The biocathode of microbial electrochemical systems and microbially-influenced corrosion

Review of cathodic electroactive bacteria: Species, properties, applications and electron transfer mechanisms

Cathodic electroactive bacteria (C-EAB) which are capable of accepting electrons from solid electrodes provide fresh avenues for pollutant removal, biosensor design, and electrosynthesis. This review systematically summarized the burgeoning applications of the C-EAB over the past decade, including 1) removal of nitrate, aromatic derivatives, and metal ions; 2) biosensing based on biocathode; 3) electrosynthesis of CH4, H2, organic carbon, NH3, and protein. In addition, the mechanisms of electron transfer by the C-EAB are also classified and summarized. Extracellular electron transfer and interspecies electron transfer have been introduced, and the electron transport mechanism of typical C-EAB, such as Shewanella oneidensis MR-1, has been combed in detail. By bringing to light this cutting-edge area of the C-EAB, this review aims to stimulate more interest and research on not only exploring great potential applications of these electron-accepting bacteria, but also developing steady and scalable processes harnessing biocathodes.

Filamentous marine Gram-positive Nocardiopsis dassonvillei biofilm as biocathode and its electron transfer mechanism

This study investigated electrochemical characteristics of Gram-positive, Nocardiopsis dassonvillei B17 facultative bacterium in bioelectrochemical systems. The results demonstrated that anodic and cathodic reaction rates were catalyzed by this bacterium, especially by utilization of aluminium alloy as a substrate. Cyclic voltammogram results depicted an increase of peak current and surface area through biofilm development, confirming its importance on catalysis of redox reactions. Phenazine derivatives were detected and their electron mediating behavior was evaluated exogenously. A symmetrical redox peak in the range of -59 to -159 mV/SHE was observed in cyclic voltammogram of bacterial solution supplemented with 12 μM phenazine, a result consistent with cyclic voltammogram of a 5-d biofilm, confirming its importance as an electron mediator in extracellular electron transfer. Furthermore, the dependency of bacterial catalysis and polarization potential were studied. These results suggested that B17 biofilm behaved as a biocathode and transferred electrons to bacterial cells through a mechanism associated with electron mediators.

Sequential anaerobic-aerobic treatment of rice mill wastewater and simultaneous power generation in microbial fuel cell

Microbial fuel cells (MFCs) have attracted widespread interest due to their capability to generate power while treating wastewater. In the present investigation, rice mill wastewater (RMW) was treated in a dual-chamber MFC with a biological cathode (MFCB), in which anaerobic treatment was provided in the anode compartment, and aerobic treatment was enployed in the cathode compartment. The performance was compared with an identical MFC with an abiotic cathode (MFCA). During continuous operation, the hydraulic retention time (HRT) of the anode compartments of both MFCs was kept at 12 h. The maximum volumetric power density obtained in MFCB (379.53 mW/m3) was lower than MFCA (791.72 mW/m3). Similarly, the maximum open-circuit voltage (OCV) and operating voltages were 0.519 V and 0.170 V for MFCB, while for the MFCA, they were 0.774 V and 0.251 V, respectively. The internal resistance of MFCA was 372.34 Ω while the MFCB showed a higher internal resistance of 533.89 Ω. The linear sweep voltammetry and cyclic voltammetry also demonstrated high electrochemical activity in MFCA compared to MFCB. However, MFCB has shown a higher chemical oxygen demand (COD) removal efficiency (96.8%) than MFCA (88.4%) under steady-state conditions. Both anaerobic and aerobic degradation of organic substrates significantly reduced the COD of RMW. Furthermore, the absence of an expensive catalyst in the cathode substantially reduces the cost of the system. The electrical performance of the system can be enhanced by employing novel cathode material with surface modification.

Bioelectrochemical systems-based metal removal and recovery from wastewater and polluted soil: Key factors, development, and perspective

Bioelectrochemical systems (BES) are considered efficient and sustainable technologies for bioenergy generation and simultaneously removal/recovery metal (loid)s from soil and wastewater. However, several current challenges of BES-based metal removal and recovery, especially concentrating target metals from complex contaminated wastewater or soil and their economic feasibility of engineering applications. This review summarized the applications of BES-based metal removal and recovery systems from wastewater and contaminated soil and evaluated their performances on electricity generation and metal removal/recovery efficiency. In addition, an in depth review of several key parameters (BES configurations, electrodes, catalysts, metal concentration, pH value, substrate categories, etc.) of BES-based metal removal and recovery was carried out to facilitate a deep understanding of their development and to suggest strategies for scaling up their specific application fields. Finally, the future intervention on multifunctional BES to improve their performances of mental removal and recovery were revealed.

