Biological iron-monosulfide production for efficient electricity harvesting from a deep-sea metal-reducing bacterium

Efficient sulfur cycling improved the performance of flowback water treatment in a microbial fuel cell

The sulfate-reducing mediate microbial fuel cell (MFC) shows advantages in treating recalcitrant flowback water (FW) from shale gas extraction, but the stability under fluctuant concentrations of sulfate in FW remains unknown. Herein, we investigated the impact of fluctuant sulfate concentrations on the performance of FW treatment in MFCs. Sulfate concentration showed a significant role in the MFC treating FW, with a COD removal of 69.8 ± 9.7% and a peak power density of 2164 ± 396 mW/m³ under 247.5 mg/L sulfate, but only 39.1% and 1216 mW/m³ under 50 mg/L sulfate. The fluctuation of sulfate in a short time allowed to a stable performance, but a longtime intermittent decrease of feeding sulfate concentration significantly inhibited power generation to no more than 512 mW/m³. The sulfur cycling between sulfate and sulfide existed in the system, but the cycling rate became much lower after the longtime intermittent decrease, with resulting to the decreased power generation. Abundant sulfur-oxidizing bacteria (SOB) of Desulfuromonadaceae and Helicobacteraceae in the MFC stably feeding with 247.5 mg/L sulfate supported a high sulfur cycling rate. With the cooperation of abundant sulfate-reducing bacteria (SRB) of Desulfovibrionaceae (capable of producing electricity) on the anode and Desulfobacteraceae in anolyte, this sulfur cycling endowed the MFC with high sulfate tolerance and critically contributed to recalcitrant organics removal and power generation. However, much less SOB of Helicobacteraceae and Campylobacteraceae on the anode with high S⁰ accumulation on the surface after the longtime intermittent decrease of sulfate likely led to the low sulfur cycling rate. With also less SRB of Marinilabiaceae (capable of producing electricity) and Synergistaceae in the system, this low sulfur cycling rate thus hampered power generation. This research provides an important reference for the bioelectrochemical treatment of wastewater containing recalcitrant organics and sulfate.

Enrichment of sulfur-oxidizing bacteria using S-doped NiFe2O4 nanosheets as the anode in microbial fuel cell enhances power production and sulfur recovery

Microbial fuel cells (MFCs) have great promise for power generation by oxidizing organic wastewater, yet the challenge to realize high efficiency in simultaneous energy production and resource recovery remains. In this study, we designed a novel MFC anode by synthesizing S-doped NiFe₂O₄ nanosheet arrays on carbon cloth (S10-NiFe₂O₄@CC) to build a three-dimensional (3D) hierarchically porous structure, with the aim to regulate the microbial community of sulfur-cycling microbes in order to enhance power production and elemental sulfur (S⁰) recovery. The S10-NiFe₂O₄@CC anode obtained a faster start-up time of 2 d and the highest power density of 4.5 W/m² in acetate-fed and mixed bacteria-based MFCs. More importantly, sulfide removal efficiency (98.3 %) (initial concentration of 50 mg/L S^2-) could be achieved within 3 d and sulfur (S₈) could be produced. Microbial community analysis revealed that the S10-NiFe₂O₄@CC anode markedly enriched sulfur-oxidizing bacteria (SOB) and promoted enrichment of SOB and sulfate-reducing bacteria (SRB) in the bulk solution as well, leading to the enhancement of power generation and S⁰ recovery. This study shows how carefully designing and optimizing the composition and structure of the anode can lead to the enrichment of a multifunctional microbiota with excellent potential for sulfide removal and resource recovery.

S-Doped NiFe2O4 Nanosheets Regulated Microbial Community of Suspension for Constructing High Electroactive Consortia

Iron-based nanomaterials (NMs) are increasingly used to promote extracellular electron transfer (EET) for energy production in bioelectrochemical systems (BESs). However, the composition and roles of planktonic bacteria in the solution regulated by iron-based NMs have rarely been taken into account. Herein, the changes of the microbial community in the solution by S-doped NiFe₂O₄ anodes have been demonstrated and used for constructing electroactive consortia on normal carbon cloth anodes, which could achieve the same level of electricity generation as NMs-mediated biofilm, as indicated by the significantly high voltage response (0.64 V) and power density (3.5 W m⁻²), whereas with different microbial diversity and connections. Network analysis showed that the introduction of iron-based NMs made Geobacter positively interact with f_Rhodocyclaceae, improving the competitiveness of the consortium (Geobacter and f_Rhodocyclaceae). Additionally, planktonic bacteria regulated by S-doped anode alone cannot hinder the stimulation of Geobacter by electricity and acetate, while the assistance of lining biofilm enhanced the cooperation of sulfur-oxidizing bacteria (SOB) and fermentative bacteria (FB), thus promoting the electroactivity of microbial consortia. This study reveals the effect of S-doped NiFe₂O₄ NMs on the network of microbial communities in MFCs and highlights the importance of globality of microbial community, which provides a feasible solution for the safer and more economical environmental applications of NMs.

