Archaea

For much of the 20th century, prokaryotes were regarded as a single group of organisms and classified based on their biochemistry, morphology and metabolism. Microbiologists tried to classify microorganisms based on the structures of their cell walls, their shapes, and the substances they consume.^[10] In 1965, Emile Zuckerkandl and Linus Pauling^[11] instead proposed using the sequences of the genes in different prokaryotes to work out how they are related to each other. This phylogenetic approach is the main method used today.^[12]

Archaea were first classified separately from bacteria in 1977 by

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.

A superphylum – TACK – which includes the Thaumarchaeota (now Nitrososphaerota), "Aigarchaeota", Crenarchaeota (now Thermoproteota), and "Korarchaeota" was proposed in 2011 to be related to the origin of eukaryotes.^[28] In 2017, the newly discovered and newly named Asgard superphylum was proposed to be more closely related to the original eukaryote and a sister group to TACK.^[29]

In 2013, the superphylum DPANN was proposed to group "

The classification of archaea into species is also controversial. Ernst Mayr's species definition — a reproductively isolated group of interbreeding organisms — does not apply, as archaea reproduce only asexually.^[37]

Archaea show high levels of horizontal 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 genus Ferroplasma.^[38] On the other hand, studies in Halorubrum found significant genetic transfer to/from less-related populations, limiting the criterion's applicability.^[39] Some researchers question whether such species designations have practical meaning.^[40]

Current knowledge on genetic diversity in archaeans is fragmentary, so the total number of species cannot be estimated with any accuracy.^[22] 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.^[41] This situation is also seen in the Bacteria; many uncultured microbes present similar issues with characterization.^[42]

The following phyla have been validly published according to the

• Methanobacteriota
• Microcaldota
• Nanobdellota
• Thermoproteota (syn. Nitrososphaerota)

The following phyla have been proposed, but have not been validly published according to the Bacteriological Code (including those that have candidatus status):

The

The following table compares some major characteristics of the three domains, to illustrate their similarities and differences.[66]

Archaea were split off as a third domain because of the large differences in their ribosomal RNA structure. The particular molecule

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.[13] These methanogens were much more similar to each other than to other organisms, leading Woese to propose the new domain of Archaea.^[13] His experiments showed that the archaea were genetically more similar to eukaryotes than prokaryotes, even though they were more similar to prokaryotes in structure.^[70] This led to the conclusion that Archaea and Eukarya shared a common ancestor more recent than Eukarya and Bacteria.^[70] The development of the nucleus occurred after the split between Bacteria and this common ancestor.^[70][1]

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 and salinity. Comparative analysis of archaeal genomes has also identified several molecular conserved signature indels and signature proteins uniquely present in either all archaea or different main groups within archaea.^[71][72][73] Another unique feature of archaea, found in no other organisms, is methanogenesis (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 the carbon cycle, breaking down organic carbon into methane, which is also a major greenhouse gas.^[74]

This difference in the biochemical structure of Bacteria and Archaea has been explained by researchers through evolutionary processes. It is theorized that both domains originated at deep sea alkaline hydrothermal vents. At least twice, microbes evolved lipid biosynthesis and cell wall biochemistry. It has been suggested that the last universal common ancestor was a non-free-living organism.^[75] 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".^[76]

The relationships among the

It has been proposed that the archaea evolved from gram-positive bacteria in response to antibiotic selection pressure.^[79][81][85] This is suggested by the observation that archaea are resistant to a wide variety of antibiotics that are produced primarily by gram-positive bacteria,^[79][81] 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.^[85] 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;^[85][86] Cavalier-Smith has made a similar suggestion, the Neomura hypothesis.^[87] This proposal is also supported by other work investigating protein structural relationships^[88] and studies that suggest that gram-positive bacteria may constitute the earliest branching lineages within the prokaryotes.^[89]