Integration of microbial electrochemical systems and photocatalysis for sustainable treatment of organic recalcitrant wastewaters: Main mechanisms, recent advances, and present prospects

In recent years, microbial electrochemical systems (MESs) have demonstrated to be an environmentally friendly technology for wastewater treatment and simultaneous production of value-added products or energy. However, practical applications of MESs for the treatment of recalcitrant wastewater are limited by their low power output and slow rates of pollutant biodegradation. As a novel technology, hybrid MESs integrating biodegradation and photocatalysis have shown great potential to accelerate the degradation of bio-recalcitrant pollutants and increase the system output. In this review, we summarize recent advances of photo-assisted MESs for enhanced removal of recalcitrant pollutants, and present further discussion about the synergistic effect of biodegradation and photocatalysis. In addition, we analyse in detail different set-up configurations, discuss mechanisms of photo-enhanced extracellular electron transfer, and briefly present ongoing research cases. Finally, we highlight the current limitations and corresponding research gaps, and propose insights for future research.

Desulfovibrio vulgaris as a model microbe for the study of corrosion under sulfate-reducing conditions

Corrosion of iron-containing metals under sulfate-reducing conditions is an economically important problem. Microbial strains now known as Desulfovibrio vulgaris served as the model microbes in many of the foundational studies that developed existing models for the corrosion of iron-containing metals under sulfate-reducing conditions. Proposed mechanisms for corrosion by D. vulgaris include: (1) H2 consumption to accelerate the oxidation of Fe0 coupled to the reduction of protons to H2; (2) production of sulfide that combines with ferrous iron to form iron sulfide coatings that promote H2 production; (3) moribund cells release hydrogenases that catalyze Fe0 oxidation with the production of H2; (4) direct electron transfer from Fe0 to cells; and (5) flavins serving as an electron shuttle for electron transfer between Fe0 and cells. The demonstrated possibility of conducting transcriptomic and proteomic analysis of cells growing on metal surfaces suggests that similar studies on D. vulgaris corrosion biofilms can aid in identifying proteins that play an important role in corrosion. Tools for making targeted gene deletions in D. vulgaris are available for functional genetic studies. These approaches, coupled with instrumentation for the detection of low concentrations of H2, and proven techniques for evaluating putative electron shuttle function, are expected to make it possible to determine which of the proposed mechanisms for D. vulgaris corrosion are most important.

In Situ Investigation of Under-Deposit Microbial Corrosion and its Inhibition Using a Multi-Electrode Array System

Carbon steel pipelines used in the oil and gas industry can be susceptible to the combined presence of deposits and microorganisms, which can result in a complex phenomenon, recently termed under-deposit microbial corrosion (UDMC). UDMC and its inhibition in CO₂ ambiance were investigated in real-time using a multi-electrode array (MEA) system and surface profilometry analysis. Maps from corrosion rates, galvanic currents, and corrosion potentials recorded at each microelectrode allowed the visualization of local corrosion events on the steel surface. A marine bacterium Enterobacter roggenkampii, an iron-oxidizing, nitrate-reducing microorganism, generated iron deposits on the surface that resulted in pitting corrosion under anaerobic conditions. Areas under deposits displayed anodic behavior, more negative potentials, higher corrosion rates, and pitting compared to areas outside deposits. In the presence of the organic film-forming corrosion inhibitor, 2-Mercaptopyrimidine, the marine bacterium induced local breakdown of the protective inhibitor film and subsequent pitting corrosion of carbon steel. The ability of the MEA system to locally measure self-corrosion processes, galvanic effects and, corrosion potentials across the surface demonstrated its suitability to detect, evaluate and monitor the UDMC process as well as the efficiency of corrosion inhibitors to prevent this corrosion phenomenon. This research highlights the importance of incorporating the microbial component to corrosion inhibitors evaluation to ensure chemical effectiveness in the likely scenario of deposit formation and microbial contamination in oil and gas production equipment.

