A novel barophilic, hyperthermophilic, anaerobic sulfur-metabolizing archaeon, strain MP(T = type strain), was isolated from a hydrothermal vent site (Snakepit) on the Mid-Atlantic Ridge (depth, 3550 m). Enrichments and isolation were done under 40 MPa hydrostatic pressure at 95°C. Strain MPwas barophilic at 75, 80, 85, 90, 95 and 98°C, and was an obligate barophile between 95 and 100°C ( ). For growth above 95°C a pressure of 15·0-17·5 MPa was required. The strain grew at 48-95°C under atmospheric pressure. The optimal temperature for growth was 85°C at both high (40 MPa) and low (0·3 MPa) pressures. The growth rate was twofold higher at 85°C under hydrostatic pressure compared to at low pressure. Strain MPcells were motile, coccoid, 0·8-2·0 μm in diameter and covered by a hexagonal S-layer lattice. The optimum pH and NaCI concentration for growth at low pressure were 7·0 and 20-30 gl, respectively. The new isolate was an obligate heterotroph and utilized yeast extract, beef extract and peptone for growth. Growth was optimal in the presence of elemental sulfur. Rifampicin and chloramphenicol inhibited growth. The core lipids consisted of a major archaeol and a complex lipid pattern consisting of a major phospholipid. The DNA G+C content was 37·1 mol%. Sequencing of the 16S rRNA gene revealed that strain MPbelonged to the genus and it is proposed that this isolate should be designated as a new species,


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