In this study the results from a systematic study of the oxidation kinetics of
Leptospirillum ferriphilum in continuous culture at total iron concentrations ranging from 2 to12
g/L are reported. In all experiments the steady-state concentrations of ferrous iron were small and
comparable, and at least 97% of was as ferric. Surprisingly, the specific ferrous iron utilisation rate
decreased with increasing total iron concentration, while yield coefficients increased. It was noted
that the biomass concentration in the reactor (as measured by both CO2 uptake rate and cell counts)
dramatically increased with increasing total iron concentrations, whereas it stayed more or less the
same over a wide range of dilution rates at a given total iron concentration. The experimental data
was re-analysed in terms of ferrous iron kinetics using Monod kinetics with a ferric inhibition term.
The results confirm that the maximum specific iron utilisation rate is itself a function of ferric iron
concentration, declining with increasing concentration. It thus appears that high concentrations of
ferric iron stimulate microbial growth while at the same time inhibiting the rate of ferrous iron
oxidation. It is postulated that these phenomena are related, i.e. that more growth occurs to reduce
the load on the individual cell, possibly by sharing some metabolic functions.
Synergistic Competitive Inhibition of Ferrous Iron Oxidation by Thiobacillus ferrooxidans by Increasing Concentrations of Ferric Iron and Cells
