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Bioelectrochemical systems (BESs) exploit the interaction between microbes and electrodes. A field of application thereof is bioelectrochemical remediation, an effective strategy in environments where the absence of suitable electron acceptors limits classic bioremediation approaches. Understanding the microbial community structure and genetic potential of anode biofilms is of great interest to interpret the mechanisms occurring in BESs. In this study, by using a whole metagenome sequencing approach, taxonomic and functional diversity patterns in the inoculum and on the anodes of three continuous-flow BES for the removal of phenol, toluene, and BTEX were obtained. The genus Geobacter was highly enriched on the anodes and two reconstructed genomes were taxonomically related to the Geobacteraceae family. To functionally characterize the microbial community, the genes coding for the anaerobic degradation of toluene, ethylbenzene, and phenol were selected as genetic markers for the anaerobic degradation of the pollutants. The genes related with direct extracellular electron transfer (EET) were also analyzed. The inoculum carried the genetic baggage for the degradation of aromatics but lacked the capacity of EET while anodic bacterial communities were able to pursue both processes. The metagenomic approach provided useful insights into the ecology and complex functions within hydrocarbon-degrading electrogenic biofilms.

Biodegradation of aromatic hydrocarbons in anoxic contaminated environments is typically limited by the lack of bioavailable electron acceptors. Microbial electrochemical technologies (METs) are able to provide a virtually inexhaustible electron acceptor in the form of a solid electrode.Recently, we provided first experimental evidence for the syntrophic degradation of toluene in a continuous-flow bioelectrochemical reactor known as the “bioelectric well”. Herein, we further analyzed the structure and function of the electroactive toluene-degrading microbiome using a suite of chemical, electrochemical, phylogenetic, proteomic, and functional gene-based analyses.The bioelectric well removed 83 ± 7 % of the toluene from the influent with a coulombic efficiency of 84 %. Cyclic voltammetry allowed to identify the formal potentials of four putative electron transfer sites, which ranged from −0.2 V to +0.1 V vs. SHE, consistent with outer membrane c-type cytochromes and pili of electroactive Geobacter species. The biofilm colonizing the surface of the anode was indeed highly enriched in Geobacter species. On the other hand, the planktonic communities thriving in the bulk of the reactor harbored aromatic hydrocarbons degraders and fermentative propionate-producing microorganisms, as revealed by phylogenetic and proteomic analyses. Most likely, propionate, acetate or other VFAs produced in the bulk liquid from the degradation of toluene were utilized as substrates by the electroactive biofilm. Interestingly, key-functional genes related to the degradation of toluene were found both in the biofilm and in the planktonic communities. Taken as a whole, the herein reported results highlight the importance of applying a comprehensive suite of techniques to unravel the complex cooperative metabolisms occurring in METs.

Microbial electrochemical technologies (MET) are promising for the remediation of groundwater pollutants such as petroleum hydrocarbons (PH). Indeed, MET can provide virtually inexhaustible electron donors or acceptors directly in the subsurface environment. However, the degradation mechanisms linking contaminants removal to electric current flow are still largely unknown, hindering the development of robust design criteria.Here, we analysed the degradation of toluene, a model PH, in a bioelectrochemical reactor known as “bioelectric well” operated in continuous-flow mode at various influent toluene concentrations. With increasing concentration of toluene, the removal rate increased while the current tended to a plateau, hence the columbic efficiency decreased. Operation at open circuit confirmed that the bioelectrochemical degradation of toluene proceeded via a syntrophic pathway involving cooperation between different microbial populations. First of all, hydrocarbon degraders quickly converted toluene into metabolic intermediates probably by breaking the aromatic ring upon fumarate addition. Subsequently, fermentative bacteria converted these intermediates into volatile fatty acids (VFA) and likely also H2, which were then used as substrates by electroactive microorganisms forming the anodic biofilm. As toluene degradation is faster than subsequent conversion steps, the increase in intermediate concentration could not result in a current increase. This work provides valuable insights on the syntrophic degradation of BTEX, which are essential for the application of microbial electrochemical system to groundwater remediation of petroleum hydrocarbons.

