Extracellular Electron Transfer Ability Research Articles

External electricity-powered FeS/graphene/bacterial hybrid for Cr6+ removal from water lacking organic electron donors

Microbial reduction is an economical and environmentally benign route to deal with high-toxic Cr6+ widely present in aquatic environments, but faces major challenges such as cytotoxicity and insufficient organic electron donors. Herein, we propose a route for the removal of Cr6+ from water using a ternary FeS/reduced graphene oxide/Shewanella oneidensis MR-1 (FeS/rGO/MR-1) hybrid with external electricity supply at an electrode potential of −0.36 V. The biohybrid with excellent redox activity is in-situ biosynthesized by virtue of the versatile metabolic pathway and powerful extracellular electron transfer ability of MR-1 cells without the need of organic carbon sources. The biohybrid is directly used to remove Cr6+ under the condition of continuous electricity input, the removal rate is negatively correlated with the applied electrode potential. Compared with binary FeS/MR-1 and rGO/MR-1 counterpart, the FeS/rGO/MR-1 hybrid achieves an improved ability to remove Cr6+ at an initial concentration of 20 mg L−1 in three consecutive batches due to the synergistic effect. The FeS is the main active component for rapid chemical reduction of Cr6+ because of its highly reducible Fe2+ and S2-, and rGO acts as the bridge to accelerate electron transfer from the applied electrode to MR-1 cells. Then, the MR-1 cells use the obtained electrons for FeS regeneration, which in turn reduces Cr6+ again, resulting in the precipitation of less toxic Cr3+ [Cr(OH)3 and Cr2O3] in the biohybrid. This study provides a feasible route for removal of toxic Cr6+ from water lacking organic electron donor by in-situ synthesis of biohybrid with external electricity supply.

SERS combined with the difference in bacterial extracellular electron transfer ability to distinguish Shewanella

Shewanella plays an important role in geochemical cycle, biological corrosion, bioremediation and bioenergy. The development of methods for identifying Shewanella can provide technical support for its rapid screening, in-depth research into its extracellular respiratory mechanism and its application in ecological environment remediation. As a tool for microbial classification, identification and detection, Surface-enhanced Raman scattering (SERS) has high feasibility and application potential. In this work, bio-synthesized silver nanoparticles (AgNPs) were used as SERS substrates to effectively distinguish different types of Shewanella bacteria based on the difference in bacterial extracellular electron transfer (EET) ability. AgNPs were combined with the analyzed bacteria to prepare “Bacteria-AgNPs” SERS samples, which can strongly enhance the Raman signal of the target bacteria and reliably obtain spatial information of different molecular functional groups of each bacteria. Our developed approach can effectively distinguish between non-metal reducing and metal-reducing bacteria, and can further distinguish the three subspecies of Shewanella (Shewanella oneidensis MR-1, Shewanella decolorationis S12, and Shewanella putrefaciens SP200) at the genus and species level. The Raman signal enhancement is presumably caused by the excitation of local surface plasma (LSP) and the enhancement of surrounding electric field. Therefore, our developed method can achieve interspecific and intraspecies discrimination of bacteria. The proposed method can be extended to distinguish other metal-reducing bacteria, and the novel SERS active substrates can be developed for practical applications.

Sub-inhibitory concentrations of ampicillin affect microbial Fe(III) oxide reduction

