sp. nov., a facultatively anaerobic hyperthermophilic archaeon isolated from the Unzen hot spring in Japan.

A novel hyperthermophilic, acidophilic and facultatively anaerobic archaeon, strain KN-1, was isolated from Unzen hot spring in Japan and characterized. The cells of KN-1 were irregular cocci with a diameter of 1.0-3.0 µm that grew at 55-87.5 °C (optimum: 75 °C) and pH 1.0-5.5 (optimum: 3.0). Chemolithoautotrophic growth of KN-1 occurred in the presence of S or H under oxic conditions. Under anoxic conditions, KN-1 grew with S, ferric citrate and FeCl as electron acceptors. A phylogenetic analysis of 16S rRNA gene sequences showed that the species most closely related to KN-1 was JCM 9 021, with 98.9 % sequence identity, indicating that strain KN-1 belongs to the genus . This genus has been considered to consist of obligate anaerobes since its description in 1991. However, KN-1 grew under oxic, microoxic and anoxic conditions. Moreover, KN-1utilized various complex substrates and some sugars as carbon or energy sources, which is also different from JCM 9 021. The average nucleotide identity and amino acid identity values between KN-1 and JCM 9 021 were 79.4 and 76.1 %, respectively, indicating that KN-1 represents a novel species. Its main polar lipids were calditoglycerocaldarchaeol and caldarchaeol, and its DNA G+C content was 40.1 mol%. We also found that JCM 9021 grew under microoxic conditions in the presence of H as an electron donor, indicating that this genus does not comprise obligate anaerobes. Based on this polyphasic taxonomic analysis, we propose the novel species, sp. nov., whose type strain is KN-1 (=JCM 34 622=KCTC 4 293).