Archaea (/ɑːrˈkiːə/ⓘar-KEE-ə) is adomain oforganisms. Traditionally, Archaea only included itsprokaryotic members, but this has since been found to beparaphyletic, aseukaryotes are now known to have evolved from archaea. Even though the domain Archaea cladistically includes eukaryotes, the term "archaea" (sg.: archaeon/ɑːrˈkiːɒn/ar-KEE-on, from the Greek "ἀρχαῖον", which means ancient) in English still generally refers specifically to prokaryotic members of Archaea. Archaea were initiallyclassified asbacteria, receiving the name archaebacteria (/ˌɑːrkibækˈtɪəriə/, in the Archaebacteriakingdom), but this term has fallen out of use.[5] Archaeal cells have unique properties separating them fromBacteria andEukaryota. Archaea are further divided into multiple recognizedphyla. Classification is difficult because most have not beenisolated in a laboratory and have been detected only by theirgene sequences in environmental samples. It is unknown if they are able to produceendospores.
Archaea are often similar to bacteria in size and shape, although a few have very different shapes, such as the flat, square cells ofHaloquadratum walsbyi.[6] Despite this, archaea possessgenes and severalmetabolic pathways that are more closely related to those of eukaryotes, notably for theenzymes involved intranscription andtranslation. Other aspects of archaeal biochemistry are unique, such as their reliance onether lipids in theircell membranes,[7] includingarchaeols. Archaea use more diverse energy sources than eukaryotes, ranging fromorganic compounds such as sugars, toammonia,metal ions or evenhydrogen gas. Thesalt-tolerantHaloarchaea use sunlight as an energy source, and other species of archaeafix carbon (autotrophy), but unlikecyanobacteria, no known species of archaea does both. Archaeareproduce asexually bybinary fission,fragmentation, orbudding; unlike bacteria, no known species of Archaea formendospores. The first observed archaea wereextremophiles, living in extreme environments such ashot springs andsalt lakes with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost everyhabitat, including soil,[8] oceans, andmarshlands. Archaea are particularly numerous in the oceans, and the archaea inplankton may be one of the most abundant groups of organisms on the planet.
For much of the 20th century, prokaryotes were regarded as a single group of organisms and classified based on theirbiochemistry,morphology andmetabolism. Microbiologists tried to classify microorganisms based on the structures of theircell walls, their shapes, and the substances they consume.[11] In 1965,Emile Zuckerkandl andLinus Pauling[12] instead proposed using the sequences of thegenes in different prokaryotes to work out how they are related to each other. Thisphylogenetic approach is the main method used today.[13]
Archaea were first classified separately from bacteria in 1977 byCarl Woese andGeorge E. Fox, based on theirribosomal RNA (rRNA) genes.[14] (At that time only themethanogens were known). They called these groups theUrkingdoms of Archaebacteria and Eubacteria, though other researchers treated them askingdoms or subkingdoms. Woese and Fox gave the first evidence for Archaebacteria as a separate "line of descent": 1. lack ofpeptidoglycan in their cell walls, 2. two unusual coenzymes, 3. results of16S ribosomal RNA gene sequencing. To emphasize this difference, Woese,Otto Kandler andMark Wheelis later proposed reclassifying organisms into three naturaldomains known as thethree-domain system: theEukarya, theBacteria and the Archaea,[2] in what is now known as theWoesian Revolution.[15]
The wordarchaea comes from theAncient Greekἀρχαῖα, meaning "ancient things",[16] as the first representatives of the domain Archaea weremethanogens and it was assumed that their metabolism reflected Earth's primitive atmosphere and the organisms' antiquity, but as new habitats were studied, more organisms were discovered. Extremehalophilic[17] andhyperthermophilic microbes[18] were also included in Archaea. For a long time, archaea were seen as extremophiles that exist only in extreme habitats such ashot springs andsalt lakes, but by the end of the 20th century, archaea had been identified in non-extreme environments as well. Today, they are known to be a large and diverse group of organisms abundantly distributed throughout nature.[19] This new appreciation of the importance and ubiquity of archaea came from usingpolymerase chain reaction (PCR) to detect prokaryotes from environmental samples (such as water or soil) by multiplying their ribosomal genes. This allows the detection and identification of organisms that have not beencultured in the laboratory.[20][21]
The classification of archaea, and of prokaryotes in general, is a rapidly moving and contentious field. Current classification systems aim to organize archaea into groups of organisms that share structural features and common ancestors.[22] These classifications rely heavily on the use of the sequence ofribosomal RNA genes to reveal relationships among organisms (molecular phylogenetics).[23] Most of the culturable and well-investigated species of archaea are members of two mainphyla, the "Euryarchaeota" and theThermoproteota (formerly Crenarchaeota). Other groups have been tentatively created, such as the peculiar speciesNanoarchaeum equitans — discovered in 2003 and assigned its own phylum, the "Nanoarchaeota".[24] A new phylum "Korarchaeota" has also been proposed, containing a small group of unusual thermophilic species sharing features of both the main phyla, but most closely related to the Thermoproteota.[25][26] Other detected species of archaea are only distantly related to any of these groups, such as theArchaeal Richmond Mine acidophilic nanoorganisms (ARMAN, comprisingMicrarchaeota and Parvarchaeota), which were discovered in 2006[27] and are some of the smallest organisms known.[28]
A superphylum – "TACK" (syn. kingdom Thermoproteati)– which includes the Thaumarchaeota (nowNitrososphaerota), "Aigarchaeota", Crenarchaeota (nowThermoproteota), and "Korarchaeota" was proposed in 2011 to be related to the origin of eukaryotes.[29] In 2017, the newly discovered and newly named "Asgard" (syn. kingdomPromethearchaeati) superphylum was proposed to be more closely related to the original eukaryote and a sister group to Thermoproteati/"TACK".[30]
In 2013, the superphylum "DPANN" (syn. kingdom Nanobdellati) was proposed to group "Nanoarchaeota", "Nanohaloarchaeota",Archaeal Richmond Mine acidophilic nanoorganisms (ARMAN, comprising "Micrarchaeota" and "Parvarchaeota"), and other similar archaea. This archaeal superphylum encompasses at least 10 different lineages and includes organisms with extremely small cell and genome sizes and limited metabolic capabilities. Therefore, Nanobdellati/"DPANN" may include members obligately dependent on symbiotic interactions, and may even include novel parasites. However, other phylogenetic analyses found that Nanobdellati/"DPANN" does not form a monophyletic group, and that the apparent grouping is caused bylong branch attraction (LBA), suggesting that all these lineages belong to "Euryarchaeota".[31][3]
The classification of archaea into species is also controversial.Ernst Mayr'sspecies definition — areproductively isolated group of interbreeding organisms — does not apply, as archaea reproduce only asexually.[38]
