Archaeon of the genus Sulfolobus , infected with the Sulfolobus virus STSV1. Scale = 1 μm.
|Otto Kandler & Mark L. Wheelis|
The archaea ( archaea , singular: archaeon ; from ancient Greek ἀρχαῖος archaĩos 'ancient', 'original'), formerly also called archaebacteria , archebacteria or primeval bacteria , form one of the three domains into which all cellular living beings are divided. The other two domains are the bacteria (Bacteria), which are combined with the archaea to form the prokaryotes , and the eukaryotes (eukaryotes), which, in contrast to the prokaryotes, have a cell nucleus .
Archaea are unicellular organisms that do not have a nucleus with a nuclear membrane like eukaryotic cells , but, like bacteria , self-contained DNA molecules ( circular chromosomes ) which are present in the cytoplasm as nuclear equivalent without a shell. Archaea and bacteria are therefore also called prokaryotes .
The separate position of the archaea as an independent domain is due to a number of genetic, physiological, structural and biochemical features, in particular significant differences in the sequence of the RNA contained in the ribosomes (the small ribosomal subunit, 16S rRNA ).
At the end of the 1970s, the American microbiologists Carl Woese and George Fox recognized and described the independence of archaea and their belonging to a separate systematic unit alongside bacteria ( eubacteria ) and eukaryotes. The researchers discovered striking differences from bacteria in the sequence of the ribosomal RNA. The structure of the cells and their peculiarities in metabolism also suggest a separate group of prokaryotes. These results were confirmed in the following years, and far-reaching advances in molecular biology made a general change in taxonomy necessary: eubacteria were renamed Bacteria and archaebacteria into Archaea . Both were described in 1990 as two separate domains in addition to the domain of the eukarya as part of a three-domain system . The Archaea are phylogenetically closer to the Eukarya than the Bacteria . Although no cell organelles can be found in archaea , they can develop special filaments comparable to a cytoskeleton to stabilize their shape .
Many cultivated archaea species are adapted to extreme environmental conditions. There are species that prefer to grow at temperatures above 80 ° C (hyper- thermophilic ), others live in highly concentrated salt solutions ( halophilic ) or in a strongly acidic environment (pH value below 0; acidophilic ) or a strongly basic environment ( pH value above 10; alkaliphilic ). Thermoplasmatales of the genus Picrophilus ( P. oshimae and P. torridus ) have a growth optimum at a pH of 0.7 and can even survive at a pH of −0.6.
Archaea are of interest in research because they may have retained features of early life on Earth. But their extraordinary metabolism is also of interest, for example the ability to grow at 110 ° C. The unusual metabolism is also interesting in terms of applications, for example archaea are used in soil and water remediation or for methane production in biogas plants .
So far, no pathogens from the archaea group are known.
Archaea are more similar to eukaryotes than bacteria in terms of many molecular biological properties. Nevertheless, they have typical bacterial properties, e.g. B. the cell size, the lack of a cell nucleus , the type of cell division , they have a self-contained DNA molecule, also relatively simple locomotion organs ( flagella ) and, like bacteria, ribosomes with a sedimentation coefficient of 70S (although the archaeal ribosomes are more complex in their structure). The genes of both domains are organized in so-called operons . Archaea can also carry plasmids , such as an archaeon of the Crenarchaeota genus Sulfolobus .
The central molecular processes, for example translation and transcription , on the other hand, are quite similar to those of eukaryotes: Archaea use similar RNA polymerases ( rifampicin and streptolydigin resistant) composed of several protein subunits , and very similar initiation and elongation factors occur during translation The start of transcription is marked by a so-called TATA box .
However, the archaea also have many unique properties, especially the structure of the cell wall shows clear differences to the other domains: The archaeal cell walls contain pseudopeptidoglycan (pseudomurein) and are generally very diverse in their structure: some archaea completely lack a cell wall ( thermoplasm ), others have highly complex cell walls consisting of many layers ( methanospirillum ). Due to their different structure, archaea are generally resistant to cell wall antibiotics. The composition of the archaeal plasma membrane differs: in bacteria and eukaryotes are fatty acids via an ester linkage to the glycerol molecules bound in archaea are found Glyceroldiether or even bis-glycerol tetraethers (single-layer membrane monolayer) and branched isoprene units instead of simple fatty acids. Hyperthermophilic archaea often have cell membranes (glycerol tetraether) that are stabilized in this way, which are not only more thermally stable, but can also adapt to acidic environments.
