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

SUMMARY: Thiobacillus denitrificans grew exclusively aerobically in thiosulphate-limited chemostat culture at all dissolved oxygen concentrations between 12 and 216 $mUM. 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 thiosulphate oxidized)-1] at the lowest dissolved oxygen concentration (12 $mUM) and decreased at higher dissolved oxygen concentrations, indicating oxygen 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 thiosulphate-oxidizing enzyme and sulphur-oxidizing enzyme were detected in aerobic cultures, but activities were even lower in anaerobic cultures. Efficient energy coupling mechanisms with respect to sulphur and thiosulphate oxidations are indicated.