Analysis of nucleotide sequences of ribosomal ribonucleic acid (RNA) led in the 1970s to the recognition of the existence of three domains of life, named Eukarya (Eukaryotes), Bacteria (Eubacteria) and Archaea (Archaebacteria). This classification replaced the earlier accepted Eukaryotes–Prokaryotes dichotomy. The Archaea resemble the Bacteria in cell size and cell structure, but possess many distinguishing features, including lack of peptidoglycan in their cell wall, presence of unique membrane lipids not found in the other domains of life and other unique biochemical and genetic properties. Most cultured Archaea were recovered from extreme environments (high salt concentrations, high temperatures and acidic hot springs). The unique properties of these extremophiles can be exploited in biotechnology. The strictly anaerobic methanogenic prokaryotes also belong to the archaeal domain. Culture‐independent studies show that Archaea are also widespread in nonextreme environments such as seawater and soil, and the properties of a few nonextremophilic Archaea are now being elucidated.

Key Concepts:

  • The Archaea form a second lineage of prokaryotes, evolutionarily distant from the Bacteria.

  • Most Archaea are classified within the phyla Crenarchaeota and Euryarchaeota; other archaeal phyla proposed are the Korarchaeota, the Nanoarchaeota, and the Thaumarchaeota.

  • Archaea resemble Bacteria in cell size, morphology and ultrastructure, but differ from the Bacteria in many biochemical, physiological and genetic features.

  • The cell membrane lipids of the Archaea are based on branched (isoprenoid) hydrophobic carbon chains linked to glycerol by ester bonds.

  • Peptidoglycan, the characteristic cell wall constituent of the Bacteria, does not occur in the Archaea.

  • All known methane‐forming prokaryotes belong to the Archaea.

  • Most cultured Archaea are extremophiles that live at high temperatures, low or high pH values or high salt concentrations.

  • Archaea are also widespread in environments not characterised by environmental extremes, but their function in aquatic and soil ecosystems is still poorly understood.

  • Archaea contribute to the autotrophic oxidation of ammonia to nitrite in different ecosystems.

  • Thanks to their unique biochemical features, the Archaea have considerable potential for biotechnological applications.

Keywords: Archaea (Archaebacteria); Euryarchaeota; Crenarchaeota; Korarchaeota; Nanoarchaeota; Thaumarchaeota; methanogens; halophiles; thermophiles; thermoacidophiles

Figure 1.

Phylogenetic 16S rRNA tree of Archaea (from Woese et al., ; modified). C1, Pyrodictium (Desulfurococcales, Pyrodictiaceae and Crenarchaeota); C2, Thermoproteus (Thermoproteales, Thermoprotaceae and Crenarchaeota); E1, Thermococcus (Thermococcales, Thermococcaceae and thermophilic sulfur‐metabolising Euryarchaeota); E2, Methanococcus (Methanococcales and methanogenic Euryarchaeota); E3, Methanobacterium (Methanobacteriales and methanogenic Euryarchaeota); E4, Methanosarcina (Methanosarcinales, Methanosarcinaceae and methanogenic Euryarchaeota); E5, extremely halophilic Euryarchaeota (Halobacteriales). For a more detailed tree that includes the nonextremophilic Archaea and environmental sequences from cold marine and terrestrial environments, see Brandon K Swan and David L Valentine (2009) Diversity of Archaea. Encyclopedia of Life Science. DOI no. 10.1002/9780470015902.a0000444.

Figure 2.

Electron micrographs of the diverse morphological forms of Archaea (platinum shadowing: (a), (b), (d)–(f), (h)–(j), (l)–(n); thin sections: (c) and (g)). (a) Methanobrevibacter smithii (short rod; bar, 1 μm), (b) Methanobacterium uliginosum (rod; bar, 1 μm), (c) Methanosphaera stadtmanae (coccus; bar, 1 μm), (d) Methanoplanus limicola (irregular flat cells; bar, 1 μm), (e) Methanospirillum hungatei (spirilli; bar, 4 μm), (f) Halobacterium salinarum (rod; bar, 1 μm), (g) Halococcus morrhuae (coccus; bar, 0.5 μm), (h) Thermoplasma acidophilum (irregular rod‐shaped cell; bar, 0.5 μm), (i) Methanolobus vulcani (cocci in packets; bar, 1 μm), (j) Pyrococcus furiosus (coccus with flagella, bar, 0.5 μm), (k) Haloferax mediterranei (irregular to rectangular flat cells; bar, 0.5 μm), (l) Thermofilum sp. (long filament; bar, 1 μm), (m) Pyrodictium occultum (irregular cocci with tubules; bar, 1 μm) and (n) Thermoproteus tenax (branching rods; bar, 2 μm).



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Further Reading

Cavicchioli R (ed.) (2007) Archaea: Molecular and Cellular Biology. Washington, DC: American Society for Microbiology Press.

Danson MJ, Hough DW and Lunt GG (eds) (1992) The Archaebacteria: Biochemistry and Biotechnology. London: Portland Press.

Dworkin M, Falkow S, Rosenberg E, Schleifer KH and Stackebrandt E (eds) (2006) The Prokaryotes. A Handbook on the Biology of Bacteria: Ecophysiology and Biochemistry, 3rd edn. New York: Springer.

Garrett RA and Klenk H‐P (eds) (2007) Archaea: Evolution, Physiology, and Molecular Biology. Hoboken, NJ: Wiley‐Blackwell.

Howland JL (2000) The Surprising Archaea: Discovering Another Domain of Life. New York: Oxford University Press.

Kates M, Kushner DJ and Matheson AT (eds) (1993) The Biochemistry of Archaea (Archaebacteria). Amsterdam: Elsevier.

Oren A and Papke RT (eds) (2010) Molecular Phylogeny of Microorganisms. Norfolk, UK: Caister Academic Press.

Robb FT, Place AR and Sowers KR et al. (eds) (1995) Archaea. A Laboratory Manual. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.

Woese CR and Wolfe RS (eds) (1985) The Bacteria. A Treatise on Structure and Function, Vol. VIII: Archaebacteria. New York: Academic Press.

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Oren, Aharon(Sep 2010) Archaea. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000443.pub2]