Thiobacter aerophilum sp. nov., a Thermophilic, Obligately Chemolithoautotrophic, Sulfur-Oxidizing Bacterium from a Hot Spring and Proposal of Thiobacteraceae fam. nov.

An aerobic, obligately chemolithoautotrophic, sulfur-oxidizing bacterium, strain AK1^T, was isolated from a terrestrial hot spring of the Uzon Caldera, Kamchatka, Russia. The cells of the new isolate were Gram-negative motile rods with a single polar flagellum. Strain AK1^T grew at 37-55 °C (optimum 50 °C) with 0-1.0% NaCl (optimum 0%) and within the pH range 4.8-7.0 (optimum pH 5.2-5.5). The new isolate was able to grow by aerobic respiration with sulfide, sulfur, or thiosulfate as the electron donor and HCO₃^-/CO₂ as the carbon source. The major fatty acids were C16:0, C17:1 Δ, and C16:1 ω7c. The respiratory lipoquinone was ubiquinone UQ-8. The size of the genome and genomic DNA G+C content of the strain AK1^T were 2.55 Mb and 64.0%, respectively. The closest 16S rRNA gene sequence of a validly published species belonged to <i>Thiobacter subterraneus</i> C55^T (97.94% identity). According to the 16S rRNA gene sequence-based and conserved protein sequences-based phylogenetic analyses, strain AK1^T represented a distinct lineage of the genus <i>Thiobacter</i> within a new family, <i>Thiobacteraceae</i> of the order <i>Burkholderiales</i>. As inferred from the morphology, physiology, chemotaxonomy, and phylogeny, strain AK1^T ought to be recognized as a novel species for which we propose the name <i>Thiobacter aerophilum</i> sp. nov. The type strain is AK1^T (=CGMCC 1.18099^T = UQM 41819^T).