Dissolution of Manganese (IV) Oxide Mediated by Acidophilic Bacteria, and Demonstration That Manganese (IV) Can Act as Both a Direct and Indirect Electron Acceptor for Iron-Reducing Acidithiobacillus spp.

Experiments were carried out to examine the oxido-reduction of manganese by extremely acidophilic Acidithiobacillus spp. grown with either elemental sulfur or molecular hydrogen as electron donor. While there was no evidence for manganese (II) oxidation, dissolution of solid phase manganese dioxide was observed in cultures grown aerobically on both electron donors, though this appeared not to involve reduction of the metal. Solubilization of MnO2 was much enhanced in cultures incubated anaerobically, even though biomass was smaller, and pH values increased significantly as a consequence of acid dissolution being accompanied by manganese (IV) reduction.
Increases in cell numbers correlated with concentrations of soluble manganese in anaerobic cultures grown on hydrogen, demonstrating that iron-oxidizing/reducing Acidithiobacillus spp. can grow in the absence of oxygen using manganese (IV) as sole electron acceptor. Addition of ferric iron to anaerobic cultures further enhanced the reductive dissolution of MnO2 as a result of its reduction to ferrous iron which then reacted with the solid manganese phase, and confirming
that Mn (IV) reduction by iron-reducing acidophiles can proceed both directly and indirectly, involving iron as a shuttle vector. The implications of these findings to developing technologies for bio-processing oxidized metal ore deposits are discussed.