Intracellular Electron Transfer Research Articles

Bio-heterointerface charging through ultrasound-boosted extracellular and intracellular electron transfer for rapid bacterial killing

The deep-seated drug-resistant bacterial infection is one of the most noticeable public-health threat owing to poor drug therapeutic effect, high recurrence, and devastating complication. Herein, we propose a bactericidal strategy of bio-heterointerface charging through ultrasound-boosted bacterial extracellular and intracellular electron transfer for eradicating implant-related drug-resistant bacterial infection, where a TiO2-modified porphyrin-based two-dimensional metal-organic framework (2DMOF-TiO2) is selected as a sonosensitizer. The ultrasound-boosted extracellular and intracellular electron transfer between methicillin-resistant Staphylococcus aureus and 2DMOF-TiO2 induces rapid reactive oxygen species (ROS) burst surrounding bacterial outer and inner, contributing to intracellular oxidation, membrane potential decrease (∼5 mV), membrane disruption, and pyrimidine metabolism disorder, thus causing bacterial death. The in vivo results of 2DMOF-TiO2 implant exhibit rapid sonocatalytic anti-infection and enhanced osseointegration at bone-implant interface. This platform may inspire the universal thinking about ultrasound-boosted extracellular and intracellular electron-transfer-induced ROS and provide a superior therapeutic candidate for various deep-seated infectious diseases.

Synchronously improving intracellular electron transfer in electron-donating bacteria and electron-accepting methanogens for facilitating direct interspecies electron transfer during anaerobic digestion of kitchen wastes

Two major issues generally limit the effectiveness of direct interspecies electron transfer (DIET) during anaerobic digestion: 1) lack of essential electrical connection component, electrically conductive pili (e-pili) or multi-heme c-type cytochrome (MHC); 2) the thermodynamic limitations of combining electrons with protons for reducing carbon dioxide to methane. To address both issues, a strategy for facilitating DIET via combining ethanol-type fermentation pretreatment (EFP) with NaCl addition during anaerobic digestion of kitchen wastes was proposed. Combining EFP with NaCl addition dramatically increased methane yield rates (531.3 ± 11.4 vs 410.5 ± 8.7 mL/gVSadded/d) and efficiencies of conversion of organic substrates to methane (571.2 ± 13.8 vs 488.7 ± 11.7 mL/gVSremoval). The increased performances by combining EFP with NaCl addition were higher than that by independent EFP or NaCl addition. Paludibacter sp., Petrimonas sp. and Syntrophomonas wolfei were the predominant and metabolically active electron-donating bacteria, and their expression of genes for e-pili/MHCs by combining EFP with NaCl addition was higher than that by independent EFP or NaCl addition. Methanothrix soehngenii and Methanoculleus bourgensis were the predominant and metabolically active electron-accepting methanogens, and their expression of genes for carbon dioxide reduction pathway for methanogenesis by combining EFP with NaCl addition was higher than that by independent EFP or NaCl addition. In addition, EFP specifically increased the transcript abundance of genes for NAD+/NADH transformation closely associated with the formation of e-pili/MHCs, while NaCl addition specifically increased the transcript abundance of genes for coenzyme F420/F420H2 transformation known to participate in reduction of carbon dioxide to methane.

Photoelectron-promoted metabolism of sulphate-reducing microorganisms in substrate-depleted environments.

Sulphate-reducing microorganisms, or SRMs, are crucial to organic decomposition, the sulphur cycle, and the formation of pyrite. Despite their low energy-yielding metabolism and intense competition with other microorganisms, their ability to thrive in natural habitats often lacking sufficient substrates remains an enigma. This study delves into how Desulfovibrio desulfuricans G20, a representative SRM, utilizes photoelectrons from extracellular sphalerite (ZnS), a semiconducting mineral that often coexists with SRMs, for its metabolism and energy production. Batch experiments with sphalerite reveal that the initial rate and extent of sulphate reduction by G20 increased by 3.6 and 3.2 times respectively under light conditions compared to darkness, when lactate was not added. Analyses of microbial photoelectrochemical, transcriptomic, and metabolomic data suggest that in the absence of lactate, G20 extracts photoelectrons from extracellular sphalerite through cytochromes, nanowires, and electron shuttles. Genes encoding movement and biofilm formation are upregulated, suggesting that G20 might sense redox potential gradients and migrate towards sphalerite to acquire photoelectrons. This process enhances the intracellular electron transfer activity, sulphur metabolism, and ATP production of G20, which becomes dominant under conditions of carbon starvation and extends cell viability in such environments. This mechanism could be a vital strategy for SRMs to survive in energy-limited environments and contribute to sulphur cycling.

