Kinetics of ferrous iron oxidation by Leptospirillum bacteria in continuous cultures.

The oxidation of ferrous iron by Leptospirillum bacteria was studied in a continuous culture in the dilution rate range 0.009-0.077 h-1 and could be described with a rate equation for competitive ferric iron inhibition kinetics in terms of the ferric/ferrous iron ratio in the solution. The ferrous iron oxidation in the continuous culture was followed by means of oxygen and carbon dioxide concentration analyses in reference air and off-gas. From these measurements the oxygen consumption rate, rO2, the carbon dioxide consumption rate, rCO2, the biomass concentration, Cx, and the biomass specific oxygen consumption rate, qO2, in the culture were determined. The ferrous iron concentration in the culture was below accurate levels to determine with the usual titrimetric method and was therefore derived from measuring the solution redox potential. The degree of reduction balance was used to check the theoretically expected relation between the rates of ferrous iron, -rFe2+, oxygen, -rO2, and biomass, rx. The maximum biomass yield and maintenance coefficient on oxygen are Yoxmax = 0.047 mol of C/mol of O2 and mo = 0.057 mol of O2/(mol of C.h). The maximum specific oxygen consumption rate, qO2,max = 1.7 mol of O2/(mol of C.h), the affinity coefficient, Ks/Ki = 0.0005 mol of Fe2+/mol of Fe3+, and the maximum specific growth rate, micromax = 0.069 h-1, Ks/Ki = 0.0004, were fitted from the measured data. For several dilution rates, off-line respiratory measurements with cell suspension from the continuous culture were carried out in dynamic BOM-Eh measurements. The dissolved oxygen and redox potential were measured simultaneously and monitored. The measured value of qO2,max varied between 2.3 and 1.7 mol/(mol of C.h). The value of Ks/Ki = 0.0007 was equal in all experiments. The measured values of qO2 in the continuous culture were well described with the kinetics determined in dynamic BOM-Eh measurements. It was concluded that dynamic BOM-Eh measurements are a convenient method to determine the kinetics of continuous culture grown Leptospirillum bacteria.