Biogenic formation of photoactive arsenic-sulfide nanotubes by Shewanella sp. strain HN-41

Dual-mode harvest solar energy for photothermal Cu2-xSe biomineralization and seawater desalination by biotic-abiotic hybrid

Biotic-abiotic hybrid photocatalytic system is an innovative strategy to capture solar energy. Diversifying solar energy conversion products and balancing photoelectron generation and transduction are critical to unravel the potential of hybrid photocatalysis. Here, we harvest solar energy in a dual mode for Cu2-xSe nanoparticles biomineralization and seawater desalination by integrating the merits of Shewanella oneidensis MR-1 and biogenic nanoparticles. Photoelectrons generated by extracellular Se0 nanoparticles power Cu2-xSe synthesis through two pathways that either cross the outer membrane to activate periplasmic Cu(II) reduction or are directly delivered into the extracellular space for Cu(I) evolution. Meanwhile, photoelectrons drive periplasmic Cu(II) reduction by reversing MtrABC complexes in S. oneidensis. Moreover, the unique photothermal feature of the as-prepared Cu2-xSe nanoparticles, the natural hydrophilicity, and the linking properties of bacterium offer a convenient way to tailor photothermal membranes for solar water production. This study provides a paradigm for balancing the source and sink of photoelectrons and diversifying solar energy conversion products in biotic-abiotic hybrid platforms.

Biomineralization behavior and mechanism of microbial-mediated removal of arsenate from water

The microbial-mediated removal of arsenate by biomineralization received much attention, but the molecular mechanism of Arsenic (As) removal by mixed microbial populations remains to be elucidated. In this study, a process for the arsenate treatment using sulfate-reducing bacteria (SRB) containing sludge was constructed, and the performance of As removal was investigated at different molar ratios of AsO₄^3- to SO₄^2-. It was found that biomineralization mediated by SRB could achieve the simultaneous removal of arsenate and sulfate from wastewater but only occurred when microbial metabolic processes were involved. The reducing ability of the microorganisms for the sulfate and arsenate was equivalent, so the precipitates produced at the molar ratio of AsO₄^3- to SO₄^2-of 2:3 were most significant. X-ray absorption fine structure (XAFS) spectroscopy was the first time used to determine the molecular structure of the precipitates which were confirmed to be orpiment (As₂S₃). Combined with the metagenomics analysis, the microbial metabolism mechanism of simultaneous removal of sulfate and arsenate by the mixed microbial population containing SRB was revealed, that is, the sulfate and As(V) were reduced by microbial enzymes to produce S^2- and As(III) to further form As₂S₃ precipitates. This research provided a reference and theoretical foundation for the simultaneous removal of sulfate and arsenic mediated by SRB-containing sludge in wastewater treatment.

Living electronics: A catalogue of engineered living electronic components

Biology leverages a range of electrical phenomena to extract and store energy, control molecular reactions and enable multicellular communication. Microbes, in particular, have evolved genetically encoded machinery enabling them to utilize the abundant redox-active molecules and minerals available on Earth, which in turn drive global-scale biogeochemical cycles. Recently, the microbial machinery enabling these redox reactions have been leveraged for interfacing cells and biomolecules with electrical circuits for biotechnological applications. Synthetic biology is allowing for the use of these machinery as components of engineered living materials with tuneable electrical properties. Herein, we review the state of such living electronic components including wires, capacitors, transistors, diodes, optoelectronic components, spin filters, sensors, logic processors, bioactuators, information storage media and methods for assembling these components into living electronic circuits.

Novel microbial consortia facilitate metalliferous immobilization in non-ferrous metal(loid)s contaminated smelter soil: Efficiency and mechanisms

Exposure to toxic metals from nonferrous metal(loid) smelter soils can pose serious threats to the surrounding ecosystems, crop production, and human health. Bioremediation using microorganisms is a promising strategy for treating metal(loid)-contaminated soils. Here, a native microbial consortium with sulfate-reducing function (SRB1) enriched from smelter soils can tolerate exposures to mixtures of heavy metal(loid)s (e.g., As and Pb) or various organic flotation reagents (e.g., ethylthionocarbamate). The addition of Fe²⁺ greatly increased As³⁺ immobilization compared to treatment without Fe²⁺, with the immobilization efficiencies of 81.0% and 58.9%, respectively. Scanning electronic microscopy-energy dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy confirmed that the As³⁺ immobilizing activity was related to the formation of arsenic sulfides (AsS, As₄S₄, and As₂S₃) and sorption/co-precipitation of pyrite (FeS₂). High-throughput 16S rRNA gene sequencing of SRB1 suggests that members of Clostridium, Desulfosporosinus, and Desulfovibrio genera play an important role in maintaining and stabilizing As³⁺ immobilization activity. Metal(loid)s immobilizing activity of SRB1 was not observed at high and toxic total exposure concentrations (220-1181 mg As/kg or 63-222 mg Pb/kg). However, at lower concentrations, SRB1 treatment decreased bioavailable fractions of As (9.0%) and Pb (28.6%) compared to without treatment. Results indicate that enriched native SRB1 consortia exhibited metal(loid) transformation capacities under non-toxic concentrations of metal(loid)s for future bioremediation strategies to decrease mixed metal(loid)s exposure from smelter polluted soils.

Performance and mechanisms exploration of nano zinc oxide (nZnO) on anaerobic decolorization by Shewanella oneidensis MR-1

Although the ecological safety of nanomaterials is of widespread concern, their current ambient concentrations are not yet sufficient to cause serious toxic effects. Thus, the nontoxic bioimpact of nanomaterials in wastewater treatment has attracted increasing attention. In this study, the effect of nano zinc oxide (nZnO), one of the most widely used nanomaterials, on the anaerobic biodegradation of methyl orange (MO) by Shewanella oneidensis MR-1 was comprehensively investigated. High-dosage nZnO (>0.5 mg/L) caused severe toxic stress on S. oneidensis MR-1, resulting in the decrease in decolorization efficiency. However, nZnO at ambient concentrations could act as nanostimulants and promote the anaerobic removal of MO by S. oneidensis MR-1, which should be attributed to the improvement of decolorization efficiency rather than cell proliferation. The dissolved Zn²⁺ was found to contribute to the bioeffect of nZnO on MO decolorization. Further investigation revealed that low-dosage nZnO could promote the cell viability, membrane permeability, anaerobic metabolism, as well as related gene expression, indicating that nZnO facilitated rather than inhibited the anaerobic wastewater treatment under ambient conditions. Thus, this work provides a new insight into the bioeffect of nZnO in actual environment and facilitates the practical application of nanomaterials as nanostimulants in biological process.