Some archaea species can move very quickly in relation to their size. With 400 to 500 "body lengths per second" (English " bodies per second ", abbreviated to "bps"), Methanocaldococcus jannaschii and Methanocaldococcus villosus are the fastest living beings measured so far. For comparison: A sports car with 400 bps would reach a speed of over 6000 km / h. The bacterium Escherichia coli , on the other hand, moves at around 20 bps, about as fast as a cheetah . In addition, archaea communicate with each other and thus coordinate their behavior.
Most of the archaea known to date are extremophiles ; H. adapted to the extreme conditions of their biotopes . Many representatives have the ability to live at very high temperatures (hyperthermophiles over 80 ° C), very low or very high pH values ( acidophiles or alkali philes ), high salt concentrations ( halophiles ) or high pressures ( barophilia ).
Hyperthermophilic archaea are often found in volcanic, marine ( black smoker ) and terrestrial ( geysers , solfatar fields ), e.g. B. volcanic habitats of Yellowstone National Park . Halophils thrive in high salinity environments, such as B. in the Dead Sea or in naturally occurring marine brines .
Methanogenic archaea are also “extreme” in a certain way: They only grow under anoxic conditions and often require molecular hydrogen for their metabolism . These archaea species are relatively widespread and occur in fresh water, sea and soil, but also as symbionts in the intestinal tract of animals and humans. Archaea have even been found in the folds of the human navel, although this is rare.
Because of this “extremophilia”, the ecological importance of the archaea was initially assessed as relatively minor. Only in recent years has it become clear through the use of finer molecular biological detection methods that archaea occur to a large extent in relatively cold seawater, but also in soils and freshwater biotopes. In certain oceanic areas z. B. Crenarchaeota up to 90% of the existing living things. Overall, it is estimated that around 1.3 × 10 28 archaea and 3.1 × 10 28 bacteria are found in the oceans .
The majority of the archaea species that are isolated and available as pure cultures in the laboratory are, however, still “extremophile”; in some cases cultivation has also been successful under less extreme conditions. Research to date shows that the archaea play an important role in the nitrogen , carbon and sulfur cycle in the earth's ecosystem.
Most of the archaea species known to date are autotrophic , i. H. they do not need any organic substances for growth; they obtain the carbon to build up their body components exclusively through the assimilation of carbon dioxide . But heterotrophy , the extraction of carbon from organic compounds, is also widespread.
A special feature of archaeal metabolism is methanogenesis , which can only be accomplished by methane-producing archaea, the so-called methanogens . They have a number of unique cofactors , for example coenzyme F 420 or methanofuran .
Most hyperthermophilic archaea are anaerobes ; the energy-generating metabolism is either chemoorganotrophic or chemolithotrophic (the energy is obtained from chemical conversions of organic or inorganic compounds). Sulfur compounds often play a major role here: During the metabolism, the sulfur is reduced and energy is released in the process.
However, the sulfur metabolism of the extremely thermophilic and acidophilic species Acidianus ambivalens (previously: Desulfurolobus ambivalens ), from the order Sulfolobales , is known, which can aerobically oxidize sulfur.
Halophilic archaea are mostly aerobic-chemoorganotrophic, they get their energy from chemical reactions of organic compounds. Under anoxic conditions or when there is a lack of nutrients, many extreme halophiles are even capable of using light energy: They have the protein bacteriorhodopsin , which absorbs light and catalyzes the transfer of protons through the cytoplasmic membrane; the resulting electrochemical gradient drives the ATPase and thus the ATP synthesis.
Like the bacteria, the archaea are extremely diverse in shape. The sizes or lengths of the archaeal cells vary from about 0.4 ( Nanoarchaeum equitans ) up to 100 µm ( Methanospirillum hungatei ), on average the cells are about 1 µm in size. The cells show different shapes, e.g. E.g .: cocci (e.g. Methanococcus jannaschii ), rods ( Thermoproteus neutrophilus ), spirilla- shaped ( Methanospirillum hungatei ), lobed cocci ( Archaeoglobus fulgidus ), discs ( Thermodiscus maritimus ), long filaments ( Thermofilum pendens ) or even square ( Haloquadratum walsbyi ).
The placement of the archaea in the system of taxonomy is not entirely undisputed. Initially classified only by appearance and physiology, today, due to new possibilities, the classification by means of phylogenetic analysis, as proposed by Carl Woese (1977, 1990) , is generally accepted .
There is a fixed procedure for describing gender and species. The genus and species are determined by publication or validation in the International Journal of Systematic and Evolutionary Microbiology (IJSEM). Higher taxa can also be described here. The current status can be viewed in the List of Prokaryotic names with Standing in Nomenclature (LPSN), maintained by Jean Euzéby. This corresponds to the International Code of Nomenclature of Bacteria (ICNB). Taxa that do not conform to this standard are shown in quotation marks.
