Electrochemically Active Bacteria Research Articles

Electrogenetic control of gene expression in Shewanella oneidensis MR-1 using Arc-dependent transcriptional promoters

Electrochemically active bacteria (EAB) are capable of electrically interacting with electrodes, enabling their application in bioelectrochemical systems (BESs). As the performance of BES is related to the metabolic activities of EAB, the development of methods to control their metabolic activities is important to facilitate BES applications. A recent study found that the EAB Shewanella oneidensis MR-1 uses the Arc system to regulate the expression of catabolic genes in response to electrode potentials, suggesting that a methodology for electrical control of gene expression in EAB, referred to as electrogenetics, can be developed by using electrode potential-responsive, Arc-dependent transcriptional promoters. Here, we explored Arc-dependent promoters in the genomes of S.oneidensis MR-1 and Escherichia coli to identify electrode potential-responsive promoters that are differentially activated in MR-1cells exposed to high- and low-potential electrodes. LacZ reporter assays using electrode-associated cells of MR-1 derivatives revealed that the activities of promoters located upstream of the E.coli feo gene (Pfeo) and the MR-1 nqrA2 (SO_0902) gene (Pnqr2) were significantly increased when S.oneidensis cells were exposed to electrodes poised at+0.7V and-0.4V (versus the standard hydrogen electrode), respectively. Additionally, we developed a microscopic system for in situ monitoring of promoter activity in electrode-associated cells and found that Pnqr2 activity was persistently induced in MR-1cells associated with an electrode poised at-0.4V. Our results indicate that these electrode potential-responsive promoters enable efficient regulation of gene expression in EAB, providing a molecular basis for the development of electrogenetics.

Bacterial extracellular electron transfer in plant and animal ecosystems.

Extracellular electron transfer (EET) is a bioelectrochemical process performed by electrochemically active bacteria (EAB) found in host-associated environments, including plant and animal ecosystems and fermenting plant- and animal-derived foods. Through direct or mediated electron transfer pathways, certain bacteria use EET to enhance ecological fitness with host-impacting effects. In the plant rhizosphere, electron acceptors support the growth of EAB such as Geobacter, cable bacteria, and some clostridia that can result changing iron and heavy metal uptake by plants. In animal microbiomes, EET is associated with diet-derived iron in the intestines of soil-dwelling termites, earthworms, and beetle larvae. EET is also associated with the colonization and metabolism of some bacteria in human and animal microbiomes, such as Streptococcus mutans in the mouth, Enterococcus faecalis and Listeria monocytogenes in the intestine, and Pseudomonas aeruginosa in the lungs. During the fermentation of plant tissues and bovine milk, lactic acid bacteria like Lactiplantibacillus plantarum and Lactococcus lactis may use EET to increase their growth and food acidification, as well as decrease environmental oxidation-reduction potential. Thus, EET is likely an important metabolic pathway for host-associated bacteria and has implications for ecosystem function, health and disease, and biotechnological applications.

Integrating a manganese ores-filled module with a submerged plants cathode sediment fuel cell: In-situ remediation of ammonium pollution in surface water

Ammonium removal from contaminated surface water depends highly on electron acceptors like dissolved oxygen (DO), which was limited in eutrophic water bodies. In this study, a small ecological module was constructed with manganese ores (MO) and used to remove ammonium for the overlying water by integrating sediment microbial fuel cells (SMFC) and submerged plants. Nine different microcosms were operated to assess the pollutants removal performances. Compared with the 44.1 % of the control, ammonium removal of microcosms with MO addition increased to 79.1 %. Moreover, ammonium removal of the SMFC coupling system and submerged plant SMFC coupling system reached 83.3 % and 97.1 %, respectively. Besides, under the two coupling modes mentioned above, the degradation of chemical oxygen demand (COD) and the total organic carbon (TOC) was enhanced. The maximum power density was 390.63 mW/m3. The mass balance of nitrogen indicated the introduction of SMFC increased the contribution of denitrification to nitrogen removal. By contrast, plants promoted the release of oxygen and regulated community structure for intensifying simultaneous nitrification and denitrification (SND). Additionally, the effects of storage by substrates and sediments and plant uptake were also important. Heterotrophic and autotrophic denitrifying bacteria were enriched on the substrates around electrodes. Besides, electrochemically active bacteria (EAB) and Mn cycle related bacteria triggering the enhancement of nitrogen conversion. The Mn cycle in different valence states, electron transport between electrodes and substrates, and oxygen-producing submerged plants conspired to help excellent contaminant removal performance. This study promises a new thought for remediation of surface water and sediments in situ.

