Archaea

Abstract

Abstract Analysis of the 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. Archaea resemble 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 cultivated Archaea were recovered from extreme environments characterised by high salt concentrations, high temperatures and/or very high or very low pH. Some nonextremophilic aerobic ammonia‐oxidising chemoautotrophs are abundant in the marine environment and in soils. The strictly anaerobic methanogenic prokaryotes also belong to the archaeal domain. Cultivation‐independent studies have shown the existence of many more archaeal lineages for which no cultivated representatives have yet been obtained. 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; a few cultivated nonextremophilic Archaea are classified in the phylum Thaumarchaeota. Cultivation‐independent approaches have led to proposals for many more archaeal phyla whose representatives are still awaiting to be isolated. 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 ether bonds. Peptidoglycan, the characteristic cell‐wall constituent of the Bacteria, does not occur in the Archaea. Obligately anaerobic methanogenic Archaea are responsible for the formation of nearly all biogenic methane. Most cultivated Archaea are extremophiles that live at high temperatures, sometimes combined with low pH, or at high salt concentrations, sometimes combined with high pH. 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.