Defined and unknown roles of conductive nanoparticles for the enhancement of microbial current generation: A review

The ability of various bacteria to make use of solid substrates through extracellular electron transfer (EET) or extracellular electron uptake (EEU) has enabled the development of valuable biotechnologies such as microbial fuel cells (MFCs) and microbial electrosynthesis (MES). It is common practice to use metallic and semiconductive nanoparticles (NPs) for microbial current enhancement. However, the effect of NPs is highly variable between systems, and there is no clear guideline for effectively increasing the current generation. In the present review, the proposed mechanisms for enhancing current production in MFCs and MES are summarized, and the critical factors for NPs to enhance microbial current generation are discussed. Implications for microbially induced iron corrosion, where iron sulfide NPs are proposed to enhance the rate of EEU, photochemically driven MES, and several future research directions to further enhance microbial current generation, are also discussed.

Biogenic iron sulfide functioning as electron-mediating interface to accelerate dissimilatory ferrihydrite reduction by Shewanella oneidensis MR-1

Microbially driven iron and sulfur geochemical cycles co-exist ubiquitously in subsurface environments and are of environmental relevance. Shewanella species (dissimilatory metal-reducing bacteria) are capable of reducing Fe(III)-(oxyhydr)oxide minerals and diverse sulfur sources using corresponding metabolic pathways and producing FeS secondary minerals. In spite of the ability in promoting bacterial extracellular electron transfer (EET), the specific role of FeS in mediating EET between microbe/mineral interface is still unclear. In this work, the electron-mediating function of biogenic FeS on promoting the reduction of ferrihydrite by S. oneidensis MR-1 using thiosulfate as sulfur source was investigated in terms of Fe(III) reduction percentage, X-ray diffraction and scanning electron microscopy. The results showed that the microbial ferrihydrite reduction was pH-dependent and positively correlated with the addition of thiosulfate. In the presence of thiosulfate, biogenic FeS in nano-scale were formed and deposited on the surfaces of S. oneidensis MR-1 and ferrihydrite to build an interfacial electron transfer bridge between them. The addition of either thiosulfate and in-vitro FeS could rescue the entirely inactivated ability of the mutant (△omcA/mtrC) in ferrihydrite reduction to some extent, but which was obviously inferior to the wild-type strain. Meanwhile, the effect of the biogenic FeS in-situ coating on the surfaces of S. oneidensis MR-1 cells on promoting microbial ferrihydrite reduction was significantly superior to the in-vitro ones. Thus, the in-situ formed biogenic FeS secondary minerals were demonstrated to mediate and accelerate interfacial electron transfer from S. oneidensis MR-1 cells to ferrihydrite through interfacing with the bacterial EET routes, especially Mtr pathway. This work provides an insight into the secondary minerals-mediating interfacial electron transfer between microbes and minerals in the presence of biological S (-II), which has important biogeochemical and environmental implications.

Exploring the Biological Effect of Biosynthesized Au-Pd Core-Shell Nanoparticles through an Untargeted Metabolomics Approach

The biosynthesis of Au-Pd core-shell nanoparticles (NPs) with wild-type Escherichia coli (Au-Pd/E. coli) is an excellent newly established, environmentally friendly synthetic method for the fabrication of nanomaterials compared to traditional chemosynthesis. However, there is insufficient detailed bioinformation on the compatibility, metabolic process, and mechanism of this approach. Metabolomics approaches have provided an excellent alternative to numerous bioinformatics approaches for shedding light on the biological response of an organism exposed to external stimuli at the molecular level. In this study, two different doses (8 and 80 μg/mL) of Au-Pd/E. coli were applied to treat human umbilical vein endothelial cells (HUVECs). Gas chromatography/mass spectrometry coupled with bioinformatics was used to analyze the changes in the HUVEC metabolome after treatment. The results indicated the occurrence of nonsignificant acute cytotoxicity based on cell proliferation and apoptosis analysis, while high concentrations (80 μg/mL) of Au-Pd/E. coli induced dramatic changes in energy metabolism, revealing a notable inhibition of the tricarboxylic acid (TCA) cycle along with the enhancement of glycolysis, the pentose phosphate pathway, fatty acid biosynthesis, and lipid accumulation, which was correlated with mitochondrial dysfunction. The metabolomics results obtained for this novel Au-Pd/E. coli-cell system could broaden our knowledge of the biological effect of Au-Pd/E. coli and possibly reveal material modifications and technological innovations.