Antibiotics are ubiquitous in the iron-rich environments but their roles in microbial reduction of Fe(III) oxides are still unclear. Using ampicillin and Geobacter soli, this study investigated the underlying mechanism by which antibiotic regulated microbial reduction of Fe(III) oxides. Results showed that sub-minimal inhibitory concentrations (sub-MIC) of ampicillin significantly affected ferrihydrite reduction by G. soli, with a stimulatory effect at 1/64 and 1/32 MIC and an inhibitory effect at 1/8 MIC. Increasing ampicillin concentration resulted in increasing cell length and decreasing bacterial zeta potential that were beneficial for ferrihydrite reduction, and decreasing outer membrane permeability that was unfavorable for ferrihydrite reduction. The respiratory metabolism ability was enhanced by 1/64 and 1/32 MIC ampicillin and reduced by 1/8 MIC ampicillin, which was also responsible for regulation of ferrihydrite reduction by ampicillin. The ferrihydrite reduction showed a positive correlation with the redox activity of extracellular polymeric substances (EPS) which was tied to the cytochrome/polysaccharide ratio and the content of α-helices and β-sheet in EPS. These results suggested that ampicillin regulated microbial Fe(III) oxide reduction through modulating the bacterial morphology, metabolism activity and extracellular electron transfer ability. Our findings provide new insights into the environmental factors regulating biogeochemical cycling of iron.

Enterococcus faecalis NADH Peroxidase-Defective Mutants Stain Falsely in Colony Zymogram Assay for Extracellular Electron Transfer to Ferric Ions.

Enterococcus faecalis cells can reduce ferric ions and other electron acceptors by extracellular electron transfer (EET). To find mutants with enhanced or defective EET, strain OG1RF with random transposon insertions in the chromosome was screened for ferric reductase activity by colony zymogram staining using the chromogenic ferrous-chelating compound Ferrozine. The screen revealed npr, eetB, and ndh3 mutants. The aberrant ferric reductase phenotype of Npr (NADH peroxidase)-defective mutants was found to be a property of colonies and not apparent with washed cells grown in liquid culture. EetB- and Ndh3-defective mutants, in contrast, consistently showed low ferric reductase activity. It is concluded that colony zymogram staining for ferric reductase activity using Ferrozine can be misleading, especially through false negative results. It is suggested that hydrogen peroxide produced in the colony quenches the zymogram staining. In addition, it is demonstrated that the negative effect of heme on EET to ferric ion in E. faecalis is relieved by cytochrome bd deficiency. The findings can help to identify bacteria with EET ability and contribute to our understanding of EET in Gram-positive bacteria and the physiology of E. faecalis.

Enhancing the extracellular electron transfer ability via Polydopamine@S. oneidensis MR-1 for Cr(VI) reduction

Microbial reduction of hexavalent chromium (Cr (VI)) shows better efficiency and cost-effectiveness. However, immobilization of Cr (III) remains a challenge as there is a limited supply of electron donors. A greener and cleaner option for donating external electrons was using bioelectrochemical systems to perform the microbial reduction of Cr(VI). In this system, we constructed a polydopamine (PDA) decorated Shewanella oneidensis MR-1 (S. oneidensis MR-1) bioelectrode with bidirectional electron transport, abbreviated as PDA@S. oneidensis MR-1. The conjugated PDA distributed on the intracellular and extracellular of individual S. oneidensis MR-1 has been shown to accelerate electron transfer by outer membrane C-type cytochromes and flavin-bound MtrC/OmcA pathway by various electrochemical analyses. As expected, the PDA@S. oneidensis MR-1 biofilm achieved 88.1% Cr (VI) removal efficiency (RE) and 58.1% Cr (III) immobilization efficiency (IE) within 24 h under the autotrophic conditions at the optimal voltage (−150 mV) compared with the control potential (0 mV). The PDA@S. oneidensis MR-1 biofilm showed increased RE activity was attributed to the shortening of the distance between individual bacteria by PDA. This research provides a viable strategy for in situ bioremediation of Cr(VI) polluted aquatic environment.