An overview in the development of cathode materials for the improvement in power generation of microbial fuel cells

Since the high cost and low power generation hinder the overall practical application of microbial fuel cells (MFCs), numerous attempts have been made in the field of cathode materials to enhance the electrical performance of MFCs because they directly catalyze the oxygen reduction reactions (ORR). To choose a proper cathode material, following principles such as ORR activity, conductivity, cost-efficiency, durability, surface area, and accessibility should be taken into consideration. In preparation of cathode materials, versatile materials have been chosen, synthesized, or modified to achieve an improvement in power generation of MFCs. The most widely applied cathode materials could be categorized into three classes, namely carbon-base materials, metal-based materials, and biocatalysts. This review summarizes the utilization, development, and the cost of cathode materials applied in MFCs and tries to highlight the effective modification methods of cathode materials which have helped in achieving enhanced power generation of MFCs in recent years.

Promoted oxidation of polycyclic aromatic hydrocarbons in soils by dual persulfate/calcium peroxide system

In situ chemical oxidations (ISCO) have been demonstrated as effective ways for remediating soils contaminated with organic pollutants by complete mineralization. This work aims to develop a technology for the oxidation remediation of soils contaminated with Polycyclic Aromatic Hydrocarbons (PAHs) using a dual calcium peroxide (CP)/persulfate (PS) oxidant system activated by oxalic acid (OA)-chelating Fe²⁺. The dual peroxide system was set up, and the effects of 5 single factors (i.e., CP dosage, PS dosage, Fe²⁺ dosage, OA concentration, and soil/water ratio) on PAHs degradation were studied using the single-factor experiment. The response surface method was then introduced to obtain the optimized experimental conditions (CP dosage, PS dosage, OA concentration) of the dual peroxide system. The result shows that the dual peroxide system significantly increased the PAHs degradation and the maximum PAHs degradation efficiency (70.8%) was achieved by the dual peroxide system under optimal conditions (PS dosage, CP concentration, Fe²⁺/PS ratio, and Fe²⁺/OA ratio was 8.89 g/kg, 0.18 mol/L, 1/4 and 0.62) at neutral soil condition. This study is an illustration of the promising efficiency of the dual peroxide system for PAH oxidation in the neutral soil and has great potential for remediation of PAHs contaminated farmland soils.

Microbial Cd(II) and Cr(VI) resistance mechanisms and application in bioremediation

The heavy metals cadmium (Cd) and chromium (Cr) are extensively used in industry and result in water and soil contamination. The highly toxic Cd(II) and Cr(VI) are the most common soluble forms of Cd and Cr, respectively. They enter the human body through the food chain and drinking water and then cause serious illnesses. Microorganisms can adsorb metals or transform Cd(II) and Cr(VI) into insoluble or less bioavailable forms, and such strategies are applicable in Cd and Cr bioremediation. This review focuses on the highlighting of novel achievements on microbial Cd(II) and Cr(VI) resistance mechanisms and their bioremediation applications. In addition, the knowledge gaps and research perspectives are also discussed in order to build a bridge between the theoretical breakthrough and the resolution of Cd(II) and Cr(VI) contamination problems.

Power density of microbial electrochemical system responds to mass transfer characters of non-ion-selective microbial separator

The microbial separator (MS) was promising alternative of ion exchange membrane for biocathode microbial electrochemical system (MES). Four microbial separators developed from porous matrixes were equipped in biocathode MESs. The power generation of MESs responded to cross-separator transfer characters of ions, dissolved oxygen (DO) and chemical oxygen demand (COD). The MES with carbon felt (CF) obtained 31% higher maximum power density at 70 ± 3 mW m^-2 and 51% higher current density at 271 ± 21 mA m^-2 than those of cation exchange membrane (CEM) separator. All MSs showed higher ionic conductivity than CEM. However, the power variation was mainly due to cathodic equilibrium potential changes rather than internal resistance. The power density demonstrated negative correlation with mass transfer coefficients of DO and COD. The cross-separator transfer of COD caused cathode variation and was identified as the primary parameter for further optimization of MES with microbial separators.

Electron transfer pathways and kinetic analysis of cathodic simultaneous nitrification and denitrification process in microbial fuel cell system

Microbial fuel cell (MFC) is an innovative bioconversion technology for wastewater treatment accompanied with electricity recovery. In this study, a kinetic model was developed base on Activated Sludge Model No.1 (ASM1) to describe electron transfer pathways during the simultaneous nitrification and denitrification (SND) process in the biocathode system of a dual-chamber MFC. The batch running of the dual-chamber MFC system showed that it produced a power density up to 2.96 W m^-3 within 48 h, the achieved SND efficiency and autotrophic denitrification ratio in the cathodic denitrification process were up to 87.3 ± 0.8% and 69.5 ± 6.6%, respectively. Meanwhile, by integrating nitrification, autotrophic denitrification, heterotrophic denitrification, organic carbon oxidation, and oxygen reduction in the cathode, the model was able to precisely fit the concentration variations of NH₃-N, dissolved oxygen (DO) and chemical oxygen demand (COD) during the cathodic SND process (R² ≥ 0.9876). The cathode electrons tended to be completely utilized with the increase of autotrophic denitrification ratio in the cathodic denitrification process. When the nitrification rate was enhanced, the autotrophic denitrification would prevail in the competition with the heterotrophic denitrification. In summary, the developed model was confirmed to be effective and reliable for describing the electron transfer pathways and predicting the performance of the nitrogen removal reactions during the cathodic SND process in a double-chamber MFC.

