Extracellular electron transfer (EET) and bioluminescence are both important for microbial growth and metabolism, but the mechanism of interaction between EET and bioluminescence is poorly understood. Herein, we demonstrate an exclusively respiratory luminous bacterium, Shewanella woodyi, which possesses EET ability and electron communication at the interface of S. woodyi and solid substrates via charge and discharge methods. Using an electro-chemiluminescence apparatus, our results confirmed that the FMN/FMNH2 content and the redox status of cytochrome c conjointly regulated the bioluminescence intensity when the potential of an indium-tin oxide electrode was changed. More importantly, this work revealed that there is an interaction between the redox reaction of single cells and bioluminescence of group communication via the EET pathway.

•An overview of conductivity and redox properties of extracellular polymeric substances.•Extracellular polymeric substances play important roles in microbial electron transfer processes.•Hopping may be a possible way for electron transfer in extracellular polymeric substances.Most microbial cells in nature are surrounded by extracellular polymeric substances (EPS), which are fundamental components and determine the physiochemical properties of a biofilm. This review highlights the EPS properties of conductivity and redox ability from an electrochemical perspective, emphasizing recent findings that EPS play important roles in microbial extracellular electron transfer (EET). Basic information regarding EPS structure and components is required to better understand their effects on EET; future directions are briefly discussed.

•Progress of air-breathing cathode in microbial fuel cells was summarized.•Effects of each component on performance of air-breathing cathode were presented.•Main factors to performance of air-breathing cathode were addressed.•Valuable information provided to develop high-efficient air-breathing cathodes.Microbial fuel cell (MFC) is an emerging technology to produce green energy and vanquish the effects of environmental contaminants. Cathodic reactions are vital for high electrical power density generated from MFCs. Recently tremendous attentions were paid towards developing high performance air-breathing cathodes. A typical air-breathing cathode comprises of electrode substrate, catalyst layer, and air-diffusion layer. Prior researches demonstrated that each component influenced the performance of air-breathing cathode MFCs. This review summarized the progress in development of the individual component and elaborated main factors to the performance of air-breathing cathode.

Nitrogen is one of major contaminants in wastewater; however, nitrogen, as bio-elements for crop growth, is the indispensable fertilizer in agriculture. In this study, two-chamber microbial fuel cells (MFCs) were first operated with microorganisms in anode chamber and potassium ferricyanide as catholyte. After being successfully startup, the two-chamber MFCs were re-constructed to three-chamber MFCs which were used to recover the NO3−-N and NH4+ -N of synthetic wastewater into valueadded nitrogenous fertilizer from cathode chamber and anode chamber, respectively. Ferric nitrate was used as the sole electron acceptor in cathode, which also was used to evaluate the NO3− -N recover efficiency in the case major anion of NO3− in cathode. The output voltage of these MFCs was about 600–700 mVat an external load of 500 Ω. About 47% NH4+ -N in anode chamber and 83% NO3− -N in cathode chamber could be recovered. Higher current density can selectively improve the recovery efficiency of both NH4+ -N and NO3− — N. The study demonstrated a nitrogen recovery process from synthetic wastewater using three-chamber MFCs.

•A novel 3D structure with freely standing threads twisted with fibers was developed.•It has larger surface area and capacitance than conventional materials.•It performed better when used as bioanode substrate.•This study provided new thoughts of preparing electrode material.Efficiency of bioelectrochemical systems (BESs) is generally limited by the performance of bioanode, resulted from the nature of microbial electron transfer and the character of the anode substrate. In the present study, a 3D structured anode material is fabricated using a towel as precursor through high-temperature carbonization. The 3D electrode is resulted from freely standing threads, twisted by fibers with diameter at micrometer scale, on a woven textile substrate. The open structure provides easy accesses for microbial to attach on the fiber surface. Furthermore, the prepared materials possess a high capacitive character which is beneficial for electron storage and contributes to the performance of bioanode. When tested in BESs, the prepared material achieves a current density of 0.80 ± 0.06 mA cm−2, larger than conventional anodes, e.g. graphite felt (0.55 ± 0.01 mA cm−2), carbon cloth (0.06 ± 0.01 mA cm−2), and carbon mesh (0.02 ± 0.00 mA cm−2). The present study provides a novel 3D anode substrate that can effectively promote the performance of BESs.