Archaea show high levels ofhorizontal gene transfer between lineages. Some researchers suggest that individuals can be grouped into species-like populations given highly similar genomes and infrequent gene transfer to/from cells with less-related genomes, as in the genusFerroplasma.[39] On the other hand, studies inHalorubrum found significant genetic transfer to/from less-related populations, limiting the criterion's applicability.[40] Some researchers question whether such species designations have practical meaning.[41]
Current knowledge on genetic diversity in archaeans is fragmentary, so the total number of species cannot be estimated with any accuracy.[23] Estimates of the number of phyla range from 18 to 23, of which only 8 have representatives that have been cultured and studied directly. Many of these hypothesized groups are known from a single rRNA sequence, so the level of diversity remains obscure.[42] This situation is also seen in the Bacteria; many uncultured microbes present similar issues with characterization.[43]
The following phyla have been proposed, but have not been validly published according to the Bacteriological Code (including those that haveCandidatus status):
Although probable prokaryotic cellfossils date to almost 3.5 billion years ago, most prokaryotes do not have distinctive morphologies, and fossil shapes cannot be used to identify them as archaea.[58] Instead,chemical fossils of uniquelipids are more informative because such compounds do not occur in other organisms.[59] Some publications suggest that archaeal or eukaryotic lipid remains are present inshales dating from 2.7 billion years ago,[60] though such data have since been questioned.[61] These lipids have also been detected in even older rocks from westGreenland. The oldest such traces come from theIsua district, which includes Earth's oldest known sediments, formed 3.8 billion years ago.[62] The archaeal lineage may be the most ancient that exists on Earth.[63]
Woese argued that the bacteria, archaea, and eukaryotes represent separate lines of descent that diverged early on from an ancestral colony of organisms.[64][65] One possibility[65][66] is that this occurred before theevolution of cells, when the lack of a typical cell membrane allowed unrestrictedlateral gene transfer, and that the common ancestors of the three domains arose by fixation of specific subsets of genes.[65][66] It is possible that thelast common ancestor of bacteria and archaea was athermophile, which raises the possibility that lower temperatures are "extreme environments" for archaea, and organisms that live in cooler environments appeared only later.[67] Since archaea and bacteria are no more related to each other than they are to eukaryotes, the termprokaryote may suggest a false similarity between them.[68] However, structural and functional similarities between lineages often occur because of shared ancestral traits orevolutionary convergence. These similarities are known as agrade, andprokaryotes are best thought of as a grade of life, characterized by such features as an absence of membrane-bound organelles.
Archaea were split off as a third domain because of the large differences in their ribosomal RNA structure. The particular molecule16S rRNA is key to the production of proteins in all organisms. Because this function is so central to life, organisms with mutations in their 16S rRNA are unlikely to survive, leading to great (but not absolute) stability in the structure of this polynucleotide over generations. 16S rRNA is large enough to show organism-specific variations, but still small enough to be compared quickly. In 1977, Carl Woese, a microbiologist studying the genetic sequences of organisms, developed a new comparison method that involved splitting the RNA into fragments that could be sorted and compared with other fragments from other organisms.[14] The more similar the patterns between species, the more closely they are related.[72]
Woese used his new rRNA comparison method to categorize and contrast different organisms. He compared a variety of species and happened upon a group of methanogens with rRNA vastly different from any known prokaryotes or eukaryotes.[14] These methanogens were much more similar to each other than to other organisms, leading Woese to propose the new domain of Archaea.[14] His experiments showed that the archaea were genetically more similar to eukaryotes than prokaryotes, even though they were more similar to prokaryotes in structure.[73] This led to the conclusion that Archaea and Eukarya shared a common ancestor more recent than Eukarya and Bacteria.[73] The development of the nucleus occurred after the split between Bacteria and this common ancestor.[73][2]
One property unique to archaea is the abundant use of ether-linked lipids in their cell membranes. Ether linkages are more chemically stable than the ester linkages found in bacteria and eukarya, which may be a contributing factor to the ability of many archaea to survive in extreme environments that place heavy stress on cell membranes, such as extreme heat andsalinity. Comparative analysis of archaeal genomes has also identified several molecularconserved signature indels and signature proteins uniquely present in either all archaea or different main groups within archaea.[74][75][76] Another unique feature of archaea, found in no other organisms, ismethanogenesis (the metabolic production of methane). Methanogenic archaea play a pivotal role in ecosystems with organisms that derive energy from oxidation of methane, many of which are bacteria, as they are often a major source of methane in such environments and can play a role as primary producers.Methanogens also play a critical role in thecarbon cycle, breaking down organic carbon into methane, which is also a major greenhouse gas.[77]
This difference in the biochemical structure of Bacteria and Archaea has been explained by researchers through evolutionary processes.[vague] It is theorized that bothdomains originated at deep sea alkalinehydrothermal vents. At least twice, microbes evolved lipid biosynthesis and cell wall biochemistry. It has been suggested that thelast universal common ancestor was a non-free-living organism.[78] It may have had a permeable membrane composed of bacterial simple chain amphiphiles (fatty acids), including archaeal simple chain amphiphiles (isoprenoids). These stabilize fatty acid membranes in seawater; this property may have driven the divergence of bacterial and archaeal membranes, "with the later biosynthesis of phospholipids giving rise to the unique G1P and G3P headgroups of archaea and bacteria respectively. If so, the properties conferred by membrane isoprenoids place the lipid divide as early as the origin of life".[79]
Phylogenetic tree showing the relationship between the Archaea and other domains of life.Eukaryotes are colored red, archaea green andbacteria blue. Adapted from Ciccarelli et al. (2006)[80]
The relationships among thethree domains are of central importance for understanding the origin of life. Most of themetabolic pathways, which are the object of the majority of an organism's genes, are common between Archaea and Bacteria, while most genes involved ingenome expression are common between Archaea and Eukarya.[81] Within prokaryotes, archaeal cell structure is most similar to that ofgram-positive bacteria, largely because both have a single lipid bilayer[82] and usually contain a thick sacculus (exoskeleton) of varying chemical composition.[83] In some phylogenetic trees based upon different gene/protein sequences of prokaryotic homologs, the archaeal homologs are more closely related to those of gram-positive bacteria.[82] Archaea and gram-positive bacteria also share conservedindels in a number of important proteins, such asHsp70 andglutamine synthetase I;[82][84] but the phylogeny of these genes was interpreted to reveal interdomain gene transfer,[85][86] and might not reflect the organismal relationship(s).[87]