Treatment of wastewaters with high concentration of sulfate before discharge is an environmental imperative. However, current technologies are often highly energy and chemical-demanding. Recently, microbial electrochemical technologies (MET) have been proposed as an alternative and more sustainable approach for the treatment of wastewaters rich in sulfate. Here we compare the sulfate reduction performance of one- and two-chamber reactors. In two-chamber reactors, sulfate reduction to sulfide was achieved with high electron recovery efficiency (83.9 ±1.3 %) at a sulfate reduction rate of 9.7 ±2.6 mgSO42− L−1 d−1 cm−2, whereas in one-chamber reactors apparently no sulfate reduction took place. Non-electrochemical microcosm experiments suggested that sulfate reduction in two-chamber reactors was driven by the availability of cathodically produced H2. In one-chamber reactors, the presence of anodically produced O2 presumably resulted in chemical short circuit including aerobic hydrogen oxidation, abiotic and biotic re-oxidation of sulfide to sulfate. Microbial community analysis demonstrated that sulfate reduction was mainly performed by sulfate reducing prokaryotes (SRP) belonging to the genera Desulfomicrobium and Desulfovibrio, whereas sulfide oxidizing bacteria (SOB) (mainly Acinetobacter and Sulfuricurvum) could be responsible for re-oxidation of sulfide. Thus, fundamental and application-oriented research on microbial electrochemical sulfate reduction ought to be studied in two-chamber reactors for avoiding a potential bias of results by anodic reactions.

Petroleum hydrocarbons (PHs) are often found in groundwater due to human activities like accidental spills, causing health and environmental risks, and requiring remediation. Microbial Electrochemical Technologies (METs) have emerged as a promising alternative to conventional bioremediation techniques for the treatment of PH-contaminated groundwater. However, the field-application of these promising sustainable as well as cost-effective technologies is still scarce. One major reason is the lack of scalable reactor configurations. Herein, an upgraded version of the “bioelectric well”, a novel tubular bioelectrochemical reactor that can be installed directly within a groundwater well, was tested for the simultaneous removal of oxidableoxidizable (i.e., toluene and other PH) and reducible (i.e., sulfate) compounds from a real contaminated groundwater. After a proof-of-concept study in batch mode, the system was operated in continuous-flow mode for 48 days with the anode polarized at 0.2 V vs. SHE and a hydraulic retention time of 11 h. In these conditions, a steady-state removal rate of toluene as high as 31 ± 2 mg L−1 d−1 was achieved, which was more than double the value observed with the open circuit potential (OCP) control and one of the highest reported in literature. The electrode polarization went along with a higher abundance of key-functional genes involved in toluene degradation. This was not only showing its clear functional connection to the microbial metabolism, but further allowed to identify the involved electrogenic biodegradation pathway. In addition, the system simultaneously removed sulfate (30 ± 1 mg L−1 d−1), with bacteria likely using the H2 generated at the cathode as electron donor. Nevertheless, the apparent sulfate removal rate in the polarized and in the OCP runs was similar. The analysis of the microbial communities evidenced a high abundance of the genus Chlorobium in t

Sulfate reducing microorganisms are typically involved in hydrocarbon biodegradation in the sea sediment, with their metabolism resulting in the by-production of toxic sulfide. In this context, it is of utmost importance identifying the optimal value for anodic potential which ensures efficient toxic sulfide removal. Along this line, in this study the (bio)electrochemical removal of sulfide was tested at anodic potentials of −205 mV, +195 mV and +300 mV (vs Ag/AgCl), also in the presence of a pure culture of the sulfur-oxidizing bacterium Desulfobulbus propionicus. Current production, sulfide concentration and sulfate concentration were monitored over time. At the end of the experiment sulfur deposition on the electrodes and the microbial communities were characterized by SEM-EDS and by next generation sequencing of the 16S rRNA gene respectively. Results confirmed that current production was linked to sulfide removal and D. propionicus promoted back oxidation of deposited sulfur to sulfate. The highest electron recovery was observed at +195 mV vs Ag/AgCl, and the lowest sulfur deposition was obtained at −205 mV vs Ag/AgCl anode polarization.

Groundwater contamination by petroleum hydrocarbons (PHs) is a widespread problem which poses serious environmental and health concerns. Recently, microbial electrochemical technologies (MET) have attracted considerable attention for remediation applications, having the potential to overcome some of the limiting factors of conventional in situ bioremediation systems. So far, field-scale application of MET has been largely hindered by the limited availability of scalable system configurations. Here, we describe the 'bioelectric well' a bioelectrochemical reactor configuration, which can be installed directly within groundwater wells and can be applied for in situ treatment of organic contaminants, such as PHs. A laboratory-scale prototype of the bioelectric well has been set up and operated in continuous-flow regime with phenol as the model contaminant. The best performance was obtained when the system was inoculated with refinery sludge and the anode potentiostatically controlled at +0.2 V versus SHE. Under this condition, the influent phenol (25 mg l-1) was nearly completely (99.5 ± 0.4%) removed, with an average degradation rate of 59 ± 3 mg l-1 d and a coulombic efficiency of 104 ± 4%. Microbial community analysis revealed a remarkable enrichment of Geobacter species on the surface of the graphite anode, clearly pointing to a direct involvement of this electro-active bacterium in the current-generating and phenol-oxidizing process. © 2017 John Wiley & Sons Ltd and Society for Applied Microbiology