The evolutionary relationship between archaea and eukaryotes remains unclear. Aside from the similarities in cell structure and function that are discussed below, many genetic trees group the two.^[91]

Complicating factors include claims that the relationship between eukaryotes and the archaeal phylum

Some species form aggregates or filaments of cells up to 200 μm long.[104] These organisms can be prominent in biofilms.^[111] Notably, aggregates of Thermococcus coalescens cells fuse together in culture, forming single giant cells.^[112] Archaea in the genus Pyrodictium produce an elaborate multicell colony involving arrays of long, thin hollow tubes called cannulae that stick out from the cells' surfaces and connect them into a dense bush-like agglomeration.^[113] The function of these cannulae is not settled, but they may allow communication or nutrient exchange with neighbors.^[114] 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.^[115]

Archaea and bacteria have generally similar

Most archaea (but not

Archaeal flagella are known as

type IV pili.^[124] 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.^[125]

Membranes

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.[126] In all organisms, cell membranes are made of molecules known as phospholipids. These molecules possess both a polar part that dissolves in water (the phosphate "head"), and a "greasy" non-polar part that does not (the lipid tail). These dissimilar parts are connected by a glycerol 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 a lipid bilayer.^[127]

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-ester lipids.^[128] 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 an ester bond, whereas in ether lipids this is an ether bond.^[129]
• The stereochemistry 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, called enantiomers. Just as a right hand does not fit easily into a left-handed glove, enantiomers of one type generally cannot be used or made by enzymes adapted for the other. The archaeal phospholipids are built on a backbone of sn-glycerol-1-phosphate, which is an enantiomer of sn-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.^[126]
• Archaeal lipid tails differ from those of other organisms in that they are based upon long isoprenoid chains with multiple side-branches, sometimes with cyclopropane or cyclohexane rings.^[130] By contrast, the fatty 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.^[131]
• 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 (a bolaamphiphile); this fusion may make their membranes more rigid and better able to resist harsh environments.^[132] For example, the lipids in Ferroplasma are of this type, which is thought to aid this organism's survival in its highly acidic habitat.^[133]

Metabolism

Further information: Microbial metabolism

Archaea exhibit a great variety of chemical reactions in their

oxidisers.^[134] In these reactions one compound passes electrons to another (in a redox reaction), releasing energy to fuel the cell's activities. One compound acts as an electron donor and one as an electron acceptor. The energy released is used to generate adenosine triphosphate (ATP) through chemiosmosis, the same basic process that happens in the mitochondrion of eukaryotic cells.^[135]

Other groups of archaea use sunlight as a source of energy (they are phototrophs), but oxygen–generating photosynthesis does not occur in any of these organisms.^[135] Many basic metabolic pathways are shared among all forms of life; for example, archaea use a modified form of glycolysis (the Entner–Doudoroff pathway) and either a complete or partial citric acid cycle.^[136] These similarities to other organisms probably reflect both early origins in the history of life and their high level of efficiency.^[137]

Nutritional types in archaeal metabolism

Nutritional type	Source of energy	Source of carbon	Examples
Phototrophs	Sunlight	Organic compounds	Halobacterium
Lithotrophs	Inorganic compounds	Organic compounds or carbon fixation	Ferroglobus, Methanobacteria or Pyrolobus
Organotrophs	Organic compounds	Organic compounds or carbon fixation	Pyrococcus, Sulfolobus or Methanosarcinales

Some Euryarchaeota are

gut-dwelling archaea. Acetic acid is also broken down into methane and carbon dioxide directly, by acetotrophic archaea. These acetotrophs are archaea in the order Methanosarcinales, and are a major part of the communities of microorganisms that produce biogas.^[140]

