Characterization of Desulfovibrio salinus sp. nov., a slightly halophilic sulfate-reducing bacterium isolated from a saline lake in Tunisia

International audienceA novel slightly halophilic sulfate-reducing bacterium, designated strain P1BSR T , was isolated from water of a saline lake in Tunisia. Strain P1BSR T had motile (single polar flagellum), Gram-negative, rod-shaped, non-spore-forming cells, occurring singly or in pairs. Strain P1BSR T grew at temperatures between 15 and 45 C (optimum 40 C), and in a pH range between 6 and 8.5 (optimum pH 6.7). The strain required NaCl for growth (1 % w/v), and tolerated high NaCl concentration (up to 12 % w/v) with an optimum of 3 % (w/v). Sulfate, thiosulfate and sulfite served as terminal electron acceptors, but not elemental sulfur, fumarate, nitrate and nitrite. Strain P1BSR T utilized lactate, pyruvate, formate, D-fructose and glycerol as carbon and energy sources. The main cellular fatty acid was C 16 : 0 (50.8 %). The genomic DNA G+C content was 47.7 mol%. Phylogenetic analysis of 16S rRNA gene sequence similarity indicated that strain P1BSR T was affiliated to the genus Desulfovibrio, with the type strains Desulfovibrio salexigens (96.51 %), Desulfovibrio zosterae (95.68 %), Desulfovibrio hydrothermalis (94.81 %) and Desulfovibrio ferrireducens (94.73 %) as its closest phylogenetic relatives. On the basis of genotypic, phenotypic and phylogenetic characteristics, it is proposed to assign strain P1BSR T to a novel species of the genus Desulfovibrio, Desulfovibrio salinus sp. nov. The type strain is P1BSR T (=DSM 101510 T =JCM 31065 T). Sulfate-reducing prokaryotes (SRPs) are mainly chemohe-terotrophs, both Bacteria and Archaea (250 species of 65 genera), that can use sulfate as a terminal electron acceptor in their energy metabolism [1, 2]. Owing to their broad metabolic capacities, sulfate-reducing bacteria (SRB) are important in the mineralization of organic matter in anoxic marine sediments. Dissimilatory sulfate reduction has been observed in various hypersaline environments such as salt-erns, the Dead Sea and the Great Salt Lake [3]. However, most of the halophilic SRPs isolated so far are marine or slightly halophilic microorganisms [with optimum salinity ranging from 1 to 4 % (w/v) NaCl] belonging to several gen-era of SRPs, including Desulfovibrio, Desulfonatronovibrio and Desulfonatronobacter species [3-6]. The first SRB species belonging to the genus Desulfovibrio isolated from a hypersaline environment was Desulfovibrio salexigens, which did not grow at NaCl concentrations higher than 12 % (w/v) [7]. Cord-Ruwish [8] then isolated several strains of SRBs from hypersaline oilfield water containing about 10 % (w/v) NaCl. One isolate grew slowly up to 27 % (w/v) NaCl but has not been described in more detail since. One year later, Trüper and Galinski [9] isolated a few SRB strains from hot brines in the Red Sea that were similar to Desulfovibrio halophilus, a moderately halophilic sulfate reducer isolated by Caumette et al. [10] from the hypersa-line Solar Lake in Sinai. The latter isolate grew in salinity ranging from 3 to 18 % (w/v), and optimally at 6-7 % (w/v) NaCl. However, since 1991, no novel SRB species belonging to the genus Desulfovibrio has been characterized that grows in salt concentrations above 10 % NaCl (w/v). No pure culture of extreme halophilic SRB (i.e. growing under saturating salt conditions) has been isolated until now. In this study, we report the isolation and characterization of a mesophilic, slightly halophilic SRB isolated from water samples of a Tunisian saline lake that is able to grow in up to 12 % NaCl (w/v), but grows optimally at 3 % (w/v). This SRB isolate is proposed to represent a novel species of the genus Desulfovibrio. Water samples were collected from a continental saline lake located in the middle-east of Tunisia and transported to the laboratory at ambient temperature. Bacteria wer