Iron Oxide-Modified Carbon Electrode and Sulfate-Reducing Bacteria for Simultaneous Enhanced Electricity Generation and Tannery Wastewater Treatment

The microbial fuel cell (MFC) is emerging as a potential technology for extracting energy from wastes/wastewater while they are treated. The major hindrance in MFC commercialization is lower power generation due to the sluggish transfer of electrons from the biocatalyst (bacteria) to the anode surface and inefficient microbial consortia for treating real complex wastewater. To overcome these concerns, a traditional carbon felt (CF) electrode modification was carried out by iron oxide (Fe₃O₄) nanoparticles via facile dip-and-dry methods, and mixed sulfate-reducing bacteria (SRBs) were utilized as efficient microbial consortia. In the modified CF electrode with SRBs, a considerable improvement in the bioelectrochemical operation was observed, where the power density (309 ± 13 mW/m²) was 1.86 times higher than bare CF with SRBs (166 ± 11 mW/m²), suggesting better bioelectrochemical performance of an SRB-enriched Fe₃O₄@CF anode in the MFC. This superior activity can be assigned to the lower charge transfer resistance, higher conductance, and increased number of catalytic sites of the Fe₃O₄@CF electrode. The SRB-enriched Fe₃O₄@CF anode also assists in enhancing MFC performance in terms of COD removal (>75%), indicating efficient biodegradability of tannery wastewater and a higher electron transfer rate from SRBs to the conductive anode. These findings demonstrate that a combination of the favorable properties of nanocomposites such as Fe₃O₄@CF anodes and efficient microbes for treating complex wastes can encourage new directions for renewable energy–related applications.

Insights into the Biosynthesis of Nanoparticles by the Genus Shewanella

The exploitation of microorganisms for the fabrication of nanoparticles (NPs) has garnered considerable research interest globally. The microbiological transformation of metals and metal salts into respective NPs can be achieved under environmentally benign conditions, offering a more sustainable alternative to chemical synthesis methods. Species of the metal-reducing bacterial genus Shewanella are able to couple the oxidation of various electron donors, including lactate, pyruvate, and hydrogen, to the reduction of a wide range of metal species, resulting in biomineralization of a multitude of metal NPs. Single-metal-based NPs as well as composite materials with properties equivalent or even superior to physically and chemically produced NPs have been synthesized by a number of Shewanella species. A mechanistic understanding of electron transfer-mediated bioreduction of metals into respective NPs by Shewanella is crucial in maximizing NP yields and directing the synthesis to produce fine-tuned NPs with tailored properties. In addition, thorough investigations into the influence of process parameters controlling the biosynthesis is another focal point for optimizing the process of NP generation. Synthesis of metal-based NPs using Shewanella species offers a low-cost, eco-friendly alternative to current physiochemical methods. This article aims to shed light on the contribution of Shewanella as a model organism in the biosynthesis of a variety of NPs and critically reviews the current state of knowledge on factors controlling their synthesis, characterization, potential applications in different sectors, and future prospects.

Carbon-Based Composites as Anodes for Microbial Fuel Cells: Recent Advances and Challenges

Owing to the low price, chemical stability and good conductivity, carbon-based materials have been extensively applied as the anode in microbial fuel cells (MFCs). In this review, apart from the charge storage mechanism and anode requirements, the major work focuses on five categories of carbon-based anode materials (traditional carbon, porous carbon, nano-carbon, metal/carbon composite and polymer/carbon composite). The relationship is demonstrated in depth between the physicochemical properties of the anode surface/interface/bulk (porosity, surface area, hydrophilicity, partical size, charge, roughness, etc.) and the bioelectrochemical performances (electron transfer, electrolyte diffusion, capacitance, toxicity, start-up time, current, power density, voltage, etc.). An outlook for future work is also proposed.

Bacterial extracellular electron transfer: a powerful route to the green biosynthesis of inorganic nanomaterials for multifunctional applications

Synthesis of inorganic nanomaterials such as metal nanoparticles (MNPs) using various biological entities as smart nanofactories has emerged as one of the foremost scientific endeavors in recent years. The biosynthesis process is environmentally friendly, cost-effective and easy to be scaled up, and can also bring neat features to products such as high dispersity and biocompatibility. However, the biomanufacturing of inorganic nanomaterials is still at the trial-and-error stage due to the lack of understanding for underlying mechanism. Dissimilatory metal reduction bacteria, especially Shewanella and Geobacter species, possess peculiar extracellular electron transfer (EET) features, through which the bacteria can pump electrons out of their cells to drive extracellular reduction reactions, and have thus exhibited distinct advantages in controllable and tailorable fabrication of inorganic nanomaterials including MNPs and graphene. Our aim is to present a critical review of recent state-of-the-art advances in inorganic biosynthesis methodologies based on bacterial EET using Shewanella and Geobacter species as typical strains. We begin with a brief introduction about bacterial EET mechanism, followed by reviewing key examples from literatures that exemplify the powerful activities of EET-enabled biosynthesis routes towards the production of a series of inorganic nanomaterials and place a special emphasis on rationally tailoring the structures and properties of products through the fine control of EET pathways. The application prospects of biogenic nanomaterials are then highlighted in multiple fields of (bio-) energy conversion, remediation of organic pollutants and toxic metals, and biomedicine. A summary and outlook are given with discussion on challenges of bio-manufacturing with well-defined controllability.

