An autotrophic, hyperthermophilic methanogen (M7) was isolated from a deep-sea hydrothermal chimney sample collected on the East Pacific Rise at depth of 2600 m. The coccoid-shaped cells are flagellated and exhibit a sligh tumbling motility. The temperature range for growth at pH 6.5 was 49--89 °C, with optimum growth at 80 °C. The optimum pH for growth was 6.5, and the optimum NaCI concentration for growth was around 25 g I-. The new isolat used H and CO as the only substrates for growth and methane production. Tungsten, selenium and yeast extract stimulated growth significantly. In the presence of C0 and H the organism reduced elemental sulphur to hydrogen sulphide. Growth was inhibited by chloramphenicol and rifampicin, but not by ampicillin, kanamycin, penicillin and streptomycin. The G+C content of the genomic DNA was 31 mol%. As determined by 16S rDNA gene sequence analysis, this organism was closely related to strain JAL-. However, despite the high percentage of similarity between their 16S rDNA sequences (97.1 %), the DNA-DNA hybridization levels between these strains were less than 5%. On the basis of these observations and physiological traits, it is proposed that this organism should be placed in a new species, . The type strain is M7(= DSM 12094) During the course of this study, the 16S rDNA sequence analysis placed sp. strain AG86(= DSM 4213) as a close relative of strain JAL-1. However, the weak level of DNA-DNA hybridization with this strain (< 10%) allowed the proposal that strain AG86also constitutes a new species, .


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