A thermophilic anaerobic bacterium, designated strain BMAT (T = type strain), was isolated from the production water of Beatrice oil field in the North Sea (United Kingdom). The cells were straight to bent rods (1 to 5 by 0.3 to 0.5 pm) which stained gram negative. Strain BMAT obtained energy from the reduction of manganese(IV), iron(III), and nitrate in the presence of yeast extract, peptone, Casamino Acids, tryptone, hydrogen, malate, acetate, citrate, pyruvate, lactate, succinate, and valerate. The isolate grew optimally at 60°C (temperature range for growth, 50 to 65°C) and in the presence of 2% (wtlvol) NaCl (NaCl range for growth, 0 to 5% [wt/vol]). The DNA base composition was 34 mol% G+C. Phylogenetic analyses of the 16s rRNA gene indicated that strain BMAT is a member of the domain Bacteria. The closest known bacterium is the moderate thermophile Flexistipes sinusarabici (similarity value, 88%). Strain BMAT possesses phenotypic and phylogenetic traits that do not allow its classification as a member of any previously described genus; therefore, we propose that this isolate should be described as a member of a novel species of a new genus, Deferribacter thermophilus gen. nov., sp. nov.Metal-reducing microorganisms play important roles in the cycling of metals and organic matter, and iron-and manganese-reducing microorganisms are believed to be responsible for a substantial amount of carbon oxidation in marine sediments and other anaerobic environments (16, 17, 25).Dissimilatory manganese-and iron-reducing microorganisms which can couple Mn(1V) and/or Fe(II1) reduction to the oxidation of carbon compounds have been isolated from sediments and extreme environments, such as the deep subsurface (3, 17, 25). The isolates include strictly anaerobic bacteria belonging to the family Geobacteraceae (19, Bacillus infernus (3) and Geovibrio ferrireducens (6) strains, and facultatively anaerobic bacteria belonging to the genus Shewanella (23,28). Nearly all of the known manganese-and iron-reducing isolates are mesophiles. While thermophilic iron reduction has been found in sediment samples from thermal springs (30), the only pure culture of a thermophilic iron reducer that has been described is a culture of B. infernus, an anaerobe from a deep terrestrial subsurface environment.Petroleum reservoirs are frequently deep subsurface extreme environments having high temperatures, pressures, and levels of salinity, and several physiological groups of anaerobic bacteria, including thermophilic fermentative, sulfate-reducing, sulfur-reducing, and methanogenic microorganisms, have been isolated from these environments (7,8, 12, 13,26,27,32). Mesophilic iron-reducing bacteria have been detected in oil field fluids (24, 29) and have been identified as Shewanella putrefaciens (29). The presence of thermophilic metal-reducing microorganisms in petroleum reservoirs has not been reported previously.Recent studies in our laboratory have shown that microbial Fe-and Mn-reducing activity is found frequently in production water samples...