It has been proposed that the archaea evolved from Gram-positive bacteria in response to antibioticselection pressure.[82][84][88] This is suggested by the observation that archaea are resistant to a wide variety of antibiotics that are produced primarily by Gram-positive bacteria,[82][84] and that these antibiotics act primarily on the genes that distinguish archaea from bacteria. The proposal is that the selective pressure towards resistance generated by the gram-positive antibiotics was eventually sufficient to cause extensive changes in many of the antibiotics' target genes, and that these strains represented the common ancestors of present-day Archaea.[88] The evolution of Archaea in response to antibiotic selection, or any other competitive selective pressure, could also explain their adaptation to extreme environments (such as high temperature or acidity) as the result of a search for unoccupied niches to escape from antibiotic-producing organisms;[88][89]Cavalier-Smith has made a similar suggestion, theNeomura hypothesis.[90] This proposal is also supported by other work investigating protein structural relationships[91] and studies that suggest that gram-positive bacteria may constitute the earliest branching lineages within the prokaryotes.[92]
The evolutionary relationship between archaea andeukaryotes remains unclear. Aside from the similarities in cell structure and function that are discussed below, many genetic trees group the two.[94]
Complicating factors include claims that the relationship between eukaryotes and the archaeal phylumThermoproteota is closer than the relationship between the "Euryarchaeota" and the phylum Thermoproteota[95] and the presence of archaea-like genes in certain bacteria, such asThermotoga maritima, fromhorizontal gene transfer.[96] The standard hypothesis states that the ancestor of the eukaryotes diverged early from the Archaea,[97][98] and that eukaryotes arose throughsymbiogenesis, the fusion of an archaean and a eubacterium, which formed themitochondria; this hypothesis explains the genetic similarities between the groups.[93] Theeocyte hypothesis instead posits thatEukaryota emerged relatively late from the Archaea.[99]
A lineage of archaea discovered in 2015,Lokiarchaeum (of the proposed new phylum "Lokiarchaeota"), named for ahydrothermal vent calledLoki's Castle in the Arctic Ocean, was found to be the most closely related to eukaryotes known at that time. It has been called a transitional organism between prokaryotes and eukaryotes.[100][101]
Details of the relation of Promethearchaeati/"Asgard" members and eukaryotes are still under consideration,[103] although, in January 2020, scientists reported thatCandidatus Prometheoarchaeum syntrophicum, a type of Promethearchaeati/"Asgard" archaea, may be a possible link between simpleprokaryotic and complexeukaryotic microorganisms about two billion years ago.[104][105][106]
Individual archaea range from 0.1 micrometers (μm) to over 15 μm in diameter, and occur in various shapes, commonly as spheres, rods, spirals or plates.[107] Other morphologies in theThermoproteota include irregularly shaped lobed cells inSulfolobus, needle-like filaments that are less than half a micrometer in diameter inThermofilum, and almost perfectly rectangular rods inThermoproteus andPyrobaculum.[108] Archaea in the genusHaloquadratum such asHaloquadratum walsbyi are flat, square specimens that live in hypersaline pools.[109] These unusual shapes are probably maintained by both their cell walls and aprokaryotic cytoskeleton. Proteins related to the cytoskeleton components of other organisms exist in archaea,[110] and filaments form within their cells,[111] but in contrast with other organisms, these cellular structures are poorly understood.[112] InThermoplasma andFerroplasma the lack of acell wall means that the cells have irregular shapes, and can resembleamoebae.[113]
Some species form aggregates or filaments of cells up to 200 μm long.[107] These organisms can be prominent inbiofilms.[114] Notably, aggregates ofThermococcus coalescens cells fuse together in culture, forming single giant cells.[115] Archaea in the genusPyrodictium produce an elaborate multicell colony involving arrays of long, thin hollow tubes calledcannulae that stick out from the cells' surfaces and connect them into a dense bush-like agglomeration.[116] The function of these cannulae is not settled, but they may allow communication or nutrient exchange with neighbors.[117] Multi-species colonies exist, such as the "string-of-pearls" community that was discovered in 2001 in a German swamp. Round whitish colonies of a novel Euryarchaeota species are spaced along thin filaments that can range up to 15 centimetres (5.9 in) long; these filaments are made of a particular bacteria species.[118]
Archaea and bacteria have generally similarcell structure, but cell composition and organization set the archaea apart. Like bacteria, archaea lack interior membranes andorganelles.[68] Like bacteria, thecell membranes of archaea are usually bounded by acell wall and they swim using one or moreflagella.[119] Structurally, archaea are most similar togram-positive bacteria. Most have a single plasma membrane and cell wall, and lack aperiplasmic space; the exception to this general rule isIgnicoccus, which possess a particularly large periplasm that contains membrane-boundvesicles and is enclosed by an outer membrane.[120]
Most archaea (but notThermoplasma andFerroplasma) possess a cell wall.[113] In most archaea, the wall is assembled from surface-layer proteins, which form anS-layer.[121] An S-layer is a rigid array of protein molecules that cover the outside of the cell (likechain mail).[122] This layer provides both chemical and physical protection, and can preventmacromolecules from contacting the cell membrane.[123] Unlike bacteria, archaea lackpeptidoglycan in their cell walls.[124]Methanobacteriales do have cell walls containingpseudopeptidoglycan, which resembles eubacterial peptidoglycan in morphology, function, and physical structure, but pseudopeptidoglycan is distinct in chemical structure; it lacksD-amino acids andN-acetylmuramic acid, substituting the latter withN-Acetyltalosaminuronic acid.[123]
Archaeal flagella are known asarchaella, that operate like bacterialflagella – their long stalks are driven by rotatory motors at the base. These motors are powered by aproton gradient across the membrane, but archaella are notably different in composition and development.[119] The two types of flagella evolved from different ancestors. The bacterial flagellum shares a common ancestor with thetype III secretion system,[125][126] while archaeal flagella appear to have evolved from bacterialtype IV pili.[127] In contrast with the bacterial flagellum, which is hollow and assembled by subunits moving up the central pore to the tip of the flagella, archaeal flagella are synthesized by adding subunits at the base.[128]
Membrane structures.Top, an archaeal phospholipid:1, isoprene chains;2, ether linkages;3, L-glycerolmoiety;4, phosphate group.Middle, a bacterial or eukaryotic phospholipid:5, fatty acid chains;6, ester linkages;7, D-glycerol moiety;8, phosphate group.Bottom:9, lipid bilayer of bacteria and eukaryotes;10, lipid monolayer of some archaea.