Biosynthetic FeS/BC hybrid particles enhanced the electroactive bacteria enrichment in microbial electrochemical systems

Modifying the surface of an anode can improve electroactive bacteria (EAB) enrichment, thereby enhancing the performance of the associated microbial electrochemical systems (MESs). In this study, biosynthetic FeS nanoparticles were used to modify the anode in MESs. The experimental results demonstrated that the stable maximum voltage of the FeS composited biochar (FeS/BC)-modified anode reached 0.72 V, which is 20% higher than that of the control. The maximum power density with the FeS/BC anode was 793 mW/m², which is 46.31% higher than that obtained with the control (542 mW/m²). According to cyclic voltammetry (CV) analysis, FeS/BC facilitates the direct electron transfer between bacteria and the electrode. The biomass protein concentration of the FeS/BC anode was 841.75 μg/cm², which is almost 1.5 times higher than that of the carbon cloth anode (344.25 μg/cm²); hence, FeS/BC modification can promote biofilm formation. The composition of Geobacter species on the FeS/BC anode (75.16%) was much higher than that on the carbon cloth anode (4.81%). All the results demonstrated that the use of the biosynthetic FeS/BC anode is an environmentally friendly and efficient strategy for enhancing the electroactive biofilm formation and EAB enrichment in MESs.

Nanomaterials Facilitating Microbial Extracellular Electron Transfer at Interfaces

Electrochemically active bacteria can transport their metabolically generated electrons to anodes, or accept electrons from cathodes to synthesize high-value chemicals and fuels, via a process known as extracellular electron transfer (EET). Harnessing of this microbial EET process has led to the development of microbial bio-electrochemical systems (BESs), which can achieve the interconversion of electrical and chemical energy and enable electricity generation, hydrogen production, electrosynthesis, wastewater treatment, desalination, water and soil remediation, and sensing. Here, the focus is on the current understanding of the microbial EET process occurring at both the bacteria-electrode interface and the biotic interface, as well as some attempts to improve the EET by using various nanomaterials. The behavior of nanomaterials in different EET routes and their influence on the performance of BESs are described. The inherent mechanisms will guide rational design of EET-related materials and lead to a better understanding of EET mechanisms.

Biosynthesized iron sulfide nanoparticles by mixed consortia for enhanced extracellular electron transfer in a microbial fuel cell

The bioanode of mixed consortia was for the first time used to in-situ synthesize iron sulfide nanoparticles in a microbial fuel cell (MFC) over a long-term period (46 days). These poorly crystalline nanoparticles with an average size of 29.97 ± 7.1 nm, comprising of FeS and FeS₂, significantly promoted extracellular electron transfer and thus the electricity generation of the MFC. A maximum power density of 519.00 mW/m² was obtained from the MFC, which was 1.92 times as high as that of the control. The cell viability was promoted by a small amount of iron sulfide nanoparticles but inhibited by the thick nanoparticle "shell" covered on the bacterial cells. Some electroactive and sulfur reducing bacteria (eg. Enterobacteriaceae, Desulfovibrio, and Geobacter) were specifically enriched on the anode. This study provides a novel insight for improving the performance of bioelectrochemical systems through in-situ sustainable nanomaterials biofabrication by mixed consortia.

Simultaneous removal of organic matter and iron from hydraulic fracturing flowback water through sulfur cycling in a microbial fuel cell

The high volume of flowback water (FW) generated during shale gas exploitation is highly saline, and contains complex organics, iron, heavy metals, and sulfate, thereby posing a significant challenge for the environmental management of the unconventional natural gas industry. Herein, the treatment of FW in a sulfur-cycle-mediated microbial fuel cell (MFC) is reported. Simultaneous removal efficiency for chemical oxygen demand (COD) and total iron from a synthetic FW was achieved, at 72 ± 7% and 90.6 ± 8.7%, respectively, with power generation of 2667 ± 529 mW/m³ in a closed-circuit MFC (CC-MFC). However, much lower iron removal (38.5 ± 4.5%) occurred in the open-circuit MFC (OC-MFC), where the generated FeS fine did not precipitate because of sulfide supersaturation. Enrichment of both sulfur-oxidizing bacteria (SOB), namely Helicobacteraceae in the anolyte and the electricity-producing bacteria, namely Desulfuromonadales on the anode likely accelerated the sulfur cycle through the biological and bioelectrochemical oxidation of sulfide in the anodic chamber, and effectively increased the molar ratio of total iron to sulfide, thus alleviating sulfide supersaturation in the closed circuitry. Enrichment of SOB in the anolyte might be attributed to the formation of FeS electricity wire and likely contributed to the stable high power generation. Bacteroidetes, Firmicutes, Proteobacteria, and Chloroflexi enriched in the anodic chamber were responsible for degrading complex organics in the FW. The treatment of real FW in the sulfur-cycle-mediated MFC also achieved high efficiency. This research provides a promising approach for the treatment of wastewater containing organic matters, heavy metals, and sulfate by using a sulfur-cycle-mediated MFC.

