An interesting fact about the tRNA – or transfer RNA – in archaeans is that in the other five kingdoms, tRNA can be found within organisms and will have similar structures, no matter what organism. However in the archaeans, this standard structure is not present. Archeans do, however, at least have ribosomes within cytoplasm, that is bound within a cell membrane, and most have a cell wall surrounding all of this. With these structures, archaeans appear to blend in a bit better with the other kingdoms. It has been discovered, though, that these similarities are purely structural, and not all that similar on a chemical base. Archaeans have the same structures, but construct them from different materials as other organisms.
A particular trait of archaeans is the unique structure of their plasma membrane, as can be seen below: (almost all other organisms are made of a phospholipid bilayer with ester-bonds, while archaea use ether-bonds)
How do organisms in the kingdom:
Archaea obtain energy through a process known as chemosynthesis. Chemosynthesis is a process similar to photosynthesis, but requires bringing in chemicals from the earth rather than taking in sunlight. Carbohydrates, among other organic molecules, are made by the organism for its own use by oxidizing sulfates or ammonia. The need for this process of chemosynthesis, in conjunction with the chemicals necessary to produce organic compounds for energy through this process, is why archaea are found in such a wide variety of extreme conditions. There are some that use complex peptide mixtures as their sources of energy, carbon, and nitrogen.
Archaea reproduce asexually, or reproduce without sex. In asexual reproduction, there is no meiosis, ploidy reduction, nor fertilization. The offspring archaean is a clone of the parent because there is no exchange of genetic material during reproduction. This means less genetic variation among archaeans, but it also allows for reproduction to occur without a mate. Asexual reproduction will occur through multiple fission, fragmentation, or budding.
Archaeans are single-celled organisms, so after they are produced, there is not a great deal more to their growth and developmental processes. Archeans are able to grow in extreme environmental conditions, such as very high temperatures, or in very salty solutions.
Respond to the environment
Members of archaea require certain chemicals to be present in their surrounding so that they may obtain the necessary energy to continue to survive. Certain members of the kingdom archaea have earned the nickname “thermophiles” – a term that means “heat-loving” – because they do so well under extremely hot conditions, and room temperature could actually be too cold for them. There are even other planets which may be able to support the life forms of archaea!
How do organisms in the kingdom interact with humans
For the most part, archaeans do not interact with other organisms, especially humans. However, certain archaeans are able to live in the guts of humans. Archaea have also been found in the human oral cavity, which includes the mouth. Archaeans have not been proven to cause disease in humans, and studies are currently being run to test for any archeans’ potential to prevent disease.
At least one interesting fact about the kingdom
It was not until 1977 that archaea was found to be its own distinct kingdom, apart from Eubacteria. In fact, the name given to this kingdom by the lead researcher working on the project, Carl Woese, had to be changed from “archaeabacteria” to simply “archaea” in order to avoid any confusion and show that this kingdom was part of or related to eubacteria in some form.
Characteristics of the archaea
Although the domains Bacteria, Archaea, and Eukarya were founded on genetic criteria, biochemical properties also indicate that the archaea form an independent group within the prokaryotes and that they share traits with both the bacteria and the eukaryotes. Major examples of these traits include:
3. Complexity of RNA polymerase: transcription within all types of organisms is performed by an enzyme called RNA polymerase, which copies a DNA template into an RNA product. Bacteria contain a simple RNA polymerase consisting of four polypeptides. However, both archaea and eukaryotes have multiple RNA polymerases that contain multiple polypeptides. For example, the RNA polymerases of archaea contain more than eight polypeptides. The RNA polymerases of eukaryotes also consist of a high number of polypeptides (10–12), with the relative sizes of the polypeptides being similar to that of hyperthermophilic archaeal RNA polymerase. Therefore, the archaeal RNA polymerases more closely resemble RNA polymerases of eukaryotes rather than those of bacteria.
4. Protein synthesis: various features of protein synthesis in the archaea are similar to those of eukaryotes but not of bacteria. A prominent difference is that bacteria have an initiator tRNA (transfer RNA) that has a modified methionine, whereas eukaryotes and archaea have an initiator tRNA with an unmodified methionine.
