Redox Mediators Stimulated Chain Elongation Process in Fluidized Cathode Electro-Fermentation Systems for Caproate Production

Medium chain fatty acids (MCFAs), the secondary products of traditional anaerobic fermentation, can be produced via chain elongation (CE), a process often retarded due to the difficulty during interspecies electron transfer (IET). This study employed redox mediators, neutral red (NR), methyl viologen (MV), and methylene blue (MB) as electron shuttles to expedite the electro-fermentation for caproate production. Results showed that MV increased the MCFAs production by promoting acetate to ethanol conversion, led to the highest carbon and electron recovery efficiency. While NR was indicated to improve CE by encouraging H2 production and the biofilm of the cathode had the highest electrical activity due to the smallest internal resistance. A higher proportion of Sutterella, Prevotella, and Hydrogenophaga, linked with the H2-mediated interspecies electron transfer (MIET) during CE process, was observed across redox mediators supplied groups compared to the control. The presence of mediators led to an elevated abundance of key enzymes for enhanced CE process and electron transfer. This study provided the perspective of the stimulated electron transfer for improved MCFAs production in electro-fermentation systems