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Soluble electron shuttles have been found to facilitate the biodecolorization of azo dyes, yet their loss due to water flow can escalate costs and risk secondary pollution. This issue can be mitigated by immobilizing the shuttles. In this study, we immobilized phenazine-1-carboxylic acid (PCA) using a chitosan/polyvinyl alcohol gel carrier and investigated its effect on the degradation of disperse red S-R (DR S-R) by Shewanella oneidensis FJAT-2478. Both free and immobilized PCA significantly increased the decolorization rate within a 50–400 mg/L concentration range for DR S-R, without affecting the final efficiency. Immobilized PCA was slightly less effective than free PCA (4.18-fold at 100 mg/L DR S-R), but was 3.63-fold more effective than the control group without PCA. It also demonstrated excellent reusability, retaining 83% of its initial activity after 10 cycles. Unlike free PCA, which reduced flavin secretion of FJAT-2478 by 36.4%, immobilized PCA increased it by 19.5%, indicating potential differences in their electron transfer modes. This study highlights the potential of immobilized phenazine-based electron shuttles in biologically decolorizing disperse dye wastewater. [ABSTRACT FROM AUTHOR]

Typha angustifolia L. (TA) pollen has been utilized as a traditional Chinese medicine for treating various internal and external traumas. Moreover, bioactive compounds possess diverse pharmacological activities. This study aims to evaluate the antiviral properties of TA based on its ability to generate bioenergy, capable of inhibiting viruses. TA pollens were extracted using water and ethanol solvents. These extracts were utilized to identify the phytochemical contents and correlate with the antioxidant activity via 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays. HPLC analysis was conducted to identify its electron-shuttling compositions. The bioenergy-generating characteristics were determined via microbial fuel cells. The water extract (TA-W) showed higher antioxidant activity due to a higher phenolic and flavonoid content compared to the ethanol extract (TA-E). Quercetin-3-O-(2G-α-L-rhamnosyl)-rutinoside, quercetin-3-O-neohesperidoside, and quercetin are the electron shuttles (ES) identified out of the 11 compounds. TA obtained a 1.39 ± 0.10 amplification factor of power generation that indicates potential bioenergy-generating and associated antiviral characteristic properties. The findings may provide a foundation for developing antiviral medications specifically designed to target virus-related diseases, while minimizing the risk of drug toxicity and reducing the costs of drug development. [ABSTRACT FROM AUTHOR]

Electron shuttles are pivotal in enhancing the conversion efficiency at the biotic‐abiotic interface within microbial photoelectrochemical systems. Nevertheless, the potentially toxic effects of electron shuttles on microbial cells and their instability when exposed to light are often overlooked. This concept highlights the attention paid to electron shuttles in microbial photoelectrochemical systems, such as the cytotoxicity, the photoexcited state, and the photostability. Furthermore, it offers theoretical guidance for selecting electron shuttles to optimize sustainable energy production and environmental remediation. [ABSTRACT FROM AUTHOR]

Typha angustifolia L. (TA) pollen has been utilized as a traditional Chinese medicine for treating various internal and external traumas. Moreover, bioactive compounds possess diverse pharmacological activities. This study aims to evaluate the antiviral properties of TA based on its ability to generate bioenergy, capable of inhibiting viruses. TA pollens were extracted using water and ethanol solvents. These extracts were utilized to identify the phytochemical contents and correlate with the antioxidant activity via 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays. HPLC analysis was conducted to identify its electron-shuttling compositions. The bioenergy-generating characteristics were determined via microbial fuel cells. The water extract (TA-W) showed higher antioxidant activity due to a higher phenolic and flavonoid content compared to the ethanol extract (TA-E). Quercetin-3-O-(2G-α-L-rhamnosyl)-rutinoside, quercetin-3-O-neohesperidoside, and quercetin are the electron shuttles (ES) identified out of the 11 compounds. TA obtained a 1.39 ± 0.10 amplification factor of power generation that indicates potential bioenergy-generating and associated antiviral characteristic properties. The findings may provide a foundation for developing antiviral medications specifically designed to target virus-related diseases, while minimizing the risk of drug toxicity and reducing the costs of drug development.

Three decades ago, dye-sensitized solar cells (DSSCs) emerged as a route for harnessing the sun's energy and converting it into electricity. Since then, an impressive amount of work has been devoted to improving the global photovoltaic efficiency of DSSCs, trying to optimize all components of the device. Up to now, the best efficiencies have usually been reached with ruthenium(II) photosensitizers, even if in the last few years many classes of organic compounds have shown record efficiencies. However, the future of DSSCs is stringently connected to the research and development of cheaper materials; in particular, the replacement of rare metals with abundant ones is an important topic in view of the long-term sustainability of DSSCs intended to replace the consolidated fossil-based technology. In this context, copper is a valid candidate, being both an alternative to ruthenium in the fabrication of photosensitizers and a material able to replace the common triiodide/iodide redox couple. Thus, recently, some research papers have confirmed the great potential of copper(I) coordination complexes as a cheap and convenient alternative to ruthenium dyes. Similarly, the use of copper compounds as electron transfer mediators for DSSCs can be an excellent way to solve the problems related to the more common I3−/I− redox couple. The goal of this mini-review is to report on the latest research devoted to the use of versatile copper complexes as photosensitizers and electron shuttles in DSSCs. The coverage, from 2022 up to now, illustrates the most recent studies on dye-sensitized solar cells based on copper complexes as molecular materials. [ABSTRACT FROM AUTHOR]

Abstract Electron shuttles are pivotal in enhancing the conversion efficiency at the biotic‐abiotic interface within microbial photoelectrochemical systems. Nevertheless, the potentially toxic effects of electron shuttles on microbial cells and their instability when exposed to light are often overlooked. This concept highlights the attention paid to electron shuttles in microbial photoelectrochemical systems, such as the cytotoxicity, the photoexcited state, and the photostability. Furthermore, it offers theoretical guidance for selecting electron shuttles to optimize sustainable energy production and environmental remediation.

Greenhouse gas emissions from fossil fuel combustion have led to global ecological degradation and energy crisis. Therefore, it is urgent to explore new technologies for the conversion, reuse and clean energy production of carbon dioxide (CO_2). Compared with the conventional technology of converting CO_2 into high-value products, the conversion of CO_2 into methane (CH_4), a low-carbon energy source, using microbial electrolysis cell (MEC) has the advantages of mild reaction conditions, recyclable catalysts and green products, and has received a lot of attention from researchers. This paper reviews the current research status of CO_2 capture and high-value resource utilization, and discusses the main influencing factors of MEC-CO_2 electromethanation (such as inoculum source, reactor configuration, applied voltage, electrode material properties, etc.), briefly outlines the important role of electron shuttles, especially riboflavin, in MEC-CO_2 electromethanation and the biofilm formation process of electric methanogenic bacteria, points out the current technical bottleneck and future research direction of MEC-CO_2 electromethanation, and finally provides theoretical support for the development of MEC-CO_2 electromethanation and the realization of "dual carbon" goal.

