THIOTHRIX CALDIFONTIS SP. NOV. AND THIOTHRIX LACUSTRIS SP. NOV., NOVEL GAMMAPROTEOBACTERIA ISOLATED FROM SULFIDE SPRINGS

Five strains of filamentous, sulfur-oxidizing bacteria were isolated from sulfur mats of different sulfide springs from various regions of the Northern Caucasus, Russia. A phylogenetic analysisbased on 16S rRNA gene sequence comparison showed that all of the isolates are affiliated with the filamentous, colourless, sulfur-oxidizing bacteria of the genus Thiothrix within the Gammaproteobacteria and are closely related to Thiothrix fructosivorans. All strains are capable of growing heterotrophically, lithoautotrophically with thiosulfate or sulfide as the sole energy source and mixotrophically. Strains G1T, G2, P and K2 are able to fix molecular nitrogen, but strain BLT is not. Randomly amplified polymorphic DNA (RAPD)-PCR analysis was used to assess the level of genetic relationships among the Thiothrix isolates. The Nei and Li similarity index revealed high genetic similarity among strains G1T, G2, P and K2 (above 75 %), indicating that they are closely related. In combination with physiological and morphological data, strains G1T, G2, P and K2 can be considered as members of the same species. The lowest genetic similarity (approx.20 %) was reached between strain BLT and the other isolated Thiothrix strains. Strains BLT and G1T shared 35% DNA-DNA relatedness and showed 51 and 53% relatedness, respectively, to Thiothrix fructosivorans ATCC 49749. On the basis of this polyphasic analysis, strains G1T, G2, P and K2 represent a novel species within the genus Thiothrix, for which the name Thiothrix caldifontis sp. nov. is proposed, with strain G1T (5DSM 21228T 5VKM B-2520T) as the type strain. In addition, strain BLT represents a second novel species, Thiothrix lacustris sp. nov., with strain BLT (5DSM 21227T 5VKM B-2521T) as the type strain.