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

¹ Laboratory of Marine Microbiology, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
² Subground Animalcule Retrieval (SUGAR) Project, Frontier Research System for Extremophiles, Japan Agency for Marine–Earth Science and Technology, 2–15 Natsushima-cho, Yokosuka 237-0061, Japan

Correspondence
Satoshi Nakagawa
nakasato{at}kais.kyoto-u.ac.jp

Two novel denitrifying bacteria, designated strains MI55-1^T and E9I37-1^T, 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^T was 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 E9I37-1^T was 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 MI55-1^T and E9I37-1^T were 29·6 and 35·5 mol%, respectively. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strains MI55-1^T and E9I37-1^T belonged to groups A and F of the -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^T (=JCM 12459^T=DSM 16512^T) as the type strain, and Nitratifractor salsuginis gen. nov., sp. nov., with E9I37-1^T (=JCM 12458^T=DSM 16511^T) as the type strain.

The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene sequences of strains MI55-1^T and E9I37-1^T are AB175499 and AB175500, respectively.

Graphs showing growth rates for strains MI55-1^T and E9I37-1^T at various temperatures, pH values, NaCl concentrations and O₂ concentrations are available as supplementary figures in IJSEM Online.

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