Sulfamethoxazole (SMX) is an extensively consumed sulfonamide antimicrobial agent and is frequently detected in surface water. This work studied the degradation process of SMX in anodic chamber of microbial fuel cell (MFC) reactors. We found that the biodegradation of SMX could be achieved after acclimation and even high concentrations of SMX (e.g. 200 ppm) could be rapidly degraded. Excitation and emission matrix fluorescence spectroscopy analysis revealed that the chemical structure of SMX was altered during the process. Q-Exactive hybrid quadrupole-Orbitrap mass spectrometry was used to identify the degradation byproducts of SMX. The activity of electrode biofilm was examined afterwards and it was found that the microbe was in an active state. High-throughput sequencing analysis suggested that the microbial community structure was greatly changed during the process; some reported SMX scavengers, such as Achromobacter and Pseudomonas, were abundant in the reactors. Some metazoans were also recognized in the biofilm samples, which indicates that the operation of the MFC reactors was in a steady state. This study discusses the degradation mechanism of SMX and explores the microbial community response during the process, which provides useful information for the application of MFC in antibiotic elimination.

Few studies have been conducted to explore the community composition in denitrifying biocathode. Herein, the microbial communities of denitrifying biocathodes yielding current of 1 mA (reactor C1) and 1.5 mA (reactor C2) were characterized by 454 pyrosequencing. The nitrate removal efficiencies in C1 and C2 were about 93 and 85 %, respectively. The optimization of data generated high-quality sequences of 18509 in C1 and 14857 in C2. Proteobacteria was the predominant phylum, and Bacteroidetes, Chloroflexi, and Planctomycetes were the subdominant groups. Classes of Alphaproteobacteria, Anaerolineae, and Phycisphaerae may benefit the performance of current production and nitrate removal. Twenty-nine dominant operational taxonomic units (OTUs) accounted for 64 and 65 % of sequences in C1 and C2, respectively. A denitrifying pathway was constructed based on the phylogenetic analysis and function inferring of the dominant OTUs. Obviously, the 454 pyrosequencing provided a high-resolution profile of bacteria community in denitrifying biocathode.

Extracellular electron transfer (EET) of microorganisms represents a communicative bridge between the interior and exterior of the cells. Most prior EET studies have focused on Gram-negative bacteria. However, fungi and Gram-positive bacteria, that contain dense cellular walls, have rarely been reported. Herein, two model dense cell wall microorganisms (Bacillus sp. WS-XY1 and the yeast Pichia stipitis) were identified to be electrochemically active. Further analysis indicated that the two microorganisms were able to secrete flavins to mediate their EET. The discovery, that dense cell wall containing microorganisms can undertake mediated EET, adds to the body of knowledge towards building a comprehensive understanding of biogeochemical and bioelectrical processes.

In this paper, we demonstrate that oxygen-containing carbon nanoparticles (O-CNPs) obtained from diffusion flame display notable ORR electrocatalytic activities in alkali media. The O-CNPs were the incomplete combustion product of hydrocarbon fuels. Electrochemical results showed that the O-CNP catalyst prepared from n-hexane could display very positive ORR peak potential of -0.24 V vs. Ag/AgCl, which was higher than the reported oxygen-containing carbon materials and common carbon black of below -0.35 V. Moreover, the ORR peak potential was varied with the unsaturation degree of the hydrocarbon fuels. It was further proposed that the remarkable ORR electrocatalyic activities of the O-CNPs was attributed to semiquinone groups bonded to the edge of the graphitic carbon layer.Oxygen-containing carbon nanoparticles (O-CNPs) were synthesized from diffusion flame of hydrocarbons and display notable electrocatalytic ORR activities in alkali media.