Fe/GMP functional nanomaterial enhancing the denitrification efficiency by bi-signal regulation: Electron transfer and microbial community

A novel functional nanomaterial composed of guanosine monophosphate (GMP) and Fe enhanced denitrification efficiency by regulating electron transfer and microbial community. Fe/GMP enhanced nitrate (NO3−) degradation rates by 3.00-fold in serum vial batch experiments, with a rate constant of 17.39 mg/(L·h) in sequencing batch reactor. Fe/GMP-mediated interface promoted the secretion of redox-active substances in the extracellular polymeric substances to enhance the extracellular electron transfer. Specifically, Fe/GMP regulated electron transfer and metabolism activity by dynamic conversion of Fe3+/Fe2+ redox signal. Additionally, enzyme activity assays verified the optimized electron distribution function of Fe/GMP and thus enhanced intracellular electron transfer. High-throughput sequencing confirmed Fe/GMP selectively enriched microorganisms (especially Thauera 50.70 %). The tetraethylammonium stress experiment demonstrated Fe/GMP as an exogenous signaling molecule to restore microbial communication for microbial community regulation. The study proposes a multifaceted synergistic mechanism based on the repeater function of Fe/GMP in denitrification and offers insights for practical applications.

Carbon Steel Corrosion Induced by Sulfate-Reducing Bacteria: A Review of Electrochemical Mechanisms and Pathways in Biofilms

Microbial metal corrosion has become an important topic in metal research, which is one of the main causes of equipment damage, energy loss, and economic loss. At present, the research on microbial metal corrosion focuses on the characteristics of corrosion products, the environmental conditions affecting corrosion, and the measures and means of corrosion prevention, etc. In contrast, the main microbial taxa involved in metal corrosion, their specific role in the corrosion process, and the electron transfer pathway research are relatively small. This paper summarizes the mechanism of microbial carbon steel corrosion caused by SRB, including the cathodic depolarization theory, acid metabolite corrosion theory, and the biocatalytic cathodic sulfate reduction mechanism. Based on the reversible nature of electron transfer in biofilms and the fact that electrons must pass through the extracellular polymers layer between the solid electrode and the cell, this paper focuses on three types of electrochemical mechanisms and electron transfer modes of extracellular electron transfer occurring in microbial fuel cells, including direct-contact electron transfer, electron transfer by conductive bacterial hair proteins or nanowires, and electron shuttling mediated by the use of soluble electron mediators. Finally, a more complete pathway of electron transfer in microbial carbon steel corrosion due to SRB is presented: an electron goes from the metal anode, through the extracellular polymer layer, the extracellular membrane, the periplasm, and the intracellular membrane, to reach the cytoplasm for sulfate allosteric reduction. This article also focuses on a variety of complex components in the extracellular polymer layer, such as extracellular DNA, quinoline humic acid, iron sulfide (FeSX), Fe3+, etc., which may act as an extracellular electron donor to provide electrons for the SRB intracellular electron transfer chain; the bioinduced mineralization that occurs in the SRB biofilm can inhibit metal corrosion, and it can be used for the development of green corrosion inhibitors. This provides theoretical guidance for the diagnosis, prediction, and prevention of microbial metal corrosion.

Correlation between intracellular electron transfer and gene expression for electrically conductive pili in electroactive bacteria during anaerobic digestion with ethanol