Geobacter sulfurreducens promoted the biosynthesis of reduced graphene oxide and coupled it for nitrobenzene reduction

In order to explore an efficient and green method to deal with nitrobenzene (NB) pollutant, reduced graphene oxide (rGO) as an electron shuttle was applied to enhance the extracellular electron transfer (EET) process of Geobacter sulfurreducens, which was a typical electrochemically active bacteria (EAB). In this study, rGO biosynthesis was achieved via the reduction of graphene oxide (GO) by G. sulfurreducens PCA within 3 days. Also, the rGO-PCA combining system completely reduced 50-200 µmol/L of NB to aniline as end product within one day. SEM characterization revealed that PCA cells were partly wrapped by rGO, and therefore the distance of electron transfer between strain PCA and rGO material was reduced. Beside, the ID/IG of GO, rGO, and rGO-PCA combining system were 0.990, 1.293 and 1.31, respectively. Moreover, highest currents were observed in rGO-PCA-NB as 12.950 µA/-12.560 µA at -408 mV/156 mV, attributing to the faster electron transfer efficiency in EET process. Therefore, the NB reduction was mainly due to: (I) direct EET process from G. sulfurreducens PCA to NB; (II) rGO served as electron shuttle and accelerated electron transfer to NB, which was the main degradation pathway. Overall, the biosynthesis of rGO via GO reduction by Geobacter promoted the NB removal process, which provided a facile strategy to alleviate the problematic nitroaromatic pollution in the environment.

Enhanced performance of microbial electrochemical systems prepared with in situ electropolymerization of L-arginine-modified carbon cloth

Microbial electrochemical systems (MESs) are an emerging technology that utilizes electrochemically active bacteria (EAB) to accomplish the transformation between chemical energy and electricity. Electrode modification is expected to promote EAB biofilm formation and increase MES performance. However, typical methods for MES electrode modification depend on harsh reaction conditions, toxic agents, or complex operations. In the study, a green and simple method was established with in situ L-arginine electropolymerization. Results demonstrate L-arginine electrooxidation is the primary step of in situ L-arginine electropolymerization, and the optimal potential range is [+0.5; +2] V. The N content on the electrode surface is increased from 1.4 ± 0.2% to 11.8 ± 1.5% with electropolymerized L-arginine (PLA) formation, and the zeta potential is increased from −29.3 ± 4.6 mV to −6.7 ± 5.2 mV. Better electropositivity increases the electrostatic attraction between EAB and working electrode, resulting in a 60% increase in biofilm biomass. Higher biomass enhances MES performance, and the current generation is increased by 76%. These results demonstrate that PLA modification is capable to increase electrode electropositivity, promote biofilm formation, and increase MES performance. The study illuminates the feasibility of MES electrode modification with in situ amino acid electropolymerization, and provides a new prospect to improve MES performance.

On-site determination of water toxicity based on freeze-dried electrochemically active bacteria

Our previous studies have reported water toxicity determination with a fresh electrochemically active bacteria (EAB) suspension as the sensing element, which exhibits high sensitivity and has great prospects in providing early warning about water pollution. However, because the preparation of fresh EAB suspensions is time-consuming, these studies are not suitable for the on-site determination of water toxicity. To solve this problem, this study investigated the rapid preparation of an EAB suspension by the rehydration of freeze-dried EABs and established a novel method for the on-site determination of water toxicity based on the freeze-dried EAB model strain Shewanella oneidensis MR-1. The results demonstrate that the optimal cryoprotectant for S. oneidensis MR-1 freeze drying is 7.5 % (w/v) skimmed milk powder. Compared with fresh S. oneidensis MR-1, freeze-dried S. oneidensis MR-1 exhibits similar extracellular electron transfer (EET) performance (74.7 % ± 0.3 %) and slightly lower sensitivity for water toxicity determination (65.8 % ± 2.2 %) with the optimal cryoprotectant. On-site determination of water toxicity was realized by using freeze-dried S. oneidensis MR-1, and the detection limits of five common toxic pollutants (Cd2+, Pb2+, Cu2+, phenol and dichlorophenol) reached 0.5 mg/L. Water toxicity determination is capable of resisting common interferences, e.g., glucose, lactate, nitrate and nitrite, and shows high accuracy in practical applications.