Interplay between arsenic and selenium biomineralization in Shewanella sp. O23S

Bacteria play crucial roles in the biogeochemical cycle of arsenic (As) and selenium (Se) as these elements are metabolized via detoxification, energy generation (anaerobic respiration) and biosynthesis (e.g. selenocysteine) strategies. To date, arsenic and selenium biomineralization in bacteria were studied separately. In this study, the anaerobic metabolism of As and Se in Shewanella sp. O23S was investigated separately and mixed, with an emphasis put on the biomineralization products of this process. Multiple analytical techniques including ICP-MS, TEM-EDS, XRD, Micro-Raman, spectrophotometry and surface charge (zeta potential) were employed. Shewanella sp. O23S is capable of reducing selenate (SeO₄^2-) and selenite (SeO₃^2-) to red Se(-S)⁰, and arsenate (AsO₄^3-) to arsenite (AsO₃^3-). The release of H₂S from cysteine led to the precipitation of AsS minerals: nanorod AsS and granular As₂S₃. When As and Se oxyanions were mixed, both As-S and Se(-S)⁰ biominerals were synthesized. All biominerals were extracellular, amorphous and presented a negative surface charge (-24 to -38 mV). Kinetic analysis indicated the following reduction yields: SeO₃^2- (90%), AsO₄^3- (60%), and SeO₄^2- (<10%). The mix of SeO₃^2- with AsO₄^3- led to a decrease in As removal to 30%, while Se reduction yield was unaffected (88%). Interestingly, SeO₄^2- incubated with AsO₄^3- boosted the Se removal (71%). The exclusive extracellular formation of As and Se biominerals might indicate an extracellular respiratory process characteristic of various Shewanella species and strains. This is the first study documenting a complex interplay between As and Se oxyanions: selenite decreased arsenate reduction, whereas arsenate stimulated selenate reduction. Further investigation needs to clarify whether Shewanella sp. O23S employs multi-substrate respiratory enzymes or separate, high affinity enzymes for As and Se oxyanion respiration.

Synthesis and Structure of a Two-Dimensional Palladium Oxide Network on Reduced Graphene Oxide

New nanostructures often reflect new and exciting properties. Here, we present an two-dimensional, hitherto unreported PdO square network with lateral dimensions up to hundreds of nanometers growing on reduced graphene oxide (rGO), forming a hybrid nanofilm. An intermediate state of dissolved Pd(0) in the bacterium S. oneidensis MR-1 is pivotal in the biosynthesis and inspires an abiotic synthesis. The PdO network shows a lattice spacing of 0.5 nm and a thickness of 1.8 nm on both sides of an rGO layer and is proposed to be cubic or tetragonal crystal, as confirmed by structural simulations. A 2D silver oxide analog with a similar structure is also obtained using an analogous abiotic synthesis. Our study thus opens a simple route to a whole new class of 2D metal oxides on rGO as promising candidates for graphene superlattices with unexplored properties and potential applications for example in electronics, sensing, and energy conversion.

Astrobiological Potential of Fe/Mg Smectites with Special Emphasis on Jezero Crater, Mars 2020 Landing Site

Life is known to adapt in accordance with its surrounding environment and sustainable resources available to it. Since harsh conditions would have precluded any possible aerobic evolution of life at the martian surface, it is plausible that martian life, should it exist, would have evolved in such a way as to derive energy from more optimum resources. Iron is one of the most abundant elements present in the martian crust and occurs at about twice the amount present on Earth. Clay minerals contribute to about half the iron found in soils and sediments. On Earth, clay acts as an electron donor as well as an acceptor in the carbon cycles and thereby supports a wide variety of metabolic reactions. In this context, we consider the potential of Fe/Mg smectites, one of the most widely reported hydrated minerals on Mars, for preservation of macro- and microscopic biosignatures. We proceed by understanding the environmental conditions during the formation of smectites and various microbes and metabolic processes associated with them as indicated in Earth-based studies. We also explore the possibility of biosignatures and their identification within the Mars 2020 landing site (Jezero Crater) by using the astrobiological payloads on board the Perseverance rover.

Impact of organic matter on As sulfidation in wetlands: An in situ experiment

Arsenic incorporation into newly formed As sulfides has recently been identified as an important As sequestration pathway in both laboratory experiments and natural As-wetlands. Here, we used an in situ experimental technique with double nylon experimental bags (10-μm mesh) to study the effect of low-cost organic materials (sawdust, wood cubes and hemp shives) on As sulfidation in three naturally As-enriched wetland soils under water-saturated (~1 m depth) and neutral pH conditions. After 15 months of in situ incubation, all of the organic materials and their corresponding inner bags were covered by yellow-black mineral accumulations, dominantly composed of crystalline As₄S₄ polymorphs (realgar and bonazziite) and reactive Fe(II) sulfides (probably mackinawite); while the major fraction of As (~80%) was sequestered as AsS minerals. The amount of As accumulation in the experimental bags varied significantly (0.03-4.24 g As kg^-1) and corresponded with different levels of As (0.23-9.4 mg As L^-1) in the groundwater. Our findings suggest an authigenic formation of AsS minerals in strongly reducing conditions of experimental bags by a combination of reduced exchange of solutes through the pores of the bag and comparatively fast microbial production of dissolved sulfide. Arsenic sulfide formation, as an effective treatment mechanism for natural and human-constructed wetlands, appears to be favored for As(III)-rich waters with a low Fe(II)/As(III) molar ratio. These conditions prevent the consumption of dissolved As and sulfide by their preferential incorporation into natural organic matter, and newly-formed Fe(II) sulfides, respectively.