Extracellular electron transfer (EET) is the physiological process that enables the reduction or oxidation of molecules and minerals beyond the surface of a microbial cell. The first bacteria characterized with this capability were Shewanella and Geobacter, both reported to couple their growth to the reduction of iron or manganese oxide minerals located extracellularly. A key difference between EET and nearly every other respiratory activity on Earth is the need to transfer electrons beyond the cell membrane. The past decade has resolved how well-conserved strategies conduct electrons from the inner membrane to the outer surface. However, recent data suggest a much wider and less well understood collection of mechanisms enabling electron transfer to distant acceptors. This review reflects the current state of knowledge from Shewanella and Geobacter, specifically focusing on transfer across the outer membrane and beyond—an activity that enables reduction of highly variable minerals, electrodes, and even other organisms. [ABSTRACT FROM AUTHOR]

Three decades ago, dye-sensitized solar cells (DSSCs) emerged as a route for harnessing the sun’s energy and converting it into electricity. Since then, an impressive amount of work has been devoted to improving the global photovoltaic efficiency of DSSCs, trying to optimize all components of the device. Up to now, the best efficiencies have usually been reached with ruthenium(II) photosensitizers, even if in the last few years many classes of organic compounds have shown record efficiencies. However, the future of DSSCs is stringently connected to the research and development of cheaper materials; in particular, the replacement of rare metals with abundant ones is an important topic in view of the long-term sustainability of DSSCs intended to replace the consolidated fossil-based technology. In this context, copper is a valid candidate, being both an alternative to ruthenium in the fabrication of photosensitizers and a material able to replace the common triiodide/iodide redox couple. Thus, recently, some research papers have confirmed the great potential of copper(I) coordination complexes as a cheap and convenient alternative to ruthenium dyes. Similarly, the use of copper compounds as electron transfer mediators for DSSCs can be an excellent way to solve the problems related to the more common I3−/I− redox couple. The goal of this mini-review is to report on the latest research devoted to the use of versatile copper complexes as photosensitizers and electron shuttles in DSSCs. The coverage, from 2022 up to now, illustrates the most recent studies on dye-sensitized solar cells based on copper complexes as molecular materials.

To this day, bioelectrochemical systems are still perceived as one of the rising technologies due to their versatile applications in electricity production, bioremediation, biosensors, and production of value-added products. While the majority of bioelectrochemical applications utilize Gram-negative bacteria, Gram-positive bacteria has not received sufficient attention. The lack of adequate knowledge about their electron transfer pathways along with the presence of a thick non-conductive cell wall are among the reasons behind their limited use. In this review, the electroactivity of Gram-positive bacteria will be covered describing the different pathways of electron transfer among different electroactive Gram-positive strains. Special emphasis will be given to the role of multiheme cytochromes, quorum sensing molecules, peptide-based signalling, and pili in the extracellular electron transfer. This review will also provide an overview of possible approaches for enhancement strategies of electron transfer such as enhancing biofilm formation, biocomposites and cell perforation. Understanding the fundamentals is critical for improving the use of Gram-positive bacteria in bioelectrochemical systems and may lead to the discovery of new applications. [ABSTRACT FROM AUTHOR]

Members of the genus Aeromonas prevail in aquatic habitats and have a great potential in biological wastewater treatment because of their unique extracellular electron transfer (EET) capabilities. However, the mediated EET mechanisms of Aeromonas have not been fully understood yet, hindering their applications in biological wastewater treatment processes. In this study, the electron shuttles in Aeromonas hydrophila, a model and widespread strain in aquatic environments and wastewater treatment plants, were explored. A. hydrophila was found to produce both flavins and 2‐amino‐3‐carboxy‐1,4‐naphthoquinone (ACNQ) as electron shuttles and utilize them to accelerate its EET for the bioreduction of various pollutants. The Mtr‐like respiratory pathway was essential for the reduction of flavins, but not involved in the ACNQ reduction. The electron shuttle activity of ACNQ for pollutant bioreduction involved the redox reactions that occurred inside the cell. These findings deepen our understanding about the underlying EET mechanisms in dissimilatory metal reducing bacteria and provide new insights into the roles of the genus Aeromonas in biological wastewater treatment. [ABSTRACT FROM AUTHOR]

Expert Systems are interactive and reliable computer-based decisionmaking systems that use both facts and heuristics for solving complex decision-making problems. Generally, the cyclic voltammetry (CV) experiments are executed a random number of times (cycles) to get a stable production of power. However, presently there are not many algorithms or models for predicting the power generation stable criteria in microbial fuel cells. For stability analysis of Medicinal herbs' CV profiles, an expert system driven by the augmented K-means clustering algorithm is proposed. Our approach requires a dataset that contains voltage-current relationships from CV experiments on the related subjects (plants/herbs). This new approach uses feature engineering and augmented K-means clustering techniques to determine the cycle number beyond which the CV curve stabilizes. We obtain an excellent estimate of the required CV cycles for getting a stable Voltage versus Current curve in this approach. Moreover, this expert system would reduce the time needed and the money spent on running additional and superfluous CV experiments cycles. Thus, it would streamline the process of Bacterial Fuel Cells production using the CV of medicinal herbs. [ABSTRACT FROM AUTHOR]

Bioelectrochemical systems (BESs) are emerging as attractive routes for sustainable energy generation, environmental remediation, bio-based chemical production and beyond. Electron shuttles (ESs) can be reversibly oxidized and reduced among multiple redox reactions, thereby assisting extracellular electron transfer (EET) process in BESs. Here, we explored the effects of 14 ESs on EET in Shewanella oneidensis MR-1, and found that anthraquinone-2-sulfonate (AQS) led to the highest cathodic current density, total charge production and reduction product formation. Subsequently, we showed that the introduction of -OH or -NH2 group into AQS at position one obviously affected redox potentials. The AQS-1-NH2 exhibited a lower redox potential and a higher Coulombic efficiency compared to AQS, revealing that the ESs with a more negative potential are conducive to minimize energy losses and improve the reduction of electron acceptor. Additionally, the cytochromes MtrA and MtrB were required for optimal AQS-mediated EET of S. oneidensis MR-1. This study will provide new clues for rational design of efficient ESs in microbial electrosynthesis.

[Display omitted] • A simultaneous bioelectricity generation and complete DNRA strain was isolated. • Genome and q-PCR analysis explain the mechanism of DNRA and outward EET pathway. • The presence of NH 4 + significantly enhances output voltage and the DNRA performance. • Bioelectricity promotes the up-regulated expression of NarG and NirB related to DNRA. • The bacterial community of cathode biofilms was affected by the bioelectricity. The synchronous bioelectricity generation and dissimilatory nitrate reduction to ammonium (DNRA) pathway in Klebsiella variicola C1 was investigated. The presence of bioelectricity facilitated cell growth on the anodic biofilms, consequently enhancing the nitrate removal efficiency decreasing total nitrogen levels and causing a negligible accumulation of NO 2 – in the supernatant. Genomic analysis revealed that K. variicola C1 possessed a complete DNRA pathway and largely annotated electron shuttles. The up-regulated expression of genes narG and nirB , encoding nitrite oxidoreductase and nitrite reductase respectively, was closely associated with increased extracellular electron transfer (EET). High-throughput sequencing analysis was employed to investigate the impact of bioelectricity on microbial community composition within cathodic biofilms. Results indicated that Halomonas , Marinobacter and Prolixibacteraceae were enriched at the cathode electrodes. In conclusion, the integration of a DNRA strain with MFC facilitated the efficient removal of wastewater containing high concentrations of NO 3 – and enabled the environmentally friendly recovery of NH 4 +. [ABSTRACT FROM AUTHOR]

