We performed phenotypic and physiological studies with 20 hyperthermophilic microorganisms isolated from hydrothermal vents located in the North Fiji Basin (southwestern Pacific Ocean) at a depth of 2,000 m. These isolates were strict anaerobes that were regular to irregular coccoids and used elemental sulfur in their metabolism. Growth was observed at temperatures ranging from 50 to 101°C. The DNA base compositions varied from 43 to 60 mol%. All of these organisms were heterotrophs and fermented peptides to acetate, isovalerate, isobutyrate, and propionate. They contained both diether and tetraether lipids in their membranes, which indicates that they belong to the domain . DNA-DNA hybridization experiments revealed that there were two distinct homology groups, which correlated well with results obtained from sodium dodecyl sulfate-polyacrylamide gel electrophoresis of soluble whole-cell proteins, and these groups corresponded to the genera and . Five isolates exhibited levels of DNA-DNA relatedness with ranging from 71 to 100% and produced almost identical protein patterns. The remaining isolates formed a weakly homogeneous group based on DNA-DNA similarity data and protein patterns; the results of unweighted pair group cluster analyses suggested that these isolates were members of five new species of the genus .


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