Metabolic Changes in Thiobacillus denitrificans Accompanying the Transition from Aerobic to Anaerobic Growth in Continuous Chemostat Culture

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Abstract
T. denitrificans grew exclusively aerobically in thiosulfate-limited chemostat culture at all dissolved O2 concentrations between 12-216 .mu.M. Nitrate reduction did not occur in aerobic cultures and nitrate and nitrite reductases only reached high levels under complete anaerobiosis. Growth yield was greatest [11.50 g dry wt (mol thiosulfate oxidized)-1] at the lowest dissolved O2 concentration (12 .mu.M) and decreased at higher dissolved O2 concentrations, indicating O2 to be a growth-inhibitory substrate; the anaerobic yield was only 77% of the maximum aerobic yield (all tested at a dilution rate of 0.08 h-1) in agreement with thermodynamic calculations. Low activities of thiosulfate-oxidizing enzyme and sulfur-oxidizing enzyme were detected in aerobic cultures, but activities were even lower in anaerobic cultures. Efficient energy coupling mechanisms with respect to sulfur and thiosulfate oxidations are indicated.