Nitratiruptor tergarcus gen. nov., sp. nov. and Nitratifractor salsuginis gen. nov., sp. nov., nitrate-reducing chemolithoautotrophs of the e-Proteobacteria isolated from a deep-sea hydrothermal system in the Mid-Okinawa Trough

Two novel denitrifying bacteria, designated strains MI55-1 and E9I37-1, were isolated from deep-sea hydrothermal vent chimney structures at the Iheya North hydrothermal field in the Mid-Okinawa Trough, Japan. Both isolates were strict chemolithoautotrophs growing by respiratory nitrate reduction with H2, forming N2 as a metabolic product. Oxygen (at low concentrations) could serve as an alternative electron acceptor for growth of the isolates. Growth of strain MI55-1 was observed at temperatures between 40 and 57 6C (optimum, 55 6C; doubling time, 2 h), at pH values between 5?4 and 6?9 (optimum, pH 6?4) and in the presence of between 1?5 and 4?0% (w/v) NaCl (optimum, 2?5%). Growth of strain E9I37-1 was observed at temperatures between 28 and 40 6C (optimum, 37 6C; doubling time, 2?5 h), at pH values between 5?6 and 7?6 (optimum, pH 7?0) and in the presence of between 1?5 and 3?5% (w/v) NaCl (optimum, 3?0%). The G+C contents of the genomic DNA of strains MI55-1 and E9I37-1 were 29?6 and 35?5 mol%, respectively. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strains MI55-1 and E9I37-1 belonged to groups A and F of the e-Proteobacteria, but that they had distant phylogenetic relationships with any species, within the phylogenetic groups, that had validly published names (sequence similarities were less than 91%). On the basis of the physiological and molecular characteristics of the novel isolates, it is proposed that they should each be classified in a novel genus: Nitratiruptor tergarcus gen. nov., sp. nov., with MI55-1 (=JCM 12459=DSM 16512) as the type strain, and Nitratifractor salsuginis gen. nov., sp. nov., with E9I37-1 (=JCM 12458=DSM 16511) as the type strain.