Oneidensis MR-1 Research Articles

A novel growth and isolation medium for exoelectrogenic bacteria

A microbiological isolation and growth medium that can effectively discriminate electrochemically active exoelectrogenic bacteria from other non-exoelectrogenic bacteria, is currently unavailable. In this study, we developed a novel chromogenic growth and isolation solid medium based on MnO2 that can selectively allow the growth of exoelectrogenic bacteria and change the medium colour in the process. Known exoelectrogenic bacteria such as Shewanella oneidensis MR1 and other such bacteria from functional microbial fuel cell (MFC) anodes were capable of growing and changing colour in the novel growth medium. On the contrary, non-exoelectrogenic bacteria such as Escherichia coli ATCC 25922 were incapable of growing and inducing a colour change in the novel medium. Further biochemical characterisation of these isolated exoelectrogenic bacteria by Raman micro-spectroscopy demonstrated that these bacteria over express cytochrome proteins that are vital in extracellular electron transfer events. This medium is a convenient method to isolate exoelectrogenic bacteria from complex environmental samples.

The influence of Shewanella oneidensis MR-1 on the transformation of iron oxides and phosphorus in a red soil

In this study, Shewanella oneidensis MR-1, an iron (Fe)-reducing bacterium, was inoculated to a red soil, which was then incubated. Soil samples were taken regularly to analyse the variation of iron oxides and phosphorus (P) fractions. The results showed that the MR-1 inoculation increased the content of the free iron oxides, but decreased the activity of the iron oxides in the soil, and had no significant influence on the amorphous iron oxides. The MR-1 inoculation increased the resin-P and residual-P, decreased the NaHCO<sub>3</sub>-extracted inorganic P (NaHCO<sub>3</sub>-P<sub>i</sub>) and NaOH-extracted inorganic P (NaOH-P<sub>i</sub>), but did not significantly influence the diluted HCl-extracted inorganic P (D.HCl-P<sub>i</sub>) and concentrated HCl-extracted inorganic P (C.HCl-P<sub>i</sub>). The presence of MR-1 influenced the correlation between the free iron oxides and NaOH-P<sub>i</sub>. In the CK where deactivated MR-1 was applied, there was a significant positive correlation between the free iron oxides and the NaOH-P<sub>i</sub>; in the treatment with the live MR-1 inoculation, there was no correlation between them. In addition, there was a significant positive correlation between the free iron oxides and the C.HCl-P<sub>i</sub>, and there was a significant negative correlation between the NaHCO<sub>3</sub>-P<sub>i</sub>, resin-P, and residual-P. Therefore, the MR-1 inoculation improved the P availability by decreasing the activity of the iron oxides and consequently improved the P use efficiency in the red soil.

REMEDIATION OF CR(VI) IN WATER USING BIOSYNTHESIZED PALLADIUM NANO-MATERIALS LOADED (SHEWANELLA ONEIDENSIS) MR-1

“Green” synthesis is attracted considerable interest in materials science as a reliable, durable, and environmentally friendly approach to fabricating a wide range of nanoparticles such as metal oxide. Green production of metal nanoparticles was used to host a variety of biological components (such as fungi, algae, plant extracts bacteria etc.). Shewanella oneidensis MR-1 was employed in this biosynthetic investigation of palladium nanoparticles (Bio-Pd) to produce Cr (VI) under aerobic circumstances. By adjusting the ratio of microbial biomass to palladium precursors, it was possible to control the distribution and size of Bio-Pd. The Pd ratio had the smallest average particle size at 6.33 1.69 nm. Additionally, it has a formic acid oxidation electrocatalytic potential of -0.132 V, which is 0.158 V smaller than that of commercial Pd/C (5%). The entire catalytic reduction of a 200 mg/L Cr (VI) solution could be achieved by the tiny, uniformly distributed extracellular Bio-Pd within 10 minutes, but commercially available Pd/C (5%) required at least 45 minutes. Over five cycles, the Bio-Pd material offers a high decrease rate. Microbes have a substantial effect on the entire process of effectively reducing Cr (VI), dispersing palladium nanoparticles, and adsorbing Cr (III). The findings of this study will serve as a guide for the advancement of effective and environmentally acceptable bio-Pd catalysts for pollution control in straightforward and reasonable situations.