Abstract For renewable and sustainable bioenergy utilization with cost-effectiveness, electron-shuttles (ESs) (or redox mediators (RMs)) act as electrochemical “catalysts” to enhance rates of redox reactions, catalytically accelerating electron transport efficiency for abiotic and biotic electrochemical reactions. ESs are popularly used in cellular respiratory systems, metabolisms in organisms, and widely applied to support global lives. Apparently, they are applicable to increase power-generating capabilities for energy utilization and/or fuel storage (i.e., dye-sensitized solar cell, batteries, and microbial fuel cells (MFCs)). This first-attempt review specifically deciphers the chemical structure association with characteristics of ESs, and discloses redox-mediating potentials of polyphenolics-abundant ESs via MFC modules. Moreover, to effectively convert electron-shuttling capabilities from non-sustainable antioxidant activities, environmental conditions to induce electrochemical mediation apparently play critical roles of great significance for bioenergy stimulation. For example, pH levels would significantly affect electrochemical potentials to be exhibited (e.g., alkaline pHs are electrochemically favorable for expression of such electron-shuttling characteristics). Regarding chemical structure effect, chemicals with ortho- and para-dihydroxyl substituents-bearing aromatics own convertible characteristics of non-renewable antioxidants and electrochemically catalytic ESs; however, ES capabilities of meta-dihydroxyl substituents can be evidently repressed due to lack of resonance effect in the structure for intermediate radical(s) during redox reaction. Moreover, this review provides conclusive remarks to elucidate the promising feasibility to identify whether such characteristics are non-renewable antioxidants or reversible ESs from natural polyphenols via cyclic voltammetry and MFC evaluation. Evidently, considering sustainable development, such electrochemically convertible polyphenolic species in plant extracts can be reversibly expressed for bioenergy-stimulating capabilities in MFCs under electrochemically favorable conditions.

• Τen herbal extracts were subjected to cyclic voltammetry for 100 times. • The Houttuynia cordata extract (HCE) possessed the highest CV area. • The supplement of HCE in MFC was found to have a PD value of 33.70 mW/m2. • Τhe CV areas were positively correlated to the PD ratios in the MFC tests. This first-attempt study revealed electrochemically promising strategy to provide optimal herbal species of supplementation as electron shuttling stimulator to microbial fuel cells (MFCs). First, cyclic voltammetry (CV) was used to screen the feasible electron shuttle (ES) materials after serial testing of 100 cycles. The results indicated that Houttuynia cordata extract (HCE) possessed the highest CV area of closed-loop profile (i.e., the most appropriate ES). Alternanthera paronychioides exhibited a symmetrical peak in the oxidation and reduction profiles. All of the herb extracts were subjected to the Shewanella sp. WLP72-inoculated double-chamber MFC to compare their power density (PD) ratios. The optimal supplement of HCE in MFC was found to have a PD value of 33.70 mW/m2, which was 2.67-fold higher than the blank control. It indicated that HCE could express the highest electrochemical characteristics as a potential ES. In addition, the CV areas were positively correlated to the PD ratios in the MFC tests. However, there were no correlations between CV areas to their total polyphenol contents (TPCs) and 2,2-Diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity. This study showed that HCE contains polyphenolic compounds (e.g., kaempferol, chlorogenic acid, and quercitrin), indicating that HCE possesses highly promising ES characteristics for bioenergy extraction. [ABSTRACT FROM AUTHOR]

• PM-based strategy is firstly proposed for MCFAs production from sludge. • PM promotes MCFAs to 12 g COD/L and reduces the fluxes of EA/ED to LCAs/CH 4. • Microbial metabolism and abundance related to MCFAs formation are heightened by PM. • 1O 2 (61.5 %) and RMnS (38.5 %) are the contributors for sludge disruption. • The in-situ formed MnO 2 can serves as electron shuttles to largely facilitate CE. Medium-chain fatty acids (MCFAs) production from sewage sludge is mainly restricted by the complex substrate structure, competitive metabolism and low electron transfer rate. This study proposes a novel permanganate (PM)-based strategy to promote sludge degradation and MCFAs production. Results show that PM pretreatment significantly increases MCFAs production, i.e. , attaining 12,036 mg COD/L, and decreases the carbon fluxes of electron acceptor (EA)/electron donor (ED) to byproducts. Further analysis reveals that PM oxidation enhances the release and biochemical conversion of organic components via disrupting extracellular polymers (EPS) structure and reducing viable cells ratio, providing directly available EA for chain elongation (CE). The microbial activity positively correlated with MCFAs generation are apparently heightened, while the competitive metabolism of CE (i.e. , methanogensis) can be completely inhibited. Accordingly, the functional bacteria related to critical bio-steps and dissimilatory manganese reduction are largely enriched. Further mechanism exploration indicates that the main contributors for sludge solubilization are 1O 2 (61.6 %) and reactive manganese species (RMnS), i.e. , Mn(V)/Mn(VI) (22.3 %) and Mn(III) (∼16.1 %). As the main reducing product of PM reaction, manganese dioxide (MnO 2) can enable the formation of microbial aggregates, and serve as electron shuttles to facilitate the carbon fluxes to MCFAs during CE process. Overall, this strategy can achieve simultaneous hydrogen recovery, weaken competitive metabolisms and provide electron transfer accelerator for CE reactions. [Display omitted] [ABSTRACT FROM AUTHOR]

• RZ greatly increased the electrochemical activity of Geobacter anode biofilm. • Facilitated effect of RZ remained after fresh electrolyte replacement without RZ. • RR, as an intermediate product of RZ, was the actual electron shuttle. • RR retained in the anode biofilm due to in situ deposition and binding with eDNA. Electrochemically active bacteria (EAB) have promising applications in renewable energy recovery, biofuel production, environmental remediation, and wastewater treatment. Resazurin (RZ) is an effective exogenous electron mediator to facilitate extracellular electron transfer (EET) in EAB, but it is not fully understood that how it catalyzes EET within thick electroactive biofilm. Using Geobacter sulfurreducens as a model EAB, this study investigated the effect and mechanisms of RZ-mediated electron transfer in anode biofilm in a bioelectrochemical system. It was found that the amendment of RZ substantially enhanced current generation of anode biofilm, and the facilitating effect of RZ remained even after fresh electrolyte replacement without RZ replenishment. As an intermediate product of RZ, resorufin (RR) was the actual electron shuttle retained within the entire anode biofilm, responsible for the observed stable electron shuttling efficiency. The retention of RR in anode biofilm might be the consequences of insoluble RR deposition and RR binding with extracellular DNA (eDNA) through intercalative interactions. This study provided in vivo and in vitro evidences for the first time that eDNA provided binding sites for RR in Geobacter biofilm, and proposed a novel electron shuttling mechanism of phenoxazine catalyzing efficient EET of electroactive biofilm. [Display omitted] [ABSTRACT FROM AUTHOR]

Magnetite nanoparticles (nanoFe3O4) have been reported to facilitate direct interspecies electron transfer (DIET) between syntrophic bacteria and methanogens thereby improving syntrophic methanogenesis. However, whether or how nanoFe3O4 affects acetotrophic methanogenesis remain unknown. Herein, we demonstrate the unique role of nanoFe3O4 in accelerating methane production from direct acetotrophic methanogenesis in Methanosarcina-enriched cultures, which was further confirmed by pure cultures of Methanosarcina barkeri. Compared with other nanomaterials of higher electrical conductivity such as carbon nanotubes and graphite, nanoFe3O4 with mixed valence Fe(II) and Fe(III) had the most significant stimulatory effect on methane production, suggesting its redox activity rather than electrical conductivity led to enhanced methanogenesis by M. barkeri. Cell morphology and spectroscopy analysis revealed that nanoFe3O4 penetrated into the cell membrane and cytoplasm of M. barkeri. These results provide the unprecedented possibility that nanoFe3O4 in the cell membrane of methanogens serve as electron shuttles to facilitate intracellular electron transfer and thus enhance methane production. This work has important implications not only for understanding the mechanisms of mineral-methanogen interaction but also for optimizing engineered methanogenic processes.

