Efficient caproate production from lignocellulose via single-step electro-fermentation platform without organic electron donor

Caproate production by microbial fermentation gained the advantages of sustainability and eco-friendliness, but challenged by sterile fermentation environment, necessity of organic electron donors. Here, a single-step electro-fermentation (EF) process of mixed culture was proposed for caprate production from rice straw. At the optimal potential of -0.8 V, caproate concentration, yield and selectivity in the neutral red (NR)-mediated EF system were 2.4 g/L, 0.2 g/g and 26.6%. Long-term operation accumulated 5.3 g/L caproate with the yield and selectivity of 0.2 g/g and 34.2% in the EF+NR system. Bioaugmentation by dosing chain-elongation microbial consortium further improved the caproate production, yield and selectivity to 9.1 g/L, 0.3 g/g and 41.5%, respectively. The improved caproate production in the bioaugmented EF+NR system was likely due to the enhanced interspecies electron transfer, reconstructed microbial community, multiple electron donors and suitable pH environment. Present study offers a feasible strategy for cost-effective caprate production directly from waste biomass.