A light-switch for efficient decolorization of azo dye wastewater based on bacteria–semiconductor interaction

This study investigated the interaction of Shewanella oneidensis MR-1 and CdS in azo dye decolorization, showing that CdS could act as a light-switch to achieve fast light-driven decolorization.

Enhanced Microbial Ferrihydrite Reduction by Pyrogenic Carbon: Impact of Graphitic Structures.

Electron-shuttling agents such as pyrogenic carbon (PC) can mediate long-distance electron transfer and play numerous key roles in aquatic and soil biogeochemical processes. The electron-shuttling capacity of PC relies on both the surface oxygen-containing functional groups and bulk graphitic structures. Although the impacts of oxygen-containing functional groups on the electron-shuttling performance of PC are well studied, there remains insufficient understanding on the function of graphitic structures. Here, we studied the functions of PC in mediating microbial (Shewanella oneidensis MR-1) reduction of ferrihydrite, a classic and geochemically important soil redox process. The results show that PC enhanced microbial ferrihydrite reduction by 20-115% and the reduction rates increased with PC pyrolysis temperature increasing from 500 to 900 °C. For PC prepared at low temperature (500-600 °C), the electron-shuttling capacity of PC is mainly attributed to its oxygen-containing functional groups, as indicated by a 50-60% decline in the ferrihydrite reduction rate when PC was reduced under a H2 atmosphere to remove surface oxygen-containing functional groups. In stark contrast, for PC prepared at higher temperature (700-900 °C), the formation of PC graphitic structures was enhanced, as suggested by the higher electrical conductivity; accordingly, the graphitic structure exhibits greater importance in shuttling electrons, as demonstrated by a minor decline (10-18%) in the ferrihydrite reduction rate after H2 treatment of PC. This study provides new insights into the nonlinear and combined role of graphitic structures and oxygen-containing functional groups of PC in mediating electron transfer, where the pyrolysis temperature of PC acts as a key factor in determining the electron-shuttling pathways.

Effect of nanowire conductive transfer on the performance of batch‐microbial fuel cells

Microbial fuel cells (MFCs) are a promising technology that uses microorganisms to simultaneously generate bioelectricity while treating wastewater. To further improve the performance of the MFC, it is essential to understand and evaluate the electron transfer mechanism. However, redesigning the electron transfer mechanism of MFCs through an experimental approach is costly and time-consuming. Hence, in this study, a numerical modeling approach is implemented through the Nernst-Monod kinetic equation, which is validated by experimental results. A nanowire conductive transferring pathway is considered between the microorganisms and anode electrodes of a batch-type MFC. Moreover, two types of bacteria are utilized such as the Shewanella oneidensis MR-1 and Shewanella putrefacient with substrate concentrations of 0.5 M sodium lactate. The results have shown that the limiting current density of the MFC from the computational model is 1514 mA m−2. On the other hand, the current density from the experimental approach for Shewanella oneidensis MR-1 is 497 mA m−2 while for Shewanella putrefacient is 140 mA m−2. The anode activation loss of 491 Ω is lower than the cathode activation loss of 643 Ω, which indicates the relative influence of the cathode activation loss on the bioelectricity generation of the MFC. In addition, the results revealed that the nanowire electron transfer mechanism in the anode biofilm was less affected by the concentration losses. This then indicates that the physical mechanism of the nanowire electron transfer can be effectively used to investigate the batch-type MFCs. In turn, the results of this study will contribute to the development of an improved MFC.

Effect of Shewanella oneidensis MR-1 on U(VI) sequestration by montmorillonite

Bacteria may change the physicochemical properties of montmorillonite and further effect the disposal of high-level radioactive waste. Therefore, we explored the influence of Shewanella oneidensis MR-1 on the elimination of representative radionuclide U(VI) by montmorillonite (MMT). The batch experiments showed that MR-1 significantly enhanced the removal efficiency of U(VI), the adsorption capacity of MMT improved from 8.4 to 16.1 mg/g after addition of MR-1, and the adsorption type changed from Langmuir to Freundlich. FTIR and XPS analysis revealed that hydroxyl, phosphate, carbonyl and amine in MMT + MR-1 were primary actors in the elimination of U(VI). The U 4f high-resolution XPS spectrum of MMT + MR-1 showed U(VI) and U(IV) peaks at the same time, indicating that the adsorption process was accompanied by the reduction reaction, which may be due to the extracellular respiration of MR-1. These investigations are significant to insight the potential significance of microbial processes for the transport and elimination of U(VI) in repositories, which in return will contribute to their safe disposal.

