ISOLATION AND CHARACTERIZATION OF ALKALIPHILIC CHEMOORGANOHETEROTROPHIC BACTERIA OXIDIZING REDUCED INORGANIC SULFUR-COMPOUNDS TO TETRATHIONATE

The inoculation of 14 samples from alkaline environments - soda soils, lakes, and springs - in medium with acetate yielded 7 enrichment cult ures of bacteria capable of oxidizing thiosulfate at pH values above 1 0.0. Eight strains of chemoorganoheterotrophic thiosulfate-oxidizing b acteria were isolated in pure culture. All strains are obligately alka liphilic gram-negative eubacteria able to grow in the pH range of 7.5- 10.5 with an optimum around 9.5. The strains oxidize sulfide, elementa l sulfur, and thiosulfate to tetrathionate. Despite the low pH optimum for thiosulfate oxidation, these bacteria possess considerable thiosu lfate-oxidizing activity under alkaline conditions. The optimum for su lfide oxidation is in the alkaline range of pH. Like most neutrophilic heterotrophic bacteria oxidizing thiosulfate to tetrathionate, alkali philic strains are able to realize the reverse reaction of tetrathiona te reduction to thiosulfate under anaerobic conditions in the presence of an organic electron donor. All the heterotrophic isolates are capa ble of denitrification. High concentrations of nitrite reductase of th e cytochrome cd(1) type were found in their cells grown anaerobically with nitrates or nitrites. Thiosulfate was not oxidized under the cond itions of denitrification with any of the accepters tested (nitrate, n itrite, and nitrous oxide). However, strain AG 4, the most active deni trifier, was found to be capable of anaerobic sulfide oxidation in the presence of nitrate and nitrite, but not with nitrous oxide. Polysulf ide and elemental sulfur were the intermediate products of sulfide oxi dation. The isolated strains comprise two groups of the generic level. A group of six motile strains belongs to the same genus (according to the results of DNA-DNA hybridization) and comprises two subgroups of the species level; by their formal characteristics, they are close to bacteria of the genus Deleya. Two nonmotile strains belong to the same species and are distinguished from the motile isolates at the generic level; they apparently represent a new taxon.