Other archaea use CO
₂ in the

reductive acetyl-CoA pathway.^[144] Carbon fixation is powered by inorganic energy sources. No known archaea carry out photosynthesis^[145] (Halobacterium is the only known phototroph archeon but it uses an alternative process to photosynthesis). Archaeal energy sources are extremely diverse, and range from the oxidation of ammonia by the Nitrosopumilales^[146][147] to the oxidation of hydrogen sulfide or elemental sulfur by species of Sulfolobus, using either oxygen or metal ions as electron acceptors.^[135]

plasma membrane. The energy stored in these electrochemical gradients is then converted into ATP by ATP synthase.^[104] This process is a form of photophosphorylation. The ability of these light-driven pumps to move ions across membranes depends on light-driven changes in the structure of a retinol cofactor buried in the center of the protein.^[148]

Genetics

Further information: Plasmid and Genome

Archaea usually have a single circular chromosome,^[149] but many euryarchaea have been shown to bear multiple copies of this chromosome.^[150] The largest known archaeal genome as of 2002 was 5,751,492 base pairs in Methanosarcina acetivorans.^[151] The tiny 490,885 base-pair genome of Nanoarchaeum equitans is one-tenth of this size and the smallest archaeal genome known; it is estimated to contain only 537 protein-encoding genes.^[152] Smaller independent pieces of DNA, called plasmids, are also found in archaea. Plasmids may be transferred between cells by physical contact, in a process that may be similar to bacterial conjugation.^[153][154]

STSV1.^[155] Bar is 1 micrometer

.

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.

tRNA genes and their aminoacyl tRNA synthetases.^[157]

Transcription in archaea more closely resembles eukaryotic than bacterial transcription, with the archaeal

promoter,^[159] but other archaeal transcription factors are closer to those found in bacteria.^[160] Post-transcriptional modification is simpler than in eukaryotes, since most archaeal genes lack introns, although there are many introns in their transfer RNA and ribosomal RNA genes,^[161] and introns may occur in a few protein-encoding genes.^[162][163]

Gene transfer and genetic exchange

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.^[164]

When the hyperthermophilic archaea

DNA damage. Ajon et al.^[166] showed that UV-induced cellular aggregation mediates chromosomal marker exchange with high frequency in S. acidocaldarius. Recombination rates exceeded those of uninduced cultures by up to three orders of magnitude. Frols et al.^[165][167] and Ajon et al.^[166] hypothesized that cellular aggregation enhances species-specific DNA transfer between Sulfolobus cells in order to provide increased repair of damaged DNA by means of homologous 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.^[168]

Archaeal viruses

Archaea are the target of a number of

phylogenetic 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-viral mobile genetic elements (MGE).^[170]

These viruses have been studied in most detail in thermophilics, particularly the orders

repetitive DNA sequences that are related to the genes of the viruses.^[177][178]

Reproduction

Further information: Asexual reproduction

Archaea reproduce asexually by binary or multiple fission, fragmentation, or budding; mitosis and meiosis do not occur, so if a species of archaea exists in more than one form, all have the same genetic material.^[104] Cell division is controlled in a cell cycle; after the cell's chromosome is replicated and the two daughter chromosomes separate, the cell divides.^[179] In the genus Sulfolobus, the cycle has characteristics that are similar to both bacterial and eukaryotic systems. The chromosomes replicate from multiple starting points (origins of replication) using DNA polymerases that resemble the equivalent eukaryotic enzymes.^[180]

In Euryarchaeota the cell division protein FtsZ, which forms a contracting ring around the cell, and the components of the septum that is constructed across the center of the cell, are similar to their bacterial equivalents.^[179] In cren-^[181][182] and thaumarchaea,^[183] 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.^[184]

Both bacteria and eukaryotes, but not archaea, make spores.^[185] Some species of Haloarchaea undergo phenotypic switching and grow as several different cell types, including thick-walled structures that are resistant to osmotic 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.^[186]

Behavior

Communication

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.^[187]

Ecology

Habitats

Hot Spring in Yellowstone National Park

produce a bright colour

Archaea exist in a broad range of

PGPR, Archaea are now considered as a source of plant growth promotion as well.^[7]

