Efficient caproate production from ethanol and acetate via chain elongation reinforced by Fe/Fe3O4 in mixed culture system

The mixed culture has several advantages in caproate synthesis, yet it lacks an effective approach to obtain high caproate production. In this study, the strategies for efficient caproate production in mixed culture system were explored and the mechanisms for efficient caproate fermentation were investigated. Results showed that the mixed culture could uptake both glycerol and ethanol as electron donor to synthetize caproate. With acclimation of sludge-based mixed culture with three steps, the corresponding maximum caproate productions for glycerol and ethanol achieved 4.3 and 10.6 g/L, respectively, which illustrated that the mixed culture preferred to utilize ethanol for caproate synthesis. To avoid spending a long time on cultivating particle anaerobic sludge, the mixed culture was transferred from the sludge-based system to wheat straw-based system. With reinforcement of Fe/Fe 3 O 4 , a high caproate production of 25.2 g/L was obtained from the straw-based fed-batch fermentation. High-throughput sequencing analysis revealed that the species Clostridium_kluyveri_sensu_stricto _12 played a significant role in biosynthesis of caproate. The iron-based materials facilitated the secretion of EPS and biofilm formation on cell carrier. Meanwhile, additional acetate was produced from CO 2 /H 2 and hemicellulose with the synergistic effect of several microorganisms such as genus Caldicoprobacter , Clostridium_sensu_stricto_ 15 and Staphylococcus , which promoted the improvement of caproate production. This study casts an insight that efficient caproate production could be achieved in the mixed culture system with reinforcement of iron-based materials. • Step acclimation promoted the growth of caproate-producing bacteria. • Adoption of wheat straw avoided acclimation of granular anaerobic sludge. • Fe/Fe 3 O 4 positively regulated the structure of microbial community. • The synergistic effect of bacteria contributed to the high caproate production.