Recently, a new procedure was developed which allowed for the first time the isolation of a hyperthermophilic archaeum tracked by 16S rRNA analysis from a terrestrial hot solfataric spring (‘Obsidian Pool’, Yellowstone National Park, WY, USA). This novel isolate is characterized here. Cells are round cocci with a diameter of 0·2--0·8 μm, occurring singly, in pairs, short chains and in grape-like aggregates. The aggregates exhibit a weak bluish-green fluorescence under UV radiation at 420 nm. The new isolate is an anaerobic obligate heterotroph, using preferentially yeast extract for growth. The metabolic products include CO, H, acetate and isovalerate. Growth is observed between 65 and 90 °C (optimum: 85 °C), from pH 5·0 to 7·0 (optimum: 6·5) and up to 0·7% NaCI. The apparent activation energy for growth is about 149 kJ mol. Elemental sulfur or hydrogen inhibits growth. The core lipids consist mainly of acyclic and cyclic glycerol diphytanyl tetraethers. The cell envelope contains a cytoplasmic membrane covered by an amorphous layer of unknown composition; there is no evidence for a regularly arrayed surface-layer protein. The G+C content is 46 mol%. On the basis of 16S rRNA sequence comparisons in combination with morphological, physiological and biochemical properties, the isolate represents a new genus within the , which has been named The type species is , the type strain is isolate M11TL (= DSM 11486).


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