5. Metabolism: various types of metabolism exist in both archaea and bacteria that do not exist in eukaryotes, including nitrogen fixation, denitrification, chemolithotrophy, and hyperthermophilic growth. Methanogenesis (the production of methane as a metabolic by-product) occurs only in the domain Archaea, specifically in the subdivision Euryarchaeota. Classical photosynthesis using chlorophyll has not been found in any archaea.
The metabolic strategies utilized by the archaea are thought to be extraordinarily diverse in nature. For example, halophilic archaea appear to be able to thrive in high-salt environments because they house a special set of genes encoding enzymes for a metabolic pathway that limits osmosis. That metabolic pathway, known as the methylaspartate pathway, represents a unique type of anaplerosis (the process of replenishing supplies of metabolic intermediates in this instance the intermediate is methylaspartate). Halophilic archaeans, which include Haloarcula marismortui, a model organism used in scientific research, are thought to have acquired the unique set of genes for the methylaspartate pathway via a process known as horizontal gene transfer, in which genes are passed from one species to another.
Types of Archaebacteria
There are three main types of archaebacteria. These are classified based on their phylogenetic relationship (how closely related they are to each other), and members of each type tend to have certain characteristics. The major types are:
1. Crenarchaeota –
Crenarchaeota are extremely heat-tolerant. They have special proteins and other biochemistry that can continue to function at temperatures as high as 230° Fahrenheit! Many Chrenarchaeota can also survive in very acidic environments.
Many species of Crenarchaeota have been discovered living in hot springs and around deep sea vents, where water has been superheated by magma beneath the Earth’s surface.
One theory of the origin of life suggests that life may have originally started around deep sea vents, where high temperatures and unusual chemistries could have led to the formation of the first cells.
They are able to survive in very salty habitats. They are also able to produce methane, which no other life form on Earth is able to do!
Euryarchaeota are the only form of life known to be able to perform cellular respiration using carbon as their electron acceptor.
This gives them an important ecological niche because the breakdown of complex carbon compounds into the simple molecule of methane is the final step in the decomposition of most life forms. Without methanogens, the Earth’s carbon cycle would be impaired.
Wherever methane gas is produced by life, Euryarchaeota are responsible.
Methanogen archaebacteria can be found in marshes and wetlands, where they are responsible for “swamp gas” and part of the marsh’s distinctive smell, and in the stomachs of ruminants such as cows, where they break down sugars found in grass that are undigestible to eukaryotes by themselves. Some methanogens live in the human gut and assist us in the same way.
They can also be found in deep sea sediments, where they produce pockets of methane beneath the ocean floor.
They are the least-understood and thought to be the oldest lineage of archaebacteria. This makes them possibly the oldest surviving organisms on Earth!
Korarchaeota can be found in hydrothermal environments much like Crenarchaeota. However, Korarchaeota have many genes found in both Crenarchaeota and Euryarcheaota, and also genes which are different from both groups. To scientists, this suggests that both other types of archaebacteria may have descended from a common ancestor similar to Korarchaeota.
Korarchaeota are rare in nature, perhaps because other, newer forms of life are better adapted to survive in modern environments than they are. Still, Korearchaeota can be found in hot springs, around deep-sea vents.
The Archaea constitute one of the three domains into which all known life may be divided. There are two other domains of life. One of these is the Eukaryota, which includes the plants, animals, fungi, and protists. Except for the protists, these organisms have been known and studied since the time of Aristotle, and are the organisms with which you are most likely familiar. The second domain to be discovered was the Bacteria, first observed in the 17th century under the microscope by people such as the Dutch naturalist Antony van Leeuwenhoek.
The tiny size of bacteria made them difficult to study. Early classifications depended on the shape of individuals, the appearance of colonies in laboratory cultures, and other physical characteristics. When biochemistry blossomed as a modern science, chemical characteristics were also used to classify bacterial species, but even this information was not enough to reliably identify and classify the tiny microbes. Reliable and repeatable classification of bacteria was not possible until the late 20th century when molecular biology made it possible to sequence their DNA.
