Insights into Chain Elongation Mechanisms of Weak Electric-Field-Stimulated Continuous Caproate Biosynthesis: Key Enzymes, Specific Species Functions, and Microbial Collaboration

Chain elongation (CE) technology can recover high-valuebiochemicalssuch as caproate, contributing to the realization of carbon neutrality.Electro-assisted fermentation shows extraordinary potential to enhancemicrobial activity in anaerobic fermentation systems. However, littleis known of the effects of key functional genes of enzymes, specificspecies functions, and microbial collaboration on caproate biosynthesisin the continuously fed expanded granular sludge bed (EGSB) reactorwith mixed culture under a weak electric field. In this study, caproateproduction was enhanced by 21% with 0.05 V electric field introductionin the CE bioreactor continuously operated for 150 days. Mechanisminvestigation revealed the abundance of functional genes involvedin converting substrates to key intermediates (acetyl CoA and malonylCoA) and in the fatty acid biosynthesis pathway (FAB), and the activitiesof transmembrane transport and energy metabolism were upregulated. Ruminococcaceae_bacterium played the most significant rolein enhancing caproate biosynthesis with electric field introduction.Some essential functional genes were undetected within Ruminococcaceae_bacterium, which implied that harmonious microbialcollaboration existed to supplement the lacking functional genes tocomplete CE processes. Firmicutes_bacterium, Pseudomonas_aeruginosa, Caproiciproducens_sp._NJN-50 and Candidatus_Neoanaerotignum_tabaqchaliae played complementary and collaborativeroles in constructing the microbial collaboration mechanisms in theCE process with the electric field. This study offered a deep understandingof the CE mechanisms in the mixed-culture continuously fed reactorand unveiled the respective roles of different species and the microbialinteraction and collaboration mechanisms with weak electric fieldstimulation and provided a promising strategy for enhancing caproatebiosynthesis.