Two strains of moderately thermophilic bacteria, which reduce elemental sulfur to hydrogen sulfide, were isolated from volcanic sources in Kamchatka. Strain K-119was obtained from a thermophilic microbial community associated with Thermothrix thiopara, and strain U–8was isolated from a cyanobacterial mat inhabiting a sulfide-rich hot spring. Cells of both strains are short Gram-negative rods, motile with one polar flagellum (strain K–119) or non-motile (strain U–8). Both strains are obligate anaerobes, have temperature optima of 54–55 °C and pH optima of 6.9-7.2. Molecular hydrogen, acetate, fumarate, malate, pyruvate, lactate and long-chain saturated fatty acids served as growth substrates for both species; strain U-8was also able to grow on propionate. All substrates were oxidized completely, HS and CO being the only metabolic products. Elemental sulfur was obligately required for growth of strain K–119, whereas strain U-8was able to grow also with thiosulfate as electron acceptor and on pyruvate without an external electron acceptor. The DNA G+C contents of strains K–119and U–8were 31.6 and 32.2 mol%, respectively. Phenotypic features and the results of 16S rRNA sequencing indicate the affiliation of the new isolates to the genus . The DNA-DNA hybridization with Desulfurella acetivorans was 40% for strain K–119and 55% for strain U-8; the DNA-DNA hybridization between the new isolates was 32%. Based on the results of morphological, physiological and phylogenetic studies the following two new species are proposed: sp. nov. with the type strain K–119(= DSM 10409) and Desulfurella propionica sp. nov. with the type strain U–8(=DSM 10410).


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