The relationship between energy production and simultaneous nitrification and denitrification via bioelectric derivation of microbial fuel cells at different anode numbers

In this study, three microbial fuel cells (MFCs) with different numbers of anodes (i.e., 1A, 3A, and 5A) were constructed to study the effects of a multi-anode (MA) system on power generation performance and nitrogen (N) removal from low carbon (C)/N wastewater. The maximum power density of 3A-MFC was 236.7 mW m^-3, which was 2.6-fold and 1.2-fold that of 1A-MFC and 5A-MFC, respectively. The 3A-MFC system produced the highest total energy output in one cycle, approximately 41.7 mW h, which was 1.5-fold and 1.3-fold that of 1A-MFC and 5A-MFC, respectively. 3A-MFC also had the highest total N (TN) removal efficiency (71.1 ± 3.9%) and simultaneous nitrification and denitrification (SND) rate (93.5 ± 2.4%). An analysis of electron flow distribution in the 3A-MFC biocathode showed that electro-autotrophic denitrification accounted for 19% of the total denitrification in the last 135 h. Thereafter, the relationships between TN removal, anode number, and bioelectricity were systematically evaluated. TN removal efficiency had a good linear relationship with energy production (R² = 0.97539); TN removal was mainly dependent on SND. Generally, the MA-MFC configuration proposed in this study produced more electrical energy and improved TN removal by enhancing nitrification and heterotrophic and electro-autotrophic denitrification of the biocathode. The proposed method is therefore effective for enhancing N removal.

Enhanced sulfate reduction accompanied with electrically-conductive pili production in graphene oxide modified biocathodes

This study aimed to investigate the graphene oxide (GO) conversion by the sulfate-reducing biocathode and its modified effects on performance of the microbial electrolysis cell (MEC). Biocathodes were acclimated with autotrophic sulfate-reducing cultures using medium containing 500 mg L^-1 sulfate. Sulfate reductive rate in the MEC was 230 and 135 g m^-3 d^-1, respectively, with and without 30 mg L^-1 GO addition. Raman measurements showed that GO was efficiently reduced to graphene by the biocathode within 24 h. Higher electrochemical activity and smaller charge transfer resistance were detected on biofilm with GO affected. With high electrical conductivity of 307 ± 36 μS cm^-1, pili substance were observed on GO affected biofilm. As dominated by Desulfovibrio sp., the biocathode could use GO as the sole electron acceptor and maintained high activity. The results from this study should provide useful information for applications of nanomaterials in the biocathode MEC.

Effects of Applied Potential and Reactants to Hydrogen-Producing Biocathode in a Microbial Electrolysis Cell

Understanding the mechanism of electron transfer between the cathode and microorganisms in cathode biofilms in microbial electrolysis cells (MECs) for hydrogen production is important. In this study, biocathodes of MECs were successfully re-enriched and subjected to different operating parameters: applied potential, sulfate use and inorganic carbon consumption. It was hypothesized that biocathode catalytic activity would be affected by the applied potentials that initiate electron transfer. While inorganic carbon, in the form of bicarbonate, could be a main carbon source for biocathode growth, sulfate could be a terminal electron acceptor and thus reduced to elemental sulfurs. It was found that potentials more negative than -0.8 V (vs. standard hydrogen electrode) were required for hydrogen production by the biocathode. In additional, a maximum hydrogen production was observed at sulfate and bicarbonate concentrations of 288 and 610 mg/L respectively. Organic carbons were found in the cathode effluents, suggesting that microbial interactions probably happen between acetogens and sulfate reducing bacteria (SRB). The hydrogen-producing biocathode was sulfate-dependent and hydrogen production could be inhibited by excessive sulfate because more energy was directed to reduce sulfate (E° SO 4 2 - /H2S = -0.35 V) than proton (E° H+/H2 = -0.41 V). This resulted in a restriction to the hydrogen production when sulfate concentration was high. Domestic wastewaters contain low amounts of organic compounds and sulfate would be a better medium to enrich and maintain a hydrogen-producing biocathode dominated by SRB. Besides the risks of limited mass transport and precipitation caused by low potential, methane contamination in the hydrogen-rich environment was inevitable in the biocathode after long term operation due to methanogenic activities.