Ethanol feeding has been widely documented as an economical and effective strategy for establishing direct interspecies electron transfer (DIET) during anaerobic digestion. However, the mechanisms involved are still unclear, especially on correlation between intracellular electron transfer in electroactive bacteria and their gene expression for electrically conductive pili (e-pili), the most essential electrical connection component for DIET. Upon cooling from room temperature, the conductivity of digester aggregates with ethanol exponentially increased by an order of magnitude (from 45.5 to 125.4 μS/cm), whereas which with its metabolites (acetaldehyde [from 40.5 to 54.4 μS/cm] or acetate [from 32.1 to 50.4 μS/cm]) did not increase significantly. In addition, the digester aggregates only with ethanol were observed with a strong dependence of conductivity on pH. Metagenomic and metatranscriptomic analysis showed that Desulfovibrio desulfuricans was the most dominant and metabolically active bacterium that contained and highly expressed the genes for e-pili. Abundance of genes encoding the total type IV pilus assembly proteins (6.72E-04 vs 1.24E-03, P < 0.05), PilA that determined the conductive properties (2.22E-04 vs 2.44E-04, P > 0.05), and PilB that proceeded the polymerization of pilin (1.56E-04 vs 3.52E-03, P < 0.05) with ethanol was lower than that with acetaldehyde. However, transcript abundance of these genes with ethanol was generally higher than that with acetaldehyde. In comparison to acetaldehyde, ethanol increased the transcript abundance of genes encoding the key enzymes involved in NADH/NAD+ transformation on complex I and ATP synthesis on complex V in intracellular electron transport chain. The improvement of intracellular electron transfer in D. desulfuricans suggested that electrons were intracellularly energized with high energy to activate e-pili during DIET.

Degradation of pyrene at high concentrations in sediment and the implications for the microbiome in microbial electrochemical systems

The polycyclic aromatic hydrocarbons (PAHs) frequently accumulate in sediments, particularly those near industrial sites. The efficacy of microbial electrochemical systems (MESs) in electricity generation and PAHs degradation is intricately linked to the microbiome’s response to high contaminant levels. Metabolomics and metagenomics were applied to elucidate the impact of a high pyrene concentration on the functional structure and metabolic dynamics of sediment microbiomes within MES. In sediment with a high pyrene concentration, both the electrical output of MES and efficiency of their microbiomes in degrading organic substrates and pyrene were markedly reduced. These conditions were associated with the enrichment of PAH-degrading microorganisms, such as Hydrogenophaga and Flavobacterium. Analyses of metabolites and genetic profiles revealed that metabolic pathways were primarily influenced in MES exposed to low pyrene concentrations whereas fewer metabolic pathways were activated in MES exposed to high concentrations; instead, pathways representing functions critical for microbial survival and communication were activated as well. According to enzymatic analyses, low pyrene levels promoted carbohydrate metabolism by sediment microorganisms, whereas high levels had an inhibitory effect. An examination of electron transport enzymes revealed that intracellular electron transfer was facilitated by both low and high concentrations. However, high levels impeded enzymes associated with outer membrane electron transfer, hindering electron movement from intracellular to extracellular electron acceptors and thus negatively affecting the electrical output of MES. Our study provides crucial theoretical insight and mechanistic understanding for addressing the challenges that may be encountered when MES are used to treat pyrene-contaminated sediment near industrial sites.

Pyrite-stimulated bio-reductive immobilization of perrhenate: Insights from integrated biotic and abiotic perspectives

Microbes possessing electron transfer capabilities hold great promise for remediating subsurface contaminated by redox-active radionuclides such as technetium-99 (99TcO4−) through bio-transformation of soluble contaminants into their sparingly soluble forms. However, the practical application of this concept has been impeded due to the low electron transfer efficiency and long-term product stability under various biogeochemical conditions. Herein, we proposed and tested a pyrite-stimulated bio-immobilization strategy for immobilizing ReO4− (a nonradioactive analogue of 99TcO4−) using sulfate-reducing bacteria (SRB), with a focus on pure-cultured Desulfovibrio vulgaris. Pyrite acted as an effective stimulant for the bio-transformation of ReO4−, boosting the removal rate of ReO4− (50 mg/L) in a solution from 2.8 % (without pyrite) to 100 %. Moreover, the immobilized products showed almost no signs of remobilization during 168 days of monitoring. Dual lines of evidence were presented to elucidate the underlying mechanisms for the pyrite-enhanced bio-activity. Transcriptomic analysis revealed a global upregulation of genes associated with electron conductive cytochromes c network, extracellular tryptophan, and intracellular electron transfer units, leading to enhanced ReO4− bio-reduction. Spectroscopic analysis confirmed the long-term stability of the bio-immobilized products, wherein ReO4− is reduced to stable Re(IV) oxides and Re(IV) sulfides. This work provides a novel green strategy for remediation of radionuclides- or heavy metals-contaminated sites.