Extremophile archaea are members of four main

acidophiles.^[189] 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.^[190]

Halophiles, including the genus

Methanopyrus kandleri Strain 116 can even reproduce at 122 °C (252 °F), the highest recorded temperature of any organism.^[192]

Other archaea exist in very acidic or alkaline conditions.[189] For example, one of the most extreme archaean acidophiles is Picrophilus torridus, which grows at pH 0, which is equivalent to thriving in 1.2 molar sulfuric acid.^[193]

This resistance to extreme environments has made archaea the focus of speculation about the possible properties of extraterrestrial life.^[194] Some extremophile habitats are not dissimilar to those on Mars,^[195] leading to the suggestion that viable microbes could be transferred between planets in meteorites.^[196]

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.

pure culture.^[199] Consequently, our understanding of the role of archaea in ocean ecology is rudimentary, so their full influence on global biogeochemical cycles remains largely unexplored.^[200] Some marine Thermoproteota are capable of nitrification, suggesting these organisms may affect the oceanic nitrogen cycle,^[146] although these oceanic Thermoproteota may also use other sources of energy.^[201]

Vast numbers of archaea are also found in the

sea floor, with these organisms making up the majority of living cells at depths over 1 meter below the ocean bottom.^[202][203] It has been demonstrated that in all oceanic surface sediments (from 1000- 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.^[204]

Role in chemical cycling

Further information: Biogeochemical cycle

Archaea recycle elements such as carbon, nitrogen, and sulfur through their various habitats.^[205] Archaea carry out many steps in the nitrogen cycle. This includes both reactions that remove nitrogen from ecosystems (such as nitrate-based respiration and denitrification) as well as processes that introduce nitrogen (such as nitrate assimilation and nitrogen fixation).^[206][207] Researchers recently discovered archaeal involvement in ammonia oxidation reactions. These reactions are particularly important in the oceans.^[147][208] The archaea also appear crucial for ammonia oxidation in soils. They produce nitrite, which other microbes then oxidize to nitrate. Plants and other organisms consume the latter.^[209]

In the sulfur cycle, archaea that grow by oxidizing sulfur compounds release this element from rocks, making it available to other organisms, but the archaea that do this, such as Sulfolobus, produce sulfuric acid as a waste product, and the growth of these organisms in abandoned mines can contribute to acid mine drainage and other environmental damage.^[210]

In the carbon cycle, methanogen archaea remove hydrogen and play an important role in the decay of organic matter by the populations of microorganisms that act as decomposers in anaerobic ecosystems, such as sediments, marshes, and sewage-treatment works.^[211]

Interactions with other organisms

Further information: Biological interaction

The well-characterized interactions between archaea and other organisms are either

parasites,^[212][213] but some species of methanogens have been suggested to be involved in infections in the mouth,^[214][215] and Nanoarchaeum equitans may be a parasite of another species of archaea, since it only survives and reproduces within the cells of the Crenarchaeon Ignicoccus hospitalis,^[152] and appears to offer no benefit to its host.^[216]

Mutualism

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 of mutualism is the interaction between protozoa and methanogenic archaea in the digestive tracts of animals that digest cellulose, such as ruminants and termites.^[217] 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.^[218]

In anaerobic protozoa, such as Plagiopyla frontata, Trimyema, Heterometopus and Metopus contortus, archaea reside inside the protozoa and consume hydrogen produced in their hydrogenosomes.^{[219][220][221][222][223]} Archaea associate with larger organisms, too. For example, the marine archaean Cenarchaeum symbiosum is an endosymbiont of the sponge Axinella mexicana.^[224]

Commensalism

Some archaea are commensals, benefiting from an association without helping or harming the other organism. For example, the methanogen

rhizosphere).^[228][229]

Parasitism

Although Archaea do not have a historical reputation of being pathogens, Archaea are often found with similar genomes to more common pathogen like E. coli,[230] 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.^[231]