Nonconductive anode-associated biocarrier achieved differentiated functional bacteria enrichment and relieved the competitive pressure on anodic biofilm of microbial electrolysis cells

Microbial electrolysis cell (MEC) was an emerging technology for efficiently converting multifarious organic wastes to biogases (e.g. hydrogen and methane) with positive energy yield. The anode respiring process mediated by electrochemical active bacteria (EAB) in electrode biofilm was the key reaction in MECs. Yet, the anode has the trend to selectively enrich EAB (e.g. Geobacter sp.), which preferred simple organics. Feeding with relatively complex substrates, the EAB and fermentative bacteria competed on anode surface and formed biofilm stratification, which was adverse to the adjustment and synergistic interaction of those two functional bacteria. In this work, compensatory nonconductive biocarriers were equipped into MECs fed with sucrose to relieve the competitive pressure on anode by enriching differentiated functional bacteria on anode and nonconductive biocarriers, thus improving the overall organic degradation rate and current density. The addition of biocarrier reduced the cycle time by 35 % and improve the current density from 3.6 ± 0.3 to 5.3 ± 0.4 A/m−2 with non-fermented sucrose influent, while the COD removal and volatile fatty acids' utilization were also accelerated. With relatively complex substrates (fermented sucrose) in MECs, the fermentative bacteria were enriched on biocarriers, while the anode biofilm was domesticated for electrochemical activity with a much higher abundance of obligate EAB of 45.2 %. The nonconductive biocarriers in MECs could achieve biofilm differentiation and improve the synergistic interaction between EAB and fermentative bacteria. Achievements in this work provided a feasible method to improve the applicability of MEC technology in wastewater treatment of complex organics.

Livestock’s Urine-Based Plant Microbial Fuel Cells Improve Plant Growth and Power Generation

Plant microbial fuel cells (P-MFCs) are sustainable and eco-friendly technologies, which use plant root exudates to directly nourish the electrochemically active bacteria (EABs) to generate sustainable electricity. However, their use in evaluating plant growth has been insufficiently studied. In this study, interconnection between plant growth and the production of bioelectricity was evaluated by using P-MFCs inoculated with 642.865 mL ≅ 643 mL of livestock’s urine such as cow urine, goat urine, and sheep urine. The greatest mean stem diameter of 0.52 ± 0.01 cm was found in P-MFC-3 inoculated with goat urine, while the P-MFC-2 treated with cow urine reached a higher average number of roots with a value of 86 ± 2.50 (95% improvement) (p < 0.05). Besides, P-MFC-4 presented greater height of 50.08 ± 0.67 cm. For polarization curve experiment a higher maximum power density of 132 ± 11.6 mW m−2 (931 mA m−2) was reached with cow urine; in turn, with regard to the long-term operation, the same reactor indicated a higher maximum average power density of 43.68 ± 3.05 mW m−2. The study’s findings indicated that Stevia P-MFC inoculated with urine was a good option to increase the biomass amount for the agricultural plants along with power generation. Further, this study opens the way for more investigation of evaluating the impact of P-MFC on plant growth.

Unraveling the influence of magnetic field on microbial and electrogenic activities in bioelectrochemical systems: A comprehensive review

Bioelectrochemical systems (BESs), such as microbial fuel/electrolysis cells, are promising wastewater treatment and energy generation approaches that use electrochemically active bacteria (EAB). Bacteria growth in BES is a critical factor that controls the performance of the overall system. A magnetic field (MF) is an effective way to accelerate biofilm formation and extracellular electron transfer (EET). The performance is highly dependent on the MF intensity, exposure time, shape and orientation of the magnets, and the microbial structure of the inoculum. Despite the increasing number of investigations into each factor, there is an insufficient comprehensive understanding of the mechanism of MFs in BESs. In this review, the basic mechanism of MFs, as well as the various attempts to use MFs in BESs, and their effect on the obtained performances are introduced. Particularly, the empirical effects of MF on the EAB growth, EET, enzyme activity, and BES performance. Moreover, the influence of MF on radical pairs was also interpreted to explain how MF affects EET. This review is the first attempt at understanding the background and current trends in the application of MF technologies in BESs.