Reversing Electron Transfer Chain for Light-Driven Hydrogen Production in Biotic-Abiotic Hybrid Systems

The biotic-abiotic photosynthetic system integrating inorganic light absorbers with whole-cell biocatalysts innovates the way for sustainable solar-driven chemical transformation. Fundamentally, the electron transfer at the biotic-abiotic interface, which may induce biological response to photoexcited electron stimuli, plays an essential role in solar energy conversion. Herein, we selected an electro-active bacterium Shewanella oneidensis MR-1 as a model, which constitutes a hybrid photosynthetic system with a self-assembled CdS semiconductor, to demonstrate unique biotic-abiotic interfacial behavior. The photoexcited electrons from CdS nanoparticles can reverse the extracellular electron transfer (EET) chain within S. oneidensis MR-1, realizing the activation of a bacterial catalytic network with light illumination. As compared with bare S. oneidensis MR-1, a significant upregulation of hydrogen yield (711-fold), ATP, and reducing equivalent (NADH/NAD⁺) was achieved in the S. oneidensis MR-1-CdS under visible light. This work sheds light on the fundamental mechanism and provides design guidelines for biotic-abiotic photosynthetic systems.

Biogenic Sulfur-Based Chalcogenide Nanocrystals: Methods of Fabrication, Mechanistic Aspects, and Bio-Applications

Bio-nanotechnology has emerged as an efficient and competitive methodology for the production of added-value nanomaterials (NMs). This review article gathers knowledge gleaned from the literature regarding the biosynthesis of sulfur-based chalcogenide nanoparticles (S-NPs), such as CdS, ZnS and PbS NPs, using various biological resources, namely bacteria, fungi including yeast, algae, plant extracts, single biomolecules, and viruses. In addition, this work sheds light onto the hypothetical mechanistic aspects, and discusses the impact of varying the experimental parameters, such as the employed bio-entity, time, pH, and biomass concentration, on the obtained S-NPs and, consequently, on their properties. Furthermore, various bio-applications of these NMs are described. Finally, key elements regarding the whole process are summed up and some hints are provided to overcome encountered bottlenecks towards the improved and scalable production of biogenic S-NPs.

Antibacterial effect of copper nanoparticles produced in a Shewanella-supported non-external circuit bioelectrical system on bacterial plant pathogens

The inhibition of copper nanoparticles produced in a nec_BES to plant pathogens was investigated in this study.

Diversity of Microbial Metal Sulfide Biomineralization

Since the emergence of life on Earth, microorganisms have contributed to biogeochemical cycles. Sulfate-reducing bacteria are an example of widespread microorganisms that participate in the metal and sulfur cycles by biomineralization of biogenic metal sulfides. In this work, we review the microbial biomineralization of metal sulfide particles and summarize distinctive features from exemplary cases. We highlight that metal sulfide biomineralization is highly metal- and organism-specific. The properties of metal sulfide biominerals depend on the degree of cellular control and on environmental factors, such as pH, temperature, and concentration of metals. Moreover, biogenic macromolecules, including peptides and proteins, help cells control their extracellular and intracellular environments that regulate biomineralization. Accordingly, metal sulfide biominerals exhibit unique features when compared to abiotic minerals or biominerals produced by dead cell debris.

Regulating the synthesis rate and yield of bio-assembled FeS nanoparticles for efficient cancer therapy

Biosynthesis has gained growing interest due to its energy efficiency and environmentally benign nature. Recently, biogenic iron sulfide nanoparticles (FeS NPs) have exhibited excellent performance in environmental remediation and energy recovery applications. However, their biosynthesis regulation strategy and application prospects in the biomedical field remain to be explored. Herein, biogenic FeS NPs are controllably synthesized by Shewanella oneidensis MR-1 and applied for cancer therapy. Tuning the synthesis rate and yield of biogenic FeS NPs is realized by altering the initial iron precursor dosage. Notably, increasing the precursor concentration decreases and delays FeS NP biosynthesis. The biogenic FeS NPs (30 nm) are homogeneously anchored on the cell surface of S. oneidensis MR-1. Moreover, the good hydrophilic nature and outstanding Fenton properties of the as-prepared FeS NPs endow them with good cancer therapy performance. The intracellular location of the FeS NPs taken up is visualized with a soft X-ray microscope (SXM). Highly efficient cancer cell killing can be achieved at extremely low concentrations (<12 μg mL^-1), lower than those in reported works. Such good performance is attributed to the Fe²⁺ release, elevated ROS, reduced glutathione (GSH) consumption, and lipid hydroperoxide (LPO) generation. The resulting FeS NPs show excellent in vivo therapeutic performance. This work provides a facile, eco-friendly, and scalable approach to produce nanomedicine, demonstrating the potential of biogenic nanoparticles for use in cancer therapy.

Insights into the Biosynthesis of Nanoparticles by the Genus Shewanella

The exploitation of microorganisms for the fabrication of nanoparticles (NPs) has garnered considerable research interest globally. The microbiological transformation of metals and metal salts into respective NPs can be achieved under environmentally benign conditions, offering a more sustainable alternative to chemical synthesis methods. Species of the metal-reducing bacterial genus Shewanella are able to couple the oxidation of various electron donors, including lactate, pyruvate, and hydrogen, to the reduction of a wide range of metal species, resulting in biomineralization of a multitude of metal NPs. Single-metal-based NPs as well as composite materials with properties equivalent or even superior to physically and chemically produced NPs have been synthesized by a number of Shewanella species. A mechanistic understanding of electron transfer-mediated bioreduction of metals into respective NPs by Shewanella is crucial in maximizing NP yields and directing the synthesis to produce fine-tuned NPs with tailored properties. In addition, thorough investigations into the influence of process parameters controlling the biosynthesis is another focal point for optimizing the process of NP generation. Synthesis of metal-based NPs using Shewanella species offers a low-cost, eco-friendly alternative to current physiochemical methods. This article aims to shed light on the contribution of Shewanella as a model organism in the biosynthesis of a variety of NPs and critically reviews the current state of knowledge on factors controlling their synthesis, characterization, potential applications in different sectors, and future prospects.

