Characterization of a membrane-separated and a membrane-less electrobioreactor for bioelectrochemical syntheses

Microbial electrosynthesis: opportunities for microbial pure cultures

Microbial electrosynthesis (MES) is an emerging technology that couples renewable electricity to microbial production processes. Although advances in MES performance have been driven largely by microbial mixed cultures, we see a great limitation in the diversity, and hence value, of products that can be achieved in undefined mixed cultures. By contrast, metabolic control of pure cultures and genetic engineering could greatly expand the scope of MES, and even of broader electrobiotechnology, to include targeted high-value products. To leverage this potential, we advocate for more efforts and activities to develop engineered electroactive microbes for synthesis, and we highlight the need for a standardized electrobioreactor infrastructure that allows the establishment and engineering of electrobioprocesses with these novel biocatalysts.

Novel electrochemical strategies for the microbial conversion of CO2 into biomass and volatile fatty acids using a fluid-like bed electrode in a three-phase reactor

Microbial electrosynthesis (MES) constitutes a bioelectrochemical process where bacteria uptake electrons extracellularly from a polarized electrode to incorporate them into their anabolic metabolism. However, the efficiency of current MES reactor designs can be lower than expected due to limitations regarding electron transfer and mass transport. One of the most promising bioreactor configurations to overcome these bottlenecks is the Microbial Electrochemical Fluidized Bed Reactor (ME-FBR). In this study, microbial CO2 fixation is investigated for the first time in a ME-FBR operated as a 3-phase reactor (solid-liquid-gas). An electroconductive carbon bed, acting as a working electrode, was fluidized with gas and polarized at different potentials (-0.6, -0.8 and -1 V vs. Ag/AgCl) so it could act as an electron donor (biocathode). Under these potentials, CO2 fixation and electron transfer were evaluated. Autotrophic electroactive microorganisms from anaerobic wastewater were enriched in a ME-FBR in the presence of 2-bromoethanosulfonic acid (BES) to inhibit the growth of methanogens. Cyclic voltammetry analysis revealed interaction between the microorganisms and the cathode. Furthermore, volatile fatty acids like propionate, formate and acetate were detected in the culture supernatant. Acetate production had a maximum rate of ca. 1 g L-1  day-1 . Planktonic cell biomass was produced under continuous culture at values as high as ca. 0.7 g L-1 dry weight. Overall, this study demonstrates the feasibility of employing a fluidized electrode with gaseous substrates and electricity as the energy source for generating biomass and carboxylic acids.

Electro-fermentation: Sustainable bioproductions steered by electricity

The market of biobased products obtainable via fermentation processes is steadily increasing over the past few years, driven by the need to create a decarbonized economy. To date, industrial fermentation (IF) employs either pure or mixed microbial cultures (MMC) whereby the type of the microbial catalysts and the used feedstock affect metabolic pathways and, in turn, the type of product(s) generated. In many cases, especially when dealing with MMC, the economic viability of IF is hindered by factors such as the low attained product titer and selectivity, which ultimately challenge the downstream recovery and purification steps. In this context, electro-fermentation (EF) represents an innovative approach, based on the use of a polarized electrode interface to trigger changes in the rate, yield, titer or product distribution deriving from traditional fermentation processes. In principle, the electrode in EF can act as an electron acceptor (i.e., anodic electro-fermentation, AEF) or donor (i.e., cathodic electro-fermentation, CEF), or simply as a mean to control the oxidation-reduction potential of the fermentation broth. However, the molecular and biochemical basis underlying the EF process are still largely unknown. This review paper provides a comprehensive overview of recent literature studies including both AEF and CEF examples with either pure or mixed microbial cultures. A critical analysis of biochemical, microbiological, and engineering aspects which presently hamper the transition of the EF technology from the laboratory to the market is also presented.