Enhancement of methanogenesis by electric syntrophy with biogenic iron-sulfide minerals

Recent studies have shown that interspecies electron transfer between chemoheterotrophic bacteria and methanogenic archaea can be mediated by electric currents flowing through conductive iron oxides, a process termed electric syntrophy. In this study, we conducted enrichment experiments with methanogenic microbial communities from rice paddy soil in the presence of ferrihydrite and/or sulfate to determine whether electric syntrophy could be enabled by biogenic iron sulfides. Although supplementation with either ferrihydrite or sulfate alone suppressed methanogenesis, supplementation with both ferrihydrite and sulfate enhanced methanogenesis. In the presence of sulfate, ferrihydrite was transformed into black precipitates consisting mainly of poorly crystalline iron sulfides. Microbial community analysis revealed that a methanogenic archaeon and iron- and sulfate-reducing bacteria (Methanosarcina, Geobacter, and Desulfotomaculum, respectively) predominated in the enrichment culture supplemented with both ferrihydrite and sulfate. Addition of an inhibitor specific for methanogenic archaea decreased the abundance of Geobacter, but not Desulfotomaculum, indicating that Geobacter acquired energy via syntrophic interaction with methanogenic archaea. Although electron acceptor compounds such as sulfate and iron oxides have been thought to suppress methanogenesis, this study revealed that coexistence of sulfate and iron oxide can promote methanogenesis by biomineralization of (semi)conductive iron sulfides that enable methanogenesis via electric syntrophy.

FeS2 Nanoparticles Decorated Graphene as Microbial-Fuel-Cell Anode Achieving High Power Density

Microbial fuel cells (MFCs) have received great attention worldwide due to their potential in recovering electrical energy from waste and inexhaustible biomass. Unfortunately, the difficulty of achieving the high power, especially in real samples, remains a bottleneck for their practical applications. Herein, FeS₂ nanoparticles decorated graphene is fabricated via a simple hydrothermal reaction. The FeS₂ nanoparticles decorated graphene anode not only benefits bacterial adhesion and enrichment of electrochemically active Geobacter species on the electrode surface but also promotes efficient extracellular electron transfer, thus giving rise to a fast start-up time of 2 d, an unprecedented power density of 3220 mW m^-2 and a remarkable current density of 3.06 A m^-2 in the acetate-feeding and mixed bacteria-based MFCs. Most importantly, the FeS₂ nanoparticles decorated graphene anode successfully achieves a power density of 310 mW m^-2 with simultaneous removal of 1319 ± 28 mg L^-1 chemical oxygen demand in effluents from a beer factory wastewater. The characteristics of improved power generation and enhanced pollutant removal efficiency opens the door toward development of high-performance MFCs via rational anode design for practical application.

Nanostructured interfaces for probing and facilitating extracellular electron transfer

Probing and facilitating microbial extracellular electron transfer through nanotechnology enabled platforms are transforming bioenergetic, bioelectronic, and other related research areas.

Stay connected: Electrical conductivity of microbial aggregates

The discovery of direct extracellular electron transfer offers an alternative to the traditional understanding of diffusional electron exchange via small molecules. The establishment of electronic connections between electron donors and acceptors in microbial communities is critical to electron transfer via electrical currents. These connections are facilitated through conductivity associated with various microbial aggregates. However, examination of conductivity in microbial samples is still in its relative infancy and conceptual models in terms of conductive mechanisms are still being developed and debated. The present review summarizes the fundamental understanding of electrical conductivity in microbial aggregates (e.g. biofilms, granules, consortia, and multicellular filaments) highlighting recent findings and key discoveries. A greater understanding of electrical conductivity in microbial aggregates could facilitate the survey for additional microbial communities that rely on direct extracellular electron transfer for survival, inform rational design towards the aggregates-based production of bioenergy/bioproducts, and inspire the construction of new synthetic conductive polymers.

Electron Transfer between Electrically Conductive Minerals and Quinones

Long-distance electron transfer in marine environments couples physically separated redox half-reactions, impacting biogeochemical cycles of iron, sulfur and carbon. Bacterial bio-electrochemical systems that facilitate electron transfer via conductive filaments or across man-made electrodes are well-known, but the impact of abiotic currents across naturally occurring conductive and semiconductive minerals is poorly understood. In this paper I use cyclic voltammetry to explore electron transfer between electrodes made of common iron minerals (magnetite, hematite, pyrite, pyrrhotite, mackinawite, and greigite), and hydroquinones—a class of organic molecules found in carbon-rich sediments. Of all tested minerals, only pyrite and magnetite showed an increase in electric current in the presence of organic molecules, with pyrite showing excellent electrocatalytic performance. Pyrite electrodes performed better than commercially available glassy carbon electrodes and showed higher peak currents, lower overpotential values and a smaller separation between oxidation and reduction peaks for each tested quinone. Hydroquinone oxidation on pyrite surfaces was reversible, diffusion controlled, and stable over a large number of potential cycles. Given the ubiquity of both pyrite and quinones, abiotic electron transfer between minerals and organic molecules is likely widespread in Nature and may contribute to several different phenomena, including anaerobic respiration of a wide variety of microorganisms in temporally anoxic zones or in the proximity of hydrothermal vent chimneys, as well as quinone cycling and the propagation of anoxic zones in organic rich waters. Finally, interactions between pyrite and quinones make use of electrochemical gradients that have been suggested as an important source of energy for the origins of life on Earth. Ubiquinones and iron sulfide clusters are common redox cofactors found in electron transport chains across all domains of life and interactions between quinones and pyrite might have been an early analog of these ubiquitous systems.