Forced Biomineralization: A Review

Biologically induced and controlled mineralization of metals promotes the development of protective structures to shield cells from thermal, chemical, and ultraviolet stresses. Metal biomineralization is widely considered to have been relevant for the survival of life in the environmental conditions of ancient terrestrial oceans. Similar behavior is seen among extremophilic biomineralizers today, which have evolved to inhabit a variety of industrial aqueous environments with elevated metal concentrations. As an example of extreme biomineralization, we introduce the category of "forced biomineralization", which we use to refer to the biologically mediated sequestration of dissolved metals and metalloids into minerals. We discuss forced mineralization as it is known to be carried out by a variety of organisms, including polyextremophiles in a range of psychrophilic, thermophilic, anaerobic, alkaliphilic, acidophilic, and halophilic conditions, as well as in environments with very high or toxic metal ion concentrations. While much additional work lies ahead to characterize the various pathways by which these biominerals form, forced biomineralization has been shown to provide insights for the progression of extreme biomimetics, allowing for promising new forays into creating the next generation of composites using organic-templating approaches under biologically extreme laboratory conditions relevant to a wide range of industrial conditions.

Engineering Biological Electron Transfer and Redox Pathways for Nanoparticle Synthesis

Many species of bacteria are naturally capable of types of electron transport not observed in eukaryotic cells. Some species live in environments containing heavy metals not typically encountered by cells of multicellular organisms, such as arsenic, cadmium, and mercury, leading to the evolution of enzymes to deal with these environmental toxins. Bacteria also inhabit a variety of extreme environments, and are capable of respiration even in the absence of oxygen as a terminal electron acceptor. Over the years, several of these exotic redox and electron transport pathways have been discovered and characterized in molecular-level detail, and more recently synthetic biology has begun to utilize these pathways to engineer cells capable of detecting and processing a variety of metals and semimetals. One such application is the biologically controlled synthesis of nanoparticles. This review will introduce the basic concepts of bacterial metal reduction, summarize recent work in engineering bacteria for nanoparticle production, and highlight the most cutting-edge work in the characterization and application of bacterial electron transport pathways.

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.

Arsenic and the gastrointestinal tract microbiome

Arsenic is a toxin, ranking first on the Agency for Toxic Substances and Disease Registry and the Environmental Protection Agency Priority List of Hazardous Substances. Chronic exposure increases the risk of a broad range of human illnesses, most notably cancer; however, there is significant variability in arsenic-induced disease among exposed individuals. Human genetics is a known component, but it alone cannot account for the large inter-individual variability in the presentation of arsenicosis symptoms. Each part of the gastrointestinal tract (GIT) may be considered as a unique environment with characteristic pH, oxygen concentration, and microbiome. Given the well-established arsenic redox transformation activities of microorganisms, it is reasonable to imagine how the GIT microbiome composition variability among individuals could play a significant role in determining the fate, mobility and toxicity of arsenic, whether inhaled or ingested. This is a relatively new field of research that would benefit from early dialogue aimed at summarizing what is known and identifying reasonable research targets and concepts. Herein, we strive to initiate this dialogue by reviewing known aspects of microbe-arsenic interactions and placing it in the context of potential for influencing host exposure and health risks. We finish by considering future experimental approaches that might be of value.

Realgar (As4S4) bioprecipitation in microcosm fed by a natural groundwater and organic matter

Sequestration of arsenic to biogenic sulfide minerals is known from As-contaminated anoxic environments. Despite numerous successful laboratory experiments, the process remains difficult to predict in moderate arsenic conditions. We performed microcosm experiments using naturally contaminated groundwater (containing ca. 6 mg/L As) and natural organic matter (NOM) particles both collected from wetland soil. Macroscopic realgar precipitates, occasionally accompanied by bonazziite, a FeS phase, elementary S, calcite, and whewellite, appeared after 4 to 18 months. Realgar only precipitated in microcosms moderately poisoned by azide or antibiotics and those in which oxidation of hydrogen sulfide to sulfur took place. The biomineralization process was not affected by the presence of additional carbon sources or the diversity, community structure, and functional composition of the microbial community. Hydrogen sulfide concentration was greater in the realgar-free microcosms, suggesting that arsenic thiolation prevented precipitation of realgar. We compared our data to available microbial community data from soils with different rates of realgar precipitation, and found that the communities from realgar-encrusted NOM particles usually showed limited sulfate reduction and the presence of fermentative metabolisms, whereas communities from realgar-free NOM particles were strongly dominated by sulfate reducers. We argue that the limited sulfate supply and intensive fermentation amplify reducing conditions, which make arsenic sulfide precipitation plausible in high-sulfate, low-arsenic groundwaters.

Biogenic Hematite from Bacteria: Facile Synthesis of Secondary Nanoclusters for Lithium Storage Capacity

Ferrihydrite, or iron(III) (oxyhydr)oxide (Fe(OH)₃), a representative scavenger of environmentally relevant toxic elements, has been repurposed as a low-cost and scalable precursor of well-developed hematite (α-Fe₂O₃) secondary nanoclusters with a hierarchically structured morphology for lithium-ion anode materials. Here, we report that the bacteria Clostridium sp. C8, isolated from a methane-gas-producing consortium, can synthesize self-assembled secondary hematite nanoclusters (∼150 nm) composed of small nanoparticles (∼15 nm) through the molecular structural rearrangement of amorphous ferrihydrite under mild conditions. The biogenic hematite particles, wrapped with graphene oxide reduced in situ by the reducing bacteria Shewanella sp. HN-41 via one-pot synthesis, deliver an excellent reversible capacity of ∼1000 mA h g^-1 after 100 cycles at a current density of 1 A g^-1. Furthermore, the heat-treated hematite/rGO exhibits a capacity of 820 mA h g^-1 at a high current density of 5 A g^-1 and a reversible capacity of up to 1635 mA h g^-1 at a current density of 100 mA g^-1. This study provides an easy, eco-efficient, and scalable microbiological synthetic route to produce hierarchical hematite/rGO secondary nanoclusters with potential as high-performance Li-ion anode materials.