Soil acidification has been a threat to sustainable agricultural development as a global environmental problem. The bioelectrochemistry system (BES) is an eco-friendly and low-cost technique for ameliorating acidic soil when compared with traditional technologies. Here, operational parameters from soil load and external resistance were investigated through mapping the pH changes of the soil. The result showed that approximately a quarter of the soil load in the operating unit was an optimized choice based on the performance at the unit and overall levels. The lower external resistance was more conducive to enhance the electric force for the increase of the soil pH. This study not only proved the feasibility of BES for ameliorating acidic soil, but also comprehensively described the feedback network of soil acidity, Al3+ removal and microbial community. After BES-amelioration, the exchangeable Al3+ obviously reduced and the microbial community structure shifted. The exploration of operational parameters and a feedback network provided theoretical support for the application of BES-based technologies in reclaiming soil acidity.

The polarization behavior of microbial fuel cells (MFCs) was evaluated under different stack operation modes, including series, parallel, series–parallel, and parallel–series. During the stack operation, voltages of individual MFCs, subunit stacks, and overall stacks were recorded as a function of the current. Meanwhile, the potentials of individual MFCs’ anode and cathode were also determined via Ag/AgCl electrodes to study the change in potentials under stack operations. The results demonstrated that the MFCs with relatively low ability to generate current were easier to suffer polarity reversal in the series stack, which was confirmed in the series subunit of the series–parallel stack. MFCs with high electroactivity would be enhanced to generate larger maximum power; however, MFCs with low electroactivity outputted smaller maximum power in a parallel stack. The changes in individual MFCs’ behavior under stack operation mode were determined primarily caused by the influences on the behavior of the anode. Results of the present study provide valuable information for optimization of stack operation of MFCs.

Hexavalent chromium [Cr(VI)] is a priority pollutant causing serious environmental issues. Microbial reduction provides an alternative strategy for Cr(VI) remediation. The dissimilatory metal-reducing bacterium, Shewanella oneidensis MR-1, was employed to study Cr(VI) reduction and toxicity in this work. To understand the effect of membrane cytochromes on Cr(VI) response, a comparative protein profile analysis from S. oneidensis MR-1 wild type and its mutant of deleting OmcA and MtrC (△omcA/mtrC) was conducted using two-dimensional electrophoresis (2-DE) technology. The 2-DE patterns were compared, and the proteins with abundant changes of up to twofold in the Cr(VI) treatment were detected. Using mass spectrometry, 38 and 45 differentially abundant proteins were identified in the wild type and the mutant, respectively. Among them, 25 proteins were shared by the two strains. The biological functions of these identified proteins were analyzed. Results showed that Cr(VI) exposure decreased the abundance of proteins involved in transcription, translation, pyruvate metabolism, energy production, and function of cellular membrane in both strains. There were also significant differences in protein expressions between the two strains under Cr(VI) treatment. Our results suggest that OmcA/MtrC deletion might result in the Cr(VI) toxicity to outer membrane and decrease assimilation of lactate, vitamin B12, and cystine. When carbohydrate metabolism was inhibited by Cr(VI), leucine and sulfur metabolism may act as the important compensatory mechanisms in the mutant. Furthermore, the mutant may regulate electron transfer in the inner membrane and periplasm to compensate for the deletion of OmcA and MtrC in Cr(VI) reduction.

Microbial fuel cells (MFCs) in negative pressure environment can provide an increase of 20% in voltage output and a maximum power generation of 7 times compared with that in normal environment. As showed by denaturing gradient gel electrophoresis, the culture in negative pressure environment for 30 days had little effect on diversity of bacterial communities. However, scanning electronic microscope indicated that negative pressure strengthened the attachment between extracellular polymeric substances and carbon felt electrode, it is the main reason for the increase in electrogenic ability of biofilm. The study suggested negative pressure-culture to be a method for promoting the performance of MFCs. The positive effect of negative pressure on the adhesion between biofilm and attachment materials also provides a potential method to improve the performance of biofilm-based environmental technology.Highlights► Effect of negative pressure on the performance of electrode biofilm was firstly investigated. ► The MFCs in negative pressure generated power of 7 times higher than that in normal environment. ► Electrogenic ability of biofilm was promoted by strengthening attachment of EPS to electrode.

