Effects of bio-Au nanoparticles on electrochemical activity of Shewanella oneidensis wild type and ΔomcA/mtrC mutant

Shewanella oneidensis: Biotechnological Application of Metal-Reducing Bacteria

What is an unconventional organism in biotechnology? The γ-proteobacterium Shewanella oneidensis might fall into this category as it was initially established as a laboratory model organism for a process that was not seen as potentially interesting for biotechnology. The reduction of solid-state extracellular electron acceptors such as iron and manganese oxides is highly relevant for many biogeochemical cycles, although it turned out in recent years to be quite relevant for many potential biotechnological applications as well. Applications started with the production of nanoparticles and dramatically increased after understanding that electrodes in bioelectrochemical systems can also be used by these organisms. From the potential production of current and hydrogen in these systems and the development of biosensors, the field expanded to anode-assisted fermentations enabling fermentation reactions that were - so far - dependent on oxygen as an electron acceptor. Now the field expands further to cathode-dependent production routines. As a side product to all these application endeavors, S. oneidensis was understood more and more, and our understanding and genetic repertoire is at eye level to E. coli. Corresponding to this line of thought, this chapter will first summarize the available arsenal of tools in molecular biology that was established for working with the organism and thereafter describe so far established directions of application. Last but not least, we will highlight potential future directions of work with the unconventional model organism S. oneidensis.

Deciphering the Role of Inorganic Nanoparticles' Surface Functionalization on Biohybrid Microbial Photoelectrodes

Shedding light on the interaction between inorganic nanoparticles (NPs) and living microorganisms is at the basis of the development of biohybrid technologies with improved performance. Au NPs have been shown to be able to improve the extracellular electron transfer (EET) in intact bacterial cells interfaced with an electrode; however, detailed information on the role of NP-surface properties in their interaction with bacterial membranes is still lacking. Herein, we unveil how the surface functionalization of Au NPs influences their interaction with photosynthetic bacteria, focusing on cell morphology, growth kinetics, NPs localization, and electrocatalytic performance. We show that functionalization of Au NPs with cysteine in the zwitterionic form results in a uniform NPs distribution in purple bacteria, specifically locating the NPs within the outer-membrane/periplasmic space of bacterial cells. These biohybrid cells, when coupled with an electrode, exhibit enhanced EET and increased (photo)current generation, paving the way for the future development of rationally designed biohybrid electrochemical systems.

In Vivo Voltammetric Imaging of Metal Nanoparticle-Catalyzed Single-Cell Electron Transfer by Fermi Level-Responsive Graphene

Metal nanomaterials can facilitate microbial extracellular electron transfer (EET) in the electrochemically active biofilm. However, the role of nanomaterials/bacteria interaction in this process is still unclear. Here, we reported the single-cell voltammetric imaging of Shewanella oneidensis MR-1 at the single-cell level to elucidate the metal-enhanced EET mechanism in vivo by the Fermi level-responsive graphene electrode. Quantified oxidation currents of ~20 fA were observed from single native cells and gold nanoparticle (AuNP)-coated cells in linear sweep voltammetry analysis. On the contrary, the oxidation potential was reduced by up to 100 mV after AuNP modification. It revealed the mechanism of AuNP-catalyzed direct EET decreasing the oxidation barrier between the outer membrane cytochromes and the electrode. Our method offered a promising strategy to understand the nanomaterials/bacteria interaction and guide the rational construction of EET-related microbial fuel cells.

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.

Combined Gold Recovery and Nanoparticle Synthesis in Microbial Systems Using Fractional Factorial Design