Significance in technology and industry

Further information: Biotechnology

Extremophile archaea, particularly those resistant either to heat or to extremes of acidity and alkalinity, are a source of enzymes that function under these harsh conditions.^[232][233] These enzymes have found many uses. For example, thermostable DNA polymerases, such as the Pfu DNA polymerase from Pyrococcus furiosus, revolutionized molecular biology by allowing the polymerase chain reaction to be used in research as a simple and rapid technique for cloning DNA. In industry, amylases, galactosidases and pullulanases in other species of Pyrococcus that function at over 100 °C (212 °F) allow food processing at high temperatures, such as the production of low lactose milk and whey.^[234] Enzymes from these thermophilic archaea also tend to be very stable in organic solvents, allowing their use in environmentally friendly processes in green chemistry that synthesize organic compounds.^[233] This stability makes them easier to use in structural biology. Consequently, the counterparts of bacterial or eukaryotic enzymes from extremophile archaea are often used in structural studies.^[235]

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 of sewage treatment, since they are part of the community of microorganisms that carry out anaerobic digestion and produce biogas.^[236] In mineral processing, acidophilic archaea display promise for the extraction of metals from ores, including gold, cobalt and copper.^[237]

Archaea host a new class of potentially useful antibiotics. A few of these archaeocins have been characterized, but hundreds more are believed to exist, especially within Haloarchaea and Sulfolobus. These compounds differ in structure from bacterial antibiotics, so they may have novel modes of action. In addition, they may allow the creation of new selectable markers for use in archaeal molecular biology.^[238]

See also

• Aerobic methane production
• Earliest known life forms
• List of Archaea genera
• List of sequenced archaeal genomes
• Nuclear localization sequence
• Stirrup protein domain
• The Surprising Archaea (book)
• Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya
• Unique properties of hyperthermophilic archaea
• Branching order of bacterial phyla (Genome Taxonomy Database, 2018)

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Further reading

• Howland JL (2000). The Surprising Archaea: Discovering Another Domain of Life. Oxford University.
  ISBN 978-0-19-511183-5
  .
• Martinko JM, Madigan MT (2005). Brock Biology of Microorganisms (11th ed.). Englewood Cliffs, N.J: Prentice Hall.
  ISBN 978-0-13-144329-7
  .
• Garrett RA, Klenk H (2005). Archaea: Evolution, Physiology and Molecular Biology. WileyBlackwell.
  ISBN 978-1-4051-4404-9
  .
• Cavicchioli R (2007). Archaea: Molecular and Cellular Biology. American Society for Microbiology.
  ISBN 978-1-55581-391-8
  .
• Blum P, ed. (2008). Archaea: New Models for Prokaryotic Biology. Caister Academic Press.
  ISBN 978-1-904455-27-1
  .
• Lipps G (2008). "Archaeal Plasmids". Plasmids: Current Research and Future Trends. Caister Academic Press.
  ISBN 978-1-904455-35-6
  .
• Sapp J (2009). The New Foundations of Evolution: On the Tree of Life. New York: Oxford University Press.
  ISBN 978-0-19-538850-3
  .
• Schaechter M (2009). Archaea (Overview) in The Desk Encyclopedia of Microbiology (2nd ed.). San Diego and London: Elsevier Academic Press.
  ISBN 978-0-12-374980-2
  .

External links

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General

• Introduction to the Archaea, ecology, systematics and morphology
• Oceans of Archaea – E.F. DeLong, ASM News, 2003

Classification

• NCBI taxonomy page on Archaea
• Genera of the domain Archaea – list of Prokaryotic names with Standing in Nomenclature
• Shotgun sequencing finds nanoorganisms – discovery of the ARMAN group of archaea

Genomics

• Browse any completed archaeal genome at UCSC
• Comparative Analysis of Archaeal Genomes Archived 16 February 2013 at the Wayback Machine (at DOE's IMG system)