Sulfur pollutants have negative impact on natural ecosystems and biological health in dynamic processes of migration and transformation on Earth. Microbial fuel cells (MFCs) have been shown to be a promising technique for the removal of sulfur pollutants in wastewater, and research efforts have been made for the development of this area. To date, some controversies still exist in the mechanism of sulfur pollutants transformation involved in electrochemical reactions and microbial effects, limiting the practical application. This review respectively discussed the roles of electrode chemical reaction and microorganisms in sulfur pollutants treatment using MFCs, demonstrated the treatment mechanism and effecting factors, and summarized the configurations, separator types, electrode materials and catalysts, as well as sulfur recovery and electrode regeneration. Furthermore, the feasibility of sulfur pollutants removal in MFCs was assessed by capital cost between MFCs and typical anaerobic biological treatment.

There is limited information about the factors that affect the power generation of single-chamber microbial fuel cells (MFCs) using soil organic matter as a fuel source. We examined the effect of soil and water depths, and temperature on the performance of soil MFCs with anode being embedded in the flooded soil and cathode in the overlaying water. Results showed that the MFC with 5 cm deep soil and 3 cm overlaying water exhibited the highest open circuit voltage of 562 mV and a power density of 0.72 mW m–2. The ohmic resistance increased with more soil and water. The polarization resistance of cathode increased with more soil while that of anode increased with more water. During the 30 d operation, the cell voltage positively correlated with temperature and reached a maximum of 162 mV with a 500 Ω external load. After the operation, the bacterial 16S rRNA gene from the soil and anode was sequenced. The bacteria in the soil were more diverse than those adhere to the anode where the bacteria were mainly affiliated to Escherichia coli and Deltaproteobacteria. In summary, the two bacterial groups may generate electricity and the electrical properties were affected by temperature and the depth of soil and water.

•The DO within alga biofilm under different light intensities was investigated.•Effects of illumination on biocathode performance was investigated from the electrochemical perspective.•Power generation of photo-MFC and DO change within the biofilm exhibited a similar trend with light intensity.The performance of photo microbial fuel cells (photo-MFCs) with Desmodesmus sp. A8 as cathodic microorganism under different light intensities (0, 1500, 2000, 2500, 3000, 3500 lx) was investigated. The results showed that illumination enhanced the output of the photo-MFC three-fold. When light intensity was increased from 0 to 1500 lx, cathode resistance decreased from 3152.0 to 136.7 Ω while anode resistance decreased from 13.9 to 11.3 Ω. In addition, the cathode potential increased from −0.44 to −0.33 V (vs. Ag/AgCl) and reached a plateau as the light intensity was increased from 1500 lx to 3500 lx. Accompanied with the potential change, dissolved oxygen (DO) within the cathode biofilm increased to 13.2 mg L−1 under light intensity of 3500 lx and dropped to 7.5 mg L−1 at 1500 lx. This work demonstrated that light intensity profoundly impacted the performance of photo-MFC with Desmodesmus sp. A8 through changing the DO.

Extracellular electron transfer (EET) and bioluminescence are both important for microbial growth and metabolism, but the mechanism of interaction between EET and bioluminescence is poorly understood. Herein, we demonstrate an exclusively respiratory luminous bacterium, Shewanella woodyi, which possesses EET ability and electron communication at the interface of S. woodyi and solid substrates via charge and discharge methods. Using an electro-chemiluminescence apparatus, our results confirmed that the FMN/FMNH2 content and the redox status of cytochrome c conjointly regulated the bioluminescence intensity when the potential of an indium-tin oxide electrode was changed. More importantly, this work revealed that there is an interaction between the redox reaction of single cells and bioluminescence of group communication via the EET pathway.