Biosynthesis of Nanomaterials by Shewanella Species for Application in Lithium Ion Batteries

Nanomaterials exhibit extraordinary properties based on their size, shape, chemical composition, and crystal structure. Owing to their unique properties nanomaterials are preferred over their bulk counterparts for a number of applications. Although conventional physical and chemical routes were established for the massive production of nanomaterials, there are some drawbacks such as environmental burden and high cost that cannot be disregarded. Recently, there has been great interest toward the green synthesis of inorganic nanomaterials. It has been reported that dissimilatory metal reduction by microorganisms is a cost-effective process to remediate toxic organic and inorganic compounds under anaerobic conditions. Particularly, members of the Shewanella genus have been utilized to produce various biogenic nanomaterials with unique micro/nanostructured morphologies through redox transformations as well as to remove harmful metals and metalloids in eco-efficient and environment-friendly methods under ambient conditions. In the present mini-review, we specifically address the active utilization of microbial respiration processes for the synthesis of novel functional biogenic nanomaterials by the members of the Shewanella genus. This biosynthetic method may provide alternative approaches to produce electrode materials for sustainable energy storage applications.

Mitigation of arsenic toxicity and accumulation in hydroponically grown rice seedlings by co-inoculation with arsenite-oxidizing and cadmium-tolerant bacteria

Arsenic (As) contamination of rice grain is a serious problem worldwide. The objective of this study was to mitigate As toxicity and accumulation in hydroponically grown KDML105 rice seedlings using bacteria isolated from heavy metal-contaminated soils. Seven strains (KKU2500-1, -2, -3, -9, -12, -16 and -22) of 24 cadmium (Cd)-tolerant bacteria produced high levels of inorganic sulfide and thiol-rich compounds in As-supplemented media. The strains were allowed to colonize rice seedlings growing in arsenite [As(III)]- or arsenate [As(V)]-supplemented Hoagland's nutrient solutions. Colonization by strains KKU2500-3 and -12 led to increases in plant growth parameters and similarly reduced As translocation into shoots [translocation factor (TF) = 0.05] in the As(V)-supplemented solution. Strains KKU2500-1 and - 12 also greatly reduced As translocation into shoots (TF = 0.16-0.20) in As(III)-supplemented solution. KKU2500-3 and - 12 co-colonized onto seedlings with the As(III)-oxidizing isolates 4.25, 4.27, 4.40 and 4.44, and the strain combinations KKU2500-12/4.25, KKU2500-3/4.25, KKU2500-3/4.27 and KKU2500-3/4.44 resulted in higher growth parameters for plants grown in As [As(III)+As(V)]-supplemented solution than other combinations. Moreover, the combinations KKU2500-3/4.25 and KKU2500-3/4.44 greatly reduced As translocation (TF = 0.15 and 0.12, respectively), and this decreased As accumulation in shoots was significantly correlated with increased sulfide stimulation in roots and nutrient solution. These results indicate that these co-inoculated bacteria can mitigate As toxicity, translocation and accumulation in KDML105 seedlings and thus demonstrate synergistic activity in rice plants, and this effect can be further developed in field trials.

Engineering bacteria for biogenic synthesis of chalcogenide nanomaterials

Microbes naturally build nanoscale structures, including structures assembled from inorganic materials. Here, we combine the natural capabilities of microbes with engineered genetic control circuits to demonstrate the ability to control biological synthesis of chalcogenide nanomaterials in a heterologous host. We transferred reductase genes from both Shewanella sp. ANA-3 and Salmonella enterica serovar Typhimurium into a heterologous host (Escherichia coli) and examined the mechanisms that regulate the properties of biogenic nanomaterials. Expression of arsenate reductase genes and thiosulfate reductase genes in E. coli resulted in the synthesis of arsenic sulfide nanomaterials. In addition to processing the starting materials via redox enzymes, cellular components also nucleated the formation of arsenic sulfide nanomaterials. The shape of the nanomaterial was influenced by the bacterial culture, with the synthetic E. coli strain producing nanospheres and conditioned media or cultures of wild-type Shewanella sp. producing nanofibres. The diameter of these nanofibres also depended on the biological context of synthesis. These results demonstrate the potential for biogenic synthesis of nanomaterials with controlled properties by combining the natural capabilities of wild microbes with the tools from synthetic biology.

Controlled growth of gold nanocrystals on biogenic As-S nanotubes by galvanic displacement

Traditional methods for fabricating nanoscale arrays are usually based on lithographic techniques while alternative new approaches rely on the use of nanoscale templates made of synthetic or biological materials. Here, gold (Au) nanocrystals were grown on the surface of the microbiologically formed As-S nanotubes through the process of galvanic displacement. The size and organization of the synthesized Au nanocrystals were affected by the pH dependent speciation of HAuCl₄ precursors as well as the initial ratio of As-S/HAuCl₄. We found that as pH increased, the Au nanocrystals grown on As-S nanotubes had smaller sizes but were more likely to assemble in one-dimension along the nanotubes. At a proper initial ratio of As-S/HAuCl₄, Au nanotubes were formed at pH 6.0. The mechanism of Au nanostructures formation and the synthesis process at different pHs were proposed. The resulting Au nanoparticle/As-S nanotube and Au nanotube/As-S nanotube hetero-structures may provide important properties to be used for novel nano-electronic devices.

