Heliobacterium sulfidophilum sp nov and Heliobacterium undosum sp nov.: Sulfide-oxidizing heliobacteria from thermal sulfidic springs

Two new species of heliobacteria isolated from cyanobacterial mats of two a lkaline sulfidic hot springs are formally described. Strains BR4 and BG29 a re assigned to anoxygenic phototrophic bacteria of the family Heliobacteria ceae, since they possess the unique properties of this taxon: strict anaero biosis, formation of bacteriochlorophyll g, the lack of extensive intracyto plasmic membranes and chlorosomes, an unusual cell wall structure, and phyl ogenetic relatedness to the low G+C gram-positive eubacteria. Based on the 16S rDNA sequence similarity, strains BR4 and BG29 are assigned to the genu s Heliobacterium and described as two new species of this genus: Heliobacte rium sulfidophilum sp. nov. and Heliobacterium undoosum sp. nov. The G+C co ntent of the DNA is 51.3 mol % in Hbt. sulfidophilum and 57.2-57.7 mol % in Hbt. undosum. The cells of Hbt, sulfidophilum are rods, and the cells of H bt. undosum are slightly twisted spirilla or short rods. Both new bacteria are motile by peritrichous flagella. Hbt. sulfidophilum produces endospores . The new bacteria are strict anaerobes growing photoheterotrophically on a limited range of organic compounds. In the dark, they can switch from phot osynthesis to the slow fermentation of pyruvate. Biotin is required as a gr owth factor. Both species are highly tolerant to sulfide (up to 2 mM at pH 7.5) acid oxidize it photoheterotrophically to elemental sulfur; photoautot rophic growth was not observed. The temperature optimal for growth of Hbt. sulfidophilum and Hbt undosum is 30-35 degrees C, and the optimal pH is 7-8 .