Summary: This study proposed an expert system approach on the basis of artificial intelligence (AI) in the modeling of cyclic voltammogram (CV) profiles of green tea extracts. AI approach of artificial neural networks is applied to generate the model phase‐plane portraits of current output versus applied voltage through CV scan cycles. The predicted current values were validated using experiments, and generic ability of approach was examined by testing on the CV scan cycles generated from Syzygium aromaticum and Citrus reticulate. It was concluded that AI approach can be employed to reveal stable point (cycle and voltage) in CV profiles for bioenergy applications. [ABSTRACT FROM AUTHOR]

• Unveil dual role of epinephrine (EP) and dopamine (DA) as neurotransmitters and electron shuttles. • Uncover bioelectrochemical stimulation of EP and DA for power generation in microbial fuel cells. • Decipher more significant electrochemical characteristics of EP and DA than vitamin B 2 , vitamin C and gallic acid. This first-attempt study provided novel electrochemical exploration via microbial fuel cells (MFCs), suggesting why catecholamine hormone-neurotransmitters epinephrine (EP) and dopamine (DA) could effectively trigger acute stress responses for humans. According to bioelectrochemical engineering, EP and DA are ortho-diphydroxyl (o -diOH) substituent(s)-bearing electron shuttles (ESs) that could effectively mediate electron-transfer capabilities for bioenergy extraction. This study quantitatively compared redox-mediating characteristics of EP and DA with other antioxidants and ESs, suggesting possible mechanisms of bioenergy-driven responses to organisms. Although some electrochemical activities of EP and DA were characterized in medical literature, this work clarified that EP and DA were also electrochemically favorable ESs for efficient bioenergy extraction. Compared to vitamin B 2 (VB), vitamin C (VC) and gallic acid (GA), quantitative evaluation upon bioenergy-stimulating capabilities of EP and DA were clearly revealed via MFC modules. Apparently, both EP and DA as ESs significantly exhibited stable and reversible redox potential peaks in cyclic voltammetry. Evidently, power density performances of MFCs supplemented with EP and DA considerably increased ca. 127–385%, suggesting that DA and EP would be the most appropriate ESs to effectively stimulate electron-mediating capabilities in multicellular organisms. It was also suspected that highly efficient power-stimulating capabilities of DA and EP could be strongly associated with their electrochemically-steered disease-curative and life-threatening capabilities to humans. [ABSTRACT FROM AUTHOR]

The present study describes anoxic Mn bioleaching of low grade ferromanganese ores using a dissimilatory Mn reducing consortia (MRC). Effect of external addition of electron shuttles such as humic acid and anthraquinone sulphonic acid was studied under same conditions. The MRC were found to be capable of utilizing MnO 2 as electron acceptor as evidenced by Mn dissolution in media. In the presence of electron shuttles, Mn extraction was found to be enhanced by 2 to 4 times compared to the case without. A maximum of 58 (±3%) Mn was found to be leached from an amorphous ore while 38 (±2%) Mn was leached from a crystalline ore, in 50 days in presence of 100 μM electron shuttle at 20 g/L S:L ratio. A significant finding was consistently low leaching of Fe under all conditions studied (<50 mg/L or < 5%) and in-situ oxidation of Fe(II) by MnO 2 to form insoluble ferric compounds as observed in residues. The presence of electron shuttle enhances reductive dissolution by MRC which favors dissolution of Mn more than Fe under anoxic conditions. X-ray fluorescence spectroscopy and X-ray diffraction peaks of residues confirm that MnO 2 is preferentially dissolved than Fe 2 O 3. These findings indicate that anoxic bioleaching can help to selectively recover Mn from low grade ferromanganese ores. Unlabelled Image • Anoxic bioleaching of Mn from low garde ores by dissimilatory Mn reducing organisms (MRC). • Enhanced Mn dissolution and restricted Fe dissolution in presence of electron shuttles HA and AQS. • Preferred Mn bioreduction over Fe and poor solubility of Fe(II) attributed to observed phenomenon. [ABSTRACT FROM AUTHOR]

Abstract A novel Shewanella @graphene core-shell composite material was fabricated following one-step bioreduction of graphene oxide (GO) by Shewanella putrefaciens CN-32. The surface properties and microstructures were characterized by FTIR, TG-DTG, SEM and TEM, which indicate that GO was effectively reduced to rGO and subsequently loaded onto the outer surface of the microbe Shewanella. CLSM was performed to get insight into the growth of the bacteria after core-shell materials formation. The reduction properties of Shewanella @graphene materials were evaluated using nitrobenzene, a representative model pollutant, as an electron acceptor. The reduction efficiency of Shewanella was improved by strengthening the contact among electron donors, electron shuttles and electron acceptors and changed with the proportions of core-shell materials. The optimal proportion of the core-shell material was OD600 = 0.6:GO = 10 mg/L, which was enhanced by the wrapped rGO and improved adsorption capability. The reduction rate was elevated 30% in comparison with pure Shewanella. In addition, the core-shell material exhibited a favorable recycling performance, which can be reused for at least five times. Facile fabrication and enhanced reduction performance of Shewanella @graphene core-shell composite endows this material with considerable potential in environmental remediation. Graphical abstract Unlabelled Image Highlights • A novel Shewanella @graphene core-shell material was fabricated via a one-step method. • Graphene oxide (GO) was effectively converted into reduced GO by Shewanella CN-32. • rGO was subsequently loaded onto the outer surface of the microbe Shewanella. • The reduction efficiency of Shewanella was improved by the presence of rGO. • rGO was functioned as additional sites and electron shuttles for nitrobenzene reduction. [ABSTRACT FROM AUTHOR]

Highlights • Unveil extraction of electron shuttles (ESs) from medicinal herb for MFC uses. • Disclose electrochemical criteria to switch flavonoid antioxidants to be ESs. • Suggest chemical structure and toxicity potency as keystone factors of ESs. Abstract Prior studies mentioned that extracts of polyphenolics-abundant edible flora might provide compositions of reversible electron shuttles for bioenergy extraction; however, detailed mysteries behind such phenomena still remained open to be explored. This first-attempt study selected a medicinal herb Scutellaria baicalensis Georgi and its main flavonoid ingredients to quantitatively decipher their electron-mediating activities for bioenergy applications. The main ingredients with strong shuttling activities (e.g. , baicalin, baicalein and wogonin) were chemical structure-associated to disclose electrochemical and bioenergetics characteristics for comparative analysis. Evidently, baicalin owned the most promising electron shuttling capabilities. Although extract of flavonoids-rich S. baicalensis Georgi also showed promising redox-mediating characteristics, typical ingredient-baicalein still exhibited high biotoxicity to bacterial CO 2 respirometric production, thereby resisting fully expression of electron transfer capabilities. Supplementation of baicalin and extract of S. baicalensis Georgi to microbial fuel cells (MFCs) was conducted to quantify such stimulating capabilities of bioelectricity generation. That is, with toxicity attenuation, processed flavonoids-rich medicinal herbs were possibly electroactive for bioenergy extraction due to favorable electron-shuttling activities. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