Recent developments and key barriers to advanced biofuels: A short review

Biofuels are regarded as one of the most viable options for reduction of CO₂ emissions in the transport sector. However, conventional plant-based biofuels (e.g., biodiesel, bioethanol)'s share of total transportation-fuel consumption in 2016 was very low, about 4%, due to several major limitations including shortage of raw materials, low CO₂ mitigation effect, blending wall, and poor cost competitiveness. Advanced biofuels such as drop-in, microalgal, and electro biofuels, especially from inedible biomass, are considered to be a promising solution to the problem of how to cope with the growing biofuel demand. In this paper, recent developments in oxy-free hydrocarbon conversion via catalytic deoxygenation reactions, the selection of and lipid-content enhancement of oleaginous microalgae, electrochemical biofuel conversion, and the diversification of valuable products from biomass and intermediates are reviewed. The challenges and prospects for future development of eco-friendly and economically advanced biofuel production processes also are outlined herein.

Oxygen Reduction Reaction Affected by Sulfate-Reducing Bacteria: Different Roles of Bacterial Cells and Metabolites

Sulfate-reducing bacteria (SRB) were found to be capable of tolerating a certain amount of oxygen (O₂), but how they affect oxygen reduction reaction (ORR) has not been clear. The present work investigated the impact of SRB on ORR in 3.5 wt% sodium chloride solution with the cyclic voltammetry method. The addition of SRB culture solution hampered both the reduction of O₂ to superoxide (O₂^·-) and hydrogen peroxide (H₂O₂) to water (H₂O), and the influence of SRB metabolites was much larger than that of bacterial cells. Sulfide and extracellular polymeric substances (EPS), typical inorganic and organic metabolic products, had great impact on ORR. Sulfide played an important role in the decrease of cathodic current for H₂O₂ reduction due to its hydrolysis and chemical reaction activity with H₂O₂. EPS were sticky, easy to adsorb on the electrode surface and abundant in functional groups, which hindered the transformation of O₂ into O₂^·- and favored the reduction of H₂O₂ to H₂O.

Bioanode as a limiting factor to biocathode performance in microbial electrolysis cells

The bioanode is important for a microbial electrolysis cell (MEC) and its robustness to maintain its catalytic activity affects the performance of the whole system. Bioanodes enriched at a potential of +0.2V (vs. standard hydrogen electrode) were able to sustain their oxidation activity when the anode potential was varied from -0.3 up to +1.0V. Chronoamperometric test revealed that the bioanode produced peak current density of 0.36A/m² and 0.37A/m² at applied potential 0 and +0.6V, respectively. Meanwhile hydrogen production at the biocathode was proportional to the applied potential, in the range from -0.5 to -1.0V. The highest production rate was 7.4L H₂/(m² cathode area)/day at -1.0V cathode potential. A limited current output at the bioanode could halt the biocathode capability to generate hydrogen. Therefore maximum applied potential that can be applied to the biocathode was calculated as -0.84V without overloading the bioanode.

The Role of Localized Acidity Generation in Microbially Influenced Corrosion

Microbially influenced corrosion is of great industrial concern. Microbial coupling of metal oxidation to sulfate-, nitrate-, nitrite-, or CO₂-reduction is proton-mediated, and some sulfate-reducing prokaryotes are capable of regulating extracellular pH. The analysis of the corrosive processes catalyzed by nitrate reducing bacteria and methanogenic archaea indicates that these microorganisms may be capable of regulating extracellular pH as well. It is proposed that nutrient limitation at metal-biofilm interfaces may induce activation of enzymatic proton-producing/proton-secreting functions in respiratory and methanogenic microorganisms to make them capable of using Fe⁰ as the electron donor. This can be further verified through experiments involving measurements of ion and gas concentrations at metal-biofilm interfaces, microscopy, and transcriptomics analyses.