Performance of electrical energy monitoring data acquisition system for plant-based microbial fuel cell

Plant microbial fuel cell (Plant-MFC) is an emerging technology that uses the metabolic activity of electrochemically active bacteria (EABs) to continue the production of bioelectricity. Since its invention and to date, great efforts have been made for its application both in real-time and large-scale. However, the construction of platforms or systems for automatic voltage monitoring has been insufficiently studied. Therefore, this study aimed to develop an automatic real-time voltage data acquisition system, which was coupled with an ATMEGA2560 connected to a personal computer. Before the system operation started it was calibrated to obtain accurate data. During this experiment, the power generation performance of two types of reactors i.e. (i) Plant-MFC and (ii) control microbial fuel cell (C-MFC), was evaluated for 15 days. The Plant-MFC was planted with an herbaceous perennial plant (Stevia rebaudiana), electrode system was placed close to the plant roots at the depth of 20 cm. The results of the study have indicated that the Plant-MFC, was more effective and achieved higher bioelectricity generation than C-MFC. The maximum voltage reached with Plant-MFC was 850 mV (0.85 V), whereas C-MFC achieved a maximum voltage of 762 mV (0.772 V). Furthermore, the same reactor demonstrated a maximum power generation of 66 mW m¯2 on 10 min of polarization, while a power density with C-MFC was equal to 13.64 mW m¯2. S.rebaudiana showed a great alternative for power generation. In addition, the monitoring acquisition system was suitable for obtaining data in real-time. However, more studies are recommended to enhance this type of system.

Improved microbial electrolysis desalination and chemical-production cell for a high-value product: Hydrogen peroxide

A high-value product of microbial electrolysis desalination and chemical-production cell (MEDCC) is a promising method for reducing costs and broadening the cell’s application. This study proposed a modified MEDCC in terms of hydrogen peroxide (H2O2) production, electrochemical performance, desalination, energy consumption by comparing it with conventional MEDCC. At the optimal voltage (1.0 V), the improved system achieved the H2O2 concentration of 1183 ± 32 mg/L, the energy consumption of 8.42 ± 1.39 kWh/kg H2O2, the desalination efficiency of 60.36 ± 9.24% within 12 h. Although the MEDCC-H2O2 system had a lower transfer rate of electrons, the system had analogous performances on current density, desalination, and acid and alkali productions with the conventional system. These results meant that the ideal voltage for achieving bipolar membrane and electrochemically active bacteria (EAB) synergism was 1.0 V in the improved MEDCC. In terms of chemical production in the cathode chamber of the MEDCC, the application of H2O2 production increased the rate of return by four times when compared to conventional alkali production. Only about 55% of total consumption was attributed to bioenergy in the substrate, suggesting the system was energy-saving. Therefore, the revised MEDCC had great potential in the field of energy-efficient and green H2O2 generation and desalination whilst maintaining acid and alkali productions.

Microbial community dynamics in electroactive biofilms across time under different applied anode potentials

In bioelectrochemical wastewater treatment systems, electrochemically active bacteria (EAB) in the anode can simultaneously treat wastewater and produce electricity via extracellular electron transfer. The anode potential has been reported as one way for selecting EAB; though, conflicting results of the relationship between applied potentials and the performance and community composition of EAB have been reported. In this study, we investigated the cultivation time and applied anode potentials (+0.2, 0, −0.2, and −0.4 V vs. Ag/AgCl) on the performance of current production and the compositions of the microbial community. Our results showed that the applied potentials affected the performance of current production, but the effect was substantially reduced with cultivation time. Particularly, the current gradually increased from negative to positive values with time for the applied anode potential at −0.4 V, implying the anode biofilm shifted from accepting electrons to producing electrons. In addition, principal coordinates analysis results indicated that microbial community compositions became closer to each other after long-term enrichment. Subsequently, principal component analysis demonstrated that systems with applied potentials from +0.2, 0 to −0.2 V and at −0.4 V were, respectively, reclassified into principal component 1 (higher-energy-harvesting group) and principal component 2 (lower-energy-harvesting group), implying in addition to cultivation time, the amount of energy available for bacterial growth is another key factor that influences EAB populations. Overall, this study has demonstrated that the selected cultivation time and the particular anode potentials applied in the study determine whether the applied anode potentials would affect the community and performance of EAB.

Enhancing the sensitivity of water toxicity detection based on suspended Shewanella oneidensis MR-1 by reversing extracellular electron transfer direction.

Water toxicity detection is of great significance to ensure the safety of water supply. With suspended electrochemically active bacteria (EAB) as the sensing element, a novel microbial electrochemical sensor (MES) has recently been reported for the real-time detection of water toxicity, but its practical applications need to further improve the sensitivity. Extracellular electron transfer (EET) is an important factor affecting MES performance. In the study, the EET of suspended EAB-based MES was optimized to further enhance the sensitivity. Firstly, by using a model EAB stain Shewanella oneidensis MR-1, it was revealed that the sensitivity was increased at most 2.7 times with inward EET (i.e., cathodic polarization). Then, a novel conjecture based on electron transfer and energy fluxes was proposed and testified to explain this phenomenon. Finally, three key operating parameters of inward EET were orthogonally optimized. The optimized parameters of inward EET included a potential of - 0.5V, a cell density of 1.8 × 108CFU/mL, and an electron acceptor concentration of 15mM.

