Existence of two kinds of sulfur-reducing systems in iron-oxidizing bacterium Thiobacillus ferrooxidans

Intact cells of Thiobacillus ferrooxidans NASF-1 incubated under anaerobic conditions in a reaction mixture containing 0.5% colloidal sulfur produced hydrogen sulfide (H2S) extracellularly. The amount of H2S produced by cells increased corresponding to the cell amounts and colloidal sulfur. Two acti vity peaks of H2S production were observed at pH 1.5 and 7.5. We tentativel y called the enzyme activities pH 1.5- and pH 7.5-sulfur reducing systems, respectively. Seven strains of T. ferrooxidans tested had both the activiti es of pH 1.5- and pH 7.5-sulfur reducing systems, but at different levels. T. ferrooxidans NASF-1 showed the highest activity of the pH 1.5-sulfur red ucing system and strain 13598 from ATCC showed the highest activity of the pH 7.5-sulfur reducing system. Further characteristics of H2S production we re studied with intact cells of NASF-1. The optimum temperatures for pH 1.5 - and pH 7.5-sulfur reducing systems of NASF-1 were 40 degrees C. Hydrogen sulfide production continued for 8 days and total amounts of H2S produced a t pH 7.5 and 1.5 were 832 and 620 nmol/mg protein, respectively. The pH 7.5 -sulfur reducing system used only colloidal sulfur as the electron acceptor . However, the pH 1.5-sulfur reducing system used both colloidal sulfur and tetrathionate. Thiosulfate, dithionate, and sulfite could not be used as t he electron acceptor for both of the sulfur reducing systems. Potassium cya nide activated by 3- fold the pH 1.5-sulfur reducing system activity at 0.5 mM but did not affect the activity of the pH 7.5-sulfur reducing system. A n inhibitor of sulfite reductase, p-chloromercuribenzene sulfonic acid, did not affect either enzyme activity. Sodium molybdate and monoiodoacetic aci d strongly inhibited the activity of the pH 1.5-sulfur reducing system at 1 .0 mM, but not the activity of pH 7.5-sulfur reducing system.