Biofilm matrix and artificial mediator for efficient electron transport in CO2 microbial electrosynthesis

Microbial electrosynthesis (MES) has been highlighted as a means to valorize inorganic gaseous carbon, such as CO2, into value-added chemicals using electricity as the reducing power. Electrode-based electron transfer delivers respiratory electrons to a live-cell biocatalyst through a biofilm matrix or via electron shuttle molecules. The addition of artificial mediators, such as neutral red (NR) and 2-hydroxy-1,4-naphthoquinone (HNQ), increased acetate synthesis significantly, suggesting that these mediators improve the electron transport capability between the suspended cells and electrode. Regular media replacement also improves MES by adapting mediator-utilizing species in the reactor. MES without a mediator initially produced acetate at a reasonable rate (5.1 +/- 0.2 mmol/l/day), but the rate became negligible in the later stage. In contrast, MES with NR or HNQ showed a higher acetate production rate (4.6 +/- 0.4 vs. 7.4 +/- 0.4 mmol/l/day, respectively) as the media replacement progressed. Confocal laser scanning microscopy and 3D imaging showed that the biofilm matrix consisted of live and dead cells, while the composition was different in the MES with and without a mediator. Microbial community analysis by next-generation sequencing (NGS) showed that acetogenic Acetobacterium (in suspension) and Sporomusa (in both suspension and biofilm) were dominant during the 96 days of operation. The biofilm and planktonic community interacted dynamically under MES conditions. This result provides a realistic model of biofilms and planktonic cells in the MES. The interaction of the suspended cells with the biofilmforming electrode via electron shuttles could improve volumetric acetate production and stabilize the MES performance.