Nitrogen-doped graphene oxide with enhanced bioelectricity generation from microbial fuel cells for marine sewage treatment

With the increasing demand for clean water and energy, microbial fuel cell (MFC) as a promising technology for obtaining energy from wastewater has attracted great research interest in the last two decades. The performance of the anode electrode is the most critical factor limiting the large-scale application of MFC. Graphene materials as a suitable candidate have been successfully used as the anode due to their excellent biocompatibility and efficient extracellular electron transfer (EET) ability. Here, nitrogen-doped graphene oxide (NGO) was prepared by a simple one-step hydrothermal method. X-ray photoelectron spectroscopy (XPS) was used to analyse the valence states of the surface chemical elements and their associated molecular species. Fourier transform infrared spectroscopy (FTIR) was used to identify the surface functional groups, and Raman spectroscopy was used to analyse the information about surface defects. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) revealed the increased electrochemical activity and rapid EET ability from the NGO electrodes. Scanning electron microscopy demonstrated the two-dimensional layered structure of the NGO with some wrinkled texture. MFCs equipped with the modified NGO anode achieved the highest power density of 708.3 mW/m2 with an output voltage of 498.6 mV in comparison with the other graphene-based electrodes, i.e., graphene and graphene oxide. Moreover, the chemical oxygen demand (COD) removal rate increased significantly from 18.1% to 45.6%. The analysis of the bacterial community using a high-throughput sequencing indicated that the relative abundance of the electricigens increased on the NGO electrode biofilim, and the relative expression of ccoN gene coding cytochrome-c oxidase (Cco) was markedly up-regulated. These results demonstrated that NGO modification effectively enhanced the bio-electrocatalytic activity of MFC with improved wastewater treatment capacity.

An electrochemical system for the rapid and accurate quantitation of microbial exoelectrogenic ability

Microbial extracellular electron transfer (EET) plays a vital role in globally important environmental phenomena, including bioremediation, bioenergy generation, and biofuel production. The quantitation of microbial exoelectrogenic ability is fundamental to studying the process of EET. However, there is no accurate and time-saving protocol to directly evaluate EET ability, hindering our understanding and application of EET. In this work, we proposed an accurate and rapid quantitation system for measuring EET ability using a gold-coated membrane filter as a working electrode. The quantitation signals could be recorded within 1 h and accurately normalized by the number of cells with outstanding repeatability and reproducibility. Further, this method could be distinguished microbial direct EET performances of different growth stages, and the results showed the middle logarithmic growth stage of Shewanella onedensis MR-1 had the best electrochemical activity. This method can be widely used for different types of electroactive microorganisms, including gram-negative bacteria, gram-positive bacteria, and fungi. Due to its time savings, accurate quantification and easy operation, this method provides a standard way to assess the role of EET ability.

Repurposing CRISPR RNA-guided integrases system for one-step, efficient genomic integration of ultra-long DNA sequences.

Genomic integration techniques offer opportunities for generation of engineered microorganisms with improved or even entirely new functions but are currently limited by inability for efficient insertion of long genetic payloads due to multiplexing. Herein, using Shewanella oneidensis MR-1 as a model, we developed an optimized CRISPR-associated transposase from cyanobacteria Scytonema hofmanni (ShCAST system), which enables programmable, RNA-guided transposition of ultra-long DNA sequences (30 kb) onto bacterial chromosomes at ∼100% efficiency in a single orientation. In this system, a crRNA (CRISPR RNA) was used to target multicopy loci like insertion-sequence elements or combining I-SceI endonuclease, thereby allowing efficient single-step multiplexed or iterative DNA insertions. The engineered strain exhibited drastically improved substrate diversity and extracellular electron transfer ability, verifying the success of this system. Our work greatly expands the application range and flexibility of genetic engineering techniques and may be readily extended to other bacteria for better controlling various microbial processes.

Novel Methanobacterium Strain Induces Severe Corrosion by Retrieving Electrons from Fe0 under a Freshwater Environment.