Enhanced Pb(II) removal from water using conductive carbonaceous nanomaterials as bacterial scaffolds: An experimental and modelling approach

This study investigated the abilities of electrochemically active bacteria (EAB) as bio-catalysts for Pb(II) bioreduction in an attached growth reactor. Electrically conductive carbon nanofibers (CNF) were used as bacterial scaffolds to facilitate electron transfer between EAB and Pb(II). Our results demonstrated that the CNF attached growth reactor can remove > 80% and > 90% of Pb(II) from solution at 0.5 mg/L and 5.0 mg/L concentrations, respectively, and maintained Pb(II) levels < 0.10 mg/L over repeated Pb(II) dosages dosed at all concentrations. Experimental measurements by SEM-EDX and ICP-OES provided evidence that Pb(II) was reduced to Pb0, suggesting that Pb(II) was adsorbed onto CNFs, and subsequently bioreduced. Numerical modelling was used to estimate Pb(II) concentrations along the CNFs via capturing Pb(II)’s diffusive transport, and the removal mechanisms governed by EAB and CNF adsorption. The model simulation results demonstrated that Pb(II) bioreduction was dominant in the reactor and the significance of Monod kinetics, biofilm thickness and Pb(II) dosage concentration on its removal. This study is the first to prove EAB’s ability to treat Pb(II) from contaminated waters in an attached growth configuration. The findings here demonstrate that CNFs can boost biological treatment efficacies in wastewater and/or water treatment to meet stringent water guidelines.

Removal of microcystin (MC-LR) in constructed wetlands integrated with microbial fuel cells: Efficiency, bioelectricity generation and microbial response

Microcystins (MCs) pollution caused by cyanobacteria harmful blooms (CHBs) has posed short- and long-term risks to aquatic ecosystems and public health. Constructed wetlands (CWs) have been verified as an effective technology for eutrophication but the removal performance for MCs did not achieve an acceptable level. CWs integrated with microbial fuel cell (MFC-CWs) were developed to intensify the nutrient and Microcystin-LR (MC-LR) removal efficiencies in this study. The results indicated that closed-circuit MFC-CWs (T1) exhibited a better NO3−-N, NH4+-N, TP and MC-LR removal efficiency compared to that of open-circuit MFC-CWs (CK, i.e., traditional CWs). Therein, a MC-LR removal efficiency of greater than 95% was observed in both trials in T1. The addition of sponge iron to the anode layer of MFC-CWs (T2) improved only the NO3−-N removal and efficiency bioelectricity generation performance compared to T1, and the average effluent MC-LR concentration of T2 (1.14 μg/L) was still higher than the provisional limit concentration (1.0 μg/L). The microbial community diversity of T1 and T2 was simplified compared to CK. The relative abundance of Sphingomonadaceae possessing the degradation capability for MCs increased in T1, which contributed to the higher MC-LR removal efficiency compared to CK and T2. While the relative abundance of electrochemically active bacteria (EAB) (i.e., Desulfuromonadaceae and Desulfomicrobiaceae) in the anode of T2 was promoted by the addition of sponge iron. Overall, this study suggests that integrating MFC into CWs provides a feasible intensification strategy for eutrophication and MCs pollution control.

The trade-off between nitrogen removal and current generation in an air-cathode bioelectrochemically assisted osmotic membrane bioreactor

One of the key challenges for the application of an osmotic membrane bioreactor (OMBR) is low nitrogen removal efficiency. Herein, a novel integrated configuration by combining a single-chamber air-cathode bioelectrochemical system (BES) with an OMBR was proposed to facilitate nitrogen removal due to simultaneous nitrification and denitrification in the bioreactor, and carbon source trade-off between nitrogen removal and current generation were also investigated under different hydraulic retention times (HRTs) and draw solute (DS) concentrations. By applying sodium acetate as DS, the supplementary carbon source from reverse-fluxed solute promoted nitrogen removal (i.e., total nitrogen removal efficiency of 93.08 ± 0.93%) and current generation (i.e., current density of 110.67 ± 4.99 A/m3). Sequencing analyses revealed that nitrifying bacteria (62.7%), denitrifying bacteria (33.82%), and electrochemically-active bacteria (EAB, 4.31%) were abundant in the bioreactor. Both a long HRT or a high DS concentration improved nitrogen removal, but promoted the carbon source competition among denitrifying bacteria, EAB, and other heterotrophic bacteria. Cost balance of 0.32–1.13 $USD/kgCOD or 3.60–9.03 $USD/kgN was obtained, and a lower DS concentration was more cost-saving for the system operation. Those results have demonstrated that the proposed system could be an alternative approach for simultaneous enhancement of nitrogen removal and current generation.