Facile One-Step Strategy for Highly Boosted Microbial Extracellular Electron Transfer of the Genus Shewanella

High performance of bacterial extracellular electron transfer (EET) is essentially important for its practical applications in versatile bioelectric fields. We developed a facile one-step approach to dramatically boost the bacterial EET activity 75-fold by exogenous addition of ethylenediamine tetraacetic acid disodium salt (EDTA-2Na, 1 mM) into the electrochemical cells, where the anodic process of microbial EET was monitored. We propose that EDTA-2Na enables both the alternation of the local environment around the c-type cytochromes located on the outer membrane channels (OMCs), which therefore changes the redox behavior of OMCs in mediating the EET process, and the formation of densely packed biofilm that can further facilitate the EET process. As a synergistic effect, the highly boosted bacterial EET activity was achieved. The method shows good generality for versatile bioelectrical bacteria. We envision that the method is also applicable for constructing various bioelectric devices.

Millimeter scale electron conduction through exoelectrogenic mixed species biofilms

The functioning of many natural and engineered environments is dependent on long distance electron transfer mediated through electrical currents. These currents have been observed in exoelectrogenic biofilms and it has been proposed that microbial biofilms can mediate electron transfer via electrical currents on the centimeter scale. However, direct evidence to confirm this hypothesis has not been demonstrated and the longest known electrical transfer distance for single species exoelectrogenic biofilms is limited to 100 μm. In the present study, biofilms were developed on electrodes with electrically non-conductive gaps from 50 μm to 1 mm and the in situ conductance of biofilms was evaluated over time. Results demonstrated that the exoelectrogenic mixed species biofilms in the present study possess the ability to transfer electrons through electrical currents over a distance of up to 1 mm, 10 times further than previously observed. Results indicate the possibility of interspecies interactions playing an important role in the spatial development of exoelectrogenic biofilms, suggesting that these biological networks might remain conductive even at longer distance. These findings have significant implications in regards to future optimization of microbial electrochemical systems.

Microbial extracellular electron transfer and its relevance to iron corrosion

Extracellular electron transfer (EET) is a microbial metabolism that enables efficient electron transfer between microbial cells and extracellular solid materials. Microorganisms harbouring EET abilities have received considerable attention for their various biotechnological applications, including bioleaching and bioelectrochemical systems. On the other hand, recent research revealed that microbial EET potentially induces corrosion of iron structures. It has been well known that corrosion of iron occurring under anoxic conditions is mostly caused by microbial activities, which is termed as microbiologically influenced corrosion (MIC). Among diverse MIC mechanisms, microbial EET activity that enhances corrosion via direct uptake of electrons from metallic iron, specifically termed as electrical MIC (EMIC), has been regarded as one of the major causative factors. The EMIC-inducing microorganisms initially identified were certain sulfate-reducing bacteria and methanogenic archaea isolated from marine environments. Subsequently, abilities to induce EMIC were also demonstrated in diverse anaerobic microorganisms in freshwater environments and oil fields, including acetogenic bacteria and nitrate-reducing bacteria. Abilities of EET and EMIC are now regarded as microbial traits more widespread among diverse microbial clades than was thought previously. In this review, basic understandings of microbial EET and recent progresses in the EMIC research are introduced.

Hydrophilicity boosted extracellular electron transfer in Shewanella loihica PV-4

A superhydrophilic electrode enables the drastically boosted bacterial EET activity ofShewanella loihicaPV-4. It is proposed that a hydrophilic electrode favors the reduced state of OMCs, and consequently both the EET activity and cell proliferation are highly facilitated.

Nanotechnology to rescue bacterial bidirectional extracellular electron transfer in bioelectrochemical systems

Advanced nanostructured electrode materials largely improve the bacterial bidirectional extracellular electron transfer in bioelectrochemical systems.

Facilitated extracellular electron transfer of Shewanella loihica PV-4 by antimony-doped tin oxide nanoparticles as active microelectrodes

Dissimilatory metal reducing bacteria are capable of extracellular electron transfer (EET) to insoluble metal oxides as external electron acceptors for their anaerobic respiration, which is recognized as an important energy-conversion process in natural and engineered environments, such as in mineral cycling, bioremediation, and microbial fuel/electrolysis cells. However, the low EET efficiency remains one of the major bottlenecks for its practical application. We report firstly that the microbial current generated by Shewanella loihica PV-4 (S. loihica PV-4) could be greatly improved that is up to ca. 115 fold, by adding antimony-doped tin oxide (ATO) nanoparticles in the electrochemical reactor. The results demonstrate that the biocompatible, electrically conductive ATO nanoparticles acted as active microelectrodes could facilitate the formation of a cells/ATO composite biofilm and the reduction of the outer membrane c-type cytochromes (OM c-Cyts) that are beneficial for the electron transfer from cells to electrode. Meanwhile, a synergistic effect between the participation of OM c-Cyts and the accelerated EET mediated by cell-secreted flavins may play an important role for the enhanced current generation in the presence of ATO nanoparticles. Moreover, it is worth noting that the TCA cycle in S. loihica PV-4 cells is activated by adding ATO nanoparticles, even if the potential is poised at +0.2 V, thereby also improving the EET process. The results presented here may provide a simple and effective strategy to boost the EET of S. loihica PV-4 cells, which is conducive to providing potential applications in bioelectrochemical systems.

