The Order Thermococcales
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The Order Thermococcales COSTANZO BERTOLDO AND GARABED ANTRANIKIAN
Among the hyperthermophilic archaea, representatives of order Thermococcales form the most numerous group to date. Members of this group are the most frequently isolated hyperthermophiles. They are heterotrophic and as such regarded as the major constituents of organic matter within marine hot water ecosystems (Canganella et al., 1997). They belong to the branch of Euryarchaeota that contains the methanogens, the genus Thermoplasma, and the extremely halophilic archaea. The Thermococcales order is actually represented by three genera: Pyrococcus (Fiala and Stetter, 1986), Thermococcus (Achenbach-Richter et al., 1988) and the newly described Paleococcus (Takai et al., 2000). Phylogenetic analysis based on 16 rDNA sequences indicates that the Paleococcus strains are members of an ancient lineage of Thermococcales that diverged prior to the formation of the genera Pyrococcus and Thermococcus (Takai et al., 2000). These three genera include at present 38 species: 2 belonging to the genus Paleococcus, 6 belonging to the genus Pyrococcus, and 30 to the genus Thermococcus. The optimal growth temperature is 95–100°C for members of the genus Pyrococcus and 80–90°C for those of the genus Thermococcus. Pyrococcus strains have been isolated only from marine hydrothermal vents, whereas species belonging to the genus Thermococcus have been isolated also from terrestrial fresh water (Ronimus et al., 1997), marine solfataric ecosystems, deep-sea hydrothermal vents (Stetter, 1996) and offshore oil wells (Takahata et al., 2001). Representatives of the order Thermococcales have coccoid cells with or without flagella; they are obligate anaerobic organotrophic thermophiles with a fermentative metabolism using peptides, polysaccharides, or other sugars as carbon sources. Elemental sulfur is either stimulatory or necessary for the growth of these microorganisms. Molecular hydrogen that is produced during fermentation reduces elemental sulfur to H2S (Schönheit and Schäfer, 1995). Most of Thermococcales are neutrophiles growing optimally at pH 6.0–7.0; only the two species Thermococcus alcaliphilus (Keller et al., 1995) and T. acidoam-
inovorans (Dirmeier et al., 1998) are able to grow optimally at pH 9.0.
Ecology The Thermococcales are generally found in natural biotopes that are typical for thermophilic microoganisms. They were originally discovered in terrestrial and submarine hot vents and they were then found also in deep subsurface environments. For example, Thermococcus celer (Zillig et al., 1983), T. litoralis (Britton et al., 1995) and Pyrococcus sp. were discovered in an offshore oil production platform in the North Sea (Stetter, 1996), and T. litoralis (Neuner et al., 1990) was isolated from a continental oil well (Paris Basin, France). This fact probably indicates the indigenous origin of hyperthermophilic archaea in the deep subsurface biosphere. Another strain, T. sibiricus, was isolated from a high temperature oil reser
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