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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.

A novel hydrothermal site was discovered in March 2007, on the mid-Atlantic ridge during the cruise 'Serpentine'. At a depth of 4100 m, the site 'Ashadze' is the deepest vent field known so far. Smoker samples were collected with the ROV 'Victor 6000' and processed in the laboratory for the enrichment of anaerobic heterotrophic microorganisms under high-temperature and high-hydrostatic pressure conditions. Strain CH1 was successfully isolated and assigned to the genus Pyrococcus, within the Euryarchaeota lineage within the Archaea domain. This organism grows within a temperature range of 80 to 108 1C and a pressure range of 20 to 120 MPa, with optima for 98 1C and 52 MPa respectively. Pyrococcus CH1 represents the first obligate piezophilic hyperthermophilic microorganism known so far. Comparisons of growth yields obtained under hightemperature/high-pressure conditions for relative organisms isolated from various depths, showed clear relationships between depth at origin and responses to hydrostatic pressure.