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

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Abstract
Two novel denitrifying bacteria, designated strains MI55-1Tand E9I37-1T, 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 N2as a metabolic product. Oxygen (at low concentrations) could serve as an alternative electron acceptor for growth of the isolates. Growth of strain MI55-1Twas observed at temperatures between 40 and 57 °C (optimum, 55 °C; 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 E9I37-1Twas observed at temperatures between 28 and 40 °C (optimum, 37 °C; 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 MI55-1Tand E9I37-1Twere 29·6 and 35·5 mol%, respectively. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strains MI55-1Tand E9I37-1Tbelonged to groups A and F of theε-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 tergarcusgen. nov., sp. nov., with MI55-1T(=JCM 12459T=DSM 16512T) as the type strain, andNitratifractor salsuginisgen. nov., sp. nov., with E9I37-1T(=JCM 12458T=DSM 16511T) as the type strain.

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