Reduction of S⁰ deposited on electroactive biofilm under an oxidative potential

The inevitable deposition of S⁰ on the electroactive biofilm (EAB) via anodic sulfide oxidation affects the stability of bioelectrochemical systems (BESs) when an accidental discharge of sulfide occurred, leading to the inhibition of electroacitivity, because the potential of anode (e.g., 0 V versus Ag/AgCl) is ~500 mV more positive than the redox potential of S^2-/S⁰. Here we found that S⁰ deposited on the EAB can be spontaneously reduced under this oxidative potential independent of microbial community variation, leading to a self-recovery of electroactivity (> 100 % in current density) with biofilm thickening (~210 μm). Transcriptomics of pure culture indicated that Geobacter highly expressed genes involving in S⁰ metabolism, which had an additional benefit to improve the viability (25 % - 36 %) of bacterial cells in biofilm distant from the anode and the cellular metabolic activity via electron shuttle pair of S⁰/S^2-(S_x^2-). Our findings highlighted the importance of spatially heterogeneous metabolism to its stability when EABs encountered with the problem of S⁰ deposition, and that in turn improved the electroactivity of EABs.