Archaeal membranes are made of molecules that are distinctly different from those in all other life forms, showing that archaea are related only distantly to bacteria and eukaryotes.[129] In all organisms,cell membranes are made of molecules known asphospholipids. These molecules possess both apolar part that dissolves in water (thephosphate "head"), and a "greasy" non-polar part that does not (the lipid tail). These dissimilar parts are connected by aglycerol moiety. In water, phospholipids cluster, with the heads facing the water and the tails facing away from it. The major structure in cell membranes is a double layer of these phospholipids, which is called alipid bilayer.[130]
The phospholipids of archaea are unusual in four ways:
They have membranes composed of glycerol-ether lipids, whereas bacteria and eukaryotes have membranes composed mainly of glycerol-esterlipids.[131] The difference is the type of bond that joins the lipids to the glycerol moiety; the two types are shown in yellow in the figure at the right. In ester lipids, this is anester bond, whereas in ether lipids this is anether bond.[132]
Thestereochemistry of the archaeal glycerol moiety is the mirror image of that found in other organisms. The glycerol moiety can occur in two forms that are mirror images of one another, calledenantiomers. Just as a right hand does not fit easily into a left-handed glove, enantiomers of one type generally cannot be used or made byenzymes adapted for the other. The archaeal phospholipids are built on a backbone ofsn-glycerol-1-phosphate, which is an enantiomer ofsn-glycerol-3-phosphate, the phospholipid backbone found in bacteria and eukaryotes. This suggests that archaea use entirely different enzymes for synthesizing phospholipids as compared to bacteria and eukaryotes. Such enzymes developed very early in life's history, indicating an early split from the other two domains.[129]
Archaeal lipid tails differ from those of other organisms in that they are based upon longisoprenoid chains with multiple side-branches, sometimes withcyclopropane orcyclohexane rings.[133] By contrast, thefatty acids in the membranes of other organisms have straight chains without side branches or rings. Although isoprenoids play an important role in the biochemistry of many organisms, only the archaea use them to make phospholipids. These branched chains may help prevent archaeal membranes from leaking at high temperatures.[134]
In some archaea, the lipid bilayer is replaced by a monolayer. In effect, the archaea fuse the tails of two phospholipid molecules into a single molecule with two polar heads (abolaamphiphile); this fusion may make their membranes more rigid and better able to resist harsh environments.[135] For example, the lipids inFerroplasma are of this type, which is thought to aid this organism's survival in its highly acidic habitat.[136]
Other groups of archaea use sunlight as a source of energy (they arephototrophs), but oxygen–generatingphotosynthesis does not occur in any of these organisms.[138] Many basicmetabolic pathways are shared among all forms of life; for example, archaea use a modified form ofglycolysis (theEntner–Doudoroff pathway) and either a complete or partialcitric acid cycle.[139] These similarities to other organisms probably reflect both early origins in the history of life and their high level of efficiency.[140]
Some Euryarchaeota aremethanogens (archaea that produce methane as a result of metabolism) living inanaerobic environments, such as swamps. This form of metabolism evolved early, and it is even possible that the first free-living organism was a methanogen.[141] A common reaction involves the use ofcarbon dioxide as an electron acceptor to oxidizehydrogen. Methanogenesis involves a range ofcoenzymes that are unique to these archaea, such ascoenzyme M andmethanofuran.[142] Other organic compounds such asalcohols,acetic acid orformic acid are used as alternativeelectron acceptors by methanogens. These reactions are common ingut-dwelling archaea.Acetic acid is also broken down into methane and carbon dioxide directly, byacetotrophic archaea. These acetotrophs are archaea in the orderMethanosarcinales, and are a major part of the communities of microorganisms that producebiogas.[143]
Archaea usually have a singlecircular chromosome,[152] but many euryarchaea have been shown to bear multiple copies of this chromosome.[153] The largest known archaeal genome as of 2002 was 5,751,492 base pairs inMethanosarcina acetivorans.[154] The tiny 490,885 base-pair genome ofNanoarchaeum equitans is one-tenth of this size and the smallest archaeal genome known; it is estimated to contain only 537 protein-encoding genes.[155] Smaller independent pieces of DNA, calledplasmids, are also found in archaea. Plasmids may be transferred between cells by physical contact, in a process that may be similar tobacterial conjugation.[156][157]
Archaea are genetically distinct from bacteria and eukaryotes, with up to 15% of the proteins encoded by any one archaeal genome being unique to the domain, although most of these unique genes have no known function.[159] Of the remainder of the unique proteins that have an identified function, most belong to the Euryarchaeota and are involved in methanogenesis. The proteins that archaea, bacteria and eukaryotes share form a common core of cell function, relating mostly totranscription,translation, andnucleotide metabolism.[160] Other characteristic archaeal features are the organization of genes of related function – such as enzymes that catalyze steps in the samemetabolic pathway into noveloperons, and large differences intRNA genes and theiraminoacyl tRNA synthetases.[160]
Transcription in archaea more closely resembles eukaryotic than bacterial transcription, with the archaealRNA polymerase being very close to its equivalent in eukaryotes,[152] while archaeal translation shows signs of both bacterial and eukaryotic equivalents.[161] Although archaea have only one type of RNA polymerase, its structure and function in transcription seems to be close to that of the eukaryoticRNA polymerase II, with similar protein assemblies (thegeneral transcription factors) directing the binding of the RNA polymerase to a gene'spromoter,[162] but other archaealtranscription factors are closer to those found in bacteria.[163]Post-transcriptional modification is simpler than in eukaryotes, since most archaeal genes lackintrons, although there are many introns in theirtransfer RNA andribosomal RNA genes,[164] and introns may occur in a few protein-encoding genes.[165][166]
Haloferax volcanii, an extreme halophilic archaeon, forms cytoplasmic bridges between cells that appear to be used for transfer of DNA from one cell to another in either direction.[167]