[Display omitted] • CHA was produced to enhance HBu/HAc-DF pathway for more H 2. • H 2 evolution via dark fermentation (DF) raised by 35.27% with CHA. • CHA acted as an electron shuttle to facilitate electron transfer. • Ca2+ and Fe2+ ions were released from CHA to stimulate microbial activity. • CHA could promote the expression of functional genes from DF. For low yield in dark fermentation (DF), in this study, the carbonized humic acid (CHA) was produced and added to DF for enhancing biohydrogen (bioH 2) yield at mesophilic condition. The highest bioH 2 yield was 151.08 mL/g glucose with the addition of CHA at 80 mg/L, which was 35.27% and 16.53% higher than those of 0 mg/L CHA and 80 mg/L mineral humic acid (MHA) groups, respectively. Electrons preferentially conducted via the butyrate pathway due to CHA amendments, which corresponded to the prediction of relevant functional genes. Furthermore, CHA possessed distinctive advantages over MHA, which acted as an electron shuttle to facilitate electron transfer, released metal ions as an essential signal mediator and favored the reduction of ferredoxin, obtaining more H 2. The use of CHA in the field of H 2 -DF depicted the high-value utilization and industrial chain extension of MHA. [ABSTRACT FROM AUTHOR]

Electron shuttles (ESs) and Fe-bearing clay minerals are commonly found in subsurface environments and have shown potential in enhancing the bioreduction of Cr(VI). However, the synergistic effect of ESs at different redox potentials and Fe-bearing clay minerals on Cr(VI) bioreduction, as well as the fundamental principles governing this process, remain unclear. In our study, we investigated the role of ESs and Fe(III) in Cr(VI) bioreduction. We found that the acceleration of ESs and Fe(III) are crucial factors in this process. Interestingly, the promotion of ESs on Cr(VI) and Fe(III) showed opposite trends. Electrochemical methods confirmed the limited steps are the extent of reduced ESs and the redox potential difference between ESs and Fe(III), separately. Furthermore, we investigated the combined effect of ESs and NAu-2 on Cr(VI) bioreduction. Our results revealed two segments: in the first segment, the ES (5-HNQ) and NAu-2 did not synergistically enhance Cr(VI) reduction. However, in the second segment, ESs and NAu-2 demonstrated a synergistic effect, significantly increasing Cr(VI) reduction by MR-1. These bioreduction processes all follow linear free energy relationships (LFERs). Overall, our study highlights the fundamental principles governing multivariate systems and presents a promising approach for the remediation of Cr(VI)-contaminated sites. [Display omitted] • ESs can enhance the bioreduction of Cr(VI) and nontronite NAu-2. • ESs promote bioreduction of Cr(VI) and NAu-2 in opposite trends following LFERs. • The combination of ESs and NAu-2 can synergistically enhance Cr(VI) bioreduction. • The combined effect of ESs and NAu-2 on Cr(VI) bioreduction conforms to LFERs. • LFERs can predict reaction rates and screen ESs for use in remediation technologies. [ABSTRACT FROM AUTHOR]

Anaerobes respire extracellular electron acceptors by extracellular electron transfer (EET). It is widely recognized that flavins can act as electron shuttles to facilitate this process. Flavin synthesis genes are widely distributed in Geobacter species. However, the functions of flavins in the EET of Geobacter species are unclear. Here, we demonstrate that G. uraniireducens can secrete abundant riboflavin (up to 270 nM) to facilitate EET. When an electrode was used as the electron acceptor, the quick recovery of anodizing current after anolyte replacement and the electrochemical behavior of the G. uraniireducens biofilm characterized by differential pulse voltammetry suggest that the self-secreted riboflavin promoted EET by serving as bound redox cofactors for cytochromes. On the contrary, when Fe(III) oxide was the electron acceptor, free riboflavin acted as electron shuttle to mediate the reduction of Fe(III) oxide. The results demonstrate the flexibility of flavins in EET, suggesting that the properties of electron acceptors can affect the binding mode of extracellular flavins, and broaden the knowledge of the EET of Geobacter species.

Conductive materials facilitate direct interspecies electron transfer between acidogens and methanogens during methane (CH4) production. Soluble microbial products (SMP) and extracellular polymeric substances (EPS) produced by microorganisms might act as the electron shuttle between microorganisms and conductive materials. In this study, effects of conductive ferroferric oxide (Fe3O4) on anaerobic treatment process and the production of SMP and EPS were investigated. The maximum CH4 production rate was enhanced by 23.3% with the dosage of Fe3O4. The concentrations of proteins, polysaccharides, and humic substances in tightly bound EPS (T-EPS) were promoted, suggesting that extracellular metabolisms were induced by conductive materials. Distribution of potential electron shuttles such as quinone-like substances, flavins, aromatic amino acids, and dipeptides in SMP and EPS phases were comprehensively investigated and these electron shuttles were significantly affected by Fe3O4. Dipeptides consisting of phenylalanine were widely detected in T-EPS of the Fe3O4 reactor, indicating a potential different extracellular electron exchange pattern with the addition of conductive materials.

Three decades ago, dye-sensitized solar cells (DSSCs) emerged as a route for harnessing the sun's energy and converting it into electricity. Since then, an impressive amount of work has been devoted to improving the global photovoltaic efficiency of DSSCs, trying to optimize all components of the device. Up to now, the best efficiencies have usually been reached with ruthenium(II) photosensitizers, even if in the last few years many classes of organic compounds have shown record efficiencies. However, the future of DSSCs is stringently connected to the research and development of cheaper materials; in particular, the replacement of rare metals with abundant ones is an important topic in view of the long-term sustainability of DSSCs intended to replace the consolidated fossil-based technology. In this context, copper is a valid candidate, being both an alternative to ruthenium in the fabrication of photosensitizers and a material able to replace the common triiodide/iodide redox couple. Thus, recently, some research papers have confirmed the great potential of copper(I) coordination complexes as a cheap and convenient alternative to ruthenium dyes. Similarly, the use of copper compounds as electron transfer mediators for DSSCs can be an excellent way to solve the problems related to the more common I 3 - redox couple. The goal of this mini-review is to report on the latest research devoted to the use of versatile copper complexes as photosensitizers and electron shuttles in DSSCs. The coverage, from 2022 up to now, illustrates the most recent studies on dye-sensitized solar cells based on copper complexes as molecular materials.- redox couple. The goal of this mini-review is to report on the latest research devoted to the use of versatile copper complexes as photosensitizers and electron shuttles in DSSCs. The coverage, from 2022 up to now, illustrates the most recent studies on dye-sensitized solar cells based on copper complexes as molecular materials.

