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.

Elucidating the microbial ecological mechanisms on the electro-fermentation of caproate production from acetate via ethanol-driven chain elongation.

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 (RA/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 RA/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.

Quorum sensing shaped microbial consortia and enhanced hydrogen recovery from waste activated sludge electro-fermentation on basis of free nitrous acid treatment.

In this study, the feasibility of free nitrous acid (FNA) pretreatment coupled with quorum sensing (QS) was investigated to enhance hydrogen recovery from waste activated sludge (WAS) via electro-fermentation (EF). 3-oxo-hexanoyl-homoserine lactone (3OC6-HSL), as the signal molecule, was only added in the first three cycles of sludge inoculation at the phase of microbial electrolysis cells (MECs) startup. Results showed that QS combined FNA (AHL-FMEC) enabled highest hydrogen yield and current (4.3 mg/g VSS and 4.5 mA), while that generated from sole FNA/QS treated WAS (FMEC/AHL-RMEC) were only 3.5/3.0 mg/g VSS and 1.5/1.5 mA, respectively. Fourier transform infrared (FT-IR) spectra illustrated the effective conversion of organics in AHL-FMEC, the utilization efficiencies of proteins and carbohydrates achieved to 75.0% and 79.7%, respectively. Besides, the internal resistance decreased from 34.5 Ω (FMEC) to 22.9 Ω (AHL-RMEC), further to 18.0 Ω, indicating the promoted bioelectrochemical activity of electroactive bacteria (EAB) in AHL-FMEC. Correspondingly, both EAB (21.7%), e.g., Geobacter (9.3%) and Pseudomonas (3.2%) and anaerobic fermentation bacteria (AFB, 28.6%), e.g., Proteiniclasticum (14.2%) and Petrimonas (3.6%) enriched to peaks in AHL-FMEC. Moreover, molecular ecological network (MEN) analysis revealed the underling relationships among AFB, EAB and homo-acetogen in EF system, suggesting the possible cooperative QS has been constructed. The results obtained in this study may provide a new insight for efficient hydrogen recovery from electro-fermentation of WAS.