In Situ Spectral Kinetics of Cr(VI) Reduction by c-Type Cytochromes in A Suspension of Living Shewanella putrefaciens 200

Effects of exogenous riboflavin or cytochrome addition on the cathodic reduction of Cr(VI) in microbial fuel cell with Shewanella putrefaciens

Bioreduction of Cr(VI) is recognized as a cost-effective and environmentally friendly method, attracting widespread interest. However, the slow rate of Cr(VI) bioreduction remains a practical challenge. Additionally, the direct removal efficiency of microbes for high concentrations of Cr(VI) is not ideal due to the toxicity. Therefore, this study investigated the effects of exogenous riboflavin or cytochrome on the cathodic reduction of Cr(VI) in microbial fuel cells. The results demonstrated that the exogenous riboflavin or cytochrome effectively improved the voltage output of the cells, with riboflavin increasing the voltage by 52.08%. Within the first 24 h, the Cr(VI) removal ratio in the normal, cytochrome, and riboflavin groups was 14.3%, 29.3%, and 53.8%, respectively. And the final removal ratio was 55.1%, 69.1%, and 98.0%, respectively. These results showed different enhancement effects of riboflavin and cytochrome on Cr(VI) removal. The analysis of riboflavin and cytochrome contents revealed that the additions did not have a significant impact on the autocrine riboflavin of S. putrefaciens, but affected the autocrine cytochrome. SEM, XPS, and FTIR results confirmed the presence of reduced Cr(III) on the cathode, which formed precipitate and adhered to the cathode surface. The EDS analysis showed that the amount of Cr on the cathode in normal, cytochrome, and riboflavin groups was 4.71%, 6.37%, 7.56%, respectively, which was consistent with the voltage and Cr(VI) removal data. These findings demonstrated the significant enhancement of exogenous riboflavin or cytochrome on Cr(VI) reduction, thereby providing data reference for the future bio-assisted remediation of Cr(VI) pollution.

Mechanism and applications of bidirectional extracellular electron transfer of Shewanella

Electrochemically active microorganisms (EAMs) play an important role in the fields of environment and energy. Shewanella is the most common EAM. Research into Shewanella contributes to a deeper comprehension of EAMs and expands practical applications. In this review, the outward and inward extracellular electron transfer (EET) mechanisms of Shewanella are summarized and the roles of riboflavin in outward and inward EET are compared. Then, four methods for the enhancement of EET performance are discussed, focusing on riboflavin, intracellular reducing force, biofilm formation and substrate spectrum, respectively. Finally, the applications of Shewanella in the environment are classified, and the restrictions are discussed. Potential solutions and promising prospects for Shewanella are also provided.

Detoxification mechanisms of electroactive microorganisms under toxicity stress: A review

Remediation of environmental toxic pollutants has attracted extensive attention in recent years. Microbial bioremediation has been an important technology for removing toxic pollutants. However, microbial activity is also susceptible to toxicity stress in the process of intracellular detoxification, which significantly reduces microbial activity. Electroactive microorganisms (EAMs) can detoxify toxic pollutants extracellularly to a certain extent, which is related to their unique extracellular electron transfer (EET) function. In this review, the extracellular and intracellular aspects of the EAMs’ detoxification mechanisms are explored separately. Additionally, various strategies for enhancing the effect of extracellular detoxification are discussed. Finally, future research directions are proposed based on the bottlenecks encountered in the current studies. This review can contribute to the development of toxic pollutants remediation technologies based on EAMs, and provide theoretical and technical support for future practical engineering applications.

Sb(III) resistance mechanism and oxidation characteristics of Klebsiella aerogenes X

