Electro-fermentation regulates mixed culture chain elongation with fresh and acclimated cathode

[Display omitted] •A fresh and acclimated cathode regulates mixed culture chain elongation process.•Tunable products (propionate, butyrate, valerate, and caproate) was generated.•A fresh carbon felt cathode increased the caproate specificity by about 28%.•With an acclimated cathode, the caproate specificity was mostly non-linear. Chain elongation has been proposed as a novel secondary fermentation process for the production of medium chain fatty acids as organic waste valorization. However, a strategy to effectively control the product spectrum of the chain elongation process is required. This study evaluated the effect of electro-fermentation on the mixed culture chain elongation process in detail. The results showed that electro-fermentation with a fresh carbon felt cathode increased the caproate specificity by about 28%, compared with the open circuit control and without carbon felt control. With an acclimated cathode, the effect of electro-fermentation on caproate specificity depended on the substrate concentration. The majority of bacteria belonged to Firmicutes, Bacteroidetes, and Proteobacteria, the distribution of which under biofilm and planktonic conditions were shaped by the application of electro-fermentation. These findings suggested that microorganisms interacted electrochemically with the polarized fresh electrode for chain elongation. The synergistic effect between microorganisms and acclimated electrode was largely affected by elevated substrate concentrations, and thus resulted in a redirection of the fermentation pattern.