Anaerobes respire extracellular electron acceptors by extracellular electron transfer (EET). It is widely recognized that flavins can act as electron shuttles to facilitate this process. Flavin synthesis genes are widely distributed in Geobacter species. However, the functions of flavins in the EET of Geobacter species are unclear. Here, we demonstrate that G. uraniireducens can secrete abundant riboflavin (up to 270 nM) to facilitate EET. When an electrode was used as the electron acceptor, the quick recovery of anodizing current after anolyte replacement and the electrochemical behavior of the G. uraniireducens biofilm characterized by differential pulse voltammetry suggest that the self-secreted riboflavin promoted EET by serving as bound redox cofactors for cytochromes. On the contrary, when Fe(III) oxide was the electron acceptor, free riboflavin acted as electron shuttle to mediate the reduction of Fe(III) oxide. The results demonstrate the flexibility of flavins in EET, suggesting that the properties of electron acceptors can affect the binding mode of extracellular flavins, and broaden the knowledge of the EET of Geobacter species. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • CMs promoted phenol degradation and microbial resistance to oxygen impact. • Extracellular polymeric substance alternation, benefiting interspecies electron transfer. • CMs enriched functional genera in phenol degradation and electron transfer. • Microbial community structure varied with types of materials. Abstract Anaerobic biodegradation is a promising and economical process to remove phenol in the wastewater, although the reaction kinetics are often limited by the toxicity of phenol. This study proposed a strategy to accelerate phenol degradation and mitigate its inhibition on the bioprocess by adding conductive materials (CMs) into the anaerobic degradation system. The results showed that CMs could stimulate phenol degradation up to one–fold higher than that in control group. Certain extracellular polymeric substances (EPS) compounds that can act as electron shuttles, i.e. protein and humic substances, were greatly enriched with the existence of CMs. In particular, compared with control group, CMs supplemented groups had 2.3 and 10 to 20 folds low molecular weight protein in bound and soluble EPS, respectively. Carbon nanotube supplemented group exhibited 3 times higher humic substance in soluble EPS. The overall electron transport system activity in CMs groups was much higher than that in control groups. The addition of CMs enriched phenol degradation bacteria– Syntrophorhabdus , Brooklawnia, Treponema and Syntrophus , as well as electroactive methanogens– Methanosaeta. It is proposed that Syntrophus and Methanosaeta were the functional genera in methanogenic phenol degradation via extracellular electron transfer. This study revealed that the presence of CMs altered the EPS composition and microbial community of the system and made it become more favorable for extracellular electron transfer. [ABSTRACT FROM AUTHOR]

Conductive materials facilitate direct interspecies electron transfer between acidogens and methanogens during methane (CH4) production. Soluble microbial products (SMP) and extracellular polymeric substances (EPS) produced by microorganisms might act as the electron shuttle between microorganisms and conductive materials. In this study, effects of conductive ferroferric oxide (Fe3O4) on anaerobic treatment process and the production of SMP and EPS were investigated. The maximum CH4 production rate was enhanced by 23.3% with the dosage of Fe3O4. The concentrations of proteins, polysaccharides, and humic substances in tightly bound EPS (T-EPS) were promoted, suggesting that extracellular metabolisms were induced by conductive materials. Distribution of potential electron shuttles such as quinone-like substances, flavins, aromatic amino acids, and dipeptides in SMP and EPS phases were comprehensively investigated and these electron shuttles were significantly affected by Fe3O4. Dipeptides consisting of phenylalanine were widely detected in T-EPS of the Fe3O4 reactor, indicating a potential different extracellular electron exchange pattern with the addition of conductive materials. [ABSTRACT FROM AUTHOR]

The ability of anthraquinone-2,6-disulfonate (AQDS) and riboflavin to enhance the sulfamethoxazole (SMX) degradation coupled with the Fe(III) reduction by Shewanella oneidensis MR-1 was investigated. The results indicated that the SMX degradation rate was 38.5% with an initial SMX concentration at 0.04 mM. For the overall performance of AQDS and riboflavin mediated SMX degradation and iron reduction, the SMX degradation rate was gradually increased with the enhancement of iron reduction. Riboflavin had a stronger enhancement on SMX degradation and iron reduction than AQDS, but the enhancement was not positively correlated with electron shuttles concentration. A quantitative characterization of the electron transfer capacity (ETC) of the electron shuttles showed that the ETC was higher for riboflavin than AQDS. The S. oneidensis MR-1 16S rRNA gene copies results indicated that electron shuttles had a positive effect on the microbial activity of S. oneidensis MR-1. The LC MS result indicated that the products of the SMX biodegradation were 3-amino-5-methylisoxazole and 4-aminobenzenesulfonic acid, which suggested that the SMX biodegradation was caused by S N bond cleavage. This study indicates that the biochemical mechanisms play a vital role in SMX transformation and Fe(II) generation in this system. [ABSTRACT FROM AUTHOR]

Electron shuttles extensively exist in various environments. Some kinds of organic substances can be applied by microorganisms to produce electrons, and then the electrons can be transferred to other substances or microorganisms through electron shuttles, resulting in coexistence and interaction of diverse species of microbes. In this review, the functional mechanisms of extracellular electron transfer mediated by different electron shuttles are described. And different subtypes as well as the application of electron shuttles in microbial degradation of pollutants, microbial electricity, and the promotion of energy generation are also discussed. Summary results show that extracellular electron transfer is based on the electrogenesis microorganism with the structure of cytochromes or pili. Materials were usually used in long-distance electron transfer because of their widespread presence and abundance. Therefore, the review is beneficial to perceive the pathways of extracellular electron transfer mediated by electron shuttles and explore the contribution of different electron shuttles in extracellular electron transfer. [ABSTRACT FROM AUTHOR]

Purpose: Extracellular Fe(III) reduction plays an important role in a variety of biogeochemical processes. Several mechanisms for microbial Fe(III) reduction in pH-neutral environments have been proposed, but pathways of microbial Fe(III) reduction within alkaline conditions have not been clearly identified. Alkaline soils are vastly distributed; thus, a better understanding of microbial Fe(III) reduction under alkaline conditions is of significance. The purpose of this study is to explore the dominant mechanism of bacterial iron reduction in alkaline environments. Materials and methods: We used antraquinone-2,6-disulfonate (AQDS) as a representative of quinone moities of humic substances and elemental sulfur and sulfate as sulfur species to investigate the potential role of humic substances and sulfur species in mediating microbial Fe(III) reduction in alkaline environments. We carried out thermodynamic calculations to predict the ability of bacteria to reduce Fe(III) (oxyhydr)oxides under alkaline conditions and the ability of AQDS and sulfur species to serve as electron acceptors for microbial anaerobic respiration in an assumed alkaline soil environments. A series of incubation experiments with two model dissimilatory metal reducing bacteria, Shewanella oneidensis MR-1 and Geobacter sulfurreducens PCA as well as mixed bacteria enriched from a soil were performed to confirm the contribution of AQDS and sulfur species to Fe(III) reduction under alkaline conditions. Results and discussion: Based on thermodynamic calculations, we predicted that, under alkaline conditions, the enzymatic reduction of Fe(III) (oxyhydr)oxides would be thermodynamically feasible but very weak. In our incubation experiments, the reduction of ferrihydrite by anaerobic cultures of Shewanella oneidensis MR-1, Geobacter sulfurreducens PCA or microbes enriched from a soil was significantly increased in the presence of S or AQDS. Notably, AQDS contributed more to promoting Fe(III) reduction as a soluble electron shuttle than S did under the alkaline conditions probably because of different mechanisms of microbial utilization of AQDS and S. Conclusions: These results suggest that microbial reduction of Fe(III) (oxyhydr)oxides under alkaline conditions may proceed via a pathway mediated by electron shuttles such as AQDS and S. Considering the high ability of electron shuttling and vast distribution of humic substances, we suggest that humic substance-mediated Fe(III) reduction may potentially be the dominant mechanism for Fe(III) reduction in alkaline environments. [ABSTRACT FROM AUTHOR]

