Carbon monoxide is a gas present in natural and anthropogenic environments that is involved in several important redox reactions. CO metabolism is a significant part of the global carbon cycle. The number of known carboxydotrophic anaerobes is increasing in recent years due to their important role in CO conversion (Sokolova et al., 2009). CO is a potent electron donor (thermodynamic CO 2 /CO redox potential 2520 mV) and represents an excellent source of energy for anaerobic micro-organisms (Kochetkova et al., 2011; Oelgeschläger & Rother, 2008). Nevertheless, only few anaerobes have been described that exhibit the capacity for hydrogenogenic carboxydotrophy. Hydrogenogenic CO-oxidizing prokaryotes are able to grow on CO with the production of hydrogen and carbon dioxide, according to the water-gas shift reaction: CO+H 2 OAH 2 +CO 2 (DG: 220 kJ mol et al., 2001). The first organism, to our knowledge, described as being able to perform this reaction in the dark, was a mesophilic species of the genus Rhodopseudomonas (Uffen, 1976). Since then, other anaerobic CO-oxidizing hydrogenogenic prokaryotes have been described, isolated from a wide range of environments around the world and spread over different phylogenetic clades (Novikov et al., 2011;Sokolova et al., 2009;Techtmann et al., 2009). The first thermophilic carboxydotroph described, to our knowledge, was Carboxydothermus hydrogenoformans (Henstra & Stams, 2011;Svetlitchnyi et al., 2001). In this work, we describe a novel anaerobic thermophilic carboxydotrophic hydrogenogenic bacterium, strain E3-O T .
Strain E3-OT was isolated from a CO-degrading enrichment culture originating from anaerobic suspended sludge from of a thermophilic municipal solid waste digester in Barcelona, Spain.A phosphate-buffered mineral salt medium (20 mM, pH 7.0) was used for enrichment cultures and isolation of strain E3-O T .The phosphate-buffered mineral medium contained the following components (l 21 ): Na 2 HPO 4 , 1.63 g; NaH 2 PO 4 , 1.02 g; resazurin, 0.5 g; NH 4 Cl, 0.3 g; CaCl 2 . 2H 2 O, 0.11 g; MgCl 2 . 6H 2 O, 0.10 g; NaCl, 0.3 g; 1 ml each of acid and alkaline trace element stock and 0.2 ml vitamin stock. Medium was reduced with 0.8 mM sodium sulfide before inoculation. Trace elements and Abbreviations: AQDS, anthraquinone-2,6-disulfonate; DGGE, denaturing gradient gel electrophoresis.The GenBank/EMBL/DDBJ accession number for the 16S rRNA gene sequence of Moorella stamsii strain E3-O T is HF563589.