sp. nov., a hydrogen- and thiosulfate-oxidizing mesophilic chemolithoautotroph isolated from diffuse-flow fluids on the East Pacific Rise, and an emended description of the genus .

A novel mesophilic, hydrogen- and thiosulfate-oxidizing bacterium, strain ISO32, was isolated from diffuse-flow hydrothermal fluids from the Crab Spa vent on the East Pacific Rise. Cells of ISO32 were rods, being motile by means of a single polar flagellum. The isolate grew at a temperature range between 30 and 55 °C (optimum, 43 °C), at a pH range between 5.3 and 7.6 (optimum, pH 5.8) and in the presence of 2.0-4.0 % NaCl (optimum, 2.5 %). The isolate was able to grow chemolithoautotrophically with molecular hydrogen, thiosulfate or elemental sulfur as the sole electron donor. Thiosulfate, elemental sulfur, nitrate and molecular oxygen were each used as a sole electron acceptor. Phylogenetic analysis of 16S rRNA gene sequences placed ISO32 in the genus of the class , with EP1-55-1 % as its closest relative (95.95 % similarity). On the basis of the phylogenetic, physiological and genomic characteristics, it is proposed that the organism represents a novel species within the genus , sp. nov. The type strain is ISO32 (=JCM 39185 =KCTC 25252). Furthermore, the genomic properties of members of the genus are distinguished from those of members of other thermophilic genera in the orders ( and ) and (, and ), with larger genome sizes and lower 16S rRNA G+C content values. Comprehensive metabolic comparisons based on genomes revealed that genes responsible for the Pta-AckA pathway were observed exclusively in members of mesophilic genera in the order and of the genus . Our results indicate that the genus contributes to elucidating the evolutionary history of in terms of metabolism and transition from a thermophilic to a mesophilic lifestyle.