Abiotic reductive dechlorination of cis-DCE by ferrous monosulfide mackinawite

Cis-1,2,-dichloroethylene (cis-DCE) is a toxic, persistent contaminant occurring mainly as a daughter product of incomplete degradation of perchloroethylene (PCE) and trichloroethylene (TCE). This paper reports on abiotic reductive dechlorination of cis-DCE by mackinawite (FeS1-x), a ferrous monosulfide, under variable geochemical conditions. To assess in situ abiotic cis-DCE dechlorination by mackinawite in the field, mackinawite suspensions prepared in a field groundwater sample collected from a cis-DCE contaminated field site were used for dechlorination experiments. The effects of geochemical variables on the dechlorination rates were monitored. A set of dechlorination experiments were also carried out in the presence of aquifer sediment from the site over a range of pH conditions to better simulate the actual field situations. The results showed that the suspensions of freshly prepared mackinawite reductively transformed cis-DCE to acetylene, whereas the conventionally prepared powder form of mackinawite had practically no reactivity with cis-DCE under the same experimental conditions. Significant cis-DCE degradation by mackinawite has not been reported prior to this study, although mackinawite has been shown to reductively transform PCE and TCE. This study suggests feasibility of using mackinawite for in situ remediation of cis-DCE-contaminated sites with high S levels such as estuaries under naturally achieved or stimulated sulfate-reducing conditions.

Sulfur-Mediated Electron Shuttling Sustains Microbial Long-Distance Extracellular Electron Transfer with the Aid of Metallic Iron Sulfides

In addition to serving as an energy source for microbial growth, iron sulfides are proposed to act as naturally occurring electrical wires that mediate long-distance extracellular electron transfer (EET) and bridge spatially discrete redox environments. These hypothetical EET reactions stand on the abilities of microbes to use the interfacial electrochemistry of metallic/semiconductive iron sulfides to maintain metabolisms; however, the mechanisms of these phenomena remain unexplored. To obtain insight into EET to iron sulfides, we monitored EET at the interface between Shewanella oneidensis MR-1 cells and biomineralized iron sulfides in an electrochemical cell. Respiratory current steeply increased with the concomitant formation of poorly crystalline mackinawite (FeS) minerals, indicating that S. oneidensis has the ability to exploit extracellularly formed metallic FeS for long-distance EET. Deletion of major proteins of the metal-reduction (Mtr) pathway (OmcA, MtrC, CymA, and PilD) caused only subtle effects on the EET efficiency, a finding that sharply contrasts the majority of studies that report that the Mtr pathway is indispensable for the reduction of metal oxides and electrodes. The gene expression analyses of polysulfide and thiosulfate reductase suggest the existence of a sulfur-mediated electron-shuttling mechanism by which HS(-) ions and water-soluble polysulfides (HS(n)(-), where n ≥ 2) generated in the periplasmic space deliver electrons from cellular metabolic processes to cell surface-associated FeS. The finding of this Mtr-independent pathway indicates that polysulfide reductases complement the function of outer-membrane cytochromes in EET reactions and, thus, significantly expand the number of microbial species potentially capable of long-distance EET in sulfur-rich anoxic environments.

Enriching distinctive microbial communities from marine sediments via an electrochemical-sulfide-oxidizing process on carbon electrodes

Sulfide is a common product of marine anaerobic respiration, and a potent reactant biologically and geochemically. Here we demonstrate the impact on microbial communities with the removal of sulfide via electrochemical methods. The use of differential pulse voltammetry revealed that the oxidation of soluble sulfide was seen at +30 mV (vs. SHE) at all pH ranges tested (from pH = 4 to 8), while non-ionized sulfide, which dominated at pH = 4 was poorly oxidized via this process. Two mixed cultures (CAT and LA) were enriched from two different marine sediments (from Catalina Island, CAT; from the Port of Los Angeles, LA) in serum bottles using a seawater medium supplemented with lactate, sulfate, and yeast extract, to obtain abundant biomass. Both CAT and LA cultures were inoculated in electrochemical cells (using yeast-extract-free seawater medium as an electrolyte) equipped with carbon-felt electrodes. In both cases, when potentials of +630 or +130 mV (vs. SHE) were applied, currents were consistently higher at +630 then at +130 mV, indicating more sulfide being oxidized at the higher potential. In addition, higher organic-acid and sulfate conversion rates were found at +630 mV with CAT, while no significant differences were found with LA at different potentials. The results of microbial-community analyses revealed a decrease in diversity for both CAT and LA after electrochemical incubation. In addition, some bacteria (e.g., Clostridium and Arcobacter) not well-known to be capable of extracellular electron transfer, were found to be dominant in the electrochemical cells. Thus, even though the different mixed cultures have different tolerances for sulfide, electrochemical-sulfide removal can lead to major population changes.

