UBO/IUEM; CNRS; Ifremer; ANR; Egide ; An obligate piezophilic anaerobic hyperthermophilic archaeon, designated strain CH1(T), was isolated from a hydrothermal vent site named 'Ashadze', which is located on the Mid-Atlantic Ridge at a depth of 4100 m. Enrichment and isolation of the strain were carried out at 95 degrees C under a hydrostatic pressure of 42 MPa. Cells of strain CH1(T) were highly motile irregular cocci with a diameter of similar to 1-1.5 mu m. Growth was recorded at 80-108 degrees C (optimum 98 degrees C) and at pressures of 20-120 MPa (optimum 52 MPa). No growth was observed under atmospheric pressures at 60-110 degrees C. Growth was observed at pH 6.0-9.5 (optimum 7.5-8.0) and in 2.5-5.5 % (w/v) NaCl (optimum 3.5%). Strain CH1(T) was strictly anaerobic and grew on complex proteinaceous substrates, such as yeast extract, Peptone, and casein, as well as on sucrose, starch, chitin, pyruvate, acetate and glycerol without electron acceptors. The G+C content of the genomic DNA was 49.0 +/- 0.5 mol%. Analysis of 16S rRNA gene sequences revealed that strain CH1(T) belongs to the genus Pyrococcus. Based on its physiological properties and similarity levels between ribosomal proteins, strain CH1(T) represents a novel species, for which the name Pyrococcus yayanosii sp. nov. is proposed. The type strain is CH1(T) (=JCM 16557). This strain is also available by request from the Souchotheque de Bretagne (catalogue LMBE) culture collection (collection no. 3310).