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[Display omitted] • Specific colonization pattern of key symbionts in MES was firstly investigated. • Co-colonization of EAB and CSB promoted caproate biosynthesis in MES. • CSB and EAB mutually interacted in metabolic activity and enhanced electron transfer. • Ecological mechanism was revealed in terms of MEN analysis and Mantel test. • Economic benefit and carbon emission were preferable via co-colonization. As promising and value-added platform chemicals, the biosynthesis of medium-chain fatty acids (MCFAs, C6-C12) via chain elongation (CE) in microbial electrosynthesis systems (MES) has recently garnered significant attentions due to the unique bioelectrocatalytic behaviors and the reduction of exogenous electron donors (EDs). Caproate-synthesizing bacteria (CSB) played a crucial role in the generation of acetyl-CoA, which subsequently underwent reverse β oxidation to synthesize MCFAs. However, the presence of electrochemically active bacteria (EAB) was also pivotal for MES as they generated hydrogen in the cathodic electron transfer pathways, the specific colonization patterns of key symbionts remained unknown. The aim of this study was to investigate the selective colonization of EAB and CSB on the performance of caproate production in MES. The reactors colonized EAB and CSB achieved the maximum caproate concentration of 5332.5 mg COD/L, which was 7.9 and 1.3 times higher compared to that inoculated with either sole EAB or CSB, respectively. Moreover, the reactors colonized with mixed culture exhibited a clear redox peak and preferable electron transfer efficiency. CSB and EAB mutually interacted in metabolic activities, thereby resulting in increased substrate utilization and caproate production. The findings of this study provide a new strategy and insights for value-added chemicals recovery in microbial electrosynthesis platform. [ABSTRACT FROM AUTHOR]

Electro-fermentation (EF) is an attractive way to implement the chain elongation (CE) process, by controlling the fermentation environment and reducing the dosage of external electron donors (EDs). However, besides the coexistence performance of external EDs and electrode, applications of EF technology on the fermentation broth containing both EDs and electron acceptors during CE process, are all still limited. The current study investigated the contribution of EF to caproate production, under different acetate: ethanol ratios (R A/E). The effect of multiple EDs, both from ethanol and the bio-cathode, on caproate production, was also assessed. A proof-of-concept, based on experimental data, was presented for the EF-mediated ethanol-driven CE process. Experimental results showed that ethanol, together with the additional electron donors from the bio-cathode, was beneficial for the stable caproate production. The caproate concentration increased with the decrease of R A/E , while the bio-cathode further contributed to 10.7%–26.1 % increase of caproate concentration. Meanwhile, the hydrogen partial pressure tended to 0.10 ± 0.01 bar in all controlled EF reactors, thus favoring caproate production. This was attributed to the increased availability EDs, i.e., hydrogen and ethanol, generated by the electrode and electrochemically active bacteria (EAB), which might create multiple additional pathways to achieve caproate production. Molecular ecological networks analysis of the key microbiomes further revealed underlying cooperative relationships, beneficial to the chain elongation process. The genus Clostridium_sensu_stricto , as the dominant microbial community, was positively related to acetogens, EAB and fermenters. [Display omitted] • The coexistence performance of external EDs and electrode was firstly elucidated. •External ethanol addition was conducive to the stable caproate production. •Caproate concentration showed rising trend with the increase of ethanol. •The introduction of bio-cathode contributed to 10.7%-26.1% increase. •Synergistic relationship of Clostridium sensu stricto and other microbes was revealed. [ABSTRACT FROM AUTHOR]