Methanogens capable of accepting electrons from Fe0 cause severe corrosion in anoxic conditions. In previous studies, all iron-corrosive methanogenic isolates were obtained from marine environments. However, the presence of methanogens with corrosion ability using Fe0 as an electron donor and their contribution to corrosion in freshwater systems is unknown. Therefore, to understand the role of methanogens in corrosion under anoxic conditions in a freshwater environment, we investigated the corrosion activities of methanogens in samples collected from groundwater and rivers. We enriched microorganisms that can grow with CO2/NaHCO3 and Fe0 as the sole carbon source and electron donor, respectively, in ground freshwater. Methanobacterium sp. TO1, which induces iron corrosion, was isolated from freshwater. Electrochemical analysis revealed that strain TO1 can uptake electrons from the cathode at lower than −0.61 V vs SHE and has a redox-active component with electrochemical potential different from those of other previously reported methanogens with extracellular electron transfer ability. This study indicated the corrosion risk by methanogens capable of taking up electrons from Fe0 in anoxic freshwater environments and the necessity of understanding the corrosion mechanism to contribute to risk diagnosis.

Electricity generation in simulated benthic microbial fuel cell with conductive polyaniline-polypyrole composite hydrogel anode

As a sustainable and eco-friendly technology, the benthic microbial fuel cell (BMFC) has been emerged as alternative and potential approach to recover electrical energy by microorganisms, but lower power density and poor long-term life hinder their practical application. Modification of anodic materials is an efficient strategy to solve this problem. Here, a conductive and biocompatible hydrogel electrode, polyaniline-polypyrole-CNTs-Fe3O4 (PANI-PPy-CNTs-Fe3O4) is prepared and applied as anode in a simulated BMFC. The maximum power density of the BMFC with PANI-PPy-CNTs-Fe3O4 hydrogel anode (5901.49 mW/m3) is 1.33, 2.15 and 2.06 times higher than that of PANI-PPy (4413.03 mW/m3), PANI and PPy anodes (2737.12 and 2859.53 mW/m3), respectively. The charge transfer resistance of quaternary hydrogel bioanode in BMFC (3.922 Ω) is much lower than that of the PANI (8.682 Ω), PPy (8.262 Ω) and PANI-PPy (5.772 Ω) hydrogel bioanodes. Moreover, the extracellular electron transfer ability is also enhanced on the composite hydrogel anode, which also exhibits the high biomass. High-throughput sequencing technology indicates that the synergistic effect of bacteria on the quaternary hydrogel made full use of organic matter as available fuel to enable a high electricity-generation anode. Such foregoing mainly ascribed to the composite hydrogel keep the high biocompatibility from components, while the addition of CNTs and Fe3O4 dramatically decreases the internal resistance and provides more electrochemical active surface area on basis of macroporous 3D hydrogel structure. This work explores the composite hydrogel used as anodes to construct high-performance BMFC and provides significant information for the applications of actual BMFCs in the future.

Plate-Based Kinetic Fluorescence Tests for High-Throughput Screening of Electrochemically Active Bacteria

Electrochemically active bacteria (EAB) have attracted considerable attention in various fields, such as wastewater treatment, energy recovery, and biogeochemistry. However, a rapid, simple, and precise method for the selection of EAB remains a challenge because of the lack of sensitive probes and rapid analytical methods. To solve this problem, riboflavin was adapted as a fluorescent probe and a program was designed to evaluate the extracellular electron transfer (EET) ability using kinetic fluorescence data. With this method, the EET abilities of 1152 strains isolated from environmental sources, including a municipal wastewater treatment plant, freshwater lake sediments, and submarine sediments, were tested, and 49 strains were identified as EAB. Hence, the results provide new insights into the abundance and roles of strains of EAB in different environmental and engineered water samples. Taken together, we developed a simple and practical method for quantitative evaluation of the EET ability of microbes and further rapid screening of EAB from environments.

Enhanced Bioreduction of Radionuclides by Driving Microbial Extracellular Electron Pumping with an Engineered CRISPR Platform.

