Once thought to be restricted to extreme environments such as deep sea hydrothermal vents, hot springs & volcanic fumaroles, as well as highly acidic lakes, it now known that archaea (formerly archaebacteria) can be found in many more diverse environments. This includes soil (including rice paddies), blanket peat bogs, animal gut (rumen), fresh water, insect hindguts, protozoan ciliates, arthropods and plants (Chaban et al., 2006; Brussard etal., 2006). They hold the record for being able to live in the most extreme environments of temperature and pH (Chaban et al., 2006) and as a group are capable of utilising a wide range of energy and C sources including sunlight

The archaea are taxonomically placed in their own group away from other microbes (prokaryotes) or plants & animals (eukaryotes) and are currently grouped into two large domains based on molecular and growth studies. These are further subdivided (see table) based on their physiology. Two more groups the “Korarachaeota” and the “Nanoarchaeota” have been suggested largely from molecular studies and their very small size respectively. Molecular biology studies have revealed many more species than previously known using culturable methods. Archaea differ from other bacteria by having a different 16sRNA sequence, cell wall, membrane lipid composition & enzyme cofactors.

A group of archaea which is of importance in soil are the methanogens. Often working in consortia with other microbes, methanogens produce methane from simple substrates such as carbon dioxide/hydrogen or acetate or from more complex molecules and in doing so contribute to global methane levels, a known greenhouse gas (Garcia, 1990).  In some cases temperature has been shown to be important in methane production and which substrates are used with more methane being produced from carbon dioxide/hydrogen at thermophilic temperatures (50°C) than at 37°C (Fey et al., 2001). Whilst as a group they can tolerate a wide range of habitats such as extremes of low or high temperature (0-100°C), salt concentration and pH (over pH 9) they all require anaerobic conditions (low redox potential).

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Main groups of Archaea (Chaban et al, 2006)