When the hyperthermophilic archaeaSulfolobus solfataricus[168] andSulfolobus acidocaldarius[169] are exposed to DNA-damaging UV irradiation or to the agentsbleomycin ormitomycin C, species-specific cellular aggregation is induced. Aggregation inS. solfataricus could not be induced by other physical stressors, such as pH or temperature shift,[168] suggesting that aggregation is induced specifically byDNA damage. Ajon et al.[169] showed that UV-induced cellular aggregation mediates chromosomal marker exchange with high frequency inS. acidocaldarius. Recombination rates exceeded those of uninduced cultures by up to three orders of magnitude. Frols et al.[168][170] and Ajon et al.[169] hypothesized that cellular aggregation enhances species-specific DNA transfer betweenSulfolobus cells in order to provide increased repair of damaged DNA by means ofhomologous recombination. This response may be a primitive form of sexual interaction similar to the more well-studied bacterial transformation systems that are also associated with species-specific DNA transfer between cells leading to homologous recombinational repair of DNA damage.[171]
Archaea are the target of a number ofviruses in a diverse virosphere distinct from bacterial and eukaryotic viruses. They have been organized into 15–18 DNA-based families so far, but multiple species remain un-isolated and await classification.[172][173][174] These families can be informally divided into two groups: archaea-specific and cosmopolitan. Archaeal-specific viruses target only archaean species and currently include 12 families. Numerous unique, previously unidentified viral structures have been observed in this group, including: bottle-shaped, spindle-shaped, coil-shaped, and droplet-shaped viruses.[173] While the reproductive cycles and genomic mechanisms of archaea-specific species may be similar to other viruses, they bear unique characteristics that were specifically developed due to the morphology of host cells they infect.[172] Their virus release mechanisms differ from that of other phages.Bacteriophages generally undergo eitherlytic pathways,lysogenic pathways, or (rarely) a mix of the two.[175] Most archaea-specific viral strains maintain a stable, somewhat lysogenic, relationship with their hosts – appearing as a chronic infection. This involves the gradual, and continuous, production and release ofvirions without killing the host cell.[176] Prangishyili (2013) noted that it has been hypothesized that tailed archaeal phages originated from bacteriophages capable of infectinghaloarchaeal species. If the hypothesis is correct, it can be concluded that otherdouble-stranded DNA viruses that make up the rest of the archaea-specific group are their own unique group in the global viral community. Krupovic et al. (2018) states that the high levels ofhorizontal gene transfer, rapid mutation rates in viral genomes, and lack of universal gene sequences have led researchers to perceive the evolutionary pathway of archaeal viruses as a network. The lack of similarities amongphylogenetic markers in this network and the global virosphere, as well as external linkages to non-viral elements, may suggest that some species of archaea specific viruses evolved from non-viralmobile genetic elements (MGE).[173]
Archaea reproduce asexually by binary or multiplefission, fragmentation, orbudding;mitosis andmeiosis do not occur, so if a species of archaea exists in more than one form, all have the same genetic material.[107]Cell division is controlled in acell cycle; after the cell'schromosome is replicated and the two daughter chromosomes separate, the cell divides.[182] In the genusSulfolobus, the cycle has characteristics that are similar to both bacterial and eukaryotic systems. The chromosomes replicate from multiple starting points (origins of replication) usingDNA polymerases that resemble the equivalent eukaryotic enzymes.[183]
In Euryarchaeota the cell division proteinFtsZ, which forms a contracting ring around the cell, and the components of theseptum that is constructed across the center of the cell, are similar to their bacterial equivalents.[182] In cren-[184][185] and thaumarchaea,[186] the cell division machinery Cdv fulfills a similar role. This machinery is related to the eukaryotic ESCRT-III machinery which, while best known for its role in cell sorting, also has been seen to fulfill a role in separation between divided cell, suggesting an ancestral role in cell division.[187]
Both bacteria and eukaryotes, but not archaea, makespores.[188] Some species ofHaloarchaea undergophenotypic switching and grow as several different cell types, including thick-walled structures that are resistant toosmotic shock and allow the archaea to survive in water at low salt concentrations, but these are not reproductive structures and may instead help them reach new habitats.[189]
Quorum sensing was originally thought to not exist in Archaea, but recent studies have shown evidence of some species being able to perform cross-talk through quorum sensing. Other studies have shown syntrophic interactions between archaea and bacteria during biofilm growth. Although research is limited in archaeal quorum sensing, some studies have uncovered LuxR proteins in archaeal species, displaying similarities with bacteria LuxR, and ultimately allowing for the detection of small molecules that are used in high density communication. Similarly to bacteria, Archaea LuxR solos have shown to bind to AHLs (lactones) and non-AHLs ligans, which is a large part in performing intraspecies, interspecies, and interkingdom communication through quorum sensing.[190]
Archaea exist in a broad range ofhabitats, and are now recognized as a major part of globalecosystems,[19] and may represent about 20% of microbial cells in the oceans.[191] However, the first-discovered archaeans wereextremophiles.[137] Indeed, some archaea survive high temperatures, often above 100 °C (212 °F), as found ingeysers,black smokers, and oil wells. Other common habitats include very cold habitats and highlysaline,acidic, oralkaline water, but archaea includemesophiles that grow in mild conditions, inswamps andmarshland,sewage, theoceans, theintestinal tract of animals, andsoils.[8][19] Similar toPGPR, Archaea are now considered as a source of plant growth promotion as well.[8]
Extremophile archaea are members of four mainphysiological groups. These are thehalophiles,thermophiles,alkaliphiles, andacidophiles.[192] These groups are not comprehensive or phylum-specific, nor are they mutually exclusive, since some archaea belong to several groups. Nonetheless, they are a useful starting point for classification.[193]
Halophiles, including the genusHalobacterium, live in extremely saline environments such assalt lakes and outnumber their bacterial counterparts at salinities greater than 20–25%.[137] Thermophiles grow best at temperatures above 45 °C (113 °F), in places such as hot springs;hyperthermophilic archaea grow optimally at temperatures greater than 80 °C (176 °F).[194] The archaealMethanopyrus kandleri Strain 116 can even reproduce at 122 °C (252 °F), the highest recorded temperature of any organism.[195]
Other archaea exist in very acidic or alkaline conditions.[192] For example, one of the most extreme archaean acidophiles isPicrophilus torridus, which grows at pH 0, which is equivalent to thriving in 1.2 molarsulfuric acid.[196]
This resistance to extreme environments has made archaea the focus of speculation about the possible properties ofextraterrestrial life.[197] Some extremophile habitats are not dissimilar to those onMars,[198] leading to the suggestion that viable microbes could be transferred between planets inmeteorites.[199]