Regulating the dechlorination and methanogenesis synchronously to achieve a win-win remediation solution for γ-hexachlorocyclohexane polluted anaerobic environment

The wish for rapid degradation of chlorinated organic pollutants along with the increase concern with respect to greenhouse effect and bioenergy methane production have created urgent needs to explore synchronous regulation approach. Microbial electrolysis cell was established under four degressive cathode potential settings (from -0.15V to -0.60V) to regulate γ-hexachlorocyclohexane (γ-HCH) reduction while CH₄ cumulation in this study. The synchronous facilitation of γ-HCH reduction and CH₄ cumulation was occurred in -0.15V treatment while the facilitation of γ-HCH reductive removal together with the inhibition of CH₄ cumulation was showed in -0.30V treatment. Electrochemical patterns via cyclic voltammetry and morphological performances via scanning electron microscopy illustrated bioelectrostimulation promoted redox reactions and helped to construct mature biofilms located on bioelectrodes. Also, bioelectrostimulated regulation pronouncedly affected the bacteria and archaeal communities and subsequently assembled distinctly core sensitive responders across bioanode, biocathode and plankton. Clostridum, Longilinea and Methanothrix relatively accumulated in the plankton, and Cupriavidus and Methanospirillum, and Perimonas and Nonoarcheaum in biocathode and bioanode, respectively; while Pseudomonas, Stenotrophomonas, Methanoculleus and Methanosarcina were diffusely enriched. Microbial interactions in the ecological network were more complicated in -0.15V and -0.30V cathodic potential treatments, coincident with the increasement of γ-HCH reduction. The co-existence between putative dechlorinators and methanogens was less significant in -0.30V treatment when compared to that in -0.15V treatment, relevant with the variations of CH₄ cumulation. In all, this study firstly corroborated the availability to synchronously regulate γ-HCH reductive removal and methanogenesis. Besides, it paves an advanced approach controlling γ-HCH reduction in cooperation with CH₄ cumulation, of which to achieve γ-HCH degradation facilitation along with biogas (CH₄) production promotion with -0.15V cathode potential during anaerobic γ-HCH contaminated wastewater digestion, or to realize γ-HCH degradation facilitation with the inhibition of CH₄ emission with -0.30V cathode potential for an all-win remediation in γ-HCH polluted anaerobic environment such as paddy soil.

Anodic electro-fermentation: Empowering anaerobic production processes via anodic respiration

In nature as well as in industrial microbiology, all microorganisms need to achieve redox balance. Their redox state and energy conservation highly depend on the availability of a terminal electron acceptor, for example oxygen in aerobic production processes. Under anaerobic conditions in the absence of an electron acceptor, redox balance is achieved via the production of reduced carbon-compounds (fermentation). An alternative strategy to artificially stabilize microbial redox and energy state is the use of anodic electro-fermentation (AEF). This emerging biotechnology empowers respiration under anaerobic conditions using the anode of a bioelectrochemical system as an undepletable terminal electron acceptor. Electrochemical control of redox metabolism and energy conservation via AEF can steer the carbon metabolism towards a product of interest and avoid the need for continuous and cost-inefficient supply of oxygen as well as the production of mixed reduced by-products, as is the case in aerobic production and fermentation processes, respectively. The great challenge for AEF is to establish efficient extracellular electron transfer (EET) from the microbe to the anode and link it to central carbon metabolism to enhance the synthesis of a target product. This article reviews the advantages and challenges of AEF, EET mechanisms, microbial energy gain, and discusses the rational choice of substrate-product couple as well as the choice of microbial catalyst. Besides, it discusses the potential of the industrial model-organism Bacillus subtilis as a promising candidate for AEF, which has not been yet considered for such an application. This prospective review contributes to a better understanding of how industrial microbiology can benefit from AEF and analyses key-factors required to successfully implement AEF processes. Overall, this work aims to advance the young research field especially by critically revisiting the fundamental aspects of AEF.

Simplified Reactor Design for Mixed Culture-Based Electrofermentation toward Butyric Acid Production

Mixed microbial culture (MMC) electrofermentation (EF) represents a promising tool to drive metabolic pathways toward the production of a specific compound. Here, the MMC-EF process has been exploited to obtain butyric acid in simplified membrane-less reactors operated by applying a difference of potential between two low-cost graphite electrodes. Ten values of voltage difference, from −0.60 V to −1.5 V, have been tested and compared with the experiment under open circuit potential (OCP). In all the tested conditions, an enhancement in the production rate of butyric acid (from a synthetic mixture of glucose, acetate, and ethanol) was observed, ranging from 1.3- to 2.7-fold relative to the OCP. Smaller enhancements in the production rate resulted in higher values of the calculated specific energy consumption. However, at all applied voltages, a low flow of current was detected in the one-chamber reactors, accounting for an average value of approximately −100 µA. These results hold a substantial potential with respect to the scalability of the electrofermentation technology, since they pinpoint the possibility to control MMC-based bioprocesses by simply inserting polarized electrodes into traditional fermenters.