Resistant bacteria are potential natural materials for the bioremediation of soil metalloid pollution. A strain isolated from farmland soil chronically exposed to Sb was identified as K. aerogenes X with high antimonite [Sb(III)] tolerance and oxidation ability. The resistance mechanism of K. aerogenes X and its extracellular polymeric substances (EPS), antioxidant enzymes, and oxidation characteristics in Sb(III) stress were investigated in this study by stress incubation experiments and FTIR. The biotoxicity of Sb was limited by the binding of the organic compounds in EPS, and the anionic functional groups (e.g., amino, carboxyl and hydroxyl groups, etc.) present in the cell envelope were the components primarily responsible for the metalloid-binding capability of K. aerogenes X. The K. aerogenes X can oxidize Sb(III), and its metabolites induce changes in reactive oxygen species (ROS), catalase (CAT), total superoxide dismutase (SOD) and glutathione s-transferase (GSH-S) activity, indicating that the resistance mechanisms of K. aerogenes X are mediated by oxidative stress, EPS restriction and cell damage. Oxidation of Sb(III) is driven by interactions in intracellular oxidation, cell electron transport, extracellular metabolism including proteins and low molecular weight components (LMWs). LMWs (molecular weight <3 kDa) are the main driving factor of Sb(III) oxidation. In addition, Sb resistance genes arsA, arsB, arsC, arsD and acr3 and potential oxidation gene arsH were identified in K. aerogenes X. Owing to its natural origin, high tolerance and oxidation ability, K. aerogenes X could serve as a potential bioremediation material for the mitigation of Sb(III) in contaminated areas.

Extracellular Electron Shuttling Mediated by Soluble c-Type Cytochromes Produced by Shewanella oneidensis MR-1

How metal-reducing bacteria transfer electrons during dissimilatory energy generation under electron acceptor-limited conditions is poorly understood. Here, we incubated the iron and manganese-reducing bacterium Shewanella oneidensis MR-1 without electron acceptors. Removal of soluble extracellular organic compounds (EOCs) dramatically retarded transfer of electrons to an experimental electron acceptor, Cr(VI), by MR-1. However, the return of either high MW (>3000 Da) or low MW (<3000 Da) soluble EOCs produced by MR-1 to washed cells restored Cr(VI) reduction though Cr(VI) reduction was fastest when both size fractions were added together. Spectral and electrochemical characterization of EOCs indicated the presence of flavins and c-type cytochromes (c-Cyts). A model of the kinetics of individual elementary reactions between cells, flavins, released c-Cyts, and Cr(VI), including the direct reduction of flavins, released c-Cyts, and Cr(VI) by cells and the indirect reduction of Cr(VI) by reduced forms of flavins and released c-Cyts, was developed. Model results suggest that released c-Cyts could act as electron mediators to accelerate electron transfer from cells to Cr(VI), and the relative contribution of this pathway was higher than that mediated by flavins. Hence, extracellular c-Cyts produced by MR-1 likely play a role in extracellular electron transfer under electron acceptor-limited conditions. These findings provide new insights into extracellular electron shuttling and the metabolic strategy of metal-reducing bacteria under electron acceptor-limited conditions.

Behaviors and fate of adsorbed Cr(VI) during Fe(II)-induced transformation of ferrihydrite-humic acid co-precipitates

The mobility of Cr(VI) in the environment is affected by the transformation of ferrihydrite (Fh) and ferrihydrite-humic acid co-precipitates (Fh-HA). However, the impacts of Fe(II)-induced transformation of Fh and Fh-HA on the mobility, speciation and partitioning of associated Cr(VI) remain unclear. In this study, the behaviors of adsorbed Cr(VI) during Fh and Fh-HA aging at 70 °C for 9 days (pH₀ = 3.0 and 7.0) in the absence and presence of Fe(II) were studied. Results revealed that the main speciation of Cr(VI) after transformation was non-desorbable Cr and its formation involved the following pathways. Firstly, Fe(II) (0.2 and 2.0 mM) induced the transformation of Fh-HA to hematite and goethite, promoting the structural incorporation of adsorbed Cr into hematite and goethite via complexation. Secondly, under neutral condition (pH₀ = 7.0), the low concentration of Fe(II) (0.2 mM) could not reduce completely Cr(VI) to Cr(III) and thus residual Cr(VI) was incorporated into the Cr(III)-Fe(III) co-precipitates. Thirdly, coprecipitated humic acid not only reduced Cr(VI) to Cr(III) via polysaccharide, but also formed complexes with incorporated Cr through carboxylic groups to sequester Cr. Our results demonstrate that Fe(II)-induced transformation of Fh-HA exerts major influences on associated Cr(VI) speciation and partitioning.

