A novel barophilic, hyperthermophilic, anaerobic sulfur-metabolizing archaeon, strain MPT (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 a t 95 "C. Strain MPT was barophilic a t 75,80,85,90,95 and 98 "C, and was an obligate barophile between 95 and 100 "C (TmaX). For growth above 95 "C, a pressure of 15-0-1 7-5 MPa was required. The strain grew a t 48-95 "C under atmospheric pressure. The optimal temperature for growth was 85 "C a t both high (40 MPa) and low (03 MPa) pressures. The growth rate was twofold higher a t 85 "C under in situ hydrostatic pressure compared to at low pressure. Strain MPT cells were motile, coccoid, 08-2-0 pm in diameter and covered b y a hexagonal S-layer lattice. The optimum pH and NaCl concentration for growth a t low pressure were 7.0 and 20-30 g I-', 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 165 rRNA gene revealed that strain MPT belonged to the genus Thermococcus and it is proposed that this isolate should be designated as a new species, Thermococcus bsrophilus.

An autotrophic, extremely thermophilic methanogen (ME3 was isolated from a deep-sea hydrothermal chimney sample collected on the Mid-Atlantic Ridge at a depth of 3000 m. The heavily flagellated cells are motile and coccoid shaped.The new strain grows between 55 and 91 "C, with an optimum growth temperature a t 85 O C . The optimum pH for growth is 65, and the optimum sea salt concentration for growth is around 25 g I-'. The organism uses H, and CO, as the only substrate for growth and methane production. Tungsten, selenium and yeast extract stimulate growth significantly. In the presence of CO, and H , , the organism reduces elemental sulphur to hydrogen sulphide. The G+C content of the genomic DNA is 33 mol%. As determined by 165 gene sequence analysis, this organism is closely related to Methanococcus jannaschii strain JAL-IT. However, no significant homology was observed between them with DNA-DNA hybridization. It is proposed that this organism should be placed in a new species, Methanococcus infernus. The type strain is MET (=DSM 118123.

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