Unveiling the bioelectrocatalyzing behaviors and microbial ecological mechanisms behind caproate production without exogenous electron donor.

Bioelectrochemical systems were proposed as a promising approach for the efficient valorization of biomass into 6–8 carbon atom medium-chain fatty acids (MCFAs), the precursors for high value-added chemicals or renewable energy, via acetyl-CoA-mediated chain elongation (CE). To achieve CE processes, exogenous electron donors (EDs), e.g., ethanol or lactic acid, were normally prerequisites. This research built a microbial electrolysis cell (MEC) for MCFAs biosynthesis from acetate without exogenous EDs addition. A wide range of applied voltages (0.6–1.2 V) was first employed to investigate the bioelectrocatalyzing response. The results show that caproate and butyrate were the main products formed from acetate under different applied voltages. Maximum caproate concentration (501 ± 12 mg COD/L) was reached at 0.8 V on day 3. Under this applied voltage, hydrogen partial pressure stabilized at about 0.1 bar, beneficial for MCFA production. Electron and carbon balances revealed that the electron-accepting capacity achieved 32% at 0.8 V, showing the highest interspecies electron transfer efficiency. Most of the carbon was recovered in the form of caproate (carbon loss was 9%). MiSeq sequencing revealed Rhodobacter and Clostridium_sensu_stricto playing the crucial role in the biosynthesis of caproate, while Acetobacterium , Acetoanaerobium , and Acetobacter represented the main ED contributors. Four available flora, i.e., homo-acetogen, anaerobic fermentation bacteria, electrode active bacteria, and nitrate-reducing bacteria, interacted and promoted caproate synthesis by molecular ecological network analysis. [Display omitted] • Optimal applied voltages for caproate production in MECs was firstly elucidated. • Hydrogen/ethanol in situ generated served as EDs without exogenous EDs addition. • Electron acceptability, carbon balance and hydrogen partial pressure were analyzed. • Caproate concentration and specificity achieved maximum under 0.8 V. • Synergistic and competitive interactions among functional microflora were revealed. [ABSTRACT FROM AUTHOR]