Directed Biofabrication of Nanoparticles through Regulating Extracellular Electron Transfer

Biofabrication of nanomaterials is currently constrained by a low production efficiency and poor controllability on product quality compared to chemical synthetic routes. In this work, we show an attractive new biosynthesis system to break these limitations. A directed production of selenium-containing nanoparticles in Shewanella oneidensis MR-1 cells, with fine-tuned composition and subcellular synthetic location, was achieved by modifying the extracellular electron transfer chain. By taking advantage of its untapped intracellular detoxification and synthetic power, we obtained high-purity, uniform-sized cadmium selenide nanoparticles in the cytoplasm, with the production rates and fluorescent intensities far exceeding the state-of-the-art biosystems. These findings may fundamentally change our perception of nanomaterial biosynthesis process and lead to the development of fine-controllable nanoparticles biosynthesis technologies.

e-Biologics: Fabrication of Sustainable Electronics with "Green" Biological Materials

The growing ubiquity of electronic devices is increasingly consuming substantial energy and rare resources for materials fabrication, as well as creating expansive volumes of toxic waste. This is not sustainable. Electronic biological materials (e-biologics) that are produced with microbes, or designed with microbial components as the guide for synthesis, are a potential green solution. Some e-biologics can be fabricated from renewable feedstocks with relatively low energy inputs, often while avoiding the harsh chemicals used for synthesizing more traditional electronic materials. Several are completely free of toxic components, can be readily recycled, and offer unique features not found in traditional electronic materials in terms of size, performance, and opportunities for diverse functionalization. An appropriate investment in the concerted multidisciplinary collaborative research required to identify and characterize e-biologics and to engineer materials and devices based on e-biologics could be rewarded with a new "green age" of sustainable electronic materials and devices.

Biosynthesis of Inorganic Nanoparticles: A Fresh Look at the Control of Shape, Size and Composition

Several methodologies have been devised for the design of nanomaterials. The "Holy Grail" for materials scientists is the cost-effective, eco-friendly synthesis of nanomaterials with controlled sizes, shapes and compositions, as these features confer to the as-produced nanocrystals unique properties making them appropriate candidates for valuable bio-applications. The present review summarizes published data regarding the production of nanomaterials with special features via sustainable methodologies based on the utilization of natural bioresources. The richness of the latter, the diversity of the routes adopted and the tuned experimental parameters have led to the fabrication of nanomaterials belonging to different chemical families with appropriate compositions and displaying interesting sizes and shapes. It is expected that these outstanding findings will encourage researchers and attract newcomers to continue and extend the exploration of possibilities offered by nature and the design of innovative and safer methodologies towards the synthesis of unique nanomaterials, possessing desired features and exhibiting valuable properties that can be exploited in a profusion of fields.

Complete removal of arsenic and zinc from a heavily contaminated acid mine drainage via an indigenous SRB consortium

Acid mine drainages (AMD) are major sources of pollution to the environment. Passive bio-remediation technologies involving sulfate-reducing bacteria (SRB) are promising for treating arsenic contaminated waters. However, mechanisms of biogenic As-sulfide formation need to be better understood to decontaminate AMDs in acidic conditions. Here, we show that a high-As AMD effluent can be decontaminated by an indigenous SRB consortium. AMD water from the Carnoulès mine (Gard, France) was incubated with the consortium under anoxic conditions and As, Zn and Fe concentrations, pH and microbial activity were monitored during 94days. Precipitated solids were analyzed using electron microscopy (SEM/TEM-EDXS), and Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy at the As K-edge. Total removal of arsenic and zinc from solution (1.06 and 0.23mmol/L, respectively) was observed in two of the triplicates. While Zn precipitated as ZnS nanoparticles, As precipitated as amorphous orpiment (am-As^III₂S₃) (33-73%), and realgar (As^IIS) (0-34%), the latter phase exhibiting a particular nanowire morphology. A minor fraction of As is also found as thiol-bound As^III (14-23%). We propose that the formation of the As^IIS nanowires results from As^III₂S₃ reduction by biogenic H₂S, enhancing the efficiency of As removal. The present description of As immobilization may help to set the basis for bioremediation strategies using SRB.

Extracellular biosynthesis of copper sulfide nanoparticles by Shewanella oneidensis MR-1 as a photothermal agent

Photothermal therapy (PTT) is a minimally invasive and effective cancer treatment method and has a great potential for innovating the conventional chemotherapy approaches. Copper sulfide (CuS) exhibits photostability, low cost, and high absorption in near infrared region, and is recognized as an ideal candidate for PTT. However, CuS, as a photothermal agent, is usually synthesized with traditional chemical approaches, which require high temperature, additional stabilization and hydrophilic modification. Herein, we report, for the first time, the preparation of CuS nanoparticles as a photothermal agent by a dissimilatory metal reducing bacterium Shewanella. oneidensis MR-1. The prepared nanoparticles are homogenously shaped, hydrophilic, small-sized (∼5nm) and highly stable. Furthermore, the biosynthesized CuS nanoparticles display a high photothermal conversion efficiency of 27.2% because of their strong absorption at 1100nm. The CuS nanoparticles could be effectively used as a PTT agent under the irradiation of 1064nm. This work provides a simple, eco-friendly and cost-effective approach for fabricating PTT agents.

Arsenic remediation by formation of arsenic sulfide minerals in a continuous anaerobic bioreactor

Arsenic (As) is a highly toxic metalloid that has been identified at high concentrations in groundwater in certain locations around the world. Concurrent microbial reduction of arsenate (As(V) ) and sulfate (SO4 (2-) ) can result in the formation of poorly soluble arsenic sulfide minerals (ASM). The objective of this research was to study As biomineralization in a minimal iron environment for the bioremediation of As-contaminated groundwater using simultaneous As(V) and SO4 (2-) reduction. A continuous-flow anaerobic bioreactor was maintained at slightly acidic pH (6.25-6.50) and fed with As(V) and SO4 (2-) , utilizing ethanol as an electron donor for over 250 d. A second bioreactor running under the same conditions but lacking SO4 (2-) was operated as a control to study the fate of As (without S). The reactor fed with SO4 (2-) removed an average 91.2% of the total soluble As at volumetric rates up to 2.9 mg As/(L · h), while less than 5% removal was observed in the control bioreactor. Soluble S removal occurred with an S to As molar ratio of 1.2, suggesting the formation of a mixture of orpiment- (As2 S3 ) and realgar-like (AsS) solid phases. Solid phase characterization using K-edge X-ray absorption spectroscopy confirmed the formation of a mixture of As2 S3 and AsS. These results indicate that a bioremediation process relying on the addition of a simple, low-cost electron donor offers potential to promote the removal of As from groundwater with naturally occurring or added SO4 (2-) by precipitation of ASM.