Nanoscale zero-valent iron coupling with Shewanella oneidensis MR-1 for enhanced reduction/removal of aqueous Cr(VI)

The surface passivation of nZVI is considered one of the most challenging in successfully applying nZVI to environmental remediation. Shewanella oneidensis MR-1 (MR-1) are widely distributed in the various aquatic environments, which can use Fe(III) (hydr)oxides as electron acceptor, converting Fe(III) to Fe(II). In this study, the nZVI coupling with MR-1 process is proposed for efficient reduction/removal of high concentration of hexavalent chromium [Cr(VI)]. By studying the experimental conditions such as initial Cr(VI) concentrations, nZVI dosages, and cell concentrations, the optimal experimental conditions for Cr(VI) removal were obtained. Under the conditions of nZVI dosage of 1.32 mM, cell concentration of 3 × 107 cells/mL and Cr(VI) concentration of 80 mg/L, the removal efficiency of Cr(VI) and total chromium [Cr(total)] can reach 98.65% and 98.23% within 6 days, respectively. The key point of this study is to demonstrate that the addition of MR-1 can promote dissolution of the passivation layer on the nZVI surface and prolong its reactivity, significantly improving Cr(VI) reduction/removal efficiency. In this process, most Cr(VI) is reduced to Cr(III), which then is removed by forming Cr2O3 and Cr(CH3C(O)CHC(O)CH3)3 complexes, and a small amount of the unreduced Cr(VI) is removed through adsorption and complexation with oxygen-containing functional groups. The extracellular polymeric substances (EPS) derived from MR-1 play a crucial role in removing the passivation layer on the nZVI surface and Cr(VI) removal. This method may have great potential application for reduction and detoxification of Cr(VI) in the aqueous environment.

Biomineralization of struvite by Shewanella oneidensis MR-1 for phosphorus recovery: Cr(VI) effect and behavior

Bacterial mineralization of struvite from wastewaters is one of the promising approaches to recover phosphorus. However, the information about whether and how the heavy metal Cr(VI) affect the bacterial mineralization system is still lack. Here a model microbe Shewanella oneidensis MR-1 was used to induce struvite mineralization in simulated Cr(VI)-bearing wastewaters and the composition, morphology, and Cr content of precipitates were characterized by a wide range of techniques. Our results reveal that the yield of struvite and Mg transformation efficiency are almost not affected by the concomitant Cr(VI), but pitting and roughening of struvite surfaces are observed. Meanwhile, the struvite mineralized from model wastewaters with 0.1 or 0.2 mM Cr(VI) can meet the regulatory standard of Cr content as fertilizer. Strain MR-1 can not only create an alkaline environment for struvite precipitation, but also completely reduce Cr(VI) to Cr(III), helping to form soluble organo-Cr(III) complexes in the supernatant, hence significantly reducing the Cr content in struvite. And the loaded Cr(III) exists in the form of amorphous Cr(III) species (e.g., Cr(OH)3) on the struvite surface instead of the crystal lattice substitution in abiotic case. Current results serve as a guidance for further development of struvite biomineralization and P recovery from Cr(VI)-containing wastewaters.

