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A moderately thermophilic, strictly anaerobic, chemoautotrophic bacterium, designated strain HS1857^T, was isolated from a deep-sea hydrothermal vent at the Noho site in the Mid-Okinawa Trough. Strain HS1857^T grew between 35 and 63 °C (optimum 55 °C), in the presence of 10–55 g l⁻¹ NaCl (optimum 25 g l⁻¹), and pH 5.5–7.1 (optimum 6.4). Growth occurred with molecular hydrogen as the electron donor and elemental sulfur, nitrate, or selenate as the electron acceptors. Formate could serve as an alternative electron donor with nitrate as an electron acceptor. During growth with nitrate as the electron acceptor, strain HS1857^T produced ammonium and formed a biofilm. CO₂ was utilized as the sole carbon source. The G + C content of the genomic DNA was 33.2 mol%. Phylogenetic analysis of the 16S rRNA gene sequence indicated that strain HS1857^T is a member of the order Nautiliales, showing a …