Ammonium as a Carbon-Free Electron and Proton Source in Microbial Electrosynthesis Processes

Extracellular-proton-transfer driving high energy-conserving methanogenesis in anaerobic digestion

Anaerobic digestion (AD) is a promising technology to realize the conversion from organic matters to methane, which is highly mediated by syntrophic microbial community via mutualistic interactions. However, small energy available in methanogenic conversion usually limits the metabolic activity. To adapt such energy-limited environment, efficient energy conservation is critical to support active physiological functions of anaerobic consortia for methanogenic metabolism. In this study, the contribution of extracellular proton transfer (EPT) enhancement to achieving energy-conserving methanogenesis in AD was explored. Proton-conductive medium (PCM) was applied to construct efficient proton transport pathway, and a large number of protons from extracellular water were found available to upregulate methanogenesis in AD, as indicated by the increase in the content of 2H (D) in methane molecules (over 40.7%), among which CO2-reduction-to-CH4 was effectively enhanced. The increases of adenosine triphosphate (ATP) concentration (+54.1%) and gene expression activities related to ATPase (+100.0%) and proton pump (+580.1%) revealed that enhanced EPT by PCM promoted transmembrane proton motive force generation to facilitate ATP synthesis. Based on genome-centric metatranscriptomic analyses, MAG14, MAG63 and MAG61 with high energy conservation activity displayed most pronounced positive response to the EPT enhancement. In these core MAGs, the metabolic pathway reconstruction and the key genes activity identification further proved that EPT enhancement-driven efficient ATP synthesis stimulated the cross-feeding of carbon and proton/electron to facilitate microbial mutualism, thereby resulting in the high energy-conserving methanogenesis. Overall, our work provides new insights into how EPT enhancement drives high energy-conserving methanogenesis, expanding our understanding of the ecological role of EPT in AD.

Progress and perspectives on microbial electrosynthesis for valorisation of CO2 into value-added products

Microbial electrosynthesis (MES) is a neoteric technology that facilitates biocatalysed synthesis of organic compounds with the aid of homoacetogenic bacteria, while feeding CO₂ as an inorganic carbon source. Operating MES with surplus renewable electricity further enhances the sustainability of this innovative bioelectrochemical system (BES). However, several lacunae exist in the domain knowledge, stunting the widespread application of MES. Despite significant progress in this area over the past decade, the product yield efficiency is not on par with other contemporary technologies. This bottleneck can be overcome by adopting a holistic approach, i.e., applying innovative and integrated solutions to ensure a robust MES operation. Further, the widespread deployment of MES exclusively relies on its ability to mature a sessile biofilm over a biocompatible electrode, while offering minimal charge transfer resistance. Additionally, operating MES preferably at H₂-generating reduction potential and valorising industrial off-gas as carbon substrate is crucial to accomplish economic sustainability. In light of the aforementioned, this review collates the latest progress in the design and development of MES-centred systems for valorisation of CO₂ into value-added products. Specifically, it highlights the significance of inoculum pre-treatment for promoting biocatalytic activity and biofilm growth on the cathodic surface. In addition, it summarizes the diverse materials that are commonly used as electrodes in MES, with an emphasis on the importance of inexpensive, robust, and biocompatible electrode materials for the practical application of MES technology. Further, the review presents insights into media conditions, operational factors, and reactor configurations that affect the overall performance of MES process. Finally, the product range of MES, downstream processing requirements, and integration of MES with other environmental remediation technologies are also discussed.

Syngas fermentation and microbial electrosynthesis integration as a single process unit

Industrially relevant syngas (15 % CO, 15% H₂, 20% N₂ in 50% CO₂) fermentation and microbial electrosynthesis were integrated as a single process unit in open and closed-circuit modes. This study examined the impact of electrochemical reducing power from -50 to -400 mV on the acetic acid synthesis and CO inhibition on fermentation. -150 mV vs. Ag/AgCl (3.0 NaCl) was identified as the lowest benchmark potential for improved acetic acid synthesis rate (0.263 mmol L^-1h^-1), which is 15-fold higher than the open circuit mode's rate. No significant inhibition by CO in the fermentation was observed, while 60% of the gas was consumed. Anodic potential above 2.0 V substantially lowered the product formation. Superseding the fermentation medium with fresh inoculum through a fed-batch operation helped lower the anodic potential.

Impact of electrochemical reducing power on homoacetogenesis

Homoacetogenesis was performed in a microbial electrosynthesis single-chamber reactor at open and closed circuits modes. The aim is to investigate how an applied reducing power affects acetic acid synthesis and H₂ gas-liquid mass transfer. At a cathode voltage of -175 mV vs. Ag/AgCl (3.0 NaCl), the acetic acid synthesis rate ramped up to 0.225 mmol L^-1h^-1 due to additional electrons and protons liberation from carbon-free sources such as water and ammonium via anodic oxidation. The study sets a new lowest benchmark that acetic acid can be bioelectrochemical synthesized at - 175 mV. The applied reducing power did not increase the H₂ gas-liquid mass transfer because the direct electron transfer from cathode to microorganisms reduced the demand for H₂ in the fermentation medium. Microbial analysis shows a high presence of Veillonellaceae spore-forming clostridia, which are identified as homoacetogens.

Explore the difference between the single-chamber and dual-chamber microbial electrosynthesis for biogas production performance

Microbial electrosynthesis (MES) is an advanced technology for efficient treatment of organic wastewater and recovery of new energy, with the advantages and disadvantages of single-chamber and dual-chamber MES reactors being less understood. Therefore, we explored the effects of single-chamber and dual-chamber structures on the methane production performance and microbial community structure of MES. Results indicated that methane concentration and current density of single-chamber MES were higher than those of dual-chamber MES, and the system stability was better, while chemical oxygen demand (COD) removal rate and cumulative methane production were not significantly different. Analysis of microbial community structure showed the abundance of acidogens and H₂-producing bacteria was higher in single-chamber MES, while fermentation bacteria and methanogens was lower. The abundance of methanogens of dual-chamber MES (21.74-24.70%) was superior to the single-chamber MES (8.23-10.10%). Moreover, in dual-chamber MES, methane was produced primarily through acetoclastic methanogenic pathway, while in single-chamber MES cathode, methane production was mainly by hydrogenotrophic methanogenic pathway. Information provided will be useful to select suitable reactors and optimize reaction design.

Integrating Syngas Fermentation into a Single-Cell Microbial Electrosynthesis (MES) Reactor

This study presents a series of experiments to test the integration of syngas fermentation into a single-cell microbial electrosynthesis (MES) process. Minimal gas–liquid mass transfer is the primary bottleneck in such gas-fermentation processes. Therefore, we hypothesized that MES integration could trigger the thermodynamic barrier, resulting in higher gas–liquid mass transfer and product-formation rates. The study was performed in three different phases as batch experiments. The first phase dealt with mixed-culture fermentation at 1 bar H2 headspace pressure. During the second phase, surface electrodes were integrated into the fermentation medium, and investigations were performed in open-circuit mode. In the third phase, the electrodes were poised with a voltage, and the second phase was extended in closed-circuit mode. Phase 2 demonstrated three times the gas consumption (1021 mmol) and 63% more production of acetic acid (60 mmol/L) than Phase 1. However, Phase 3 failed; at –0.8 V, acetic acid was oxidized to yield hydrogen gas in the headspace.