Pyrococcus horikoshii sp. nov., a hyperthermophilic archaeon isolated from a hydrothermal vent at the Okinawa Trough

A hyperthermophilic, anaerobic archaeon was isolated from hydrothermal fluid samples obtained at the Okinawa Trough vents in the NE Pacific Ocean, at a depth of 1395m. The strain is obligately heterotrophic, and utilizes complex proteinaceous media (peptone, tryptone, or yeast extract), or a 21-amino-acid mixture supplemented with vitamins, as growth substrates. Sulfur greatly enhances growth. The cells are irregular cocci with a tuft of flagella, growing optimally at 98 degrees C (maximum growth temperature 102 degrees C), but capable of prolonged survival at 105 degrees C. Optimum growth was at pH 7 (range 5-8) and NaCl concentration 2.4% (range 1%-5%). Tryptophan was required for growth, in contrast to the closely related strains Pyrococcus furiosus and P. abyssi. Thin sections of the cell, viewed by transmission electron microscopy, revealed a periplasmic space similar in appearance to the envelope of P. furiosus. The predominant cell membrane component was tetraether lipid, with minor amounts of diether lipids. Treatment of the cells by mild osmotic shock released an extract that contained a Zn2+-dependent alkaline phosphatase. Phylogenetic analysis of the sequences encoding 16S rRNA and glutamate dehydrogenase places the isolate with certainty within the genus Pyrococcus although there is relatively low DNA-DNA hybridization (<63%) with described species of this genus. Based on the reported results, we propose a new species, to be named Pyrococcus horikoshii sp. nov.