Ferrous iron oxidation kinetics of Acidiplasma cupricumulans, a key archaeon in the mineral biooxidation consortium: Impact of nutrient availability, ferric iron and thiocyanate

Abstract

The kinetics of Fe2+ oxidation by a strain of Acidiplasma cupricumulans (Ap. cupricumulans) isolated from a commercial biooxidation tank was evaluated over a Fe2+ concentration range of 0.3 g/L – 5.0 g/L in batch experiments. Despite Fe2+ availability, substrate utilisation was incomplete at initial concentrations greater than 1 g/L Fe2+. Additionally, the maximum biomass concentration achieved remained constant above 1 g/L Fe2+. These findings suggested that the microbial system was limited by a secondary nutrient on using a minimal basal salts medium. The kinetic expression−rFe2+=dCFe2+dt=qFe2+max.CX1+KFe2+CFe3+CFe2+developed by Hansford (1997) was found to fit the experimental data (R2 > 0.96) when the system was limited by a single substrate (Fe2+), achieved at Fe2+ concentrations between 0.3 g/L – 1.0 g/L. A maximum specific Fe2+ oxidation rate (qFe2+max) of 4.088 × 10−10 mg Fe2+ cell−1 h−1 and kinetic constant (KFe2+) of 1.397 were determined. At higher Fe2+ concentrations, the Hansford model required modification through the inclusion of a lumped parameter (Kapp), to account for secondary nutrient limitation. On culturing Ap. cupricumulans in a richer nutrient medium (0 K medium), the Fe2+ substrate specificity and microbial growth improved, affirming the speculated secondary nutrient limitation.The impact of thiocyanate (0 mg/L – 3.0 mg/L) on Fe2+ oxidation kinetics was also investigated and the mechanism of SCN− inhibition discerned. Inhibition kinetic studies illustrated that Fe2+ oxidation was non-competitively inhibited by the presence of SCN− in solution, with a corresponding inhibition constant (Ki) of 32.90 mg/L. However, the presence of SCN− at concentrations above 0.5 mg/L necessitated re-evaluation of the affinity coefficient, KFe2+. It was postulated that the re-assessment of the kinetic parameter (KFe2+) was warranted due to the lowered proton motive force and the resultant decline in biomass yield from substrate (YXS).