Effects of biochar/AQDS on As(III)-adsorbed ferrihydrite reduction and arsenic (As) and iron (Fe) transformation: Abiotic and biological conditions

Microbe induced iron (Fe) reduction play an important role in arsenic (As) transformation and the related secondary mineral formation. Meanwhile biochar could react as electron shuttle for this process. Impact of biochar and model electron shuttle anthraquinone-2,6-disulfonate (AQDS) on the chemical/biological iron reduction of As(III)-adsorbed ferrihydrite and the solid-liquid redistribution of As in M1 buffer were studied. Fe reduction results in the release of As adsorbed on ferrihydrite into the solution. Under abiogenic conditions, both biochar and AQDS promoted ferrous production, the chemical oxidation of As(III) and As release. Inoculate with Shewanella oneidensis MR-1, AQDS has greater electronic shuttle function than biochar (with the maximum Fe(II) contents: 154 mg/L > 76.6 mg/L respectively). However, only 12.8 mg/L As was released in the presence of AQDS, which was much lower than that in the presence of biochar (21.6 mg/L), and may be associated with the transformation of As speciation and the formation of secondary minerals. XRD and EDX-SEM confirmed that the As could be fixed by the generated secondary mineral vivianite. The relative contents of vivianite in biological control and AQDS addition were 2.7% and 18.4%, respectively. This study provides information on the transformation and migration of As and Fe with the addition of biochar under anaerobic conditions, which is potential to understand the mechanism of As(III)-contaminated soil remediation.

Biological removal of sulfamethoxazole enhanced by S. oneidensis MR-1 via promoting NADH generation and electron transfer and consumption

The bio-removal efficiency of sulfamethoxazole (SMX) from wastewater is usually very poor. In this paper a new efficient method to biodegrade SMX was reported. The SMX biodegradation efficiency by Paracoccus denitrificans was observed to be remarkably enhanced from 48.9% to 94.2% after Shewanella oneidensis MR-1 addition. The mechanisms investigation revealed that P. denitrificans was the dominant microbe for SMX biodegradation. Although SMX biodegradation by S. oneidensis MR-1 alone was negligible, its presence advanced NADH generation. The proteomics assay revealed that the expression of key proteins relevant with complex I and III and cytochrome c in electron transfer chain were increased due to P. denitrificans acquiring iron from periplasm to cytoplasm being improved. In addition, the extracellular electron transfer capability was enhanced as S. oneidensis MR-1 not only produced flavin, but caused P. denitrificans to secret more extracellular polymeric substances. Further investigation indicated that the expression of key enzymes related to electron consumption in SMX biodegradation was up-regulated. Based on these findings, the pathways of S. oneidensis MR-1 promoting SMX biodegradation were proposed. As all nitrate could be removed with almost no nitrite accumulation, this study would also provide an attractive way for simultaneous bio-removal of multiple pollutants from wastewater.

Construction of an Acetate Metabolic Pathway to Enhance Electron Generation of Engineered Shewanella oneidensis.

Background: Microbial fuel cells (MFCs) are a novel bioelectrochemical devices that can use exoelectrogens as biocatalyst to convert various organic wastes into electricity. Among them, acetate, a major component of industrial biological wastewater and by-product of lignocellulose degradation, could release eight electrons per mole when completely degraded into CO2 and H2O, which has been identified as a promising carbon source and electron donor. However, Shewanella oneidensis MR-1, a famous facultative anaerobic exoelectrogens, only preferentially uses lactate as carbon source and electron donor and could hardly metabolize acetate in MFCs, which greatly limited Coulombic efficiency of MFCs and the capacity of bio-catalysis. Results: Here, to enable acetate as the sole carbon source and electron donor for electricity production in S. oneidensis, we successfully constructed three engineered S. oneidensis (named AceU1, AceU2, and AceU3) by assembling the succinyl-CoA:acetate CoA-transferase (SCACT) metabolism pathways, including acetate coenzyme A transferase encoded by ato1 and ato2 gene from G. sulfurreducens and citrate synthase encoded by the gltA gene from S. oneidensis, which could successfully utilize acetate as carbon source under anaerobic and aerobic conditions. Then, biochemical characterizations showed the engineered strain AceU3 generated a maximum power density of 8.3 ± 1.2 mW/m2 with acetate as the sole electron donor in MFCs. In addition, when further using lactate as the electron donor, the maximum power density obtained by AceU3 was 51.1 ± 3.1 mW/m2, which approximately 2.4-fold higher than that of wild type (WT). Besides, the Coulombic efficiency of AceU3 strain could reach 12.4% increased by 2.0-fold compared that of WT, which demonstrated that the engineered strain AceU3 can further utilize acetate as an electron donor to continuously generate electricity. Conclusion: In the present study, we first rationally designed S. oneidensis for enhancing the electron generation by using acetate as sole carbon source and electron donor. Based on synthetic biology strategies, modular assembly of acetate metabolic pathways could be further extended to other exoelectrogens to improve the Coulombic efficiency and broaden the spectrum of available carbon sources in MFCs for bioelectricity production.