Marine sediments represent an important sink of harmful petroleum hydrocarbons after an accidental oil spill. Electrobioremediation techniques, which combine electrokinetic transport and biodegradation processes, represent an emerging technological platform for a sustainable remediation of contaminated sediments. Here, we describe the results of a long-term mesocosm-scale electrobioremediation experiment for the treatment of marine sediments contaminated by crude oil. A dimensionally stable anode and a stainless-steel mesh cathode were employed to drive seawater electrolysis at a fixed current density of 11 A/m 2 . This approach allowed establishing conditions conducive to contaminants biodegradation, as confirmed by the enrichment of Alcanivorax borkumensis cells harboring the alkB-gene and other aerobic hydrocarbonoclastic bacteria. Oil chemistry analyses indicated that aromatic hydrocarbons were primarily removed from the sediment via electroosmosis and low molecular weight alkanes (nC 6 to nC 10 ) via biodegradation.

Microbial electrochemical technologies (MET) are increasingly being considered for in situ remediation of contaminated groundwater. However, their application potential for the simultaneous treatment of complex mixtures of organic and inorganic contaminants, has been only marginally explored. Here we have analyzed the performance of the ‘bioelectric well’, a previously developed bioelectrochemical reactor configuration, in the treatment of benzene, toluene, ethyl-benzene and xylenes (BTEX) mixtures. Although to different extents, all BTEX were found to be degraded in the bioelectrochemical system, operated using a continuous-flow of groundwater at a hydraulic retention time of 8.8 h, with the graphite anode potentiostatically controlled at +0.200 V vs. the standard hydrogen electrode. In the case of toluene and ethyl-benzene, biodegradation was further confirmed by the GC–MS identification of fumarate-addition metabolites, previously shown to be involved in the activation of these contaminants under anaerobic conditions. Degradation rates were higher for toluene (31.3 ± 1.5 mg/L d) and lower for benzene (6.1 ± 0.3 mg/L d), ethyl-benzene (3.3 ± 0.1 mg/L d), and xylenes (4.5 ± 0.2 mg/L d). BTEX degradation was linked to electric current generation, with coulombic efficiencies falling in the range 53–69%, although methanogenesis also contributed to contaminant degradation. Remarkably, the system also allowed removal of sulfate simultaneously with toluene. Sulfate removal was likely driven by the hydrogen abiotically generated at the cathode. Taken as a whole, these findings highlight the remarkable potential of this innovative reactor configuration for application in a variety of contamination scenarios.

Marine sediments represent an important sink of harmful petroleum hydrocarbons after an accidental oil spill. Electrobioremediation techniques, which combine electrokinetic transport and biodegradation processes, represent an emerging technological platform for a sustainable remediation of contaminated sediments. Here, we describe the results of a long-term mesocosm-scale electrobioremediation experi- ment for the treatment of marine sediments contaminated by crude oil. A dimensionally stable anode and a stainless-steel mesh cathode were employed to drive seawater electrolysis at a fixed current density of 11 A/m2. This approach allowed establishing conditions conducive to contaminants biodeg- radation, as confirmed by the enrichment of Alcanivorax borkumensis cells harboring the alkB-gene and other aerobic hydrocarbonoclastic bacteria. Oil chemistry analyses indicated that aromatic hydrocarbons were primarily removed from the sediment via electroosmosis and low molecular weight alkanes (nC6 to nC10) via biodegradation.