Molecules of DNA are found in the cells of all living things, and store the information cells need to build proteins and other cell components. One of the most important components of cells is the ribosome, a large and complex molecule that converts the DNA message into a chemical product. Most of the chemical composition of a ribosome is RNA, a molecule very similar to DNA, and which has its own sequence of building blocks. With sequencing techniques, a molecular biologist can take apart the building block of RNA one by one and identify each one. The result is the sequence of those building blocks.
Because ribosomes are so critically important is the functioning of living things, they are not prone to rapid evolution. A major change in ribosome sequence can render the ribosome unable to fulfill its duties of building new proteins for the cell. Because of this, we say that the sequence in the ribosomes is conserved -- that it does not change much over time. This slow rate of molecular evolution made the ribosome sequence a good choice for unlocking the secrets of bacterial evolution. By comparing the slight differences in ribosome sequence among a wide diversity of bacteria, groups of similar sequences could be found and recognized as a related group.
In the 1970s, Carl Woese and his colleagues at the University of Illinois at Urbana-Champaign began investigating the sequences of bacteria with the goal of developing a better picture of bacterial relationships. Their findings were published in 1977, and included a big surprise. Not all tiny microbes were closely related. In addition to the bacteria and eukaryote groups in the analysis, there was a third group of methane-producing microbes. These methanogens were already known to be chemical oddities in the microbial world, since they were killed by oxygen, produced unusual enzymes, and had cell walls different from all known bacteria.
The significance of Woese's work was that he showed these bizarre microbes were different at the most fundamental level of their biology. Their RNA sequences were no more like those of the bacteria than like fish or flowers. To recognize this enormous difference, he named the group "Archaebacteria" to distinguish them from the "Eubacteria" (true bacteria). As the true level of separation between these organisms became clear, Woese shortened his original name to Archaea to keep anyone from thinking that archaeans were simply a bacterial group.
Since the discovery that methanogens belong to the Archaea and not to the Bacteria, a number of other archaeal groups have been discovered. These include some truly weird microbes that thrive in extremely salty water, as well as microbes that live at temperatures close to boiling. Even more recently, scientists have begun finding archaea in an increasing array of habitats, such as the ocean surface, deep ocean muds, salt marshes, the guts of animals, and even in oil reserves deep below the surface of the Earth. Archaea have gone from obscurity to being nearly ubiquitous in only 25 years!
Archaeans have increasingly become the study of scientific investigation. In many ways, archaeal cells resemble the cells of bacteria, but in a number of important respects, they are more like the cells of eukaryotes. The question arises whether the Archaea are closer relatives of the bacteria or our our group, the eukaryotes. This is a very difficult question to answer, because we are talking about the deepest branches of the tree of life itself we do not have any early ancestors of life around today for comparison. One novel approach used in addressing the question is to look at sequences of duplicated genes. Some DNA sequences occur in more than one copy within each cell, presumably because an extra copy was made at some point in the past. There are a very few genes known to exist in duplicate copies in all living cells, suggesting that the duplication happened before the separation of the three domains of life. In comparing the two sets of sequences, scientists have found that the Archaea may actually be more closely related to us (and the other eukaryotes) than to the bacteria.
The Difference Between Archaea and Bacteria
If you’re a biology student, you may well be wondering about the difference between Archaea and Bacteria!
Biochemically they are nearly as different from Bacteria as they are from Eukaryotes – which is why they are in a Kingdom of their own.
Scientists believe that all three groups of living things, Bacteria, Archaea and Eukaryota all arose separately from some unknown ancestor.
Of 27 distinguishing characteristics (listed in Brock and Madigan 2000):
- Bacteria and Archaea share 15,
- Eukaryotes and Archaea share 8,
- Bacteria and Eukaryotes share only 3.
One of these characteristics is the possession of Plasmids, which is common in both Bacteria and Archaea, but very rare in Eukaryotes.
Archaea differ from bacteria in that they have histone proteins associated with their DNA as we do.
Like us, they have no muramic acid in their cell walls and they use methionine as their initiator tRNA, whereas bacteria use Formylmethionine. Also like us, their ribosomes are sensitive to diphtheria toxin. Those in bacteria are not.
They are insensitive to chloramphenicol, streptomycin and kanamycin. Whereas most bacteria are sensitive to these substances.