Green synthesis of gold nanoparticles (AuNPs) using microorganisms has been generally studied aiming for high-yield production and morphologies appropriated for various applications, such as bioremediation, (bio)sensors, and (bio)catalysis. Numerous approaches showed the individual effect of factors influencing the synthesis of AuNPs with limited analysis of the governing factors enhancing the production and desired quality of the precipitates. This study proposes a fractional-factorial design to investigate the simultaneous influence of seven environmental factors (cell concentration, temperature, anoxic/oxic conditions, pH, gold concentration, electron donor type, and bacterial species) on the recovery yield and synthesis of targeted AuNPs. Various sizes and morphologies of the AuNPs were obtained by varying the environmental factors studied. The factors with significant effects (i.e., 0.2 mM Au and pH 5) were selected according to statistical analysis for optimal removal of 88.2 ± 3.5% of gold and with the production of valuable 50 nm AuNPs, which are known for their enhanced sensitivity. Implications of the cytochrome-C on the bacterial mechanisms and the provision of electron donors via an electrochemical system are further discussed. This study helps develop gold recovery and nanoparticle synthesis methods, focusing on the determining factor(s) for efficient, low-cost, green synthesis of valuable materials.

Enhanced transmembrane electron transfer in Shewanella oneidensis MR-1 using gold nanoparticles for high-performance microbial fuel cells

Low efficiency of extracellular electron transfer (EET) is a major bottleneck in developing high-performance microbial fuel cells (MFCs). Herein, we construct Shewanella oneidensis MR-1@Au for the bioanode of MFCs. Through performance recovery experiments of mutants, we proved that abundant Au nanoparticles not only tightly covered the bacteria surface, but were also distributed in the periplasm and cytoplasm, and even embedded in the outer and inner membranes of the cell. These Au nanoparticles could act as electron conduits to enable highly efficient electron transfer between S. oneidensis MR-1 and electrodes. Strikingly, the maximum power density of the S. oneidensis MR-1@Au bioanode reached up to 3749 mW m-2, which was 17.4 times higher than that with the native bacteria, reaching the highest performance yet reported in MFCs using Au or Au-based nanocomposites as the anode. This work elucidates the role of Au nanoparticles in promoting transmembrane and extracellular electron transfer from the perspective of molecular biology and electrochemistry, while alleviating bottlenecks in MFC performances.

Bio-Pd(0) diverting electron from CoQ-long chain to FDH/Hase-short chain during sulfamethoxazole degradation

Microbial electron output capacity is critical for organic contaminants biodegradation. Herein, original C. freundii JH could oxidate formate in anaerobic respiration, but lack the ability to degrade sulfamethoxazole (SMX). While the incorporation of Pd(0) could effectively improve the electron output via improving the combination between flavins and c-type cytochromes (c-Cyts), increasing the activities of key enzymes (formate dehydrogenase, hydrogenase, F₀F₁-ATPases), etc. More importantly, the presence of Pd(0) caused the NADH dehydrogenase (complex I) nearly in idle, and triggered the decrease of NADH/NAD⁺ ratio and increase of H⁺-efflux transmembrane gradient, eventually resulting in the electrons diverting from CoQ-involved long respiratory chain (decreasing from 91.67% to 36.25%) to FDH/Hases-based hydrogen-producing short chain (increasing from 22.44% to 84.88%), which further intensified the electron output. Above changes effectively launched and guaranteed the high-level SMX degradation by palladized C. freundii JH, alleviating the ecotoxicity of SMX in aquatic and terrestrial environments. These conclusions provided the new view to regulate the microbial electron output behaviors.

Versatile mechanisms and enhanced strategies of pollutants removal mediated by Shewanella oneidensis: A review

The removal of environmental pollutants is important for a sustainable ecosystem and human health. Shewanella oneidensis (S. oneidensis) has diverse electron transfer pathways and can use a variety of contaminants as electron acceptors or electron donors. This paper reviews S. oneidensis's function in removing environmental pollutants, including heavy metals, inorganic non-metallic ions (INMIs), and toxic organic pollutants. S. oneidensis can mineralize o-xylene (OX), phenanthrene (PHE), and pyridine (Py) as electron donors, and also reduce azo dyes, nitro aromatic compounds (NACs), heavy metals, and iodate by extracellular electron transfer (EET). For azo dyes, NACs, Cr(VI), nitrite, nitrate, thiosulfate, and sulfite that can cross the membrane, S. oneidensis transfers electrons to intracellular reductases to catalyze their reduction. However, most organic pollutants cannot be directly degraded by S. oneidensis, but S. oneidensis can remove these pollutants by self-synthesizing catalysts or photocatalysts, constructing bio-photocatalytic systems, driving Fenton reactions, forming microbial consortia, and genetic engineering. However, the industrial-scale application of S. oneidensis is insufficient. Future research on the metabolism of S. oneidensis and interfacial reactions with other materials needs to be deepened, and large-scale reactors should be developed that can be used for practical engineering applications.

