Palaeococcus helgesonii sp. nov., a facultatively anaerobic, hyperthermophilic archaeon from a geothermal well on Vulcano Island, Italy

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Abstract
A novel, hyperthermophilic archaeon was isolated from a shallow geothermal well that taps marine waters on the Island of Vulcano in the southern Tyrrhenian Sea, Italy. The cells were irregular cocci, 0.6–1.5 μm in diameter, with multiple polar flagella. Growth was observed at temperatures from 45 to 85 °C (optimum at ~80 °C), pH 5–8 (optimum at 6.5), and 0.5–6.0% NaCl (optimum at ~2.8%). The minimum doubling time was 50 min. The isolate was obligately chemoheterotrophic, utilizing complex organic compounds including yeast or beef extract, peptone, tryptone, or casein for best growth. The presence of elemental sulfur enhanced growth. The isolate grew anaerobically as well as microaerobically. The G+C content of the genomic DNA was 42.5 mol%. The 16S rRNA sequence indicated that the new isolate was a member of the Thermococcales within the euryarchaeota, representing the second species in the genus Palaeococcus. Its physiology and phylogeny differed in several key characteristics from those of Palaeococcus ferrophilus, justifying the establishment of a new species; the name Palaeococcus helgesonii sp. nov. is proposed, type strain PI1 (DSM 15127).