• Facet-dependent reductive dissolution is facilitated by cysteine. • As(V) reallocations is coupled to reductive dissolution of hematite. • Reductive dissolution negligibly affects As(III) immobilization. • As(V) reallocation on hematite is facet-dependent controlled by ET rates. • Hematite {001} facet exhibits highest ET and As immobilization ability. The interfacial electron transfer (ET) between electron shuttling compounds and iron (Fe) oxyhydroxides plays a crucial role in the reductive dissolution of Fe minerals and the fate of surface-bound arsenic (As). However, the impact of exposed facets of highly crystalline hematite on reductive dissolution and As immobilization is poorly understood. In this study, we systematically investigated the interfacial processes of the electron shuttling compound cysteine (Cys) on various facets of hematite and the reallocations of surface-bound As(III) or As(V) on the respective surfaces. Our results demonstrate that the ET process between Cys and hematite generates Fe(II) and leads to reductive dissolution, with more Fe(II) generated on {001} facets of exposed hematite nanoplates (HNPs). Reductive dissolution of hematite leads to significantly enhanced As(V) reallocations on hematite. Nevertheless, upon the addition of Cys, a raipd release of As(III) can be halted by its prompt re-adsorption, leaving the extent of As(III) immobilization on hematite unchanged throughout the course of reductive dissolution. This is due to that Fe(II) can form new precipitates with As(V), a process that is facet-dependent and influenced by water chemistry. Electrochemical analysis reveals that HNPs exhibit higher conductivity and ET ability, which is beneficial for reductive dissolution and As reallocations on hematite. These findings highlight the facet-dependent reallocations of As(III) and As(V) facilitated by electron shuttling compounds and have implications for the biogeochemical processes of As in soil and subsurface environments. [Display omitted] [ABSTRACT FROM AUTHOR]

As agricultural waste recycling and reuse is a top-priority task to follow "net zero by 2050″, this study selected Citrus microcarpa peels and pulps for a feasibility study to minimize bioenergy-related CO 2 emissions. In the Philippines, C. microcarpa peels and pulps are considered agricultural wastes that contain an abundance of flavonoids, which possess an array of biological activities. Herein, the exploration of power augmenting capabilities and investigation of electron shuttling species in C. microcarpa extracts was implemented for bioenergy applications of circular economy. The phytochemical and antioxidant activity analysis of hot water and ethanol extracts was carried out using a microplate reader. Evaluation of the bioenergy-generating capabilities and electrochemical characteristics of the herbal extracts was implemented through the MFC platform and 50 cycles of cyclic voltammetry. The ethanol extracts of the C. microcarpa peels and pulps exhibited superior bioelectricity stimulation in MFCs. Furthermore, the ethanol extracts performed as better antioxidants compared to the water extracts. According to 50 cycles of CV scanning, only the ethanol extracts exhibited electron shuttling characteristics as indicated by the symmetry of their reduction and oxidation peaks. These results are attributed to the abundance of flavonoids and condensed tannins in the extracts, as revealed by principal component analysis. • First revealed the potential of C. microcarpa wastes as a bioenergy resource. • Ethanol extracts possess highest TPC, TFC, TTC, and antioxidant activity. • Ethanol extracts yielded higher bioelectricity generation in MFCs. • Electron shuttles in the extracts were rutin, caffeic acid, neoeriocitrin, quercetin. • PCA reveals that TFC is responsible for high antioxidant activity and power generation. [ABSTRACT FROM AUTHOR]

Reduced graphene oxide (rGO) can enhance the bioreduction of nitrobenzene by Shewanella oneidensis MR-1. The effects of surface modification and coexisting soluble electron shuttles on rGO-enhanced bioreduction of nitrobenzene were investigated here. The results showed that rGO enhanced the secretion of extracellular polymeric substance and the bioreduction of nitrobenzene of several folds. No inhibition effect on nitrobenzene bioreduction was observed even when the concentration of rGO was as high as 200 mg/L. The surface modification of rGO might affect the electrical conductivity which was assumed as one of the main factors that contributed to the enhancement of nitrobenzene bioreduction by rGO materials. Moreover, the coexisting electron shuttles further enhanced the rGO-mediated nitrobenzene bioreduction. After the simultaneous addition of flavin mononucleotide (10 μM) and rGO (50 mg/L), the reduction rate increased 7.8 times to 424.98 ± 7.84 mg (nitrobenzene)/(g (dry cell)∙h), which was higher than those ever reported. [ABSTRACT FROM AUTHOR]

As antioxidants could scavenge free radicals and active oxygen groups, their electrochemical characteristics were also closely related to electron transfer (ET) capabilities. Prior studies revealed that decolorized metabolites (DM) of textile dyes as electron shuttles (ESs) could stimulate ET capabilities of reductive decolorization and bioelectricity generation (RD&BG). Cyclic voltammetric analysis showed that several medicinal herbs owned promising redox-mediating capabilities to be antioxidants or ESs. Due to such dual characteristics, supplementation of intact tea extracts could stimulate power generation of microbial fuel cells, no matter what degree of tea fermentation was treated. With the abundance of polyphenolics, antioxidant potency of tea extracts was positively associated to redox mediating characteristics. Electron-shuttling mechanisms of compositions of tea extract ( e.g. , epigallocatechin gallate, gallocatechin, gallic acid) were also proposed. [ABSTRACT FROM AUTHOR]

As known, naturally-present edible flora contained antioxidants ( e.g ., polyphenolic compounds) that are significant to human health. Prior studies revealed that decolorized metabolites (DM) of textile dyes might stimulate electron transfer (ET) capabilities of reductive decolorization and bioelectricity generation (RD&BG) due to their roles as electron shuttles (ESs). Such ET stimulating phenomena were also suspected to be associated with characteristics of antioxidant. This first-attempt study selected 6 edible flora to consider whether antioxidant-containing herbs were feasible to be used for wastewater decolorization. Apparently, DM of Gynura bicolor showed promising electron-shuttling capabilities to increase ET efficiency of RD&BG. Moreover, according to cyclic voltammetric profiles, the dosage should be above threshold level to trigger effective ET performance. In addition, supplementation of sufficient DM of G. bicolor could significantly enhance the efficiency of RD of Reactive Black 5 (RBk5) and BG. This study suggested that supplementation of extracts of naturally present edible flora may act as ESs to stimulate RD and BG for environmentally-friendly bioremediation. [ABSTRACT FROM AUTHOR]

The reduction of uranium hexavalent (U(VI)) to tetravalent (U(IV)) is an important reaction because of the change in its mobility in the natural environment. Although the flavin mononucleotide (FMN) has acted as an electron shuttle for the U(VI) reduction in vivo system, which is called an electron mediator, only the rate constant for the electron transfer from FMN to U(VI) has been determined. This study examined the rate constant for the U(VI) reduction process by three flavin analogues (riboflavin, flavin mononucleotide, flavin adenine dinucleotide) to elucidate their substituent group effect on the U(VI) reduction rate by electrochemical methods. The formation of the U(IV) was monitored by UV-vis spectrometry at 660 nm during the constant potential electrolysis of the U(VI) solution in the presence of the mediator. The cyclic voltammograms indicated that the three flavin analogues behaved as electron mediator to reduce U(VI). The logarithmic rate constant for the U(VI) reduction was related to the standard redox potential of the mediators. This linear relationship indicated that the redox-active group of the mediator and the substituent group of the mediator dominate capability of the U(VI) reduction and its rate, respectively. The apparent reduction potential of U(VI) increased about 0.2 V in the presence of the mediators, which strongly suggests that the biological electron mediator makes the U(VI) reduction possible even under more oxidative conditions. [ABSTRACT FROM AUTHOR]