Au(III)-induced extracellular electron transfer by Burkholderia contaminans ZCC for the bio-recovery of gold nanoparticles

The biorecovery of gold (Au) by microbial reduction has received increasing attention, however, the biomolecules involved and the mechanisms by which they operate to produce Au nanoparticles have been not resolved. Here we report that Burkholderia contaminans ZCC is capable of reduction of Au(III) to Au nanoparticles on the cell surface. Exposure of B. contaminans ZCC to Au(III) led to significant changes in the functional group of cell proteins, with approximately 11.1% of the (C-C/C-H) bonds being converted to CO (8.1%) and C-OH (3.0%) bonds and 29.4% of the CO bonds being converted to (C-OH/C-O-C/P-O-C) bonds, respectively. In response to Au(III), B. contaminans ZCC also displayed the ability of extracellular electron transfer (EET) via membrane proteins and could produce reduced riboflavin as verified by electrochemical and liquid chromatography-mass spectrometric results, but did not do so without Au(III) being present. Addition of exogenous reduced riboflavin to the medium suggested that B. contaminans ZCC could utilize indirect EET via riboflavin to enhance the rate of reduction of Au(III). Transcriptional analysis of the riboflavin genes (ribBDEFH) supported the view of the importance of riboflavin in the reduction of Au(III) and its importance in the biorecovery of gold.

Extraction of Au(III) by Microbially Reduced Metal-Organic Frameworks

Gold is a critical resource in the jewelry and electronics industries and is facing increased consumer demand. Accordingly, methods for its extraction from waste effluents and environmental water sources have been sought to supplement existing mining infrastructure. Redox-mediated treatments, such as Fe(II)-based platforms, offer promise for precipitating soluble Au(III). We hypothesized that microbial generation of Fe(II) in the presence of sorbent metal-organic frameworks could capitalize on the advantages of both biological- and chemical-driven extraction approaches. Toward this aim, we tested Au(III) removal by Shewanella oneidensis cultured with Fe(III)-based materials (ferrihydrite, Fe-BTC, MIL-100, or MIL-127). Across all tested materials, S. oneidensis generated the highest levels of redox-active Fe(II) (1.99 ± 0.27 mM) when cultured with MIL-127 as a respiratory substrate in a bicarbonate-buffered medium. This translated into superior Au(III) removal performance in terms of both removal rate and capacity (k = 2.55 ± 0.60 h^-1; Q = 183 mg g^-1). Unlike other materials tested, MIL-127 also maintained cell viability following repeated Au(III) challenges, enabling the regeneration of Fe(II) in the framework. Together, these effects facilitated the treatment of multiple cycles of Au(III) by S. oneidensis-reduced MIL-127. Overall, this work demonstrates that microbial generation of Fe(II) can facilitate the removal of Au(III), augmenting purely adsorptive platforms. Given the biological and chemical modularity of our system, our results suggest that future optimizations to microbial Fe(II) generation may offer promise for improving Au(III) extraction processes.

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

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

Intra/extracellular electron transfer for aerobic denitrification mediated by in-situ biosynthesis palladium nanoparticles