Recently, several studies have shown that archaea exist not only in mesophilic and thermophilic environments but are also present, sometimes in high numbers, at low temperatures as well. For example, archaea are common in cold oceanic environments such as polar seas.[200] Even more significant are the large numbers of archaea found throughout the world's oceans in non-extreme habitats among theplankton community (as part of thepicoplankton).[201] Although these archaea can be present in extremely high numbers (up to 40% of the microbial biomass), almost none of these species have been isolated and studied inpure culture.[202] Consequently, our understanding of the role of archaea in ocean ecology is rudimentary, so their full influence on globalbiogeochemical cycles remains largely unexplored.[203] Some marine Thermoproteota are capable ofnitrification, suggesting these organisms may affect the oceanicnitrogen cycle,[149] although these oceanic Thermoproteota may also use other sources of energy.[204]
Vast numbers of archaea are also found in thesediments that cover thesea floor, with these organisms making up the majority of living cells at depths over 1 meter below the ocean bottom.[205][206] It has been demonstrated that in all oceanic surface sediments (from 1,000- to 10,000-m water depth), the impact of viral infection is higher on archaea than on bacteria and virus-induced lysis of archaea accounts for up to one-third of the total microbial biomass killed, resulting in the release of ~0.3 to 0.5 gigatons of carbon per year globally.[207]
Archaea recycle elements such ascarbon,nitrogen, andsulfur through their various habitats.[208] Archaea carry out many steps in thenitrogen cycle. This includes both reactions that remove nitrogen from ecosystems (such asnitrate-based respiration anddenitrification) as well as processes that introduce nitrogen (such as nitrate assimilation andnitrogen fixation).[209][210]
Researchers recently discovered archaeal involvement inammonia oxidation reactions. These reactions are particularly important in the oceans.[150][211] The archaea also appear crucial for ammonia oxidation in soils. They producenitrite, which other microbes then oxidize tonitrate. Plants and other organisms consume the latter.[212]
In thesulfur cycle, archaea that grow by oxidizingsulfur compounds release this element from rocks, making it available to other organisms, but the archaea that do this, such asSulfolobus, producesulfuric acid as a waste product, and the growth of these organisms in abandoned mines can contribute toacid mine drainage and other environmental damage.[213]
In thecarbon cycle, methanogen archaea remove hydrogen and play an important role in the decay of organic matter by the populations of microorganisms that act asdecomposers in anaerobic ecosystems, such as sediments, marshes, andsewage-treatment works.[214]
Mutualism is an interaction between individuals of different species that results in positive (beneficial) effects on per capita reproduction and/or survival of the interacting populations. One well-understood example ofmutualism is the interaction betweenprotozoa andmethanogenic archaea in the digestive tracts of animals that digestcellulose, such asruminants andtermites.[220] In these anaerobic environments, protozoa break down plant cellulose to obtain energy. This process releases hydrogen as a waste product, but high levels of hydrogen reduce energy production. When methanogens convert hydrogen to methane, protozoa benefit from more energy.[221]
Some archaea are commensals, benefiting from an association without helping or harming the other organism. For example, the methanogenMethanobrevibacter smithii is by far the most common archaean in thehuman flora, making up about one in ten of the prokaryotes in the human gut.[228] In termites and in humans, these methanogens may in fact be mutualists, interacting with other microbes in the gut to aid digestion.[229] Archaean communities associate with a range of other organisms, such as on the surface ofcorals,[230] and in the region of soil that surrounds plant roots (therhizosphere).[231][232]
Although Archaea do not have a historical reputation of being pathogens, Archaea are often found with similar genomes to more common pathogens likeE. coli,[233] showing metabolic links and evolutionary history with today's pathogens. Archaea have been inconsistently detected in clinical studies because of the lack of categorization of Archaea into more specific species.[234]
Extremophile archaea, particularly those resistant either to heat or to extremes of acidity and alkalinity, are a source ofenzymes that function under these harsh conditions.[235][236] These enzymes have found many uses. For example, thermostableDNA polymerases, such as thePfu DNA polymerase fromPyrococcus furiosus, revolutionizedmolecular biology by allowing thepolymerase chain reaction to be used in research as a simple and rapid technique forcloning DNA. In industry,amylases,galactosidases andpullulanases in other species ofPyrococcus that function at over 100 °C (212 °F) allowfood processing at high temperatures, such as the production of lowlactose milk andwhey.[237] Enzymes from these thermophilic archaea also tend to be very stable in organic solvents, allowing their use in environmentally friendly processes ingreen chemistry that synthesize organic compounds.[236] This stability makes them easier to use instructural biology. Consequently, the counterparts of bacterial or eukaryotic enzymes from extremophile archaea are often used in structural studies.[238]
In contrast with the range of applications of archaean enzymes, the use of the organisms themselves in biotechnology is less developed.Methanogenic archaea are a vital part ofsewage treatment, since they are part of the community of microorganisms that carry outanaerobic digestion and producebiogas.[239] Inmineral processing, acidophilic archaea display promise for the extraction of metals fromores, includinggold,cobalt andcopper.[240]
Archaea host a new class of potentially usefulantibiotics. A few of thesearchaeocins have been characterized, but hundreds more are believed to exist, especially withinHaloarchaea andSulfolobus. These compounds differ in structure from bacterial antibiotics, so they may have novel modes of action. In addition, they may allow the creation of newselectable markers for use in archaeal molecular biology.[241]
^Göker, Markus; Oren, Aharon (22 January 2024). "Valid publication of names of two domains and seven kingdoms of prokaryotes".International Journal of Systematic and Evolutionary Microbiology.74 (1).doi:10.1099/ijsem.0.006242. – this article represents the valid publication of the name Archaea. Although Woese proposed the name in 1990, the name was not valid until this article from 2024, hence the date.
^abcdChow C, Padda KP, Puri A, Chanway CP (September 2022). "An Archaic Approach to a Modern Issue: Endophytic Archaea for Sustainable Agriculture".Current Microbiology.79 (11): 322.doi:10.1007/s00284-022-03016-y.PMID36125558.S2CID252376815.
^Moissl-Eichinger C, Pausan M, Taffner J, Berg G, Bang C, Schmitz RA (January 2018). "Archaea Are Interactive Components of Complex Microbiomes".Trends in Microbiology.26 (1):70–85.doi:10.1016/j.tim.2017.07.004.PMID28826642.
