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. Even exposure to high levels of UV radiation doesn't bother them. This raises the possibility that other living things might be able to exist in similarly salty, rocky, dry places on other planets, meteorites or moons.
So, what's out there? Are there archaea-like living things on other planets? We still have so much to discover about the world of archaea here on Earth, but as they continue to challenge and broaden our very definitions of where life can thrive, it's an exciting time for new biological discoveries.
Like organisms in the domain Bacteria, organisms of the domain Archaea are all unicellular organisms. However, archaea differ structurally from bacteria in several significant ways, as discussed in Unique Characteristics of Prokaryotic Cells. To summarize:. Domain Archaea is as diverse as domain Bacteria, and its representatives can be found in any habitat.
Some archaea are mesophiles , and many are extremophiles , preferring extreme hot or cold, extreme salinity, or other conditions that are hostile to most other forms of life on earth. Their metabolism is adapted to the harsh environments, and they can perform methanogenesis , for example, which bacteria and eukaryotes cannot. The size and complexity of the archaeal genome makes it difficult to classify. Most taxonomists agree that within the Archaea, there are currently five major phyla: Crenarchaeota , Euryarchaeota , Korarchaeota , Nanoarchaeota , and Thaumarchaeota.
There are likely many other archaeal groups that have not yet been systematically studied and classified. With few exceptions, archaea are not present in the human microbiota, and none are currently known to be associated with infectious diseases in humans, animals, plants, or microorganisms.
However, many play important roles in the environment and may thus have an indirect impact on human health. Crenarchaeota is a class of Archaea that is extremely diverse, containing genera and species that differ vastly in their morphology and requirements for growth. All Crenarchaeota are aquatic organisms, and they are thought to be the most abundant microorganisms in the oceans. Figure 1. Sulfolobus , an archaeon of the class Crenarchaeota, oxidizes sulfur and stores sulfuric acid in its granules.
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. Each nucleotide of RNA is composed of a sugar called ribose, a phosphate group and one of four nitrogenous bases: adenine A , uracil U , guanine G or cytosine C. The T-sub-1 enzyme therefore digests an RNA "text" into short "words," called oligonucleotides.
The oligonucleotides made in this way were short enough to be sequenced by the available techniques. When 16 S RNAs from different organisms include the same six-letter sequence, it almost always reflects a true homology. S-sub-AB ranges from 1 when dictionaries A and B are identical to less than.
By compiling the S-sub-AB values for a number of organisms in a matrix one can discern a pattern of relatedness or unrelatedness among organisms. Most of the bacteria form a coherent but very large which is to say ancient group. In collaboration with Ralph S. Indeed, they appear to represent an evolutionary branching that far antedated the common ancestor of all true bacteria. They are the size of bacteria, they have no nuclear membrane, they have a low DNA content and so on.
Our analysis showed they are not. Methanogens are related as closely to eukaryotes as to eubacteria. How could this be? Hence there is no reason two prokaryotic lines of descent cannot be just as distinct from each other as either one is from the eukaryotic line. This idea was too novel to be easily accepted, and initially some biologists rejected out of hand the notion of a "third form of life.
The supposed great antiquity of the archaebacteria remains an unproved prejudice, but it is a plausible one. This implies that the methanogens are as old as or older than any other bacterial group. The name archaebacteria implies that these organisms were the dominant ones in the primeval biosphere. Bacteria that give off methane have been known for some time. In ancient times methanogens could have existed almost anywhere. Methanogens are found in stagnant water and in sewage-treatment plants in amounts that have made it commercially feasible to manufacture methane.
Methanogens can be isolated from the ocean bottom and from hot springs. In spite of their intolerance of oxygen they are obviously globally distributed. The extreme halophiles are bacteria that require high concentrations of salt in order to survive; some of them grow readily in saturated brine.
They can give a red color to salt evaporation ponds and can discolor and spoil salted fish. The extreme halophiles grow in salty habitats along the ocean borders and in inland waters such as the Great Salt Lake and the Dead Sea. They maintain large gradients in the concentration of certain ions across their cell membrane and exploit the gradients to move a variety of substances into and out of the cell.
In addition the extreme halophiles have a comparatively simple photosynthetic mechanism based not on chlorophyll but on a membrane-bound pigment, bacterial rhodopsin, that is remarkably like one of the visual pigments. Sulfolobus , one of the two genera of thermoacidophiles, is found in hot sulfur springs.
Its various species generally grow at temperatures near 80 degrees Celsius degrees Fahrenheit ; growth at temperatures above 90 degrees has been observed for some varieties. It is a mycoplasma: it has no cell wall but merely the limiting cell membrane. Although archaebacterial thermoacidophiles can grow only in an acidic environment, the internal milieu of the cell has a quite moderate pH, near neutrality; this requires that a sizable pH gradient be maintained across the cell membrane.
As in the extreme halophiles the gradient may play a role in pumping other molecules into and out of the cell. It is interesting that when the temperature is reduced and as a consequence Sulfolobus stops metabolizing, the cell's internal pH can no longer be maintained near neutrality and the cell dies. For some time it had been recognized that various organisms now classified as archaebacteria are individually somewhat peculiar.
In each instance the idiosyncrasy has been seen as just that: an adaptation to some peculiar niche or biochemical quirk. The ribosomal-RNA phylogenetic measurement, however, showed at least some of the idiosyncrasies might instead be general characteristics of a new group of organisms.
Thus informed, investigators in many countries have undertaken to find the general properties that link archaebacteria to one another and to see how those properties either distinguish the archaebacteria from the other two major forms or relate them specifically to one or the other of those forms.
All of them turned out to be atypical. The lipids of the extreme halophiles and the thermoacidophiles are also composed of a glycerol group linked to two long hydrocarbon chains, but the connection between the glycerol and the chains is an ether -O- link rather than an ester link. The differences are of two kinds. Nothing whatever is known about the control of gene expression in archaebacteria.
The recognition of three lines of descent equidistant from one another gives a much better perspective for judging which properties are ancestral and which have evolved recently. At what stage in the evolution of the cell did the fundamental division into the primary kingdoms take place? What was the nature of the universal ancestor?
Long ago, however, there must have been still simpler forms of the cell. Consider the following argument. The translation mechanism is complex, comprising on the order of large molecular components. For such a mechanism to have evolved in a single step is clearly impossible. The primitive version of the mechanism must have been far simpler, smaller and less accurate.
Otherwise the probability of error in making a protein strand would have been too great.
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