Promoting Shewanella Bidirectional Extracellular Electron Transfer for Bioelectrocatalysis by Electropolymerized Riboflavin Interface on Carbon Electrode

Promotion of direct electron transfer between Shewanella putrefaciens CN32 and carbon fiber electrodes via in situ growth of α-Fe2O3 nanoarray

The iron transport system plays a crucial role in the extracellular electron transfer process of Shewanella sp. In this study, we fabricated a vertically oriented α-Fe2O3 nanoarray on carbon cloth to enhance interfacial electron transfer in Shewanella putrefaciens CN32 microbial fuel cells. The incorporation of the α-Fe2O3 nanoarray not only resulted in a slight increase in flavin content but also significantly enhanced biofilm loading, leading to an eight-fold higher maximum power density compared to plain carbon cloth. Through expression level analyses of electron transfer-related genes in the outer membrane and core genes in the iron transport system, we propose that the α-Fe2O3 nanoarray can serve as an electron mediator, facilitating direct electron transfer between the bacteria and electrodes. This finding provides important insights into the potential application of iron-containing oxide electrodes in the design of microbial fuel cells and other bioelectrochemical systems, highlighting the role of α-Fe2O3 in promoting direct electron transfer.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

Carbon source priority and availability limit bidirectional electron transfer in freshwater mixed culture electrochemically active bacterial biofilms

This study investigated, if a mixed electroactive bacterial (EAB) culture cultivated heterotrophically at a positive applied potential could be adapted from oxidative to reductive or bidirectional extracellular electron transfer (EET). To this end, a periodic potential reversal regime between - 0.5 and 0.2 V vs. Ag/AgCl was applied. This yielded biofilm detachment and mediated electroautotrophic EET in combination with carbonate, i.e., dissolved CO2, as the sole carbon source, whereby the emerged mixed culture (S1) contained previously unknown EAB. Using acetate (S2) as well as a mixture of acetate and carbonate (S3) as the main carbon sources yielded primarily alternating electrogenic organoheterotropic metabolism with the higher maximum oxidation current densities recorded for mixed carbon media, exceeding on average 1 mA cm-2. More frequent periodic polarization reversal resulted in the increase of maximum oxidative current densities by about 50% for S2-BES and 80% for S3-BES, in comparison to half-batch polarization. The EAB mixed cultures developed accordingly, with S1 represented by mostly aerobes (84.8%) and being very different in composition to S2 and S3, dominated by anaerobes (96.9 and 96.5%, respectively). S2 and S3 biofilms remained attached to the electrodes. There was only minor evidence of fully reversible bidirectional EET. In conclusion the three triplicates fed with organic and/or inorganic carbon sources demonstrated two forms of diauxie: Firstly, S1-BES showed a preference for the electrode as the electron donor via mediated EET. Secondly, S2-BES and S3-BES showed a preference for acetate as electron donor and c-source, as long as this was available, switching to CO2 reduction, when acetate was depleted.

Retracted: The bidirectional extracellular electron transfer process aids iron cycling by Geoalkalibacter halelectricus in a highly saline-alkaline condition