The slow electron transfer rate is the bottleneck to the biological wastewater treatment process, and the nanoparticles (NPs) has been verified as a feasible strategy to improve the biological degradation efficiency by accelerating the electron transfer. Here, we employed the Gram-positive Bacillus megaterium Y-4, capable of synthetizing Pd(0), to investigate the intra/extracellular electron transfer (IET/EET) mechanisms mediated by NPs in aerobic denitrification for the first time. Kinetic and thermodynamic results showed that the bio-Pd(0) could significantly promote the removal of both nitrate and nitrite by improving affinity and decreasing activation energy. The enzymic activity and the respiration chain inhibition experiment indicated that the bio-Pd(0) could facilitate the nitrate biotic reduction by improving the Fe-S center activity and serving as parallel H carriers to replace coenzyme Q to selectively increase the electron flux toward nitrate in IET, while promoting the nitrite reduction by abiotic catalysis. Most importantly, the detection of DPV peak at -226~-287 mV proved that the one-electron EET via multiheme cytochrome-bound flavins also occurred in Gram-positive bacteria and enhanced in Pd-loaded cells. In addition, the remarkable increase of the formal charge in EPS indicated that the bio-Pd(0) could act as an electron shuttle to increase the redox site in EPS, eventually accelerating the electron hopping in long-distance electron transfer. Overall, this study expanded our understanding of the roles of bio-Pd(0) on the aerobic denitrification process and provided an insight into the IET/EET of Gram-positive strains.

Single cell electron collectors for highly efficient wiring-up electronic abiotic/biotic interfaces

By electronically wiring-up living cells with abiotic conductive surfaces, bioelectrochemical systems (BES) harvest energy and synthesize electric-/solar-chemicals with unmatched thermodynamic efficiency. However, the establishment of an efficient electronic interface between living cells and abiotic surfaces is hindered due to the requirement of extremely close contact and high interfacial area, which is quite challenging for cell and material engineering. Herein, we propose a new concept of a single cell electron collector, which is in-situ built with an interconnected intact conductive layer on and cross the individual cell membrane. The single cell electron collector forms intimate contact with the cellular electron transfer machinery and maximizes the interfacial area, achieving record-high interfacial electron transfer efficiency and BES performance. Thus, this single cell electron collector provides a superior tool to wire living cells with abiotic surfaces at the single-cell level and adds new dimensions for abiotic/biotic interface engineering.

Carbon dots-fed Shewanella oneidensis MR-1 for bioelectricity enhancement

Bioelectricity generation, by Shewanella oneidensis (S. oneidensis) MR-1, has become particularly alluring, thanks to its extraordinary prospects for energy production, pollution treatment, and biosynthesis. Attempts to improve its technological output by modification of S. oneidensis MR-1 remains complicated, expensive and inefficient. Herein, we report on the augmentation of S. oneidensis MR-1 with carbon dots (CDs). The CDs-fed cells show accelerated extracellular electron transfer and metabolic rate, with increased intracellular charge, higher adenosine triphosphate level, quicker substrate consumption and more abundant extracellular secretion. Meanwhile, the CDs promote cellular adhesion, electronegativity, and biofilm formation. In bioelectrical systems the CDs-fed cells increase the maximum current value, 7.34 fold, and power output, 6.46 fold. The enhancement efficacy is found to be strongly dependent on the surface charge of the CDs. This work demonstrates a simple, cost-effective and efficient route to improve bioelectricity generation of S. oneidensis MR-1, holding promise in all relevant technologies.

Bacterial fuel cells have generated attention with the prospect of green energy production; current research is focused on optimising the system to improve efficiency. Here, the authors report on the feeding of carbon dots to S. oneidensis MR-1 to enhance metabolic activity and bioelectric generation.

Microbe-mediated sustainable bio-recovery of gold from low-grade precious solid waste: A microbiological overview

In an era of electronics, recovering the precious metal such as gold from ever increasing piles of electronic-wastes and metal-ion infested soil has become one of the prime concerns for researchers worldwide. Biological mining is an attractive, economical and non-hazardous to recover gold from the low-grade auriferous ore containing waste or soil. This review represents the recent major biological gold retrieval methods used to bio-mine gold. The biomining methods discussed in this review include, bioleaching, bio-oxidation, bio-precipitation, bio-flotation, bio-flocculation, bio-sorption, bio-reduction, bio-electrometallurgical technologies and bioaccumulation. The mechanism of gold biorecovery by microbes is explained in detail to explore its intracellular mechanistic, which help it withstand high concentrations of gold without causing any fatal consequences. Major challenges and future opportunities associated with each method and how they will dictate the fate of gold bio-metallurgy from metal wastes or metal infested soil bioremediation in the coming future are also discussed. With the help of concurrent advancements in high-throughput technologies, the gold bio-exploratory methods will speed up our ways to ensure maximum gold retrieval out of such low-grade ores containing sources, while keeping the gold mining clean and more sustainable.

