Anaerobic oxidation of thiosulfate to tetrathionate by obligately heterotrophic bacteria, belonging to the Pseudomonas stutzeri group

A number of strains of heterotrophic bacteria were isolated from various en vironments on the basis of their potential to oxidize inorganic sulfur comp ounds to tetrathionate. The isolates were screened for the ability to oxidi ze thiosulfate under denitrifying conditions. Many of them could grow anaer obically with acetate and nitrate, and eight strains could oxidize thiosulf ate to tetrathionate under the same conditions. In batch cultures with acet ate as carbon and energy source, most active anaerobic thiosulfate oxidatio n occurred with N2O as electron acceptor. The level of anaerobic thiosulfat e-oxidizing activity in cultures and cell suspensions supplied with nitrate correlated with the activity of nitrite reductase in cell suspensions. Som e strains converted thiosulfate to tetrathionate equally well with nitrite, nitrate and N2O as electron accepters. Others functioned best with N2O dur ing anaerobic thiosulfate oxidation. The latter strains appeared to have a lower level of nitrite reductase activity. Thiosulfate oxidation under anae robic conditions was much slower than in the presence of oxygen, and was ob viously controlled by the availability of organic electron donor. The strai ns had DNA-DNA similarity levels higher than 30%. Sequence analysis of the 16S rRNA gene of four selected isolates showed their affiliation to specifi c genomovars of Pseudomonas stutzeri and the proposed new species, Pseudomo nas balearica. As shown by 16S rRNA sequence analysis and DNA-DNA hybridiza tion, the previously misnamed 'Flavobacterium lutescans' (ATCC 27951) is al so a P. stutzeri strain which can oxidize thiosulfate to tetrathionate aero bically and anaerobically in the presence of N2O. The data suggest that tet rathionate-forming heterotrophic bacteria, in particular those belonging to the P. stutzeri 'superspecies', can play a much mole significant role in t he biogeochemical cycles than was previously recognized. (C) 1999 Federatio n of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.