Like bacteria, some of them are capable of denitrification and nitrogen fixation. But unlike bacteria none of them are capable of nitrification.
Also – like bacteria – some of them are capable of growth at temperatures above 80 Celsius.
Hot Springs, in Yellow Stone National Park edged with microbial mats
No eukaryotes (us, plants, etc.) are capable of this.
They differ from both bacteria and eukaryotes in that their membrane lipids are ether-linked not ester-linked and in that they are capable of methanogenesis.
Far fewer Archaea are known to science than Bacteria. In fact only, 209 species were listed in 1999.
But now that more people are aware of (and looking for) them, many more species are sure to be found.
Most Archaea are anaerobic (living in the absence of Oxygen) and many live in uncommon and extreme environments, i.e. hot springs, Arctic ice floes, highly saline waters and highly acidic or alkaline soils.
Many of the world record holders for extreme environments are Archaea.
Their tolerances range from 4°C to 110°C and from pH -0.06 to 9.5.
Hot springs water pond with algae and archaea
Nearly half of the known Archaea are Methanogenic, meaning that they give off methane as a by-product of their metabolic activity.
Though many Archaea (singular Archaeon) live in environments which are hazardous to most other organisms, some live much closer to us.
Species of Archaea have in fact been found in animal – including in human – digestive tracts!
Well, I hope this has given you some insight into the question of archaea vs. bacteria.
Scientists Improve Evolutionary Tree of Life for Archaea
An international group of researchers from UK, France, Hungary and Sweden has provided new insights into the origins of the Archaea, the group of simple cellular organisms that are the ancestors of all complex life.
According to Williams et al, the earliest metabolisms of the Archaea were based on the anaerobic reduction of carbon dioxide, and likely evolved during the earliest period of Earth’s evolutionary history. This is an artist’s impression of an Archean landscape. Image credit: Tim Bertelink / CC BY-SA 4.0.
The Archaea are one of the primary domains of cellular life, and are possibly the most ancient form of life: putative fossils of archaean cells in stromatolites have been dated to almost 3.5 billion years ago.
Like bacteria, these microorganisms are prokaryotes, meaning that they have no cell nucleus or any other organelles in their cells.
They thrive in a bewildering variety of habitats, from the familiar – soils and oceans – to the inhospitable and bizarre.
They play major roles in modern-day biogeochemical cycles, and are central to debates about the origin of eukaryotic cells. However, understanding their origins and evolutionary history is challenging because of the huge time spans involved.
To find the root of the archaeal tree and to resolve the metabolism of the earliest archaeal cells, University of Bristol researcher Dr. Tom Williams and co-authors applied a new statistical approach that harnesses the information in patterns of gene family evolution.
“With the development of new technologies for sequencing genomes directly from the environment, many new groups of the Archaea have been discovered,” Dr. Williams said.
“But while these genomes have greatly improved our understanding of the diversity of the Archaea, they have so far failed to bring clarity to the evolutionary history of the group.”
“This is because, like other microorganisms, the Archaea frequently obtain DNA from distantly related organisms by lateral gene transfer, which can greatly complicate the reconstruction of evolutionary history.”
Left: a rooted tree of the Archaea. Right: an ML (maximum likelihood) reconstruction of archaeal gene family evolution the analysis was performed with (A) and without (B) the inclusion of the DPANN (Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanoarchaeota, Nanohaloarchaea) lineages. Image credit: Williams et al, doi: 10.1073/pnas.1618463114.
By determining which genes appeared first during the evolution of the Archaea, the new evolutionary tree makes clear predictions about the basic biochemistry of the earliest Archaea, cells which may have lived over 3.5 billion years ago.
“Metabolic reconstructions on the rooted tree suggest that early Archaea were anaerobes that may have had the ability to reduce carbon dioxide to acetate via the Wood-Ljungdahl pathway, a biochemical pathway that today is found not only in the Archaea but also in bacteria, another major group of microorganisms,” the authors said.