• First study to electrochemically decipher potential mechanism of anti-COVID-19 TCM formulae. • Jing Guan Fang owns bioenergy-stimulating and electron-shuttling characteristics as well as anti-inflammatory activity. • S. baicalensis and baicalin are the "main-effect" herb and major compound of JGF. • Reversible catalysis of electron shuttles (i.e., flavonoids) enhances anti-disease and anti-inflammatory efficacy. • Electron-shuttling characteristics are strongly associated with therapeutic potential. Jing Guan Fang (JGF) is an anti-COVID-19 Chinese Medicine decoction comprised of five medicinal herbs to possess anti-inflammatory and antiviral properties for treatment. This study aims to electrochemically decipher the anti-coronavirus activity of JGF and show that microbial fuel cells may serve as a platform for screening efficacious herbal medicines and providing scientific bases for the mechanism of action (MOA) of TCMs. Electrochemical techniques (e.g., cyclic voltammetry) and MFCs were adopted as the bioenergy-based platforms to assess the bioenergy-stimulating characteristics of JGF. Phytochemical analysis correlated polyphenolic and flavonoid content with antioxidant activity and bioenergy-stimulating properties. Network pharmacology on the active compounds was employed to identify anti-inflammatory and anti-COVID-19 protein targets, and molecular docking validated in silico results. This first-attempt results show that JGF possesses significant reversible bioenergy-stimulation (amplification 2.02 ± 0.04) properties suggesting that its antiviral efficacy is both bioenergy-steered and electron mediated. Major flavonoids and flavone glycosides identified by HPLC (e.g., baicalein and baicalin, respectively) possess electron-shuttling (ES) characteristics that allow herbal medicines to treat COVID-19 via (1) reversible scavenging of ROS to lessen inflammation; (2) inhibition of viral proteins; and (3) targeting of immunomodulatory pathways to stimulate the immune response according to network pharmacology. [Display omitted] [ABSTRACT FROM AUTHOR]

The coexistence of multiple pollutants and lack of carbon sources are challenges for the biological treatment of wastewater. To achieve simultaneous removal of nitrate (NO 3 −-N) and cadmium (Cd2+) at low carbon to nitrogen (C/N) ratios, 2-hydroxy-1,4-naphthoquinone (HNQ) was selected from three redox mediators as an accelerator for denitrification of heterotrophic strain Pseudomonas stutzeri sp. GF2 and autotrophic strain Zoogloea sp. FY6. Then, halloysite nanotubes immobilized with 2-hydroxy-1,4-naphthoquinone (HNTs-HNQ) were prepared and a bioreactor was constructed with immobilized redox mediator granules (IRMG) as the carrier, which was immobilized with HNTs-HNQ and inoculated with the two strains. The immobilized HNQ and the inoculated strains jointly improved the removal ability of NO 3 −-N and Cd2+ and the removal efficiency of NO 3 −-N (25.0 mg L−1) and Cd2+ (5.0 mg L−1) were 92.81% and 93.94% at C/N = 1.5 and hydraulic retention time (HRT) = 4 h. The Cd2+ was removed by adsorption of iron oxides (FeO(OH) and Fe 3 O 4) and IRMG. The electron transport system activity (ETSA) of bacteria was improved and the composition of dissolved organic matter in the effluent was not affected by HNQ. The HNQ promoted the production of FeO(OH) and up-regulated the proportion of Zoogloea (54.75% in the microbial community), indicating that Zoogloea sp. FY6 was dominant in the microbial community. In addition, HNQ influenced the metabolic pathways and improved the relative abundance of some genes involved in nitrogen metabolism and the iron redox cycle. [Display omitted] • Simultaneous removal of nitrate and cadmium at low C/N ratio was achieved. • Immobilized redox mediator granules were prepared to accelerate electron transfer. • Halloysite nanotubes were employed to immobilize and release the redox mediator. • Zoogloea sp. FY6 showed a high proportion and an important role in the bioreactor. • The 2-hydroxy-1,4-naphthoquinone affects bacterial metabolism and gene abundance. [ABSTRACT FROM AUTHOR]

The reduction of selenite and formation of selenium nanoparticles tuned by electron shuttles was investigated using Shewanella oneidensis MR-1. Results showed that anthraquinone-2,6-disulfonate (AQDS) and riboflavin profoundly accelerated the reduction of selenite ranging from 0.5 to 5.0 mM. Selenite of 2.5 mM was completely removed from the solution after 12-h reduction by S. oneidensis MR-1 with OD 600 of 0.5. The acceleration heavily depended on extracellular cytochromes OmcA and MtrC and bypassed the fumarate reductase FccA in the periplasm. These results indicated that electron shuttles diverted the selenite reduction from inside to outside cells. Consistently, extracellular formation of Se(0) nanoparticles was observed in the presence of AQDS, which will benefit for the extraction and purification of these nanoparticles. This study indicates that electron shuttles are readily controllable parameter for selenite reduction and biosynthesis of Se(0) nanoparticles. [ABSTRACT FROM AUTHOR]

Dissimilatory metal reducing organisms play key roles in the biogeochemical cycle of metals as well as in the durability of submerged and buried metallic structures. The molecular mechanisms that support electron transfer across the microbe-metal interface in these organisms remain poorly explored. It is known that outer membrane proteins, in particular multiheme cytochromes, are essential for this type of metabolism, being responsible for direct and indirect, via electron shuttles, interaction with the insoluble electron acceptors. Soluble electron shuttles such as flavins, phenazines and humic acids are known to enhance extracellular electron transfer. In this work, this phenomenon was explored. All known outer membrane decaheme cytochromes from Shewanella oneidensis MR-1 with known metal terminal reductase activity and a undecaheme cytochrome from Shewanella sp. HRCR-6 were expressed and purified. Their interactions with soluble electron shuttles were studied using stopped-flow kinetics, NMR spectroscopy and molecular simulations. The results show that despite the structural similarities, expected from the available structural data and sequence homology, the detailed characteristics of their interactions with soluble electron shuttles are different. MtrC and OmcA appear to interact with a variety of different electron shuttles in the close vicinity of some of their hemes, and with affinities that are biologically relevant for the concentrations typical found in the medium for this type of compounds. All data support a view of a distant interaction between the hemes of MtrF and the electron shuttles. For UndA a clear structural characterization was achieved for the interaction with AQDS a humic acid analogue. These results provide guidance for future work of the manipulation of these proteins toward modulation of their role in metal attachment and reduction.

Accelerating the rate-limiting Fe 3+ /Fe 2+ circulation in Fenton reactions through the addition of reducing agents (or co-catalysts) stands out as one of the most promising technologies for rapid water decontamination. However, conventional reducing agents such as hydroxylamine and metal sulfides are greatly restricted by three intractable challenges: (1) self-quenching effects, (2) heavy metal dissolution, and (3) irreversible capacity decline. To this end, we, for the first time, introduced redox-active polymers as electron shuttles to expedite the Fe 3+ /Fe 2+ cycle and promote H 2 O 2 activation. The reduction of Fe 3+ mainly took place at active N-H or O-H bonds through a proton-coupled electron transfer process. As electron carriers, H atoms at the solid phase could effectively inhibit radical quenching, avoid metal dissolution, and maintain long-term reducing capacity via facile regeneration. Experimental and density functional theory (DFT) calculation results indicated that the activity of different polymers shows a volcano curve trend as a function of the energy barrier, highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap, and vertical ionization potential. Thanks to the appropriate redox ability, polyaniline outperforms other redox-active polymers (e.g., poypyrrole, hydroquinone resin, poly(2,6-diaminopyridine), and hexaazatrinaphthalene framework) with a highest iron reduction capacity up to 5.5 mmol/g, which corresponds to the state transformation from leucoemeraldine to emeraldine. Moreover, the proposed system exhibited high pollutant removal efficiency in a flow-through reactor for 8000 bed volumes without an obvious decline in performance. Overall, this work established a green and sustainable oxidation system, which offers great potential for practical organic wastewater remediation.