Biochar modulates intracellular electron transfer for nitrate reduction in denitrifying anaerobic methane oxidizing archaea

Denitrifying anaerobic methane oxidizing (DAMO) archaea plays a significant role in simultaneously nitrogen removal and methane mitigation, yet its limited metabolic activity hinders engineering applications. This study employed biochar to explore its potential for enhancing the metabolic activity and nitrate reduction capacity of DAMO microorganisms. Sawdust biochar (7 g/L) was found to increase the nitrate reduction rate by 2.85 times, although it did not affect the nitrite reduction rate individually. Scanning electron microscopy (SEM) and fluorescence excitation-emission matrix (EEM) analyses revealed that biochar promoted microbial aggregation, and stimulated the secretion of extracellular polymeric substances (EPS). Moreover, biochar bolstered the redox capacity and conductivity of the biofilm, notably enhancing the activity of the electron transfer system by 1.65 times. Key genes involved in intracellular electron transport (Hdr, MHC, Rnf) and membrane transport proteins (BBP, ABC, NDH) of archaea were significantly up-regulated. These findings suggest that biochar regulates electrons generated by reverse methanogenesis to the membrane for nitrate reduction.

In-situ biogenic FeS boosted acetate accumulation through CO2 capture and valorization using microbial electrosynthesis (MES)

Biosynthetic strategy of ferrous sulfide (FeS) through Shewanella oneidensis MR-1 in bioelectrochemical systems has been extensively explored to maintain bettered electron transfer performance. In this study, the simultaneous FeS biosynthesis and CO2 reduction were conducted in the microbial electrosynthesis (MES) system, aiming at improving electron transfer efficiency as well as acetate production. It was shown that the FeS was successfully biomineralized and then loaded onto the sludge in the Fe3+/S2O32−/MR-1 group, of which the acetate accumulation increased by 87.50%, 64.63% and 33.66% than the control, MR-1 and Fe3+/S2O32− groups. Also, electrochemical performance revealed by the CV analysis was enhanced, whereas the increased EPS accumulation and intracellular electron transfer activity by the biosynthesized FeS could also be proved. Moreover, the biogenic FeS enriched the abundances of acetogenic and sulfur-utilizing species as Acetobacterium, Clostridium_sensu_stricto_13, Sulfuricurvum and Desulfovibrio. Meanwhile, strongly positive correlations between Shewanella, the sulfur utilizers and acetogens were confirmed in the Fe3+/S2O32−/MR-1 group. With improved electron transfer efficiency and the boosted acetate accumulation, the in-situ biogenic FeS might be promising for CO2 capture and utilization.

Biosynthesis of silver nanoparticles using Burkholderia contaminans ZCC and mechanistic analysis at the proteome level

The biogenic synthesis of silver nanoparticles (AgNPs) by microorganisms has been a subject of increasing attention. Despite extensive studies on this biosynthetic pathway, the mechanisms underlying the involvement of proteins and enzymes in AgNPs production have not been fully explored. Herein, we reported that Burkholderia contaminans ZCC was able to reduce Ag+ to AgNPs with a diameter of (10±5) nm inside the cell. Exposure of B. contaminans ZCC to Ag+ ions led to significant changes in the functional groups of cellular proteins, with approximately 5.72% of the (C-OH) bonds being converted to (C-C/C-H) (3.61%) and CO (2.11%) bonds, and 4.52% of the CO (carbonyl) bonds being converted to (C-OH) bonds. Furthermore, the presence of Ag+ and AgNPs induced the ability of extracellular electron transfer for ZCC cells via specific membrane proteins, but this did not occur in the absence of Ag+ ions. Proteomic analysis of the proteins and enzymes involved in heavy metal efflux systems, protein secretion system, oxidative phosphorylation, intracellular electron transfer chain, and glutathione metabolism suggests that glutathione S-transferase and ubiquinol-cytochrome c reductase iron-sulfur subunit play importance roles in the biosynthesis of AgNPs. These findings contribute to a deeper understanding of the functions exerted by glutathione S-transferase and ferredoxin-thioredoxin reductase iron-sulfur subunits in the biogenesis of AgNPs, thereby hold immense potential for optimizing biotechnological techniques aimed at enhancing the yield and purity of biosynthetic AgNPs.