Dissimilatory metal-reducing bacteria (DMRB) with extracellular electron transfer (EET) capability show great potential in bioremediating the subsurface environments contaminated by uranium through bioreduction and precipitation of hexavalent uranium [U(VI)]. However, the low EET efficiency of DMRB remains a bottleneck for their applications. Herein, we develop an engineered CRISPR platform to drive the extracellular electron pumping of Shewanella oneidensis, a representative DMRB species widely present in aquatic environments. The CRISPR platform allows for highly efficient and multiplex genome editing and rapid platform elimination post-editing in S. oneidensis. Enabled by such a platform, a genomic promoter engineering strategy (GPS) for genome-widely engineering the EET-encoding gene network was established. The production of electron conductive Mtr complex, synthesis of electron shuttle flavin, and generation of NADH as intracellular electron carrier are globally optimized and promoted, leading to a significantly enhanced EET ability. Applied to U(VI) bioreduction, the edited strains achieve up to 3.62-fold higher reduction capacity over the control. Our work endows DMRB with an enhanced ability to remediate the radionuclides-contaminated environments and provides a gene editing approach to handle the growing environmental challenges of radionuclide contaminations.

Disentangling the syntrophic electron transfer mechanisms of Candidatus geobacter eutrophica through electrochemical stimulation and machine learning

Interspecies hydrogen transfer (IHT) and direct interspecies electron transfer (DIET) are two syntrophy models for methanogenesis. Their relative importance in methanogenic environments is still unclear. Our recent discovery of a novel species Candidatus Geobacter eutrophica with the genetic potential of IHT and DIET may serve as a model species to address this knowledge gap. To experimentally demonstrate its DIET ability, we performed electrochemical enrichment of Ca. G. eutrophica-dominating communities under 0 and 0.4 V vs. Ag/AgCl based on the presumption that DIET and extracellular electron transfer (EET) share similar metabolic pathways. After three batches of enrichment, Geobacter OTU650, which was phylogenetically close to Ca. G. eutrophica, was outcompeted in the control but remained abundant and active under electrochemical stimulation, indicating Ca. G. eutrophica’s EET ability. The high-quality draft genome further showed high phylogenomic similarity with Ca. G. eutrophica, and the genes encoding outer membrane cytochromes and enzymes for hydrogen metabolism were actively expressed. A Bayesian network was trained with the genes encoding enzymes for alcohol metabolism, hydrogen metabolism, EET, and methanogenesis from dominant fermentative bacteria, Geobacter, and Methanobacterium. Methane production could not be accurately predicted when the genes for IHT were in silico knocked out, inferring its more important role in methanogenesis. The genomics-enabled machine learning modeling approach can provide predictive insights into the importance of IHT and DIET.

Engineering a Rhamnose-Inducible System to Enhance the Extracellular Electron Transfer Ability of Shewanella Genus for Improved Cr(VI) Reduction

Members of the Shewanella genus play important roles in bioenergy generation and environmental bioremediation, but their practical applications in engineered systems are still limited due to the lack of effective synthetic biology tools and parts such as promoters and expression systems. In this work, the rhaBAD promoter of Escherichia coli was introduced into various Shewanella species including S. oneidensis MR-1, S. decolorationis S12, and S. aquimarina SW-120 to enhance their extracellular pollutant transformation ability with Cr(VI) reduction as an example. The rhaBAD promoter had low leaky expression without l-rhamnose induction and exhibited a linear response to a wide concentration range. The inducer was nontoxic and could not be metabolizable by the Shewanella species. In addition, a unimodal induction of the rhaBAD promoter was found at the single-cell level. Applying this rhamnose-inducible system to the Shewanella species allowed for precise control on their extracellular electron transfer abilities to promote Cr(VI) transformation. The engineered strain induced by l-rhamnose reduced 98% Cr(VI) within 9 h, while only 59.1% Cr(VI) reduction was achieved by the control strain. Moreover, a 3-fold higher Cr(VI) reduction rate for the engineered strain induced by l-rhamnose confirms the improvement of its biotransformation ability. Our work provides an efficient regulation tool for engineering Shewanella species and may hopefully bring them a step further toward practical applications.

Energy Taxis toward Redox-Active Surfaces Decreases the Transport of Electroactive Bacteria in Saturated Porous Media.