Evidence for fungi and gold redox interaction under Earth surface conditions

Microbial contribution to gold biogeochemical cycling has been proposed. However, studies have focused primarily on the influence of prokaryotes on gold reduction and precipitation through a detoxification-oriented mechanism. Here we show, fungi, a major driver of mineral bioweathering, can initiate gold oxidation under Earth surface conditions, which is of significance for dissolved gold species formation and distribution. Presence of the gold-oxidizing fungus TA_pink1, an isolate of Fusarium oxysporum, suggests fungi have the potential to substantially impact gold biogeochemical cycling. Our data further reveal that indigenous fungal diversity positively correlates with in situ gold concentrations. Hypocreales, the order of the gold-oxidizing fungus, show the highest centrality in the fungal microbiome of the auriferous environment. Therefore, we argue that the redox interaction between fungi and gold is critical and should be considered in gold biogeochemical cycling.

Mechanism study of photo-induced gold nanoparticles formation by Shewanella oneidensis MR-1

Shewanella oneidensis MR-1, a bioelectricity generating bacterium, is broadly used in bioremediation, microbial fuel cell and dissimilatory reduction and recovery of precious metals. Herein, we report for the first time that photo induction as a trigger to stimulate gold nanoparticles (Au@NPs) formation by MR-1, with wavelength and light intensity as two key variables. Results indicated that sigmoidal model is the best fit for Au@NPs formation at various wavelengths (with R² > 0.97). Light intensity in terms of photosynthetic photon flux density (PPFD) critically influences the rate constant in the low-light intensity region (PPFD < 20), while wavelength controls the maximum rate constant in the high-light region (PPFD > 20). By deletion of Mtr pathway genes in MR-1, we proposed the mechanism for light induced Au@NP formation is the excitation effect of light on certain active groups and extracellular polymeric substances (EPS) on the cell surface. Also, the release of electrons from proteins and co-enzyme complexes enhance electron generation. To the best of our knowledge, this is the first-attempt to explore the effect of photo-induction on Au@NPs production by MR-1, which provides an alternative cost-effective and eco-friendly process in green chemical industry.

Extracellular electron transfer of Enterobacter cloacae SgZ-5T via bi-mediators for the biorecovery of palladium as nanorods

In nature, microbes use extracellular electron transfer (EET) to recover noble metals. Most attention has been paid to the biorecovery process occurring intracellularly and on the cell surface. In this work, we report that Pd nanorods could be biosynthesized by Enterobacter cloacae SgZ-5T in the extracellular space. This bacterium possesses both a direct EET pathway through membrane redox systems and an indirect EET pathway via the self-secreted electron carrier hydroquinone (HQ). When exposed to Pd(II), the bacteria adjusted their metabolic pathway and membrane-bound proteins to secrete riboflavin (RF). However, no HQ was detected in the supernatant in presence of Pd(II). No significant change was observed through metabolomic analysis regarding the abundance of HQ in presence of Pd(II) compared to Pd(II)-free supernatant. Similar results were also obtained through transcriptomic analysis of YqjG gene encoding glutathionyl-HQ reductase synthase. X-ray photoelectron spectroscopic evidence indicated that HQ may adsorb to the surface of Pd nanorods. Moreover, the gene encoding RF synthase (ribE) was up-regulated in the present of Pd(II), suggesting that this bioreduction process induced RF synthase, which had been shown in previous results. The UV-vis spectroscopy data demonstrated that the Pd(II) reduction rate was enhanced by 5%, 5.5% and 30% by the addition of 3.33 μM HQ, 3.33 μM RF and the both, respectively. All these results revealed that the bi-mediators secreted by bacteria were beneficial for biorecovery of Pd. This work is of significance for understanding metal biorecovery processes and natural biogeochemical processes.

Polydopamine coating on individual cells for enhanced extracellular electron transfer

The double-mediator electron transport pathway was achieved by PDA coating on the surface of individual cells to facilitate EET.