Enhanced short-chain fatty acids production from food waste with magnetic biochar via anaerobic fermentation: Linking interfacial, extracellular, and intracellular electron transfer

Electron transfer is vital for microbial communities in anaerobic fermentation systems for food waste treatment. Considerable endeavors highlight the importance of interspecific electron transfer between acid-producing bacteria and methanogenic archaea. However, the contribution of electron transfer network from intracellular to extracellular in these microbes with conducive materials is unclear, and deserves more attention. This work demonstrated that magnetic biochar (as a typical conducive material) could enhance interfacial, extracellular and intracellular electron transfer networks during food waste fermentation, resulting in the improved production of short-chain fatty acids, accompanied by synchronous enhancements in solubilization, hydrolysis, and acidification steps. Further study revealed that magnetic biochar exhibited large surface area (28.5 m2/g) and high conductivity (97.6 μS/cm) with low charge transfer resistance and high catalytic activity, resulting in enriching acid-forming bacteria associated with electroactivity (e.g., Clostridium, Parabacteroides, and Fonticella). Also, magnetic biochar up-regulated the gene expressions involved in extracellular electron transfer, including membrane proteins (e.g., MtrA and MtrB) and conductive flagella (especially type IV pilus, e.g., PilA, PilB, and PilC), and intracellular electron transfer, particularly in the central pathway TCA cycle (e.g., acnA and sucA). In response to the evident stimulation of electron transfer networks (electron transport system activity increased by 204.1 % compared with the control), the critical gene expressions involved in fatty acid biosynthesis, requiring sufficient electrons and energy, were corresponding up-regulated (e.g., accA, accC, and accD), resulting in the promotion of short-chain fatty acids generation. This work would improve the in-depth understanding of electron transfer impacts on anaerobic systems and provide useful guidance for FW disposal.

Effect of sludge humic acid-derived nano-biochars on anaerobic degradation of sulfamethoxazole by Shewanella oneidensis MR-1

Some nano-biochars (nano-BCs) as electron mediators could enter into cells to directly promote intracellular electron transfer and cell activities. However, little information was available on the effect of nano-BCs on SMX degradation. In this study, nano-BCs were prepared using sludge-derived humic acid (SHA) and their effects on SMX degradation by Shewanella oneidensis MR-1 were investigated. Results showed that nano-BCs (Carbon dots, CDs, <10 nm) synthesized using SHA performed a better accelerating effect than that of the nano-BCs with a larger size (10–100 nm), which could be attributed to the better electron transfer abilities of CDs. The degradation rate of 10 mg/L SMX in the presence of 100 mg/L CDs was significantly increased by 84.6% compared to that without CDs. Further analysis showed that CDs could not only be combined with extracellular Fe(III) to accelerate its reduction, but also participate in the reduction of 4-aminobenzenesulphonic acid as an intermediate metabolite of SMX via coupling with extracellular Fe(III) reduction. Meanwhile, CDs could enter cells to directly participate in intracellular electron transfer, resulting in 32.2% and 25.2% increases of electron transfer system activity and ATP level, respectively. Moreover, the activities of SMX-degrading enzymes located in periplasm and cytoplasm were increased by around 2.2-fold in the presence of CDs. These results provide an insight into the accelerating effect of nano-BCs with the size of <10 nm on SMX degradation and an approach for SHA utilization.

Daily sonic toothbrush triggered biocompatible BaTiO3/chitosan multiporous coating with enhanced piezocatalysis for intraoral antibacterial activity

Pure titanium and its alloys are widely used in dental implants because of their excellent physical chemistry and biocompatibility properties, but their negligible antibacterial activity may lead to bacterial infection and implants failure. Inspired by the piezoelectricity and porous structure of living bone, a multiporous piezo-coating with barium titanate (BaTiO3, BTO) nanoparticles embedded in chitosan film is modified on acid-etched pure titanium. The surface of coating shows antibacterial effects and can be further improved under daily sonic toothbrush stimulation with an antibacterial rate of 90.41%. Interestingly, the planktonic bacteria around this coating also have low proliferation. The antibacterial mechanism may be BTO-mediated intracellular and extracellular reactive oxygen species (ROS). On the one hand, the contact potential difference (CPD) caused by the spontaneous polarization of BTO nanoparticles may destroy the intracellular electron transfer and lead to ROS burst. On the other hand, BTO could transfer mechanical energy to electrical energy under the stimulation of sonic toothbrushing (ST), then driving piezocatalytic effect to generate extracellular ROS. In addition, the piezo-coating also exhibits excellent biocompatibility to MC3T3-E1 cells, and promotes more hydroxyapatite deposition and plasma albumin adhesion. In summary, during the stimulation of daily sonic toothbrush, the bioinspired piezo-coating can effectively inhibit the formation of bacterial biofilm, which is a promising strategy to prevent dental implants infection in daily life.