^Parks DH, Chuvochina M, Waite DW, Rinke C, Skarshewski A, Chaumeil PA, Hugenholtz P (November 2018). "A standardized bacterial taxonomy based on genome phylogeny substantially revises the tree of life".Nature Biotechnology.36 (10):996–1004.doi:10.1038/nbt.4229.PMID30148503.S2CID52093100.
^Stetter KO (1996). "Hyperthermophiles in the history of life".Ciba Foundation Symposium.202:1–10, discussion 11–8.PMID9243007.
^abcDeLong EF (December 1998). "Everything in moderation: archaea as 'non-extremophiles'".Current Opinion in Genetics & Development.8 (6):649–54.doi:10.1016/S0959-437X(98)80032-4.PMID9914204.
^Theron J, Cloete TE (2000). "Molecular techniques for determining microbial diversity and community structure in natural environments".Critical Reviews in Microbiology.26 (1):37–57.doi:10.1080/10408410091154174.PMID10782339.S2CID5829573.
^abRobertson CE, Harris JK, Spear JR, Pace NR (December 2005). "Phylogenetic diversity and ecology of environmental Archaea".Current Opinion in Microbiology.8 (6):638–642.doi:10.1016/j.mib.2005.10.003.PMID16236543.
^Guy L, Ettema TJ (December 2011). "The archaeal 'TACK' superphylum and the origin of eukaryotes".Trends in Microbiology.19 (12):580–87.doi:10.1016/j.tim.2011.09.002.PMID22018741.
^Göker, Markus; Oren, Aharon (11 September 2023). "Valid publication of four additional phylum names".International Journal of Systematic and Evolutionary Microbiology.73 (9).doi:10.1099/ijsem.0.006024.PMID37695645.
^Imachi, Hiroyuki; Nobu, Masaru K.; Kato, Shingo; Takaki, Yoshihiro; Miyazaki, Masayuki; Miyata, Makoto; Ogawara, Miyuki; Saito, Yumi; Sakai, Sanae; Tahara, Yuhei O.; Takano, Yoshinori; Tasumi, Eiji; Uematsu, Katsuyuki; Yoshimura, Toshihiro; Itoh, Takashi; Ohkuma, Moriya; Takai, Ken (5 July 2024). "Promethearchaeum syntrophicum gen. nov., sp. nov., an anaerobic, obligately syntrophic archaeon, the first isolate of the lineage 'Asgard' archaea, and proposal of the new archaeal phylum Promethearchaeota phyl. nov. and kingdom Promethearchaeati regn. nov".International Journal of Systematic and Evolutionary Microbiology.74 (7). Society for General Microbiology.doi:10.1099/ijsem.0.006435.ISSN1466-5034.PMC 11316595.PMID38967634.
^"Age of the Earth". U.S. Geological Survey. 1997.Archived from the original on 23 December 2005. Retrieved10 January 2006.
^abcGupta RS (August 1998). "What are archaebacteria: life's third domain or monoderm prokaryotes related to gram-positive bacteria? A new proposal for the classification of prokaryotic organisms".Molecular Microbiology.29 (3):695–707.doi:10.1046/j.1365-2958.1998.00978.x.PMID9723910.S2CID41206658.
^Gupta RS (2005). "Molecular Sequences and the Early History of Life". In Sapp J (ed.).Microbial Phylogeny and Evolution: Concepts and Controversies. New York: Oxford University Press. pp. 160–183.
^abLatorre A, Durban A, Moya A, Pereto J (2011)."The role of symbiosis in eukaryotic evolution". In Gargaud M, López-Garcìa P, Martin H (eds.).Origins and Evolution of Life: An astrobiological perspective. Cambridge: Cambridge University Press. pp. 326–339.ISBN978-0-521-76131-4.Archived from the original on 24 March 2019. Retrieved27 August 2017.
^Nelson KE, Clayton RA, Gill SR, Gwinn ML, Dodson RJ, Haft DH, et al. (May 1999). "Evidence for lateral gene transfer between Archaea and bacteria from genome sequence of Thermotoga maritima".Nature.399 (6734):323–29.Bibcode:1999Natur.399..323N.doi:10.1038/20601.PMID10360571.S2CID4420157.
^Hall-Stoodley L, Costerton JW, Stoodley P (February 2004). "Bacterial biofilms: from the natural environment to infectious diseases".Nature Reviews. Microbiology.2 (2):95–108.doi:10.1038/nrmicro821.PMID15040259.S2CID9107205.
^Nickell S, Hegerl R, Baumeister W, Rachel R (January 2003). "Pyrodictium cannulae enter the periplasmic space but do not enter the cytoplasm, as revealed by cryo-electron tomography".Journal of Structural Biology.141 (1):34–42.doi:10.1016/S1047-8477(02)00581-6.PMID12576018.
^Engelhardt H, Peters J (December 1998). "Structural research on surface layers: a focus on stability, surface layer homology domains, and surface layer-cell wall interactions".Journal of Structural Biology.124 (2–3):276–302.doi:10.1006/jsbi.1998.4070.PMID10049812.
^Howland JL (2000).The Surprising Archaea: Discovering Another Domain of Life. Oxford: Oxford University Press. p. 32.ISBN978-0-19-511183-5.
^Gophna U, Ron EZ, Graur D (July 2003). "Bacterial type III secretion systems are ancient and evolved by multiple horizontal-transfer events".Gene.312:151–63.doi:10.1016/S0378-1119(03)00612-7.PMID12909351.
^Nguyen L, Paulsen IT, Tchieu J, Hueck CJ, Saier MH (April 2000). "Phylogenetic analyses of the constituents of Type III protein secretion systems".Journal of Molecular Microbiology and Biotechnology.2 (2):125–44.PMID10939240.
^Ng SY, Chaban B, Jarrell KF (2006). "Archaeal flagella, bacterial flagella and type IV pili: a comparison of genes and posttranslational modifications".Journal of Molecular Microbiology and Biotechnology.11 (3–5):167–91.doi:10.1159/000094053.PMID16983194.S2CID30386932.
^Youssefian S, Rahbar N, Van Dessel S (May 2018). "Thermal conductivity and rectification in asymmetric archaeal lipid membranes".The Journal of Chemical Physics.148 (17): 174901.Bibcode:2018JChPh.148q4901Y.doi:10.1063/1.5018589.PMID29739208.