The fate and transport of bacteria in porous media are essential for bioremediation and water quality control. However, the influence of biological activities like extracellular electron transfer (EET) and swimming motility toward granular media on cell transport remains unknown. Here, electroactive bacteria with higher Fe(III) reduction abilities were found to demonstrate greater retention in ferrihydrite-coated sand. Increasing the concentrations of the electron donor (1-10 mM lactate), shuttle (0-50 μM anthraquinone-2,6-disulfonate), and acceptor (ferrihydrite, MnO2, or biochar) under flow conditions significantly reduced Shewanella oneidensis MR-1's mobility through redox-active porous media. The deficiency of EET ability or flagellar motion and inhibition of intracellular proton motive force, all of which are essential for energy taxis, enhanced MR-1's transport. It was proposed that EET could facilitate MR-1 to sense, tactically move toward, and attach on redox-active media surface, eventually improving its retention. Positive linear correlations were established among parameters describing MR-1's energy taxis ability (relative taxis index), cell transport behavior (dispersion coefficient and relative change of effluent percentage), and redox activity of media surface (reduction potential or electron-accepting rate), providing novel insights into the critical impacts of bacterial microscale motility on macroscale cell transport through porous media.

Developing a population-state decision system for intelligently reprogramming extracellular electron transfer in Shewanella oneidensis

The unique extracellular electron transfer (EET) ability has positioned electroactive bacteria (EAB) as a major class of cellular chassis for genetic engineering aimed at favorable environmental, energy, and geoscience applications. However, previous efforts to genetically enhance EET ability have often impaired the basal metabolism and cellular growth due to the competition for the limited cellular resource. Here, we design a quorum sensing-based population-state decision (PSD) system for intelligently reprogramming the EET regulation system, which allows the rebalanced allocation of the cellular resource upon the bacterial growth state. We demonstrate that the electron output from Shewanella oneidensis MR-1 could be greatly enhanced by the PSD system via shifting the dominant metabolic flux from initial bacterial growth to subsequent EET enhancement (i.e., after reaching a certain population-state threshold). The strain engineered with this system achieved up to 4.8-fold EET enhancement and exhibited a substantially improved pollutant reduction ability, increasing the reduction efficiencies of methyl orange and hexavalent chromium by 18.8- and 5.5-fold, respectively. Moreover, the PSD system outcompeted the constant expression system in managing EET enhancement, resulting in considerably enhanced electron output and pollutant bioreduction capability. The PSD system provides a powerful tool for intelligently managing extracellular electron transfer and may inspire the development of new-generation smart bioelectrical devices for various applications.

Probing Microbial Extracellular Respiration Ability Using Riboflavin.

Electrochemically active bacteria (EAB) are capable of extracellular electron transfer (EET) to insoluble metal oxides, and thus play a great role in the fields of environment, energy, and geosciences. However, rapid and accurate quantification of the EET ability of EAB is still challenging. In this work, we develop a riboflavin-based fluorescence method for facile, accurate, and in situ measurement of the EET ability of EAB. This method is successfully used to quantify the single-cellular EET ability of Geobacter sulfurreducens DL-1 (60.29 ± 13.02 fA) and Shewanella oneidensis MR-1 (2.11 ± 0.47 fA), the two widely present EAB in the environment. It also enables quantitative identification of EET-related c-type cytochromes in the outer membrane of S. oneidensis MR-1. This method provides a useful tool to rapidly identify EAB in diverse environments and elucidate their electron transfer mechanisms.

Comparative facilitation of activated carbon and goethite on methanogenesis from volatile fatty acids

To provide insight into direct interspecies electron transfer (DIET) via carbon-based materials and ferric oxides, the effects of three conductive materials (i.e. activated carbon (AC), iron modified activated carbon (FEAC) and goethite (FEOOH)), on methanogenesis from volatile fatty acids (VFAs) were evaluated. Under the acid stress (~4 g/L VFAs), the maximum methane yield of 266 mL/g-chemical oxygen demand (COD) was found in the FEOOH supplemented reactor, which was 48% higher than that of AC reactor. The reasons for the enhanced activity of the electron transport chain and extracellular electron transfer ability by FEOOH include: 1) the activation on iron-containing enzymes that involved in methanogenesis and acidogenesis; 2) selective enrichment on functional microorganism. The higher electron donating capacities (EDC) value of FEOOH may be a triggering factor on the growth of Syntrophomonadaceae, which perform DIET with methanogens (Methanosaeta and Methanosarcina) for the syntrophic degradation of VFAs.