Artificial Cytochrome c Mimics: Graphene Oxide-Fe(III) Complex-Coated Molecularly Imprinted Colloidosomes for Selective Photoreduction of Highly Toxic Pollutants

Our previous works showed that the molecularly imprinted TiO₂ photocatalyst has been one of the most efficient materials for selective photooxidation of highly toxic organic pollutants (HTOPs) from complicated wastewater. However, some highly toxic pollutants (e.g., heavy metals) are very stable under the oxidizing environment. Therefore, the design of enzyme-like catalysts to selectively reduce highly toxic pollutants is extremely needed. In this work, inspired by the bioreduction ability of cytochrome c (cyt c, a heme containing metalloprotein), we presented a simple and efficient way to generate an artificial cyt c mimic (ACM) using graphene oxide (GO)-Fe(III) complex-coated molecularly imprinted colloidosomes. Prior to loading of Fe(III) centers to GO for constructing ACMs, GO-coated molecularly imprinted colloidosomes were synthesized via Pickering emulsion polymerization. Similar to a nature cyt c, the ACM contained both the molecular recognition element (molecularly imprinted cavity) and the non-heme electron sink (GO-Fe(III) complex), which resulted in the ACMs having good selectivity toward the enzyme-like reduction of the highly toxic target Cr(VI). Moreover, by using HepG2 cells as model cells, the Cr(VI) solution after treating by ACMs was proved to be safe and nontoxic. To confirm that the present method was universal for constructing ACMs, various GO-Fe(III) complex-coated molecularly imprinted colloidosomes, which could selectively photoreduce other highly toxic inorganic ions and organic pollutants, were also investigated. The ACMs described herein will act as a vector that encourages the design of more functional non-heme enzymes in sensing, environmental separation, clinical diagnose, and delivery of therapeutic agents.

Coupled Kinetics Model for Microbially Mediated Arsenic Reduction and Adsorption/Desorption on Iron Oxides: Role of Arsenic Desorption Induced by Microbes

The dynamic behavior of arsenic (As) species is closely associated with iron mineral dissolution/transformation in the environment. Bacterially induced As(V) desorption from iron oxides may be another important process that facilitates As(V) release from iron oxides without significant reductive dissolution of iron oxides. Under the impact of bacterially induced desorption, As kinetic behavior is controlled by both the microbial reduction of As(V) and the As(III)&As(V) reactions on iron oxide surfaces. However, there is still a lack of quantitative understanding on the coupled kinetics of these processes in complex systems. We developed a quantitative model that integrated the time-dependent microbial reduction of As(V) with nonlinear As(III)&As(V) adsorption/desorption kinetics on iron oxides under the impact of bacterially induced As(V) desorption. We collected and modeled literature data from 11 representative studies, in which microbial reduction reactions occurred with minimal iron oxide dissolution/transformation. Our model highlighted the significance of microbially induced As(V) desorption and time-dependent changes of microbial reduction rates. The model can quantitatively assess the roles and the coupling of individual reactions in controlling the overall reaction rates. It provided a basis for developing comprehensive models for As cycling in the environment by coupling with other chemical, physical, and microbial processes.

Biodegradation of metal complex Naphthol Green B and formation of iron-sulfur nanoparticles by marine bacterium Pseudoalteromonas sp CF10-13

Treatment of metal complex dye wastewater has attracted growing attention due to the degradation-resistant, high cost and potential hazards of current techniques. This study reported a marine bacterium (Pseudoalteromonas sp CF10-13) with potential performance in decolorization and degradation of a metal complex dye-Naphthol Green B (NGB) at wide ranges of salinity, dye concentration and alkalinity under anaerobic conditions. It was inferred that the secretion of electron mediators in soluble extracellular metabolites by P. sp CF10-13 played important roles in NGB decolorization and degradation through extracellular electron transfer. Naphthalenesulfonate, the major structure in NGB molecule, was further degraded into low-toxic benzamide. Black stable iron-sulfur nanoparticles were formed endogenously avoiding H₂S releasing, exogenous sulfur addition and metal sludge in accumulation. Accordingly, this study provided a cost-effective and eco-friendly biodegradation method to refractory NGB, further promoting the understanding of dye resources recovery.

