Thiobacillus ferrooxidans JCM 7811 was cultivated in a basal salts medium containing thiosulfate and ferric ion (Fe3+). T. ferrooxidans grew with consuming thiosulfate. Appropriate concentrations of thiosulfate and Fe3+ were 20 and 30 mM, respectively. When o-phenanthroline, which chelates ferrous ion (Fe2+), was added to the culture, Fe2+ reduced from Fe3+ was gradually accumulated. In the cultures with 25 and 50 mM of o-phenanthroline, the bacterial growth and thiosulfate consumption were

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... bited. These results indicated that thiosulfate was oxidized through the sulfur oxidation route with a ferric ion-reducing (FIR) system. During 5 days of cultivation, the cells grew up to 5.0 X 108 cells/ml, and the grown cells had the iron oxidizing activity of 2.2,umol Fe2+/mg protein • min. These values were comparable with those of the cells grown on ferrous sulfate. The cell yield against thiosulfate (Yx/s) was 250 X 108 cells/mmol-S203 2-, which was about 20-fold higher than that against ferrous sulfate. Iron ions were cyclically utilized in the FIR system, therefore, the usage of ferrous sulfate could be reduced to obtain a desired cell mass. Thiobacillus ferrooxidans is a chemoautotrophic, acidophilic bacterium that generates its energy from the oxidation of ferrous iron or various inorganic sulfur compounds. Studies of the biochemistry and physiology of this microorganism have been stimulated by the ever increasing development of the bioleaching processes that are used industrially to recover metals from ores and to desulfurize coal (2, 4). Usually T ferrooxidans is cultivated on ferrous sulfate as an energy substrate, but its growth rate and cell yield are not sufficient to obtain a desirable amount of cells for industrial use. Taya et al. (10) have reported that the ultimate