^Hanford MJ, Peeples TL (January 2002). "Archaeal tetraether lipids: unique structures and applications".Applied Biochemistry and Biotechnology.97 (1):45–62.doi:10.1385/ABAB:97:1:45.PMID11900115.S2CID22334666.
^Macalady JL, Vestling MM, Baumler D, Boekelheide N, Kaspar CW, Banfield JF (October 2004). "Tetraether-linked membrane monolayers in Ferroplasma spp: a key to survival in acid".Extremophiles.8 (5):411–19.doi:10.1007/s00792-004-0404-5.PMID15258835.S2CID15702103.
^abcValentine DL (April 2007). "Adaptations to energy stress dictate the ecology and evolution of the Archaea".Nature Reviews. Microbiology.5 (4):316–23.doi:10.1038/nrmicro1619.PMID17334387.S2CID12384143.
^Zillig W (December 1991). "Comparative biochemistry of Archaea and Bacteria".Current Opinion in Genetics & Development.1 (4):544–51.doi:10.1016/S0959-437X(05)80206-0.PMID1822288.
^Klocke M, Nettmann E, Bergmann I, Mundt K, Souidi K, Mumme J, et al. (August 2008). "Characterization of the methanogenic Archaea within two-phase biogas reactor systems operated with plant biomass".Systematic and Applied Microbiology.31 (3):190–205.Bibcode:2008SyApM..31..190K.doi:10.1016/j.syapm.2008.02.003.PMID18501543.
^Bryant DA, Frigaard NU (November 2006). "Prokaryotic photosynthesis and phototrophy illuminated".Trends in Microbiology.14 (11):488–96.doi:10.1016/j.tim.2006.09.001.PMID16997562.
^Yoshinari S, Itoh T, Hallam SJ, DeLong EF, Yokobori S, Yamagishi A, et al. (August 2006). "Archaeal pre-mRNA splicing: a connection to hetero-oligomeric splicing endonuclease".Biochemical and Biophysical Research Communications.346 (3):1024–32.doi:10.1016/j.bbrc.2006.06.011.PMID16781672.
^Fröls S, White MF, Schleper C (February 2009). "Reactions to UV damage in the model archaeon Sulfolobus solfataricus".Biochemical Society Transactions.37 (Pt 1):36–41.doi:10.1042/BST0370036.PMID19143598.
^abBlohs M, Moissl-Eichinger C, Mahnert A, Spang A, Dombrowski N, Krupovic M, Klingl A (January 2019). "Archaea – An Introduction". In Schmidt TM (ed.).Encyclopedia of Microbiology (Fourth ed.). Academic Press. pp. 243–252.doi:10.1016/B978-0-12-809633-8.20884-4.ISBN978-0-12-811737-8.S2CID164553336.
^Kelman LM, Kelman Z (September 2004). "Multiple origins of replication in archaea".Trends in Microbiology.12 (9):399–401.doi:10.1016/j.tim.2004.07.001.PMID15337158.
^Ciaramella M, Napoli A, Rossi M (February 2005). "Another extreme genome: how to live at pH 0".Trends in Microbiology.13 (2):49–51.doi:10.1016/j.tim.2004.12.001.PMID15680761.
^Davies PC (1996). "The Transfer of Viable Microorganisms Between Planets". In Bock GR, Goode JA (eds.).Ciba Foundation Symposium 202 – Evolution of Hydrothermal Ecosystems on Earth (And Mars?). Novartis Foundation Symposia. Vol. 202. pp. 304–14, discussion 314–17.doi:10.1002/9780470514986.ch16.ISBN9780470514986.PMID9243022.
^López-García P, López-López A, Moreira D, Rodríguez-Valera F (July 2001). "Diversity of free-living prokaryotes from a deep-sea site at the Antarctic Polar Front".FEMS Microbiology Ecology.36 (2–3):193–202.Bibcode:2001FEMME..36..193L.doi:10.1016/s0168-6496(01)00133-7 (inactive 1 November 2024).PMID11451524.{{cite journal}}: CS1 maint: DOI inactive as of November 2024 (link)
^Coolen MJ, Abbas B, van Bleijswijk J, Hopmans EC, Kuypers MM, Wakeham SG, et al. (April 2007). "Putative ammonia-oxidizing Crenarchaeota in suboxic waters of the Black Sea: A basin-wide ecological study using 16S ribosomal and functional genes and membrane lipids".Environmental Microbiology.9 (4):1001–16.Bibcode:2007EnvMi...9.1001C.doi:10.1111/j.1462-2920.2006.01227.x.hdl:1912/2034.PMID17359272.
^Chaban B, Ng SY, Jarrell KF (February 2006). "Archaeal habitats—from the extreme to the ordinary".Canadian Journal of Microbiology.52 (2):73–116.doi:10.1139/w05-147.PMID16541146.
^abEgorova K, Antranikian G (December 2005). "Industrial relevance of thermophilic Archaea".Current Opinion in Microbiology.8 (6):649–55.doi:10.1016/j.mib.2005.10.015.PMID16257257.
^Synowiecki J, Grzybowska B, Zdziebło A (2006). "Sources, properties and suitability of new thermostable enzymes in food processing".Critical Reviews in Food Science and Nutrition.46 (3):197–205.doi:10.1080/10408690590957296.PMID16527752.S2CID7208835.
^Shand RF, Leyva KJ (2008). "Archaeal Antimicrobials: An Undiscovered Country". In Blum P (ed.).Archaea: New Models for Prokaryotic Biology. Caister Academic Press.ISBN978-1-904455-27-1.
Garrett RA, Klenk H (2005).Archaea: Evolution, Physiology and Molecular Biology. WileyBlackwell.ISBN978-1-4051-4404-9.
Cavicchioli R (2007).Archaea: Molecular and Cellular Biology. American Society for Microbiology.ISBN978-1-55581-391-8.
Blum P, ed. (2008).Archaea: New Models for Prokaryotic Biology. Caister Academic Press.ISBN978-1-904455-27-1.
Lipps G (2008). "Archaeal Plasmids".Plasmids: Current Research and Future Trends. Caister Academic Press.ISBN978-1-904455-35-6.
Sapp J (2009).The New Foundations of Evolution: On the Tree of Life. New York: Oxford University Press.ISBN978-0-19-538850-3.
Schaechter M (2009).Archaea (Overview) in The Desk Encyclopedia of Microbiology (2nd ed.). San Diego and London: Elsevier Academic Press.ISBN978-0-12-374980-2.
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