Dependence of particle concentration effect on pH and redox for arsenic removal by FeS-coated sand under anoxic conditions

FeS has been recognized as a good scavenger for arsenic under anoxic conditions. To create a suitable adsorbent for flow-through reactors such as permeable reactive barriers, it has been suggested that this material may be coated onto sand. However, previous work on FeS-coated sand has focused on batch reactors, while flow-through reactors usually have higher solid-solution ratios. To ascertain whether differences in the solid-solution ratio (SSR) are important in this system, batch sorption experiments were conducted as a function of pH using As(III) and FeS-coated sands at various solid-solution ratios. The results showed little variation in the distribution coefficient with SSR at pH 7 and 9. However, at pH 5, the results showed lower values of the distribution coefficient at lower SSRs, the reverse of typically reported SSR effects. Measured pe values showed a dependence on SSR, which, when coupled with chemical modeling of the Fe-As-S-H2O system, suggested a change in the removal mechanism with SSR, from adsorption to a reduced Fe(II) oxyhydroxide phase (represented by Fe2(OH)5) to precipitation as As2S3 or AsS. On the other hand, at pH 7 and 9, arsenite adsorption is the most probable removal mechanism regardless of the pe. Thus, this study identified variations in pH and redox conditions, and the removal mechanisms that these parameters govern, as the reason for the apparent SSR effect.

Microbial synthesis of Pd/Fe3O4, Au/Fe3O4 and PdAu/Fe3O4 nanocomposites for catalytic reduction of nitroaromatic compounds

Magnetically recoverable noble metal nanoparticles are promising catalysts for chemical reactions. However, the chemical synthesis of these nanocatalysts generally causes environmental concern due to usage of toxic chemicals under extreme conditions. Here, Pd/Fe3O4, Au/Fe3O4 and PdAu/Fe3O4 nanocomposites are biosynthesized under ambient and physiological conditions by Shewanella oneidensis MR-1. Microbial cells firstly transform akaganeite into magnetite, which then serves as support for the further synthesis of Pd, Au and PdAu nanoparticles from respective precursor salts. Surface-bound cellular components and exopolysaccharides not only function as shape-directing agent to convert some Fe3O4 nanoparticles to nanorods, but also participate in the formation of PdAu alloy nanoparticles on magnetite. All these three kinds of magnetic nanocomposites can catalyze the reduction of 4-nitrophenol and some other nitroaromatic compounds by NaBH4. PdAu/Fe3O4 demonstrates higher catalytic activity than Pd/Fe3O4 and Au/Fe3O4. Moreover, the magnetic nanocomposites can be easily recovered through magnetic decantation after catalysis reaction. PdAu/Fe3O4 can be reused in at least eight successive cycles of 4-nitrophenol reduction. The biosynthesis approach presented here does not require harmful agents or rigorous conditions and thus provides facile and environmentally benign choice for the preparation of magnetic noble metal nanocatalysts.

Shewanella sp. O23S as a Driving Agent of a System Utilizing Dissimilatory Arsenate-Reducing Bacteria Responsible for Self-Cleaning of Water Contaminated with Arsenic

The purpose of this study was a detailed characterization of Shewanella sp. O23S, a strain involved in arsenic transformation in ancient gold mine waters contaminated with arsenic and other heavy metals. Physiological analysis of Shewanella sp. O23S showed that it is a facultative anaerobe, capable of growth using arsenate, thiosulfate, nitrate, iron or manganite as a terminal electron acceptor, and lactate or citrate as an electron donor. The strain can grow under anaerobic conditions and utilize arsenate in the respiratory process in a broad range of temperatures (10–37 °C), pH (4–8), salinity (0%–2%), and the presence of heavy metals (Cd, Co, Cr, Cu, Mn, Mo, Se, V and Zn). Under reductive conditions this strain can simultaneously use arsenate and thiosulfate as electron acceptors and produce yellow arsenic (III) sulfide (As₂S₃) precipitate. Simulation of As-removal from water containing arsenate (2.5 mM) and thiosulfate (5 mM) showed 82.5% efficiency after 21 days of incubation at room temperature. Based on the obtained results, we have proposed a model of a microbially mediated system for self-cleaning of mine waters contaminated with arsenic, in which Shewanella sp. O23S is the main driving agent.

Photocatalytic properties of zinc sulfide nanocrystals biofabricated by metal-reducing bacterium Shewanella oneidensis MR-1

Accumulation and utilization of heavy metals from wastewater by biological treatment system has aroused great interest. In the present study, a metal-reducing bacterium Shewanella oneidensis MR-1 was used to explore the biofabrication of ZnS nanocrystals from the artificial wastewater. The biogenic H2S produced via the reduction of thiosulfate precipitated the Zn(II) as sulfide extracellularly. Characterization by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and field emission scanning electron microscope (FESEM) confirmed the precipitates as ZnS nanocrystals. The biogenic ZnS nanocrystals appeared spherical in shape with an average diameter of 5 nm and mainly aggregated in the medium and cell surface of S. oneidensis MR-1. UV-vis DRS spectra showed ZnS nanoparticles appeared a strong absorption below 360 nm. Thus, the photocatalytic activity of ZnS was evaluated by the photodegradation of rhodamine B (RhB) under UV irradiation. The biogenic ZnS nanocrystals showed a high level of photodegradation efficiency to RhB coupled with a significant blue-shift of maximum adsorption peak. A detailed analysis indicated the photogenerated holes, rather than hydroxyl radicals, contributed to the photocatalytic decolorization of RhB. This approach of coupling biosynthesis of nanoparticles with heavy metal removal may offer a potential avenue for efficient bioremediation of heavy metal wastewater.

