Growth of Thiobacillus ferrooxidans on ferrous iron in chemostat culture: Influence of product and substrate inhibition

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Abstract
Thiobacillus ferrooxidans was grown at pH 1.6 in continuous flow chemostat culture on ferrous sulphate as growth limiting substrate at dilution rates between 0.02–1.33 h−1. Iron oxidation and growth were subject to product inhibition by ferric iron and under some conditions substrate inhibition by ferrous iron. Product inhibition could be predominantly competitive or non‐competitive, and the mode observed depended partly on previous steady state conditions. The inhibition phenomena resulted in unique anomalous washout curves and complex relationships between steady‐state substrate, product and biomass concentrations, for which mathematical models are developed. For the growth states subject to non‐competitive inhibition by Fe3+ at D 0.073–0.99 h−1, the growth yield coefficient corrected for maintenance (YG) was 1.33 g dry wt (g atom Fe2+ oxidized)−1 and the maintenance coefficient (m) was 0.43 g atom Fe2+ oxidized (g dry wt)−1 h−1). For predominantly competitive states (D, 0.05–0.268 h−1) with 2–70 mM Fe3+ in steady states, YG was 0.36–0.38 and m was 0–0.04. A consequence of product inhibition and substrate inhibition was the possibility of more than one steady state product value and yield for a single steady state substate concentration. This was demonstrated experimentally. Substrate saturation coefficient, Ks (giving half maximum specific growth rate) for Fe2+, was 0.7–2.4 mM and maximum specific growth rate (μm) 1.25–1.78 h−1. The results presented reveal unusual and novel properties of T. ferrooxidans relevant to describing its activities in natural environments or in mineral leaching systems.