Archaea

AbstractAnalysis of nucleotide sequences of ribosomalribonucleic 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.