Tom A. Williams et al. Integrative modeling of gene and genome evolution roots the archaeal tree of life. PNAS, published online May 22, 2017 doi: 10.1073/pnas.1618463114
Biology's big brainteaser
If you hang around the bar at a sufficiently obscure science conference you might overhear talk of one of modern biology's most intriguing puzzles. The conundrum concerns a fifth of all life on the planet, and answering it could help us to treat diseases that have evaded medicine for thousands of years.
At the very least it could point us towards a new generation of drugs. Yet perhaps the biggest puzzle is why modern science has largely ignored it. Have you even heard of the archaea? A growing number of microbiologists have, and they are beginning to turn the study of this third branch of life from bar room chat to scientific reality.
It all started back in 1990. The microbiologist Carl Woese at the University of Illinois found that life could be divided into three distinct groups, each of which had branched from a common, primitive ancestor billions of years ago. He assigned organisms to each category depending, among other things, on what their cells looked like and how their internal machinery worked. Bacteria are one such category. The other two are eukaryotes and archaea. Humans belong to the former, along with animals, plants and fungi. The latter, the archaea, lie somewhere between us and bacteria. Like bacteria, they are tiny, single-celled organisms. Their genetic machinery, however, is much closer to ours.
The greatest outstanding mystery of archaea became apparent when people looked at the organisms known to cause disease in humans and animals. Plenty of bacteria are known culprits and there are numerous disease-causing eukaryotes, mostly fungi. But archaea seemed to be entirely harmless. "What's puzzling is that we have these three kinds of life, but only two bear organisms that cause disease," says Rick Cavicchioli, a molecular biologist at the University of New South Wales. "What is it about these archaea that means they don't cause disease?"
The answer could have huge significance. If there is a fundamental reason why archaea cannot or do not cause disease, then this could be the key to "switching off" pathogenic microbes that make people ill, says Cavicchioli. But there's a problem. What if archaea aren't as benign as we might think? Perhaps they are causing disease, and have been for millennia, but we just haven't realised.
The prospect is looking ever more likely. Archaea are estimated to make up a fifth of all life, meaning they outnumber animals. And they live in all the right places to cause disease. As well as being found in extreme environments on Earth, from hot undersea vents to Antarctica, archaea have been found in the intestines, the mouth and in slimy films of mucous elsewhere in the body. A gram of human faeces can hold 10bn of them. "On the evidence we've got so far, we should be finding more than 30 types of archaea that cause disease," says Cavicchioli.
Peter Westerman, an expert in archaea at the Technical University of Denmark is hunting archaea that may be responsible for unexplained diseases. "It's surprising how many illnesses are caused by unknown pathogens," he says. Archaea might be playing a part in some of those. Westerman's team is painstakingly looking at samples of diseased tissue to look for tell-tale signs of archaea.
Other researchers say they are already starting to see links between disease and archaea, but that the results are often surprising. Paul Lepp, a microbiologist at Stanford University, California, has been looking at whether archaea play a role in gum disease. The study is not finished, but Lepp says first results show archaea are far more prevalent in the mouths of people with severe gum disease than those with healthy gums. They may simply grow better on diseased tissue, but it's a lead worth following.
Working out the real culprit of a disease is often complicated because a whole community of different bacteria and other organisms can be found thriving on diseased tissue. Lepp suspects that, as members of a wider community, archaea might sometimes play a more subtle role in progressing a disease. Most of the archaea found in humans and other animals convert hydrogen around them into methane, earning them the name "methanogens". This could be invaluable for a community of disease-causing bacteria. "Bacteria often produce hydrogen, but when it gets to a high enough level it becomes toxic so it kills them off," says Lepp. With archaea around, the toxic hydrogen will be converted into methane. "That means the pathogens could grow to a much larger population than otherwise, making them much more of a problem," says Lepp.
Lepp's colleague at Stanford, Paul Eckburg, is also hunting for pathogenic archaea. His focus is on inflammatory bowel conditions, one being Crohn's disease. Again, his work is in its early stages, but his findings are proving unusual. "In the data we've got so far, it looks like patients with Crohn's disease do not have archaea in their intestines while those without the disease do have them," he says. "In this case, it may be that archaea actually have a protective role."
