|· Thermoprotei Reysenbach 2002
· ‘Candidatus Nitrosocaldus yellowstonii‘ de la Torre et al. 2008
· Crenarchaeota Garrity and Holt 2002
· not Crenarchaeota Cavalier-Smith 2002
The Crenarchaeota (Greek for “spring old quality” as specimens were originally isolated from geothermally heated sulfuric springs in Italy) (also known as Crenarchaea or eocytes) are archaea that have been classified as a phylum of the Archaea domain. Initially, the Crenarchaeota were thought to be sulfur-dependent extremophiles but recent studies have identified characteristic Crenarchaeota environmental rRNA indicating the organisms may be the most abundant archaea in the marine environment. Originally, they were separated from the other archaea based on rRNA sequences; other physiological features, such as lack of histones, have supported this division, although some crenarchaea were found to have histones. Until recently all cultured Crenarchaea had been thermophilic or hyperthermophilic organisms, some of which have the ability to grow at up to 113 °C.These organisms stain Gram negative and are morphologically diverse having rod, cocci, filamentous and oddly shaped cells.
One of the best characterized members of the Crenarcheota is Sulfolobus solfataricus. This organism was originally isolated from geothermally heated sulfuric springs in Italy, and grows at 80 °C and pH of 2–4. Since its initial characterization by Wolfram Zillig, a pioneer in thermophile and archaean research, similar species in the same genus have been found around the world. Unlike the vast majority of cultured thermophiles, Sulfolobus grows aerobically and chemoorganotrophically (gaining its energy from organic sources such as sugars). These factors allow a much easier growth under laboratory conditions than anaerobic organisms and have led to Sulfolobus becoming a model organism for the study of hyperthermophiles and a large group of diverse viruses that replicate within them.
The Kingdom Crenarchaeota has been defined phylogenetically, based on comparative molecular sequence analyses, and its members are therefore primarily defined by sequence similarity. However, like all Archaea, Crenarchaeota are prokaryotic, and are bounded by ether-linked lipid membranes which contain isoprinoid side chains instead of fatty acids. Cells range in size from cocci <1µm in diameter to filaments over 100µm in length. Species display a wide range of cell shapes, including regular cocci clustered in grape-like aggregates (Staphylothermus), irregular, lobed cells (Sulfolobus), discs (Thermodiscus), very thin filaments (<0.5µm diameter; Thermofilum), and almost rectangular rods (Thermoproteus, Pyrobaculum). Most species possess flagella and are motile. A few members of the Crenarchaeota exhibit strange morphologies: Pyrodictium produces disk-shaped cells connected by extensive networks of proteinaceous fibers which may help it to attach to sulfur granules.
Metabolically, Crenarchaeota are quite diverse, ranging from chemoorganotrophs to chemolithoautotrophs. They are anaerobes, facultative anaerobes or aerobes, and many utilize sulfur in some way for energy metabolism. Several species are primary producers of organic matter, using carbon dioxide as sole carbon source, and gaining energy by the oxidation of inorganic substances like sulfur and hydrogen, and reduction of sulfur or nitrate. Others grow on organic substrates by aerobic or anaerobic respiration or by fermentation.
The most spectacular feature of the Crenarchaeota, however, is their tolerance to, and even preference for, extremes of acidity and temperature. While many prefer neutral to slightly acidic pH ranges, members of the crenarchaeal order Sulfolobales flourish at pH 1-2 and die above pH 7. Optimum growth temperatures range from 75° to 105°C, and the maximum temperature of growth can be as high as 113°C (Pyrobolus). Most species are unable to grow below 70°C, although they can survive for long periods at low temperatures.