Enhanced Power Generation of Oxygen-Reducing Biocathode with an Alternating Hydrophobic and Hydrophilic Surface

Most oxygen-reducing biocathodes for microbial electrochemical systems (MESs) require energy-intensive aeration of the catholyte, which negates the energy-saving benefits of MESs. To avoid aeration and enhance oxygen-utilization efficiency, columnar activated carbon with half of its surface coated by polytetrafluoroethylene (PTFE-coated CAC) was fabricated as biocathode material, and its performance was investigated using a tide-type biocathode MES (TBMES). The TBMES with PTFE-coated biocathode achieved a maximum power density of 8.2 ± 0.8 W m^-3, which was 39% higher than that of the untreated control (CAC biocathode). The PTFE-coated biocathode was able to store a cumulative total charge (Q_m) of (10.8 ± 0.2) × 10⁴ C m^-3 during one charge-discharge cycle, whereas the Q_m of CAC biocathode was only (6.9 ± 0.1) × 10⁴ C m^-3, demonstrating that the oxygen entrapment capability of PTFE-coated biocathode was 54 ± 3.8% higher than that of the control. Internal resistance analysis under both oxygen sufficient and reoxygenation conditions suggested the oxygen entrapped by this surface-hydrophobic biocathode was basically sufficient for cathodic oxygen reduction reaction. The slight difference in cathodic microbial communities of the two biocathodes further indicated that the higher accessibility of oxygen due to the hydrophobic surface was the primary cause for the better performance of the PTFE-coated biocathode, while the higher biocatalytic activity of the cathodic biofilm was a minor factor.

Low temperature acclimation with electrical stimulation enhance the biocathode functioning stability for antibiotics detoxification

Improvement of the stability of functional microbial communities in wastewater treatment system is critical to accelerate pollutants detoxification in cold regions. Although biocathode communities could accelerate environmental pollutants degradation, how to acclimate the cold stress and to improve the catalytic stability of functional microbial communities are remain poorly understood. Here we investigated the structural and functional responses of antibiotic chloramphenicol (CAP) reducing biocathode communities to constant low temperature 10 °C (10-biocathode) and temperature elevation from 10 °C to 25 °C (S25-biocathode). Our results indicated that the low temperature acclimation with electrical stimulation obviously enhanced the CAP nitro group reduction efficiency when comparing the aromatic amine product AMCl2 formation efficiency with the 10-biocathode and S25-biocathode under the opened and closed circuit conditions. The 10-biocathode generated comparative AMCl maximum as the S25-biocathode but showed significant lower dehalogenation rate of AMCl2 to AMCl. The continuous low temperature and temperature elevation both enriched core functional community in the 10-biocathode and S25-biocathode, respectively. The 10-biocathode functioning stability maintained mainly through selectively enriching cold-adapted functional species, coexisting metabolically similar nitroaromatics reducers and maintaining the relative abundance of key electrons transfer genes. This study provides new insights into biocathode functioning stability for accelerating environmental pollutants degradation in cold wastewater system.

Simultaneous inhibition of sulfate-reducing bacteria, removal of H2S and production of rhamnolipid by recombinant Pseudomonas stutzeri Rhl: Applications for microbial enhanced oil recovery

Sulfate-reducing bacteria (SRB) are widely existed in oil production system, and its H2S product inhibits rhamnolipid producing bacteria. In-situ production of rhamnolipid is promising for microbial enhanced oil recovery. Inhibition of SRB, removal of H2S and production of rhamnolipid by recombinant Pseudomonas stutzeri Rhl were investigated. Strain Rhl can simultaneously remove S(2-) (>92%) and produce rhamnolipid (>136mg/l) under S(2-) stress below 33.3mg/l. Rhl reduced the SRB numbers from 10(9) to 10(5)cells/ml, and the production of H2S was delayed and decreased to below 2mg/l. Rhl also produced rhamnolipid and removed S(2-) under laboratory simulated oil reservoir conditions. High-throughput sequencing data demonstrated that addition of strain Rhl significantly changed the original microbial communities of oilfield production water and decreased the species and abundance of SRB. Bioaugmentation of strain Rhl in oilfield is promising for simultaneous control of SRB, removal of S(2-) and enhance oil recovery.

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.

Performance and recent improvement in microbial fuel cells for simultaneous carbon and nitrogen removal: A review

Microbial fuel cells (MFCs) have become a promising technology for wastewater treatment accompanying electricity generation. Carbon and nitrogen removal can be achieved by utilizing the electron transfer between the anode and cathode in an MFC. However, large-scale power production and high removal efficiency must be achieved at a low cost to make MFCs practical and economically competitive in the future. This article reviews the principles, feasibility and bottlenecks of MFCs for simultaneous carbon and nitrogen removal, the recent advances and prospective strategies for performance improvement, as well as the involved microbes and electron transfer mechanisms.