Unravelling what role, if any, archaea play in disease is becoming a priority for teams of microbiologists around the world. Most believe it's a matter of time before they are confirmed, at least, as contributing to the progress of certain illnesses. "It's very important we get to the bottom of this, because if archaea are causing diseases we may have a problem," says Westerman. The antibiotics we have now have all been designed with bacteria in mind, and a new generation of drugs would be needed to tackle archaea. "It's all a bit scary," he says.
Structure and Dynamics of Membranes
M. Bloom , O.G. Mouritsen , in Handbook of Biological Physics , 1995
5 Archaebacterial membranes
Archaebacteria were only identified as a separate kingdom of cells a relatively short time ago and the physical properties of their membranes have been studied less completely than those of eubacteria and eucaryotes.
In the Cavalier-Smith  scenario for the evolution of archaebacteria, the integrity and form of the cell was maintained after the loss of the cell wall by the re-evolution of diverse types of cell walls  , and the evolution of a unique class of archaebacterial lipids [34, 35] in response to the extreme conditions of salt concentration, temperature and pressure (e.g., as in deep sea hot springs) under which archaebacteria occupy their special niche.
Unlike the straight-chain fatty acids and fatty acid ester-linked glycerol lipids that are characteristic of eubacteria and eucaryotes, archaebacterial lipids are distinguished by being isoprenoid and hydroisoprenoid hydrocarbon and isopranyl glycerol ether-linked lipids. Glycerol ether lipids are found in other types of cells in minor amounts, but only in archaebacteria have they been evolved as the fundamental glycerolipid component. While some ether lipids are structurally analagous to their counterparts in eubacteria and eucaryotes, in particular di-ether lipids, others such as the tetra-ether lipids are not. Some of these are about forty carbons long and have two polar ends. It would seem that such lipids would quite naturally span the lipid bilayer structure and this may well be their configuration in biological membranes. However, surprisingly little is known about the structure of archaebacterial membranes, partly, it seems, because of the small quantities of their pure lipids available for studies of physical properties  .
One of the best structural studies using X-ray diffraction [37, 38] has led to the intriguing suggestion that the plasma membrane of the archaebacterium Sulfolobus solfataricus may be composed of two interlocking cubic phases  see section 8.1 . It appears that the study of the physical properties of archaebacterial lipids is still in its early stages and capable of yielding surprises.
Archaea is found in a wide range of habitats, from extremely saline to acidic/alkaline or extremely hot to insanely cold places. They contribute nearly 20% of the earth’s biomass and play a major role in the global ecosystem.
Archaea having ability to tolerate extreme temperature and acidity is called crenarchaeota. This type is further divided into two sub-types.
- Thermophiles: Archaeans that live in extremely hot temperatures are called thermophiles. They can grow and reproduce at temperature 100 o C, and sometimes even above. They are found in acidic soils, hot springs and near volcano openings.
Example – Methanopyrus kandleri
- Mesophiles: Archaeans that live in neither too hot nor too cold, but moderate temperature are known as mesophiles. They grow and develop the best in temperature ranging from 20- 40 o C. They are typically found in cheese, yoghurt, etc.
Example – Listeria monocytogenes
- Psychrophiles: Archaeans that live in extremely cold temperatures are called psychrophiles or cycrophiles. They can grow and reproduce in temperature ranging from -20 o C to +10 o C. They are found in arctic and alpine soil, deep ocean water, high latitude, glacier, snowfields, etc.
Example – Chryseobacterium greenlandensis
Methane producing and salt loving archaea is known as euryarchaeota. This type is further divided into two sub-types.
- Methanogens: Archaeans that release methane as waste during the process of digestion or making energy. They play important role in carbon cycle and are often used in sewage treatment plant.
Example – Methanocaldococcusjannaschii, Methanobrevibactersmithii
- Halophiles: Archaeans that flourish in extremely saline locales are known as halophiles. They can be found anywhere with concentration of salt five times greater than that of the ocean such as Great Salt Lake, Owens Lake, Dead Sea, etc.
Example – Dunaliella salina, Halobacterium salinarum.
According to the researches, this group of archaeans does not belong to the lineage of crenarchaeota and euryarchaeota but is enriched with mixed culture of both the types. They are found only in high temperature hydrothermal environment. However, they are not abundant in nature.