The effect of persistent exposure to thiocyanate and the impact on inhibition of ferrous iron oxidation in an industrial strain of Acidiplasma cupricumulans

Abstract

Acidiplasma cupricumulans is recorded as an acidophilic chemomixotrophic iron-oxidising archaeal species that is prevalent within the mineral biooxidation community. The archaeal strain was persistently exposed to 1 mg/L of SCN⁻, a toxic anion present in low concentrations in remediated cyanidation tailings wastewater from gold recovery plants, to potentially facilitate adaptation. Adaptation to a concentration of 1 mg/L SCN⁻ was induced. The performance data of the thiocyanate-adapted strain was compared with that of the control strain to characterise the evolutionary change in microbial activity. Adaptation to SCN⁻ altered microbial growth, producing an elevated $Y_{\mathrm{xs}}$ which possibly denoted an upregulation within the central carbon metabolism. Furthermore, adaptation suppressed the rate of Fe²⁺ oxidation. As such, the Fe²⁺ oxidation rate expression developed previously for Ap. cupricumulans was not suitable and required modification of the kinetic constant ($K_{{\mathrm{Fe}}^{2+}}$) to account for the observed reduction in substrate affinity. The updated $K_{{\mathrm{Fe}}^{2+}}$ was ∼3 times larger which aligned with the suspected bioenergetic burden. In addition, the inhibition kinetics of the adapted strain were evaluated across a SCN⁻ range of 0–3.0 mg/L to quantify changes in microbial resilience. The inhibition mechanism remained unchanged, with Fe²⁺ oxidation activity non-competitively inhibited by SCN⁻ in the solution matrix. However, adaptation increased the sensitivity of Ap. cupricumulans SCN⁻ and resulted in an inhibition constant ($K_i$) ∼2 times smaller relative to the control strain. The presented inhibition model predicted experimental data well, particularly at concentrations between 1 and 1.5 mg/L SCN⁻; at concentrations above 1.5 mg/L SCN the deviation between repeat data sets was exacerbated and reflected an elevated degree of sensitivity to culture conditions and the potential impact of multiple stressors. Adaptation to 1 mg/L SCN⁻ did not confer any beneficial alteration within the cell and highlighted that the accumulation of SCN⁻ within biooxidation circuits should be carefully monitored to maintain the desired structure of the microbial community, and therein process efficiency.