Electronic control of redox reactions inside Escherichia coli using a genetic module.

Microorganisms regulate the redox state of different biomolecules to precisely control biological processes. These processes can be modulated by electrochemically coupling intracellular biomolecules to an external electrode, but current approaches afford only limited control and specificity. Here we describe specific electrochemical control of the reduction of intracellular biomolecules in Escherichia coli through introduction of a heterologous electron transfer pathway. E. coli expressing cymAmtrCAB from Shewanella oneidensis MR-1 consumed electrons directly from a cathode when fumarate or nitrate, both intracellular electron acceptors, were present. The fumarate-triggered current consumption occurred only when fumarate reductase was present, indicating all the electrons passed through this enzyme. Moreover, CymAMtrCAB-expressing E. coli used current to stoichiometrically reduce nitrate. Thus, our work introduces a modular genetic tool to reduce a specific intracellular redox molecule with an electrode, opening the possibility of electronically controlling biological processes such as biosynthesis and growth in any microorganism.

Influence of surface coatings on the adhesion of Shewanella oneidensis MR-1 to hematite

The adhesion of dissimilatory iron reducing bacteria (DIRB) to iron oxides is an important process to initiate direct extracellular electron transfer. Iron oxides in natural environments are often coated by organic matter or silica (SiO2) which alters their surface physicochemical properties. To investigate the influence of these surface coatings, we characterized the dynamic adhesion processes of Shewanella oneidensis MR-1 to bare hematite, humic acid-coated hematite (hematite-HA), and SiO2-coated hematite (hematite-SiO2) using Quartz Crystal Microbalance with Dissipation (QCM-D). The molecular-level process and mechanism were investigated using in situ Attenuated Total Reflectance - Fourier Transform Infrared (ATR-FTIR) spectrometry. We found that MR-1 formed a rigid bacterial layer on bare hematite. Coating with HA or SiO2 decreased the surface cell density during the initial adhesion stage, and compromised the stability of the subsequent bacterial attachment. The FTIR combined with two-dimensional correlation spectroscopy (2D-COS) analysis showed that C-moieties of polysaccharides dominated interactions in initial adhesion on HA and SiO2-coated hematite. In the longer term, the HA coating hindered the adsorption of amide, but promoted the binding of polysaccharide C-moieties to hematite. We concluded that, in general, both the HA and SiO2 coatings reduced the attachment of MR-1 on hematite. These results advance our understanding of the roles of surface coatings on microbe-mineral interactions, which has significant implications for a series of biogeochemical processes in nature.

YgfY Contributes to Stress Tolerance in Shewanella oneidensis Neither as an Antitoxin Nor as a Flavinylation Factor of Succinate Dehydrogenase.

YgfY(SdhE/CptB) is highly conserved while has controversial functions in bacteria. It works as an antitoxin and composes a type IV toxin–antitoxin system with YgfX(CptA) typically in Escherichia coli, while functions as an flavinylation factor of succinate dehydrogenase and fumarate reductase typically in Serratia sp. In this study, we report the contribution of YgfY in Shewanella oneidensis MR-1 to tolerance of low temperature and nitrite. YgfY deficiency causes several growth defects of S. oneidensis MR-1 at low temperature, while YgfX do not cause a growth defect or morphological change of S. oneidensis MR1-1 and E. coli. YgfY do not interact with FtsZ and MreB nor with YgfX examined by bacterial two-hybrid assay. YgfY effect on growth under low temperature is not attributed to succinate dehydrogenase (SDH) because a mutant without SDH grows comparably with the wild-type strain in the presence of succinate. The ygfY mutant shows impaired tolerance to nitrite. Transcription of nitrite reductase and most ribosome proteins is significantly decreased in the ygfY mutant, which is consistent with the phenotypes detected above. Effects of YgfY on growth and nitrite tolerance are closely related to the RGXXE motif in YgfY. In summary, this study demonstrates pleiotropic impacts of YgfY in S. oneidensis MR-1, and sheds a light on the physiological versatility of YgfY in bacteria.

