Figure 7: Variation of electric current in a natural soil–water system and the corresponding change of the relative percentage of A.

faecalis. In this system, sterile soil extract was used as electrolyte in both chambers, and the natural red soil was added into the C-chamber. A −0.2 V (versus saturated calomel electrode) potential was applied on the electrode in the C-chamber to simulate the light condition, while the electrode with no applied voltage simulated the dark condition. The open and solid squares correspond to electric current values. The height of column represented the relative percentage of A. faecalis. In the simulated light condition, the relative proportion of A. faecalis became enriched (filled columns) over 15 days, whereas it slightly decreased (hatched columns near the bottom) in the simulated dark condition.