Two novel, strictly aerobic, sulfur-dependent, thermoacidophilic strains, IC-006T and IC-007, were isolated from a solfataric field at Hakone Ohwaku-dani, Kanagawa, Japan. Cells of the two strains were irregular cocci with a diameter of 1.0-1.8 µm. They were strict aerobes and grew in a temperature range between 45 and 69 °C (optimally at 65 °C) and a pH range between 0.4 and 5.5 (optimally at pH 3.5). They required sulfur or a reduced sulfur compound, and sulfur was oxidized to sulfate. They grew autotrophically or mixotrophically utilizing several sugars and complex organic substances as carbon sources. The DNA G+C content was 42.4 mol%. A comparison of the 16S rRNA gene sequences among members of the order Sulfolobales indicated that they were closely related to Sulfolobus metallicus, forming an independent lineage within this order. The two isolates and Sulfolobus metallicus were also diffentiated based on their phenotypic properties from the other members of the order Sulfolobales. Detailed comparisons of the phenotypic properties and DNA-DNA hybridization study illustrated that the two isolates belong to a species different from Sulfolobus metallicus. On the basis of the phylogenetic and phenotypic comparisons, we propose a new genus and species, Sulfuracidifex tepidarius gen. nov., sp. nov. to accommodate strains IC-006T and IC-007. The type strain of Sulfuracidifex tepidarius is IC-006T (=JCM 16833T=DSM 104736T). In addition, Sulfolobus metallicus should be transferred to the new genus as Sulfuracidifex metallicus comb. nov.: the type strain is Kra23T (=DSM 6482T=JCM 9184T=NBRC 15436T).
Keywords: Archaea; iron oxidation; solfatara; sulfur oxidation; thermoacidophilic.