In addition, the global classification within the archaea and bacteria was reformed by means of phylogenetic analysis of the 16S rRNA gene. A current compilation of the taxa from this and numerous further publications appears in Bergey's Manual , and in Taxonomic Outline of the Bacteria and Archaea , whereby in addition to the 16S rRNA gene, further phylogenetic marker genes are now sometimes used. Some of these taxa are justified, but have not yet been published validly or are generally not recorded by the ICNB. Such taxa are shown in quotation marks.
The systematics presented here contains the taxa from phylum to family. There are contradicting entries for some taxa. These were checked for validity using original literature and a phylogenetic analysis.
A few years ago the description of the additional phyla " Korarchaeota " and " Nanoarchaeota " was published. A representative of the proposed phylum "Nanoarchaeota" was successfully co-cultivated and its genome sequenced, the so-called Nanoarchaeum equitans. There are enrichment cultures of the proposed phylum "Korarchaeota", initially detected using its 16S rRNA gene base sequences in samples from hot springs . From this the complete base sequence of the genome could now be published, given the informal name "Candidatus Korarchaeum cryptofilum ". Without isolated strains, the representatives of this phyla have no validated place in the taxonomy according to the current rules of the ICSB, but represent two of four known phyla of the archaea.
Superphylum " Euryarchaeota "
- Class Archaeoglobi
- Class Halobacteria
- Class Methanobacteria
- Class methanococci
- Class " Methanomicrobia "
- Class Methanopyri
- Class thermococci
- Class Thermoplasmata
Superphylum " DPANN "
Superphylum " Proteoarchaeota "
- Super group " TACK " (Filarchaeota)
- Supergroup " Asgard " (Asgardarchaeota)
Archaea in man
Archaea have been found in humans in the intestines ( colon ), in the mouth (dental flora), in the navel and in the vagina . Archaea, belonging to the genus Methanobrevibacter , especially Methanobrevibacter smithii , occur in the intestine . These belong to the methanogenic archaea . M. smithii is not found in the gut in all humans , and archaea have never been identified in infants under two years of age. In a study, archaea belonging to the phylum Thaumarchaeota were also found on the skin. Possibly the number of these archaea correlates with the frequency of sweating.
Methanogens of the species M. smithii and Methanosphaera stadtmanae live in association with syntrophic bacteria in the human digestive tract, so that they have an impact on digestion. These use the two products of bacterial fermentation hydrogen and formate for methanogenesis. A high concentration of hydrogen inhibits the ATP production of other bacteria. M. smithii also breaks down methane , which is toxic to humans. Therefore, the methanogens have a positive influence on the human intestinal flora . Whether these also influence how much energy humans can take in from food is still the subject of research.
Although archaea are in close contact with humans, there is no evidence of species pathogenic to humans . However, a correlation between the disease and the number of methanogenic archaea was demonstrated: the more archaea there were, for example in the (inflamed) gums , the more pronounced the corresponding periodontal disease . Archaea of the species Methanobrevibacter oralis occur in particular . The amount of methanogenic archaea was also increased in those areas in patients with colon cancer or diverticulosis . However, these archaea only contribute indirectly to the disease by promoting the growth of genuinely pathogenic bacteria - the archaea themselves are not. If archaea are viewed as “copathogens” or “ pathobionts ” (symbionts that become pathological under certain conditions), then the disease could be treated with medications that target these archaea. Statins, for example, inhibit the growth of Methanobrevibacter oralis, which is associated with periodontitis .
It is not yet clear why the known archaea are not pathogenic to humans. The uniqueness of many cofactors and vitamins that are not found in humans is not necessarily the cause. Even the fact that the microhabitats created by pathogenic processes are populated is not a unique selling point of the archaea - in principle, organisms with a similar (anaerobic, hydrogenotrophic) metabolism could also use these habitats.
Archaeal metabolic functions, cell components or enzymes are used industrially. Especially the extremophiles have many properties that can be used biotechnologically. Some examples that are already in the development phase or application:
- Biogas production
- Microbial ore leaching (“microbial ore leaching” or “bioleaching”): In this process, low-grade, sulfidic ores are leached (the sulfide components are microbially oxidized to sulfate, thereby converting the heavy metals into a soluble state); this is used, for example, for the extraction of copper , zinc and nickel .
- Medicine: Use of cell wall components (so-called S-Layer ) as a carrier for vaccines
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