A Self-Driven Bioreactor Based on Bacterium-Metal-Organic Framework Biohybrids for Boosting Chemotherapy via Cyclic Lactate Catabolism.

The excessive lactate in the tumor microenvironment always leads to poor therapeutic outcomes of chemotherapy. In this study, a self-driven bioreactor (defined as SO@MDH, where SO is Shewanella oneidensis MR-1 and MDH is MIL-101 metal-organic framework nanoparticles/doxorubicin/hyaluronic acid) is rationally constructed via the integration of doxorubicin (DOX)-loaded metal-organic framework (MOF) MIL-101 nanoparticles with SO to sensitize chemotherapy. Owing to the intrinsic tumor tropism and electron-driven respiration of SO, the biohybrid SO@MDH could actively target and colonize hypoxic and eutrophic tumor regions and anaerobically metabolize lactate accompanied by the transfer of electrons to Fe3+, which is the key component of the MIL-101 nanoparticles. As a result, the intratumoral lactate would undergo continuous catabolism coupled with the reduction of Fe3+ to Fe2+ and the subsequent degradation of MIL-101 frameworks, leading to an expeditious drug release for effective chemotherapy. Meanwhile, the generated Fe2+ will be promptly oxidized by the abundant hydrogen peroxide in the tumor microenvironment to reproduce Fe3+, which is, in turn, beneficial to circularly catabolize lactate and boost chemotherapy. More importantly, the consumption of intratumoral lactic acid could significantly inhibit the expression of multidrug resistance-related ABCB1 protein (also named P-glycoprotein (P-gp)) for conquering drug-resistant tumors. SO@MDH demonstrated here holds high tumor specificity and promising chemotherapeutic efficacy for suppressing tumor growth and overcoming multidrug resistance, confirming its potential prospects in cancer therapy.

New Mobilization Pathway of Antimonite: Thiolation and Oxidation by Dissimilatory Metal-Reducing Bacteria via Elemental Sulfur Respiration.

Antimony (Sb) mobilization is widely explored with dissimilatory metal-reducing bacteria (DMRB) via microbial iron(III)-reduction. Here, our study found a previously unknown pathway whereby DMRB release adsorbed antimonite (SbIII-O) from goethite via elemental sulfur (S0) respiratory reduction under mild alkaline conditions. We incubated SbIII-O-loaded goethite with Shewanella oneidensis MR-1 in the presence of S0 at pH 8.5. The incubation results showed that MR-1 reduced S0 instead of goethite, and biogenic sulfide induced the formation of thioantimonite (SbIII-S). SbIII-S was then oxidized by S0 to mobile thioantimonate (SbV-S), resulting in over fourfold greater Sb release to water compared with the abiotic control. SbIV-S was identified as the intermediate during the oxidation process by Fourier transform ion cyclotron resonance mass spectrometry and electron spin resonance analysis. The existence of SbIV-S reveals that the oxidation of SbIII-S to SbV-S follows a two-step consecutive one-electron transfer from Sb to S atoms. SbV-S then links with SbIII-S by sharing S atoms and inhibits SbIII-S polymerization and SbIII2S3 precipitation like a "capping agent". This study clarifies the thiolation and oxidation pathway of SbIII-O to SbV-S by S0 respiration and expands the role of DMRB in the fate of Sb.

Optically Accessible Microfluidic Flow Channels for Noninvasive High-Resolution Biofilm Imaging Using Lattice Light Sheet Microscopy.