Biomethane production enhancement from waste activated sludge with recycled magnetic biochar: Insights into the recycled strategies and mechanisms

Adding conductive materials for enhancing methane production from biomass has caused widespread attention. However, the discharge of digestive residues inevitably causes the loss of conductive materials, which not only weakens the roles of conductive materials but also increases the operating cost. This study aims to reveal the potentials of recycled magnetic biochar (MBC) in sludge anaerobic digestion. Results suggested that the direct utilization was better than the indirect utilization. Compared to raw MBC group, methane production further increased by 6.5–7.0 % in multi-cycle MBC groups. The mechanism analysis illustrated that the recycled MBC accelerated the conversion of intermediates. Besides, the recycled MBC promoted the content of redox-active substances and the activity of intracellular electron transfer systems. In addition, the acetoclastic methanogenesis pathway was improved in recycled MBC biofilms while the hydrogenotrophic methanogenesis was reduced. More interestingly, the microbial communities in biofilms presented directional evolutions with the increased cycle numbers of MBC. Correspondingly, the dominant direct interspecies electron transfer patterns changed from Geobacter &Methanosaeta to Clostridium &Methanosarcina, and the electron mediators converted to MBC and c-type cytochrome. The findings provided new insights into reutilization of conductive materials in anaerobic digestion.

Enhanced bioelectricity generation in thermophilic microbial fuel cell with lignocellulose as an electron donor by resazurin-mediated electron transfer

Microbial fuel cell (MFC) with lignocellulose as an electron donor is considered a sustainable biorefinery. However, low lignocellulose degradation and energy output restrict the scale of application. Herein, the extracellular electron transfer (EET) capacity of Acetivibrio thermocellus DSM 1313 with lignocellulose as substrate was shown to be mediated by the self-produced flavin, and its intracellular electron transfer went through the whole respiratory chain. Thermophilic MFC with resazurin exhibited an increase in the open circuit voltage by 37.78%, and a 2.60 folds increase in power density of 77.85 mW/m2, respectively. Differential pulse voltammetry and electrochemical impedance spectroscopy analysis indicated that resazurin decreased the solution and anode charge transfer resistance, and enhanced the extracellular electrochemical activity. Furthermore, resazurin resulted in a lower redox potential, allowing preferential electron transfer to resazurin rather than flavin. This research establishes a resazurin-mediated thermophilic MFC with lignocellulose as substrate, which provides novel idea on the biomass refinery.

Metal–organic framework derived bio-anode enhances chlorobenzene removal and electricity generation: Special Ru4+/Ru3+-bridged intracellular electron transfer

Efficient removal of chlorinated organic contaminants using the microbial fuel cell (MFC) provides a promising strategy to alleviate water pollution and energy crisis. However, bio-degradation is challenged by poor biofilm formation and sluggish intracellular electron transfer, causing unsatisfactory electricity generation. To address those problems, a metal–organic framework derivative, Ru-porous TiO2 (Ru-PT) bio-anode has been artfully designed herein for chlorobenzene removal. The Ru-PT bio-anode not only formed a compact anodic biofilm due to the large specific surface area of PT, but more importantly, it introduced special pseudocapacitance-enhanced intracellular electron transfer by slowly implanting Ru4+/Ru3+ redox pair into bacteria. Such a Ru4+/Ru3+ implantation was then found to directionally induce the enrichment of a dual-functional genus (degrader & exoelectrogen), Pseudomonas, thereby enhancing the conversion of bio-refractory chlorophenols towards biodegradable carboxylic acids. These features allowed our MFC to have a resilient chlorobenzene removal and accompanied satisfactory electricity generation, with power density, coulombic efficiency, and turnover frequency reaching 662 mW m−2, 8.7%, and 386,622 s−1, which outcompeted those of other MFCs reported. Further, benefiting from the reversible pseudocapacitance, the Ru-PT bio-anode intriguingly functioned as an internal capacitor for electricity storage. This work provided important insights into cost-effective bio-anode development and offered an avenue for engineering MFC.