Bidirectional extracellular electron transfer (EET) is crucial to upholding microbial metabolism with insoluble electron acceptors or donors in anoxic environments. Investigating bidirectional EET-capable microorganisms is desired to understand the cell-cell and microbe-mineral interactions and their role in mineral cycling besides leveraging their energy generation and conversion, biosensing, and bio-battery applications. Here, we report on iron cycling by haloalkaliphilic Geoalkalibacter halelectricus via bidirectional EET under haloalkaline conditions. It efficiently reduces Fe3+ oxide (Fe2O3) to Fe0 at a 0.75 ± 0.08 mM/mgprotein/d rate linked to acetate oxidation via outward EET and oxidizes Fe0 to Fe3+ at a 0.24 ± 0.03 mM/mgprotein/d rate via inward EET to reduce fumarate. Bioelectrochemical cultivation confirmed its outward and inward EET capabilities. It produced 895 ± 23 µA/cm2 current by linking acetate oxidation to anode reduction via outward EET and reduced fumarate by drawing electrons from the cathode (‒2.5 ± 0.3 µA/cm2) via inward EET. The cyclic voltammograms of G. halelectricus biofilms revealed redox moieties with different formal potentials, suggesting the involvement of different membrane components in bidirectional EET. The cyclic voltammetry and GC-MS analysis of the cell-free spent medium revealed the lack of soluble redox mediators, suggesting direct electron transfer by G. halelecctricus in achieving bidirectional EET. By reporting on the first haloalkaliphilic bacterium capable of oxidizing and reducing insoluble Fe0 and Fe3+ oxide, respectively, this study advances the limited understanding of the metabolic capabilities of extremophiles to respire on insoluble electron acceptors or donors via bidirectional EET and invokes the possible role of G. halelectricus in iron cycling in barely studied haloalkaline environments. IMPORTANCE Bidirectional extracellular electron transfer (EET) appears to be a key microbial metabolic process in anoxic environments that are depleted in soluble electron donor and acceptor molecules. Though it is an ecologically important and applied microbial phenomenon, it has been reported with a few microorganisms, mostly from nonextreme environments. Moreover, direct electron transfer-based bidirectional EET is studied for very few microorganisms with electrodes in engineered systems and barely with the natural insoluble electron acceptor and donor molecules in anoxic conditions. This study advances the understanding of extremophilic microbial taxa capable of bidirectional EET and its role in barely investigated Fe cycling in highly saline-alkaline environments. It also offers research opportunities for understanding the membrane components involved in the bidirectional EET of G. halelectricus. The high rate of Fe3+ oxide reduction activity by G. halelectricus suggests its possible use as a biocatalyst in the anaerobic iron bioleaching process under neutral-alkaline pH conditions.

Elongated Riboflavin-Producing Shewanella oneidensis in a Hybrid Biofilm Boosts Extracellular Electron Transfer

Shewanella oneidensis is able to carry out extracellular electron transfer (EET), although its EET efficiency is largely limited by low flavin concentrations, poor biofilm forming-ability, and weak biofilm conductivity. After identifying an important role for riboflavin (RF) in EET via in vitro experiments, the synthesis of RF is directed to 837.74 ± 11.42 µm in S. oneidensis. Molecular dynamics simulation reveals RF as a cofactor that binds strongly to the outer membrane cytochrome MtrC, which is correspondingly further overexpressed to enhance EET. Then the cell division inhibitor sulA, which dramatically enhanced the thickness and biomass of biofilm increased by 155% and 77%, respectively, is overexpressed. To reduce reaction overpotential due to biofilm thickness, a spider-web-like hybrid biofilm comprising RF, multiwalled carbon nanotubes (MWCNTs), and graphene oxide (GO) with adsorption-optimized elongated S. oneidensis, achieve a 77.83-fold increase in power (3736 mW m^-2 ) relative to MR-1 and dramatically reduce the charge-transfer resistance and boosted biofilm electroactivity. This work provides an elegant paradigm to boost EET based on a synthetic biology strategy and materials science strategy, opens up further opportunities for other electrogenic bacteria.

Wide distribution of extracellular electron transfer functionality in natural proteinaceous organic materials for microbial reductive dehalogenation

Extracellular electron transfer materials (EETMs) in the environment, such as humic substances and biochar, are formed from the humification/heating of natural organic materials. However, the distribution of extracellular electron transfer (EET) functionality in fresh natural organic materials has not yet been explored. In the present study, we reveal the wide distribution of EET functionality in proteinaceous materials for the first time using an anaerobic pentachlorophenol dechlorinating consortium, whose activity depends on EETM. Out of 11 natural organic materials and 13 reference compounds, seven proteinaceous organic materials (albumin, beef, milk, pork, soybean, yolk, and bovine serum albumin) functioned as EETMs. Carbohydrates and lipids did not function as EETMs. Comparative spectroscopic analyses suggested that a β-sheet secondary structure was essential for proteins to function as EETMs, regardless of water solubility. A high content of reduced sulfur was potentially involved in EET functionality. Although proteinaceous materials have thus far been considered simply as nutrients, the wide distribution of EET functionality in these materials provides new insights into their impact on biogeochemical cycles. In addition, structural information on EET functionality can provide a scientific basis for the development of eco-friendly EETMs.