Biosynthesis of au nanoparticles by a marine bacterium and enhancing their catalytic activity through metal ions and metal oxides

The authors report that a marine Shewanella sp. CNZ-1 is capable of producing Au NPs under various conditions. Results showed that initial concentration of Au(III), pH values and electron donors affected nucleation of Au NPs by CNZ-1, resulting in different apparent color of the as-obtained bio-Au NPs, which were further characterized by UV-Vis, TEM, XRD, and XPS analyses. Mechanism studies revealed that Au(III) was first reduced to Au(I) and eventually reduced to EPS-coated Au⁰ NPs. FTIR and FEEM analyses revealed that some amides and humic acid-like matters were involved in the production of bio-Au NPs through CNZ-1 cells. In addition, the authors also found that the catalytic activity of bio-Au NPs for 4-nitrophenol (4-NP) reduction could be enhanced by various metal ions (Ca²⁺ , Cu²⁺ , Co²⁺ , Fe²⁺ , Fe³⁺ , Ni²⁺ , Sr²⁺ , and Cr³⁺ ) and metal oxides (Fe₃ O₄ , Al₂ O₃ , and SiO₂ ), which is beneficial for their further practical application. The maximum zero-order rate constant k ₁ and first-order rate constant k₂ of all metal ions/oxides supplemented systems can reach 99.65 mg/(L^. min) and 2.419 min^-1 , which are 11.3- and 12.6-fold higher than that of control systems, respectively. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2727, 2019.

Anode modification by biogenic gold nanoparticles for the improved performance of microbial fuel cells and microbial community shift

In this study, carbon cloth anodes were modified using biogenic gold nanoparticles (BioAu) and nanohybrids of multi-walled carbon nanotubes blended with BioAu (BioAu/MWCNT) to improve the performance of microbial fuel cells (MFCs). The results demonstrated that BioAu modification significantly enhanced the electricity generation of MFCs. In particular, BioAu/MWCNT nanohybrids as the modifier displayed a better performance. The MFC with the BioAu/MWCNT electrode had the shortest start-up time (6.74 d) and highest power density (178.34 ± 4.79 mW/m²), which were 141.69% shorter and 56.11% higher compared with those of the unmodified control, respectively. These improvements were attributed to the excellent electrocatalytic activity and strong affinity towards exoelectrogens of the BioAu/MWCNT nanohybrids on the electrode. High throughput sequencing analysis indicated that the relative abundance of electroactive bacteria in the biofilm community, mostly from the classes of Gammaproteobacteria and Negativicutes, increased after anode modification.

Dual-Function Electrocatalytic and Macroporous Hollow-Fiber Cathode for Converting Waste Streams to Valuable Resources Using Microbial Electrochemical Systems

Dual-function electrocatalytic and macroporous hollow-fiber cathodes are recently proposed as promising advanced material for maximizing the conversion of waste streams such as wastewater and waste CO₂ to valuable resources (e.g., clean freshwater, energy, value-added chemicals) in microbial electrochemical systems. The first part of this progress report reviews recent developments in this type of cathode architecture for the simultaneous recovery of clean freshwater and energy from wastewater. Critical insights are provided on suitable materials for fabricating these cathodes, as well as addressing some challenges in the fabrication process with proposed strategies to overcome them. The second and complementary part of the progress report highlights how the unique features of this cathode architecture can solve one of the intrinsic bottlenecks (gas-liquid mass transfer limitation) in the application of microbial electrochemical systems for CO₂ reduction to value-added products. Strategies to further improve the availability of CO₂ to microbial catalysts on the cathode are proposed. The importance of understanding microbe-cathode interactions, as well as electron transfer mechanisms at the cathode-cell and cell-cell interface to better design dual-function macroporous hollow-fiber cathodes, is critically discussed with insights on how the choice of material is important in facilitating direct electron transfer versus mediated electron transfer.

