A novel mesophilic, anaerobic, mixotrophic bacterium, with designated strains EPR-M and HR-1, was isolated from a semi-extinct hydrothermal vent at the East Pacific Rise and from an Fe-mat at Lō’ihi Seamount, respectively. The cells were Gram-negative, pleomorphic rods of about 2.0 µm in length and 0.5 µm in width. Strain EPR-M grew between 25 and 45 °C (optimum, 37.5–40 °C), 10 and 50 g l NaCl (optimum, 15–20 g l) and pH 5.5 and 8.6 (optimum, pH 6.4). Strain HR-1 grew between 20 and 45 °C (optimum, 37.5–40 °C), 10 and 50 g l NaCl (optimum, 15–25 g l) and pH 5.5 and 8.6 (optimum, pH 6.4). Shortest generation times with H as the primary electron donor, CO as the carbon source and ferric citrate as terminal electron acceptor were 6.7 and 5.5 h for EPR-M and HR-1, respectively. Fe(OH), MnO, AsO , SO , SeO , SO , S and NO  were also used as terminal electron acceptors. Acetate, yeast extract, formate, lactate, tryptone and Casamino acids also served as both electron donors and carbon sources. G+C content of the genomic DNA was 59.4 mol% for strain EPR-M and 59.2 mol% for strain HR-1. Phylogenetic and phylogenomic analyses indicated that both strains were closely related to each other and to , within the class δ-(now within the class ). Based on phylogenetic and phylogenomic analyses in addition to physiological and biochemical characteristics, both strains were found to represent a novel species within the genus , for which the name  sp. nov. is proposed.  is represented by type strain EPR-M (=JCM 32109=KCTC 15831=ATCC TSD-173) and strain HR-1 (=JCM 32110=KCTC 15832).