On-going applications of Shewanella species in microbial electrochemical system for bioenergy, bioremediation and biosensing

Microbial electrochemical system (MES) has attracted ever-growing interest as a promising platform for renewable energy conversion and bioelectrochemical remediation. Shewanella species, the dissimilatory metal reduction model bacteria with versatile extracellular electron transfer (EET) strategies, are the well-received microorganisms in diverse MES devices for various practical applications as well as microbial EET mechanism investigation. Meanwhile, the available genomic information and the unceasing established gene-editing toolbox offer an unprecedented opportunity to boost the applications of Shewanella species in MES. This review thoroughly summarizes the status quo of the applications of Shewanella species in microbial fuel cells for bioelectricity generation, microbial electrosynthesis for biotransformation of valuable chemicals and bioremediation of environment-hazardous pollutants with synoptical discussion on their EET mechanism. Recent advances in rational design and genetic engineering of Shewanella strains for either promoting the MES performance or broadening their applications are surveyed. Moreover, some emerging applications beyond electricity generation, such as biosensing and biocomputing, are also documented. The challenges and perspectives for Shewanella-based MES are also discussed elaborately for the sake of not only discovering new scientific lights on microbial extracellular respiratory but also propelling practical applications.

Intracellular versus extracellular accumulation of Hexavalent chromium reduction products by Geobacter sulfurreducens PCA

Hexavalent chromium (Cr(VI)) reduction by Geobacter sulfurreducens PCA was evaluated in batch experiments, and the form and amounts of intracellular and extra-cellular Cr(VI) reduction products were determined over time. The first-order Cr(VI) reduction rate per unit mass of cells was consistent for different initial cell concentrations, and approximately equal to (2.065 ± 0.389) x 10^-9 mL CFU^-1 h^-1. A portion of the reduced Cr(VI) products precipitated on Geobacter cell walls as Cr(III) and was bound via carboxylate functional groups, a portion accumulated inside Geobacter cells, and another portion existed as soluble Cr(III) or organo-Cr(III) released to solution. A mass balance analysis of total chromium in aqueous media, on cell walls, and inside cells was determined as a function of time, and with different initial cell concentrations. Mass balances were between 92% and 98%, and indicated Cr(VI) reduction products accumulate more on cell walls and inside cells with time and with increasing initial cell concentration, as opposed to particulates in aqueous solution. Reduced Cr(VI) products both in solution and on cell surfaces appear to form organo-Cr(III) complexes, and our results suggest that such complexes are more stable to reoxidation than aqueous Cr(III) or Cr(OH)₃. Chromium inside cells is also likely more stable to reoxidation, both because it can form organic complexes, and it is separated by the cell membrane from solution conditions. Hence, Cr(VI) reduction products in groundwater during bioremediation may become more stable against re-oxidation, and may pose a lower risk to human health, over time and with greater initial biomass densities.

Electron shuttles enhance the degradation of sulfamethoxazole coupled with Fe(III) reduction by Shewanella oneidensis MR-1

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

Environmentally available biowastes as substrate in microbial fuel cell for efficient chromium reduction

Dual chambered microbial fuel cells with Potassium dichromate (22 g/L, MFC-1) and tannery effluent waste water containing 26 mg/L (MFC-2), 5 mg/L (MFC-3) of Cr(VI) as catholyte, sweet lime waste inoculated by cowdung as anolyte and graphite electrodes were used to reduce toxic Cr(VI) to Cr(III) with simultaneous power generation. Cr (VI) in the cathode chamber reduced to Cr₂O₃ within 24 h. Complete reduction of Cr(VI) from tannery effluents by microbial fuel cell is noticed within 10 days. The 16 s rRNA sequencing studies demonstrated presence of Geobacter Metallireducens in mixed culture bacteria in anaerobic anode. The power density of the device is 396.7 mW/m²on day1which is 7.2 times higher than literature data of 55.5 mW/m². The processes involved on the biofilm/electrolyte interface and graphite/electrolyte interface is studied by Electrochemical Impedance Spectroscopy. Electrochemical studies demonstrated the active growth of biofilm on anode which reduces charge transfer resistance from day 1 to day 25. The concentration of Cr(VI) reduced in the present studies are approximately 1000 times higher than those reported in the literature.

The Functional Mechanisms and Application of Electron Shuttles in Extracellular Electron Transfer

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