Self-produced biophotosensitizers enhance the degradation of organic pollutants in photo-bioelectrochemical systems

Bioelectrochemical systems (BESs) with integrated photoactive components have been shown to be a promising strategy for enhancing the performance for bioenergy generation and pollutant removal. This study revealed an efficient photo-BES with an enhanced pollutant degradation rate by utilizing self-produced biomolecules as photosensitizers in situ. Results showed that the BES could increase the coulombic efficiency from 60.8% to 73.0% and the degradation rate of bisphenol A (BPA) from 0.030 to 0.189 h^-1 when the suspension in the reactor was illuminated with simulated sunlight in the absence of any external photosensitizers. We identified that the regular BES released many organic substances into the reactor during operation. These substances, including dissolved biomolecules and solid cell residues, were photoactive for producing hydroxyl radicals during light illumination. Quenching experiments verified that the •OH generated from the self-produced biophotosensitizers contributed to the enhanced degradation of BPA. Additionally, the phototransformation of biophotosensitizers was also observed in photo-BES. The quantity of tyrosine protein-like components decreased, but that of the humic components remained relatively stable. Our findings imply that BESs with integrated self-produced biophotosensitizers may be promising for constructing advanced electrochemical and biological systems for synchronous bioelectricity production and degradation of organic pollutants in wastewater treatments.

Transcriptome Analysis to Identify Crucial Genes for Reinforcing Flavins-Mediated Extracellular Electron Transfer in Shewanella oneidensis

Flavins serve as the electron mediators in Shewanella oneidensis, determining the extracellular electron transfer (EET) rate. Currently, metabolic engineering of flavins biosynthetic pathway has been studied for improving EET. However, the cellular response triggered by flavins that contribute to EET remains to be elucidated. In this study, the riboflavin-overproducing strain C5 (expressing the flavins synthetic genes in plasmid PYYDT) and the PYYDT strain (harboring the empty plasmid PYYDT) in the microbial fuel cells are applied for comparative transcriptomic analyses to investigate beneficial gene targets that could improve EET. From the differentially expressed genes, we select the significantly upregulated and downregulated genes for inverse engineering in S. oneidensis. The results show that overexpression of ahpC and ccpA, and inactivation of pubA, putB, and tonB are able to improve the EET capability. Combinatorial modulation of these five genes results in the recombinant strain CM4, achieving the maximum power density of 651.78 ± 124.60 mW/m², 1.97 folds of the parental strain. These genes modulation is speculated to reduce the ROS damage and to promote cytochrome synthesis and heme accumulation, which coherently enhance EET. Our findings facilitate in-depth understanding of the mechanism of flavins-mediated EET and provide new insights in promoting EET of S. oneidensis for electricity generation.

Extracellular proteins of Desulfovibrio vulgaris as adsorbents and redox shuttles promote biomineralization of antimony

Biomineralization is the key process governing the biogeochemical cycling of multivalent metals in the environment. Although some sulfate-reducing bacteria (SRB) are recently recognized to respire metal ions, the role of their extracellular proteins in the immobilization and redox transformation of antimony (Sb) remains elusive. Here, a model strain Desulfovibrio vulgaris Hildenborough (DvH) was used to study microbial extracellular proteins of functions and possible mechanisms in Sb(V) biomineralization. We found that the functional groups (N-H, CO, O-CO, NH₂-R and RCOH/RCNH₂) of extracellular proteins could adsorb and fix Sb(V) through electrostatic attraction and chelation. DvH could rapidly reduce Sb(V) adsorbed on the cell surface and form amorphous nanometer-sized stibnite and/or antimony trioxide, respectively with sulfur and oxygen. Proteomic analysis indicated that some extracellular proteins involved in electron transfer increased significantly (p < 0.05) at 1.8 mM Sb(V). The upregulated flavoproteins could serve as a redox shuttle to transfer electrons from c-type cytochrome networks to reduce Sb(V). Also, the upregulated extracellular proteins involved in sulfur reduction, amino acid transport and protein synthesis processes, and the downregulated flagellar proteins would contribute to a better adaption under 1.8 mM Sb(V). This study advances our understanding of how microbial extracellular proteins promote Sb biomineralization in DvH.