Biosynthesis of Pd and Au as nanoparticles by a marine bacterium Bacillus sp. GP and their enhanced catalytic performance using metal oxides for 4-nitrophenol reduction

Recovery of noble metals using marine bacteria is becoming an attractive research area because the marine microbes can better adapt to unfavorable environment than terrestrial microorganisms. In this study, we first reported that a marine Bacillus sp. GP was capable of producing Pd and Au NPs in the presence of sodium lactate. Ultraviolet visible spectrometer (UV-vis), transmission electron microscopy (TEM), X-ray diffraction patterns (XRD), X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR) analyses were employed to explain the process and mechanism of Pd(II)/Au(III) reduction through GP. Additionally, we also found that bio-Pd/Au NPs could be used as catalysts in chemical reduction of 4-nitrophenol (4-NP). Moreover, the catalytic activity of bio-Pd NPs could be enhanced by Fe₃O₄, Al₂O₃ and SiO₂, which is beneficial for practical application. The k₁ (k₂) values of Fe₃O₄, Al₂O₃ and SiO₂ supplemental systems were approximately 1.28-1.69 (1.15-1.69), 1.42-1.75 (1.53-1.91) and 1.07-1.73 (1.14-1.49) fold, respectively, compared to that of control systems.

Facilitated extracellular electron transfer of Geobacter sulfurreducens biofilm with in situ formed gold nanoparticles

The conductivity of a biofilm is the key factor for the high current density of a bioelectrochemical system (BES). Most previous works have focused on electrode modification, but, this only benefits the microorganisms that directly contact the electrode. The low conductivity of biofilm limits the current density of the BES. In this work, gold nanoparticles (Au-NPs) were successfully fabricated in situ into a Geobacter sulfurreducens biofilm to increase the conductivity. 20 ppm NaAuCl₄ (the precursor) was slowly dropped into the anode chamber at a rate of 1.3 mL/h in a continuous-flow three-electrode BES. The Au(III) was transformed to Au-NPs, which then precipitated in the biofilm via biological mineralization. The current density of the anode increased by 40%. Meanwhile, the removal percentage of the organic substrate (acetate) was enhanced 2.2 times, from 24.7% to 53.3%, after the in situ fabrication of Au-NPs. This method greatly lowered the charge transfer resistance of the anode and enhanced the anodic limiting current. Our results proved that the current density and organic removal rate of the G. sulfurreducens biofilm in the anode were effectively enhanced by in situ Au-NP fabrication. This work not only provides a simple and effective strategy for enhancing the electricity generation of BES with conductive NP fabrication, but also improves the understanding of the extracellular electron transfer (EET) of exoelectrogens.

Microbial synthesis of bimetallic PdPt nanoparticles for catalytic reduction of 4-nitrophenol

Bimetallic nanoparticles are generally believed to have improved catalytic activity and stability due to geometric and electronic changes. In this work, biogenic-Pd (bio-Pd), biogenic-Pt (bio-Pt), and biogenic-PdPt (bio-PdPt) nanoparticles were synthesized by Shewanella oneidensis MR-1 in the absence or presence of quinone. Compared with direct microbial reduction process, the addition of anthraquinone-2,6-disulfonate (AQDS) could promote the reduction efficiency of Pd(II) or/and Pt(IV) and result in decrease of particles size. All kinds of nanoparticles could catalyze 4-nitrophenol reduction by NaBH₄ and their catalytic activities took the following order: bio-PdPt (AQDS) ∼ bio-PdPt > bio-Pd (AQDS) > bio-Pd > bio-Pt (AQDS) ∼ bio-Pt. Moreover, the bio-PdPt (AQDS) nanoparticles could be reused for 6 cycles. We believe that this simple and efficient biosynthesis approach for synthesizing bimetallic bio-PdPt nanocatalysts is important for preparing active and stable catalysts.

Nanotechnology to rescue bacterial bidirectional extracellular electron transfer in bioelectrochemical systems

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

Biogenic gold nanoparticles-reduced graphene oxide nanohybrid: synthesis, characterization and application in chemical and biological reduction of nitroaromatics

The biogenic AuNPs/rGO can participate in and accelerate electron transfer, and catalyze both chemical and biological reduction of nitroaromatics efficiently.

Comparative proteomics reveal the impact of OmcA/MtrC deletion on Shewanella oneidensis MR-1 in response to hexavalent chromium exposure

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