Figure 2.

Mobilization mechanisms for A. ferrooxidans. (A) Aerobic and anaerobic protein pathway for iron oxidation and reduction. Under aerobic conditions, the transition from Fe(II) to Fe(III) occurs through hydrogen oxidation, NAD⁺ reduction, ATP production, and sulfur oxidation. In anaerobic conditions, the electron transport chain is mostly understood, except for an unidentified protein (?) responsible for the transformation of Fe(III) to Fe(II). (B) Planktonic and sessile cells permanently or non-permanently attached to the substrate. In the case of planktonic or floating cells, leaching occurs in the space between the rock and the organism via a series of redox reactions, while in the case of sessile cells, adherence to the substrate occurs due to extracellular polymeric substance (EPS) production, and leaching is termed direct or contact leaching. Rocks are generally sulfides, but not exclusively. Bioleaching can also be cooperative, wherein sessile and planktonic cells collaborate in metal mobilization between the acidic medium, substrate, and cells. Cell attachment to the substrate can be irreversible if EPS has already been produced, or reversible if not yet produced, and the process by which cells can adhere or detach from the substrate is termed absorption or dispersion.