Thermodynamic and kinetic characterization using process dynamics: Acidophilic ferrous iron oxidation by Leptospirillum ferrooxidans

Abstract

Kinetic and stoichiometric properties of acidophilic aerobic ferrous iron oxidation by growing and non-growing Leptospirillum ferrooxidans cultures were investigated. The use of a continuous stirred tank reactor operated at a variable dilution rate and equipped with on-line measurement of the electron donor, acceptor and anabolic substrate uptake rate enabled detailed kinetic characterization from a single experiment. It was demonstrated that substrate conversion and microbial growth are tightly coupled processes in L. ferrooxidans, and uncoupling occurs only due to the minor impact of substrate conversion for growth-independent maintenance purposes. The tight stoichiometric coupling implies bioenergetic uncoupling of the catabolism and anabolism because the Gibbs energy change for ferrous iron oxidation as a function of the actual growth rate of the culture ranges from -45 to -25 kJ mol-FeII(-1). Bioenergetic description of the process could only be achieved by introduction of a growth rate dependent Gibbs energy dissipation term. Removal of carbon dioxide from the influent gas stopped biomass growth, but the biomass specific respiration rate was unaffected or slightly stimulated. The uncoupling of the catabolism and anabolism is suggested to induce instantaneously an energy dissipation pathway. Also dosage of a low concentration propionic acid resulted in complete inhibition of the anabolism. Propionic acid served as an uncoupler of the membrane potential and all catabolic energy is required for the increased maintenance requirements. Recovery of the anabolism after reestablishment of the normal cultivation conditions was obtained only after 1-2 days. The results obtained provide additional constraints on cultivation of L. ferrooxidans for biotechnological application.