Figure 3: Time course changes of Fe²⁺ and A. ferrooxidans concentrations using sphalerite or goethite as a semiconducting mineral under different conditions.

The cathode electrolyte is either the m9K medium (experiments with A.ferrooxidans) or a modified Fe²⁺-free m9K medium (A.ferrooxidans-free experiments) by replacing 22.4 g l⁻¹ FeSO₄·7H₂O with 6.1 g l⁻¹ FeCl₃ (equivalent to 2.1 g l⁻¹ Fe³⁺ conc.). The slightly lower Fe²⁺ concentration at time zero (that is, 4,250 mg l⁻¹ in (b)) than the expected value from 22.4 g l⁻¹ FeSO₄·7H₂O (4,500 mg l⁻¹) for the treatments with A.ferrooxidans was probably due to oxidation of Fe²⁺ and precipitation of ferric iron oxide. (a) In the presence of sphalerite in the P-chamber, photoelectrons reduced Fe³⁺, initially present in the modified m9K medium. When bacteria were added to the C-chamber in the dark, cell concentration increased (orange stars) using energy from the oxidation of Fe²⁺ (green stars initially present in the m9K medium. In visible light, sphalerite photocatalysis occurred but when the circuit was open, cell concentration only reached 8.0×10⁷ cells ml⁻¹ (orange squares). In the presence of both sphalerite and cells, the photoelectrons stimulated the cell growth due to the continuous generation of Fe²⁺ from the reduction of Fe³⁺ by photoelectrons (orange and green circles for cell growth and Fe²⁺ concentration, respectively). (b) In the presence of goethite, photoelectrons reduced Fe³⁺ in the modified m9K medium. When bacteria were added to the C-chamber in the dark, cell concentration increased from 3.5×10⁵ cells ml⁻¹ to a maximum of 1.2×10⁶ cells ml⁻¹ (orange stars) by consuming energy released from the oxidation of Fe²⁺ (green stars) present in the m9K medium. In visible light, goethite photocatalysis occurred but when the circuit was open, cell concentration only reached 5.1×10⁵ cells ml⁻¹ (orange squares), and the cell growth occurred at the expense of the m9K medium- Fe²⁺ (green squares). In the presence of both goethite and cells, the photoelectrons stimulated the cell growth (orange and green circles for cell growth and Fe²⁺ concentration, respectively).