Imaging platforms that enable long-term, high-resolution imaging of biofilms are required to study cellular level dynamics within bacterial biofilms. By combining high spatial and temporal resolution and low phototoxicity, lattice light sheet microscopy (LLSM) has made critical contributions to the study of cellular dynamics. However, the power of LLSM has not yet been leveraged for biofilm research because the open-on-top imaging geometry using water-immersion objective lenses is not compatible with living bacterial specimens; bacterial growth on the microscope’s objective lenses makes long-term time-lapse imaging impossible and raises considerable safety concerns for microscope users. To make LLSM compatible with pathogenic bacterial specimens, we developed hermetically sealed, but optically accessible, microfluidic flow channels that can sustain bacterial biofilm growth for multiple days under precisely controllable physical and chemical conditions. To generate a liquid- and gas-tight seal, we glued a thin polymer film across a 3D-printed channel, where the top wall had been omitted. We achieved negligible optical aberrations by using polymer films that precisely match the refractive index of water. Bacteria do not adhere to the polymer film itself, so that the polymer window provides unobstructed optical access to the channel interior. Inside the flow channels, biofilms can be grown on arbitrary, even nontransparent, surfaces. By integrating this flow channel with LLSM, we were able to record the growth of S. oneidensis MR-1 biofilms over several days at cellular resolution without any observable phototoxicity or photodamage.

Biogenic iron sulfide functioning as electron-mediating interface to accelerate dissimilatory ferrihydrite reduction by Shewanella oneidensis MR-1

Microbially driven iron and sulfur geochemical cycles co-exist ubiquitously in subsurface environments and are of environmental relevance. Shewanella species (dissimilatory metal-reducing bacteria) are capable of reducing Fe(III)-(oxyhydr)oxide minerals and diverse sulfur sources using corresponding metabolic pathways and producing FeS secondary minerals. In spite of the ability in promoting bacterial extracellular electron transfer (EET), the specific role of FeS in mediating EET between microbe/mineral interface is still unclear. In this work, the electron-mediating function of biogenic FeS on promoting the reduction of ferrihydrite by S. oneidensis MR-1 using thiosulfate as sulfur source was investigated in terms of Fe(III) reduction percentage, X-ray diffraction and scanning electron microscopy. The results showed that the microbial ferrihydrite reduction was pH-dependent and positively correlated with the addition of thiosulfate. In the presence of thiosulfate, biogenic FeS in nano-scale were formed and deposited on the surfaces of S. oneidensis MR-1 and ferrihydrite to build an interfacial electron transfer bridge between them. The addition of either thiosulfate and in-vitro FeS could rescue the entirely inactivated ability of the mutant (△omcA/mtrC) in ferrihydrite reduction to some extent, but which was obviously inferior to the wild-type strain. Meanwhile, the effect of the biogenic FeS in-situ coating on the surfaces of S. oneidensis MR-1 cells on promoting microbial ferrihydrite reduction was significantly superior to the in-vitro ones. Thus, the in-situ formed biogenic FeS secondary minerals were demonstrated to mediate and accelerate interfacial electron transfer from S. oneidensis MR-1 cells to ferrihydrite through interfacing with the bacterial EET routes, especially Mtr pathway. This work provides an insight into the secondary minerals-mediating interfacial electron transfer between microbes and minerals in the presence of biological S (-II), which has important biogeochemical and environmental implications.

Engineering of salt-tolerant Shewanella aquimarina XMS-1 for enhanced pollutants transformation and electricity generation

Saline wastewater poses a challenge during bio-treatment process due to salinity affecting the physiological activity of microorganisms and inhibiting their growth and metabolism. Thus, screening and engineering the salt-tolerant strains with stronger performances are urgent. Shewanella aquimarina XMS-1, a salt-tolerant dissimilated metal reducing bacteria (DMRB), was isolated from seawater environment. Its ability for reducing pollutants and generating electricity was enhanced by overexpression of riboflavin synthesis pathway encoding genes from S. oneidensis MR-1 under salt stress. Furthermore, upon contact with graphene oxide (GO), the engineered strain XMS-1/pYYDT-rib with enhanced flavins synthesis could reduce GO and self-assemble to form a three-dimensional (3D) biohybrid system named XMS-1/flavins/rGO. This 3D biohybrid system significantly enhanced the EET efficiency of S. aquimarina XMS-1. Our findings provide a feasible strategy for treatment of salt-containing industrial wastewater contaminated by metal and organic pollutants.