Quantum dot interactions with and toxicity to Shewanella oneidensis MR-1

Combining abiotic photosensitisers such as quantum dots (QDs) with non-photosynthetic bacteria presents an intriguing concept into the design of artificial photosynthetic organisms and solar-driven fuel production. Shewanella oneidensis MR-1 (MR-1) is a versatile bacterium concerning respiration, metabolism and biocatalysis, and is a promising organism for artificial photosynthesis as the bacterium’s synthetic and catalytic ability provides a potential system for bacterial biohydrogen production. MR-1’s hydrogenases are present in the periplasmatic space. It follows that for photoenergised electrons to reach these enzymes, QDs will need to be able to enter the periplasm, or electrons need to enter the periplasm via the Mtr pathway that is responsible for MR-1’s extracellular electron transfer ability. As a step towards this goal, various QDs were tested for their photo-reducing potential, nanotoxicology and further for their interaction with MR-1. CdTe/CdS/TGA, CdTe/CdS/Cysteamine, a commercial, negatively charged CdTe and CuInS2/ZnS/PMAL QDs were examined. The photoreduction potential of the QDs was confirmed by measuring their ability to photoreduce methyl viologen with different sacrificial electron donors. The commercial CdTe and CuInS2/ZnS/PMAL QDs showed no toxicity towards MR-1 as evaluated by a colony-forming units method and a fluorescence viability assay. Only the commercial negatively charged CdTe QDs showed good interaction with MR-1. With transmission electron microscopy, QDs were observed both in the cytoplasm and periplasm. These results inform on the possibilities and bottlenecks when developing bionanotechnological systems for the photosynthetic production of biohydrogen by MR-1.

Enhanced biomethane production from anthracite by application of an electric field

The utilization of biogenic coalbed methane (CBM) meets the increasing demand of energy and reduces the impact of greenhouse gases on the environment. Application of an electric field is a promising technology to increase biomethane production from various complex organic compounds. Here, the effects of an applied voltage (0.6 V, 1.2 V, and 1.8 V) on methane production due to coal biodegradation, as well as the structure of microflora and intermediates were studied by anaerobic culture, pyrosequencing, and GC–MS. The maximum methane production was observed at 1.2 V with an average of 63.0 μmol/g of coal, which was 134% higher than the control without electrodes (26.9 μmol/g of coal). The CO2 yield was negatively correlated with the methane yield, suggesting that more carbon in coal has been transferred to methane after application of an electric field. The pyrosequencing results showed that the abundance of bacteria with extracellular electron transfer abilities, such as Soehngenia, Desulfovibrio, and Deferrisoma, increased after applying a 1.2 V electric field. Moreover, the structure of the fungal communities changed significantly, especially the abundance of Mortierella, Thielavia, Humicola, and Plectosphaerella. According to the GC-MS results, most of the products detected at the end of the cultivation period in the control were not detected in the cultures cultivated at 1.2 V. New organic compounds such as alcohols, unsaturated fatty acids, and amines were produced in the cultures cultivated at 1.2 V. These results suggest that application of an electric field changes the structure of the microflora, which promotes extracellular electron transfer and the biodegradation of organic compounds, thus enhancing methane production from coal. Furthermore, the distinct organic products detected after cultivation and the significant changes in microbial communities suggest that the degrading metabolism of organic matter in coal might be altered due to the application of an electric field. Based on the results, a biogenic methane production system implemented by application of an electric field was proposed and described.