Nanoparticle facilitated extracellular electron transfer in microbial fuel cells

Microbial fuel cells (MFCs) have been the focus of substantial research interest due to their potential for long-term, renewable electrical power generation via the metabolism of a broad spectrum of organic substrates, although the low power densities have limited their applications to date. Here, we demonstrate the potential to improve the power extraction by exploiting biogenic inorganic nanoparticles to facilitate extracellular electron transfer in MFCs. Simultaneous short-circuit current recording and optical imaging on a nanotechnology-enabled platform showed substantial current increase from Shewanella PV-4 after the formation of cell/iron sulfide nanoparticle aggregates. Detailed characterization of the structure and composition of the cell/nanoparticle interface revealed crystalline iron sulfide nanoparticles in intimate contact with and uniformly coating the cell membrane. In addition, studies designed to address the fundamental mechanisms of charge transport in this hybrid system showed that charge transport only occurred in the presence of live Shewanella, and moreover demonstrated that the enhanced current output can be attributed to improved electron transfer at cell/electrode interface and through the cellular-networks. Our approach of interconnecting and electrically contacting bacterial cells through biogenic nanoparticles represents a unique and promising direction in MFC research and has the potential to not only advance our fundamental knowledge about electron transfer processes in these biological systems but also overcome a key limitation in MFCs by constructing an electrically connected, three-dimensional cell network from the bottom-up.

Isolation of acetogenic bacteria that induce biocorrosion by utilizing metallic iron as the sole electron donor

Corrosion of iron occurring under anoxic conditions, which is termed microbiologically influenced corrosion (MIC) or biocorrosion, is mostly caused by microbial activities. Microbial activity that enhances corrosion via uptake of electrons from metallic iron [Fe(0)] has been regarded as one of the major causative factors. In addition to sulfate-reducing bacteria and methanogenic archaea in marine environments, acetogenic bacteria in freshwater environments have recently been suggested to cause MIC under anoxic conditions. However, no microorganisms that perform acetogenesis-dependent MIC have been isolated or had their MIC-inducing mechanisms characterized. Here, we enriched and isolated acetogenic bacteria that induce iron corrosion by utilizing Fe(0) as the sole electron donor under freshwater, sulfate-free, and anoxic conditions. The enriched communities produced significantly larger amounts of Fe(II) than the abiotic controls and produced acetate coupled with Fe(0) oxidation prior to CH4 production. Microbial community analysis revealed that Sporomusa sp. and Desulfovibrio sp. dominated in the enrichments. Strain GT1, which is closely related to the acetogen Sporomusa sphaeroides, was eventually isolated from the enrichment. Strain GT1 grew acetogenetically with Fe(0) as the sole electron donor and enhanced iron corrosion, which is the first demonstration of MIC mediated by a pure culture of an acetogen. Other well-known acetogenic bacteria, including Sporomusa ovata and Acetobacterium spp., did not grow well on Fe(0). These results indicate that very few species of acetogens have specific mechanisms to efficiently utilize cathodic electrons derived from Fe(0) oxidation and induce iron corrosion.

Hybrid bio-organic interfaces with matchable nanoscale topography for durable high extracellular electron transfer activity

Here, we developed a novel hybrid bio-organic interface with matchable nano-scale topography between a polypyrrole nanowire array (PPy-NA) and the bacterium Shewanella, which enabled a remarkably increased extracellular electron transfer (EET) current from genus Shewanella over a rather long period. PPy-NA thus exhibited outstanding performance in mediating bacterial EET, which was superior to normal electrodes such as carbon plates, Au and tin-doped In₂O₃. It was proposed that the combined effect of the inherent electrochemical nature of PPy and the porous structured bacterial network that was generated on the PPy-NA enabled long-term stability, while the high efficiency was attributed to the enhanced electron transfer rate between PPy-NA and microbes caused by the enhanced local topological interactions.

The fuel cell model of abiogenesis: a new approach to origin-of-life simulations

In this paper, we discuss how prebiotic geo-electrochemical systems can be modeled as a fuel cell and how laboratory simulations of the origin of life in general can benefit from this systems-led approach. As a specific example, the components of what we have termed the "prebiotic fuel cell" (PFC) that operates at a putative Hadean hydrothermal vent are detailed, and we used electrochemical analysis techniques and proton exchange membrane (PEM) fuel cell components to test the properties of this PFC and other geo-electrochemical systems, the results of which are reported here. The modular nature of fuel cells makes them ideal for creating geo-electrochemical reactors with which to simulate hydrothermal systems on wet rocky planets and characterize the energetic properties of the seafloor/hydrothermal interface. That electrochemical techniques should be applied to simulating the origin of life follows from the recognition of the fuel cell-like properties of prebiotic chemical systems and the earliest metabolisms. Conducting this type of laboratory simulation of the emergence of bioenergetics will not only be informative in the context of the origin of life on Earth but may help in understanding whether life might emerge in similar environments on other worlds.