Hydrogenothermus marinus gen. nov., sp. nov., a novel thermophilic hydrogen-oxidizing bacterium, recognition of Calderobacterium hydrogenophilum as a member of the genus Hydrogenobacter and proposal of the reclassification of Hydrogenobacter acidophilus as Hydrogenobaculum acidophilum gen. nov., comb. nov., in the phylum 'Hydrogenobacter/Aquifex'.

A novel thermophilic, hydrogen-oxidizing bacterium, VM1T, has been isolated from a marine hydrothermal area of Vulcano Island, Italy. Cells of the strain were gram-negative rods, 2-4 microm long and 1-1.5 microm wide with four to seven monopolarly inserted flagella. Cells grew chemolithoautotrophically under an atmosphere of H2/CO2 (80:20) in the presence of low concentrations of O2 (optimum 1-2%). Carbohydrates and peptide substrates were not utilized, neither for energy generation nor as a source of cellular carbon. Growth of VM1T occurred between 45 and 80 degrees C with an optimum at 65 degrees C. Growth was observed between pH 5 and 7. NaCl stimulated growth in the range 0.5-6% with an optimum at 2-3%. Hydrogen could not be replaced by elemental sulfur or thiosulfate as electron donors. Nitrate and sulfate were not used as electron acceptors. The major respiratory lipoquinone was a new menathioquinone. Analysis of the fatty acids of VM1T revealed straight-chain saturated C18:0 and the unsaturated C18:1omega9c and C20:1omega9c as major components. The G+C content of the total DNA was 43 mol%. Phylogenetic analysis placed strain VM1T near the members of the genera Hydrogenobacter, Thermocrinis and Aquifex on a separate deep-branching phylogenetic lineage. Therefore, it is proposed that strain VM1T (= DSM 12046T = JCM 10974T) represents a novel species within a new genus, for which the name Hydrogenothermus marinus gen. nov., sp. nov., is proposed. In addition, it is shown that Calderobacterium hydrogenophilum should be transferred to the genus Hydrogenobacter; the name Hydrogenobacter hydrogenophilus comb. nov. (DSM 2913T = JCM 8158T) is proposed for this organism. Furthermore, on the basis of 16S rRNA sequence analysis, Hydrogenobacter acidophilus is only distantly related to Hydrogenobacter species. Owing to this finding and its growth at low pH, the name Hydrogenobaculum acidophilum gen. nov., comb. nov., is proposed for Hydrogenobacter acidophilus. The type strain is JCM 8795T (= DSM 11251T).