A slurry electrode integrated with membrane electrolysis for high-performance acetate production in microbial electrosynthesis

Microbial electrosynthesis (MES) technology employs electrotrophic microbes as biocatalysts to produce chemicals from CO₂. The application of a slurry electrode can enlarge the surface area to volume ratio, and membrane electrolysis (ME) for on-line extraction can solve the problem of product inhibition. This study constructed a novel dual-chamber ME-MES integrated system equipped with a slurry electrode, and the effect of concentration of powder-activated carbon (AC) in the catholyte on chemical production was also evaluated. The integrated system amended with 5 g L^-1 AC produced up to 13.4 g L^-1 acetate, showing a 179% increase compared with the control group without AC (4.8 g L^-1). However, further increasing the AC concentration to 10 and 20 g L^-1 resulted in decreased acetate production. A high concentration of AC showed higher antimicrobial activity to methanogens, as compared to acetogens. Amending AC exacerbated the process of electroosmosis. Also, amending AC with 0 to 10 g L^-1 decreased the electrochemical losses via both the membrane and electrolyte. The chemical production using H₂ or the electrode as electron donors showed a similar trend when amending AC. The present study provided important information for guiding future research to construct an efficient configuration of an MES bioreactor.

Extracellular Polymeric Substances from Geobacter sulfurreducens Biofilms in Microbial Fuel Cells

Bioelectrochemical systems (BESs) are hybrid systems using electroactive bacteria and solid electrodes, which serve as electron donor or acceptor for microorganisms. When forming a biofilm on the electrode, bacteria secrete extracellular polymeric substances (EPSs). However, EPS excretion of electroactive biofilms in BES has been rarely studied so far. Consequently, the aim of this study is to develop a routine including the electrochemical cultivation, biofilm harvesting, fractionation, and biochemical analysis of the EPS secreted by Geobacter sulfurreducens under electroactive conditions. G. sulfurreducens was cultivated in microbial fuel cell mode on graphite-based electrodes polarized to +400 mV versus Ag/AgCl for 8 d. A maximum current density of 172 ± 29 μA cm^-2 was reached after 7 d. The EPS secreted from the biofilms were harvested and fractioned into soluble, loosely bound, and tightly bound EPS and biochemically analyzed. Electroactive cultures secreted significantly more EPSs compared to cells grown under standard heterotrophic conditions (fumarate respiration). With 116 pg per cell, the highest amount of EPSs was measured for the soluble EPS fraction of G. sulfurreducens using anodic respiration, followed by the tightly bound (18 pg cell^-1) and loosely bound (11 pg cell^-1) fractions of the EPS. Proteins were found to dominate all EPS fractions of the biofilms grown under electrochemical conditions. To the best of the authors' knowledge, these experiments are the first approach toward a complete analysis of the main EPS components of G. sulfurreducens under anode-respiring conditions.

Same but different-Scale up and numbering up in electrobiotechnology and photobiotechnology

Facing energy problems, there is a strong demand for new technologies dealing with the replacement of fossil fuels. The emerging fields of biotechnology, photobiotechnology and electrobiotechnology, offer solutions for the production of fuels, energy, or chemicals using renewable energy sources (light or electrical current e.g. produced by wind or solar power) or organic (waste) substrates. From an engineering point of view both technologies have analogies and some similar challenges, since both light and electron transfer are primarily surface-dependent. In contrast to that, bioproduction processes are typically volume dependent. To allow large scale and industrially relevant applications of photobiotechnology and electrobiotechnology, this opinion first gives an overview over the current scales reached in these areas. We then try to point out the challenges and possible methods for the scale up or numbering up of the reactors used. It is shown that the field of photobiotechnology is by now much more advanced than electrobiotechnology and has achieved industrial applications in some cases. We argue that transferring knowledge from photobiotechnology to electrobiotechnology can speed up the development of the emerging field of electrobiotechnology. We believe that a combination of scale up and numbering up, as it has been shown for several photobiotechnological reactors, may well lead to industrially relevant scales in electrobiotechnological processes allowing an industrial application of the technology in near future.