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Enhanced current production by exogenous electron mediators via synergy of promoting biofilm formation and electron shuttling process.

Exogenous electron mediators (EMs) can facilitate extracellular electron transfer (EET) via electron shuttling processes, but it is still unclear whether and how biofilm formation is affected by the presence of EMs. Here, the impacts of EMs on EET and biofilm formation were investigated in bioelectrochemical systems (BESs) with Shewanella oneidensis MR-1, and results showed that the presence of five different EMs led to high density current production. All the EMs substantially promoted biofilm formation, with 15 ~ 36 times higher total biofilm DNA with EMs than without, and EMs also increased the production of extracellular polymeric substances, which was favorable for biofilm formation. The current decreased substantially after removing EMs from the medium or by replacing electrodes without biofilm, suggesting that both biofilm and EMs are required for high density current production. EET-related gene expression was up-regulated with EMs, resulting in the high flux of cell electron output. A synergistic mechanism was proposed: EMs in suspension were quickly reduced by the cells and re-oxidized rapidly by the electrode, resulting in a micro-environment with sufficient oxidized EMs for biofilm formation, and thus, besides the well-known electron shuttling process, the EMs-induced high biofilm formation and high Mtr gene expression could jointly contribute to the EET, and subsequently produce a high density current. This study provides a new insight into EMs-enhanced current production via regulating the biofilm formation and EET-related gene expression.