Iron oxidation response of Acidithiobacillus ferrooxidans under Ni2+ stress

Abstract:

[Objective] Acidithiobacillus ferrooxidans is one of the key bacteria involved in the bioleaching of primary and secondary ores and solid wastes. In the bioleaching environments, high levels of heavy metals are one of the major challenges encountered by A. ferrooxidans. Resistance to heavy metals stress is essential for the adaptation of microorganisms to the extreme environments. However, the tolerance and iron oxidation response of A. ferrooxidans to Ni²⁺ stress remain unclear. [Methods] In this study, we employed a two-step culture system with the Ni²⁺concentration range designed within 0-40 g/L. The ferrous oxidation efficiency, expression of rus operon genes, and extracellular electron transfer of A. ferrooxidans exposed to Ni²⁺ stress were respectively investigated by phenanthroline spectrophotometry, real-time quantitative PCR, and cyclic voltammetry. [Results] A. ferrooxidans could tolerate Ni²⁺ stress at the concentration ≤30 g/L in the two-step culture system. However, 40 g/L Ni²⁺ significantly inhibited the ferrous iron oxidation activity of A. ferrooxidans, and the ferrous oxidation inhibition efficiency at the time point of 24 h was 59.4% that of the control (0 g/L Ni²⁺). Moreover, the high concentration (40 g/L) of Ni²⁺ decreased the transcriptional levels of rus operon genes. Especially, the transcriptional levels of cyc1 and coxBACD were maximally down-regulated by 16 folds and 2.7-7.4 folds, respectively. Meanwhile, the rate of extracellular electron transfer in A. ferrooxidans exposed to 40 g/L Ni²⁺ was decelerated to 24.5 µA in comparison to the control (0 g/L Ni²⁺, 29.0 µA). [Conclusion]High concentration (40 g/L) of Ni²⁺ down-regulated the expression of rus operon genes involved in Fe²⁺oxidation, inhibited the electron transport from Fe²⁺ oxidation, and finally slowed down ferrous oxidation activity. The findings provide theoretical support for the bioleaching of Ni-containing solid waste.

Key words:Acidithiobacillus ferrooxidans;heavy metal stress;ferrous oxidation metabolism;electrochemistry;iron respiratory chain