Enhanced performance of microbial fuel cell with electron mediators from tetracycline hydrochloride degradation

Tetracycline hydrochloride (TCH) is a typical antibiotic pollutant with high toxicity and persistence. The degradation of TCH and the generation of the associated electron mediator in a dual chamber microbial fuel cells (MFCs) were studied. The results of liquid chromatography revealed that TCH could be effectively removed (>93%) in MFCs mode. The maximum COD removal was 88.14 ± 1.47% in MFCs while it was 69.57 ± 1.36% in open circuit MFCs. According to cyclic voltammetry, the presence of the relevant redox peaks clearly suggested that the intermediates from TCH degradation could act as endogenous electron mediator. The highest power density of 120.02 ± 2.76 mW/m2 and the lowest internal resistance of 18.68 Ω were achieved in MFC with 2 mg/L of TCH. Microbial community analysis illustrated that Bacteroides, Comamonas, Clostridium_sensu_stricto, Desulfovibrio and Geobacter were enriched and played a dominant role in TCH degradation and power generation. Electrochemical active bacteria had certain tolerance to TCH and the inhibiting threshold value of TCH was below 5 mg/L. This study provided a new thinking that low concentration of TCH could produce electron mediators to improve the performance of MFC system.

In situ COD monitoring with use of a hybrid of constructed wetland-microbial fuel cell

The hybrid system of constructed wetland and microbial fuel cell (CW-MFC) used as a biosensor is becoming a new research focus with the advantage of resisting the shock loading and enriching more electricigens. In this study, a structural parameter S integrating the size, the position and the spacing of the anode and the cathode was proposed. And the electrogenesis and biosensing performances of the vertical flow CW-MFC biosensors were evaluated at different S values. The results showed that all the three biosensors could achieve good monitoring for COD (R2 > 0.97). And the coulombic yield was more suitable for the response signal than output voltage. But different biosensing properties including detection signal, detection range, detection time, correlation fitting degree and sensitivity were also displayed. Further, in order to optimize the biosensing performance, the coulombic yield in stable voltage stage (Qs) was proposed which can shorten the detection time by 70% at most. On the anodes, abundant nitrogen-transforming bacteria (NTB) were enriched as well as electrochemically active bacteria (EAB). The competition of NTB for substrates and electrons with EAB disturbed the output voltage signal but not affect the stability of coulombic yield signal. Moreover, the significant linear correlation between the S values and the ratios of EAB to NTB colonized both on anodes and on cathodes indicated the differences of the electricity generation and biosensing performance at the different structural parameters.

Physiological metabolism of electrochemically active bacteria directed by combined acetate and Cd(II) in single-chamber microbial electrolysis cells

It is of great interest to explore physiological metabolism of electrochemically active bacteria (EAB) for combined organics and heavy metals in single-chamber microbial electrolysis cells (MECs). Four pure culture EAB varying degrees responded to the combined acetate (1.0–5.0 g/L) and Cd(II) (20–150 mg/L) at different initial concentrations in the single-chamber MECs, shown as significant relevance of Cd(II) removal (2.57–7.35 mg/L/h) and H2 production (0–0.0011 m3/m3/h) instead of acetate removal (73–130 mg/L/h), to these EAB species at initial Cd(II) below 120 mg/L and initial acetate below 3.0 g/L. A high initial acetate (5.0 g/L) compensated the Cd(II) inhibition and broadened the removal of Cd(II) to 150 mg/L. These EAB physiologically released variable amounts of extracellular polymeric substances with a compositional diversity in response to the changes of initial Cd(II) and circuital current whereas the activities of typical intracellular enzymes were more apparently altered by the initial Cd(II) than the circuital current. These results provide experimental validation of the presence, the metabolic plasticity and the physiological response of these EAB directed by the changes of initial Cd(II) and acetate concentrations in the single-chamber MECs, deepening our understanding of EAB physiological coping strategies in metallurgical microbial electro-ecological cycles.

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.