Interference of electron transfer chain inhibitors in bioelectrochemical systems

Substances inhibiting specific proteins involved in cellular electron transfer chains are used in biochemical research to investigate intracellular electron transfer routes and to redirect the electron fluxes. This also provides an in vivo approach to investigating the extracellular electron transfer (EET) mechanisms within and across biological membranes in bioelectrochemical research. However, the applicability of these specific inhibitors in electrochemical systems remains to be validated, particularly when aiming for (semi-)quantitative evaluation of the contribution of specific redox proteins to the EET. Here we conducted a systematic analysis of commonly used inhibitors and discovered several of them to be electrochemically active and thus capable of interfering with measurements in a bioelectrochemical reactor system in abiotic experiments. We also observed effects in vivo using a biophotovoltaics reactor with Synechocystis sp. PCC 6803 as a model system.

Enhancing bioelectrocatalytic oxidation of gaseous chlorobenzene by introducing transmembrane Ru4+/Ru3+-mediated reversible intracellular electron transfer

Microbial electrolysis cell (MEC) is a cost-effective process to eliminate chlorinated volatile organic compounds. Poor mass transfer and sluggish intracellular electron transfer remain obstacles. To overcome these, a Ru-mesoporous TiO2/macroporous carbon (Ru-MeT/MaC) bio-anode was tailored here. The hierarchically meso-macroporous structure separated biofilm formation and chlorobenzene adsorption, improving chlorobenzene mass transfer. Further, modifying Ru created a reversible intracellular electron transfer pathway by introducing transmembrane Ru4+/Ru3+ redox couple within bacteria. Such a unique transmembrane introduction induced a compelling bacteria-electricity synergism accompanied by the acclimation of dual-functional genera (degrader & exoelectrogen), promoting the decomposition of aromatic compounds toward organic acids. Those features enabled the resulting MEC to exhibit a super-prominent chlorobenzene elimination capacity (136.7 g m−3 h−1) with a short empty bed residence time (60 s), superior to those of state-of-the-art bioelectrocatalytic processes reported. Leveraging the reversible electron transfer, the Ru-MeT/MaC bio-anode was operated in the charging-discharging mode to further enhance chlorobenzene removal. SynopsisThe concept of introducing bioavailable conductive redox couple into bacteria via reasonably designing the functional structure of the bio-anode is expected to enhance the bioelectrocatalytic oxidation of hydrophobic Cl-VOCs and therefore alleviate air pollution.

Evidence for the feasibility of transmembrane proton gradient regulating oxytetracycline extracellular biodegradation mediated by biosynthesized palladium nanoparticles

Extracellular biodegradation is a promising technology for removing antibiotics and repressing the spread of resistance genes, but the strategy is limited by the low extracellular electron transfer (EET) efficiency of microorganisms. In this work, biogenic Pd0 nanoparticles (bio-Pd0) were introduced in cells in situ to enhance oxytetracycline (OTC) extracellular degradation and the effects of transmembrane proton gradient (TPG) on EET and energy metabolism mediated by bio-Pd0 were investigated. The results indicated that the intracellular OTC concentration gradually decreased with increase in pH due to the simultaneous decreases of OTC adsorption and TPG-dependent OTC uptake. On the contrary, the efficiency of OTC biodegradation mediated by bio-Pd0@B. megaterium showed a pH-dependent increase. The negligible intracellular OTC degradation, the high dependence of OTC biodegradation on respiration chain and the results on enzyme activity and respiratory chain inhibition experiments showed that NADH-dependent (rather than FADH2-dependent) EET process mediated by substrate-level phosphorylation modulated OTC biodegradation due to high energy storage and proton translocation capacity. Moreover, the results showed that altering TPG is an efficient approach to improve EET efficiency, which can be attributed to the increased NADH generation by the TCA cycle, enhanced transmembrane electron output efficiency (as evidenced by increased intracellular electron transfer system (IETS) activity, the negative shift of onset potential, and enhanced one-electron transfer through bound flavin) and stimulation of substrate-level phosphorylation energy metabolism catalyzed by succinic thiokinase (STH) under low TPG conditions. The results of structural equation model that OTC biodegradation was directly and positively modulated by the net outward proton flux as well as STH activity, and indirectly regulated by TPG through NADH level and IETS activity confirmed the previous findings. This study provides a new perspective for engineering microbial EET and application of bioelectrochemistry processes in bioremediation.