Microbial electrochemical systems (MES) represent an innovative reagent-free technology for in situ remediation of groundwater contaminated by petroleum hydrocarbons. Here we describe the long-term (>160 days) anaerobic treatment of synthetic groundwater containing toluene (25 mg L-1) in a novel laboratory-scale, continuous-flow bioelectrochemical reactor, termed the "bioelectric well". Under optimal operating conditions (i.e., anode potential potentiostatically controlled at +0.2 V vs. SHE and recycle flow-rate set at 75 mL min-1), the observed electrogenic toluene oxidation rate was 67.2 ± 5.7 mg L d-1, a value which is among the highest reported in the literature for laboratory-scale anaerobic treatment systems. Correspondingly, electric current was 5.1 ± 0.1 mA and the Coulombic efficiency (i.e., the yield of toluene conversion into electric current) was 79 ± 7%. Electrogenic toluene oxidation was most likely catalyzed by Geobacter species, which were found to dominate the surface of the graphite anode. The combined application of GC-MS (for detection and identification of metabolites) and qPCR (for quantification of functional genes) revealed that toluene degradation was initiated by fumarate addition, an activation pathway that is widely distributed among anaerobic hydrocarbon-degrading microorganisms. Collectively, the results of this study indicate that the bioelectric well holds remarkable potential for in situ treatment of groundwater contaminated by petroleum hydrocarbons.

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Background Recent studies have suggested that addition of electrically conductive biochar particles is an effective strategy to improve the methanogenic conversion of waste organic substrates, by promoting syntrophic associations between acetogenic and methanogenic organisms based on interspecies electron transfer processes. However, the underlying fundamentals of the process are still largely speculative and, therefore, a priori identification, screening, and even design of suitable biochar materials for a given biotechnological process are not yet possible. Results Here, three charcoal-like products (i.e., biochars) obtained from the pyrolysis of different lignocellulosic materials, (i.e., wheat bran pellets, coppiced woodlands, and orchard pruning) were tested for their capacity to enhance methane production from a food waste fermentate. In all biochar-supplemented (25 g/L) batch experiments, the complete methanogenic conversion of fermentate volatile fatty acids proceeded at a rate that was up to 5 times higher than that observed in the unamended (or sand-supplemented) controls. Fluorescent in situ hybridization analysis coupled with confocal laser scanning microscopy revealed an intimate association between archaea and bacteria around the biochar particles and provided a clear indication that biochar also shaped the composition of the microbial consortium. Based on the application of a suite of physico-chemical and electrochemical characterization techniques, we demonstrated that the positive effect of biochar is directly related to the electron-donating capacity (EDC) of the material, but is independent of its bulk electrical conductivity and specific surface area. The latter properties were all previously hypothesized to play a major role in the biochar-mediated interspecies electron transfer process in methanogenic consortia. Conclusions Collectively, these results of this study suggest that for biochar addition in anaerobic digester operation, the

Annually, thousands of oil spills occur across the globe. As a result, petroleum substances and petrochemical compounds are widespread contaminants causing concern due to their toxicity and recalcitrance. Many remediation strategies have been developed using both physicochemical and biological approaches. Biological strategies are most benign, aiming to enhance microbial metabolic activities by supplying limiting inorganic nutrients, electron acceptors or donors, thus stimulating oxidation or reduction of contaminants. A key issue is controlling the supply of electron donors/acceptors. Bioelectrochemical systems (BES) have emerged, in which an electrical current serves as either electron donor or acceptor for oil spill bioremediation. BES are highly controllable and can possibly also serve as biosensors for real time monitoring of the degradation process. Despite being promising, multiple aspects need to be considered to make BES suitable for field applications including system design, electrode materials, operational parameters, mode of action and radius of influence. The microbiological processes, involved in bioelectrochemical contaminant degradation, are currently not fully understood, particularly in relation to electron transfer mechanisms. Especially in sulfate rich environments, the sulfur cycle appears pivotal during hydrocarbon oxidation. This review provides a comprehensive analysis of the research on bioelectrochemical remediation of oil spills and of the key parameters involved in the process.

This chapter is dedicated to microbial electrochemical technologies (METs), a novel conception for industrial and environmental biotechnology allowing the exploitation of low-value biomasses, like wastewater as well as the production of value-added products. Microbial electrochemical technologies are realized in devices termed bioelectrochemical systems (BES). Primary METs are based on the microbially assisted oxidation or reduction of substrates and the microbially facilitated transfer of electrons between the substrate and the electrode, which serves as microbial electron acceptor or donor. In this chapter the fundamentals of the microbial extracellular electron transfer are discussed, including the different electron transfer mechanisms and their thermodynamics (i.e. their energetic efficiency). Subsequently, major BES classes are presented. These include microbial fuel cells (MFCs) for electricity generation, microbial electrolysis cells (MECs) or BES for the production of value added products as well as bioremediation and biosensor applications. As microbial electrochemical technologies face a rapid progress in their development the chapter highlights the principle concepts rather than engineering details.