Augmenting the biodegradation of recalcitrant ethinylestradiol using Rhodopseudomonas palustris in a hybrid photo-assisted microbial fuel cell with enhanced bio-hydrogen production

This study presents the biodegradation potential of ethinylestradiol (EE2) in anaerobic environments using exoelectrogenic activity of Rhodopseudomonas palustris. EE2, a basic ingredient in oral contraceptives, is a significant estrogenic micropollutant in various wastewaters and is considered highly recalcitrant. This recalcitrance of EE2 has caused anoxic areas to become repositories for these pollutants. Thus, it is essential to find the microorganisms and suitable methods to degrade this compound. An initial EE2 concentration of 1 mg/L, used in an anaerobic photobioreactor, resulted in 70% EE2 degradation over a period of 16 days with an increase of 63% in hydrogen production when EE2 was used with glycerol as the main carbon source in the culture medium. Furthermore, in the novel setup of hybrid photo-assisted microbial fuel cell (h-PMFC) employed here, EE2 degradation enhanced to 89.82% with a maximum power density of 0.633 ± 0.04 mW/m². The hybrid MFC employed here could metabolize EE2 and sustained the bio-hydrogen production for 14 days to run the hydrogen fuel cell which otherwise could not be sustained with glycerol only and thus increased the overall power output. The current work highlights the use of R. palustris and the significance of co-metabolism in bioremediation of pollutants and bioenergy generation.

Mechanistic evaluation of the exoelectrogenic activity of Rhodopseudomonas palustris under different nitrogen regimes

The operation of bioelectrochemical systems (BESs) relies on the ability of microbes to export electrons outside of their cells. However, microorganisms are not evolutionary conceived to power BESs as most of the redox processes occur within. In this study, a low cost strategy equivalent to the one used to improve hydrogen production is employed to divert electrons from the metabolism to an electrode. Varying the ratio of nitrogen to carbon concentration (0, 0.20 and 0.54) determines what fraction of the electron flux is directed towards biosynthesis, biohydrogen generation and extracellular electron transfer. The ratio of 0.54 produced a higher specific growth rate while the ratio of 0.20 resulted in combined higher maximum specific hydrogen production and exoelectrogenic activity, translating into a maximum power density of 2.39 ± 0.13 mW m^-2 in a novel hybrid hydrogen-photosynthetic microbial fuel cell. The current work sets a framework for the optimisation of R. palustris for bioenergy recovery.

Soluble electron acceptors affect bioluminescence from Shewanella woodyi

Shewanella woodyi cultures were used to correlate bioluminescence intensity with changes in the electrochemical potential of a saltwater medium using soluble electron acceptors. A relationship between the concentration of NaNO₃ or CoCl₂ to bioluminescence intensity was confirmed using aerobic cultures of S. woodyi at 20°C with glucose as the sole carbon source. In general, increasing the concentration of nitrate or Co(II) reduced the bioluminescence per cell, with complete luminescence being repressed at ≥5 mM nitrate and ≥0.5 mM Co(II). Results from cell viability fluorescent staining concluded that increasing the concentration of Co(II) or nitrate did not affect the overall viability of the cells when compared with cultures lacking Co(II) or nitrate. These data show that potentials of <0.2 V vs Normal Hydrogen Electrode (NHE) repress the luminescence from the cells, but the exact mechanism is unclear. Our results indicated that the luminescence intensity from S. woodyi could be systematically reduced using these two soluble electron acceptors, making S. woodyi a potential model bacterium for whole-cell luminescence bioelectrochemical sensor applications.