Field effect transistors based on semiconductive microbially synthesized chalcogenide nanofibers

Microbial redox activity offers a potentially transformative approach to the low-temperature synthesis of nanostructured inorganic materials. Diverse strains of the dissimilatory metal-reducing bacteria Shewanella are known to produce photoactive filamentous arsenic sulfide nanomaterials by reducing arsenate and thiosulfate in anaerobic culture conditions. Here we report in situ microscopic observations and measure the thermally activated (79 kJ mol(-1)) precipitation kinetics of high yield (504 mg per liter of culture, 82% of theoretical maximum) extracellular As2S3 nanofibers produced by Shewanella sp. strain ANA-3, and demonstrate their potential in functional devices by constructing field effect transistors (FETs) based on individual nanofibers. The use of strain ANA-3, which possesses both respiratory and detoxification arsenic reductases, resulted in significantly faster nanofiber synthesis than other strains previously tested, mutants of ANA-3 deficient in arsenic reduction, and when compared to abiotic arsenic sulfide precipitation from As(III) and S(2-). Detailed characterization by electron microscopy, energy-dispersive X-ray spectroscopy, electron probe microanalysis and Tauc analysis of UV-vis spectrophotometry showed the biogenic precipitate to consist primarily of amorphous As2S3 nanofibers with an indirect optical band gap of 2.37 eV. X-ray diffraction also revealed the presence of crystalline As8S(9-x) minerals that, until recently, were thought to form only at higher temperatures and under hydrothermal conditions. The nanoscale FETs enabled a detailed characterization of the charge mobility (∼10(-5) cm(2) V(-1) s(-1)) and gating behavior of the heterogeneously doped nanofibers. These studies indicate that the biotransformation of metalloids and chalcogens by bacteria enables fast, efficient, sustainable synthesis of technologically relevant chalcogenides for potential electronic and optoelectronic applications.

Shewanella-mediated biosynthesis of manganese oxide micro-/nanocubes as efficient electrocatalysts for the oxygen reduction reaction

Developing efficient electrocatalysts for the oxygen reduction reaction (ORR) is critical for promoting the widespread application of fuel cells and metal-air batteries. Here, we develop a biological low-cost, ecofriendly method for the synthesis of Mn2 O3 micro-/nanocubes by calcination of MnCO3 precursors in an oxygen atmosphere. Microcubic MnCO3 precursors with an edge length of 2.5 μm were fabricated by dissimilatory metal-reducing Shewanella loihica PV-4 in the presence of MnO4 (-) as the sole electron acceptor under anaerobic conditions. After calcining the MnCO3 precursors at 500 and 700 °C, porous Mn2 O3 -500 and Mn2 O3 -700 also showed microcubic morphology, while their edge lengths decreased to 1.8 μm due to thermal decomposition. Moreover, the surfaces of the Mn2 O3 microcubes were covered by granular nanoparticles with average diameters in the range of 18-202 nm, depending on the calcination temperatures. Electrochemical measurements demonstrated that the porous Mn2 O3 -500 micro-/nanocubes exhibit promising catalytic activity towards the ORR in an alkaline medium, which should be due to a synergistic effect of the overlapping molecular orbitals of oxygen/manganese and the hierarchically porous structures that are favorable for oxygen absorption. Moreover, these Mn2 O3 micro-/nanocubes possess better stability than commercial Pt/C catalysts and methanol-tolerance property in alkaline solution. Thus the Shewanella-mediated biosynthesis method we provided here might be a new strategy for the preparation of various transition metal oxides as high-performance ORR electrocatalysts at low cost.

Biologically based method for the synthesis of Hg–Se nanostructures by Shewanella spp

Living organisms, especially microorganisms, have the potential to offer cheap and benign synthetic routes for the production of nanomaterials.

Biomineralization of arsenate to arsenic sulfides is greatly enhanced at mildly acidic conditions

Arsenic (As) is an important water contaminant due to its high toxicity and widespread occurrence. Arsenic-sulfide minerals (ASM) are formed during microbial reduction of arsenate (As(V)) and sulfate (SO4(2-)). The objective of this research is to study the effect of the pH on the removal of As due to the formation of ASM in an iron-poor system. A series of batch experiments was used to study the reduction of SO4(2-) and As(V) by an anaerobic biofilm mixed culture in a range of pH conditions (6.1-7.2), using ethanol as the electron donor. Total soluble concentrations and speciation of S and As were monitored. Solid phase speciation of arsenic was characterized by x-ray adsorption spectroscopy (XAS). A marked decrease of the total aqueous concentrations of As and S was observed in the inoculated treatments amended with ethanol, but not in the non-inoculated controls, indicating that the As-removal was biologically mediated. The pH dramatically affected the extent and rate of As removal, as well as the stoichiometric composition of the precipitate. The amount of As removed was 2-fold higher and the rate of the As removal was up to 17-fold greater at pH 6.1 than at pH 7.2. Stoichiometric analysis and XAS results confirmed the precipitate was composed of a mixture of orpiment and realgar, and the proportion of orpiment in the sample increased with increasing pH. The results taken as a whole suggest that ASM formation is greatly enhanced at mildly acidic pH conditions.

New insights into hydrothermal vent processes in the unique shallow-submarine arc-volcano, Kolumbo (Santorini), Greece

We report on integrated geomorphological, mineralogical, geochemical and biological investigations of the hydrothermal vent field located on the floor of the density-stratified acidic (pH ~ 5) crater of the Kolumbo shallow-submarine arc-volcano, near Santorini. Kolumbo features rare geodynamic setting at convergent boundaries, where arc-volcanism and seafloor hydrothermal activity are occurring in thinned continental crust. Special focus is given to unique enrichments of polymetallic spires in Sb and Tl (±Hg, As, Au, Ag, Zn) indicating a new hybrid seafloor analogue of epithermal-to-volcanic-hosted-massive-sulphide deposits. Iron microbial-mat analyses reveal dominating ferrihydrite-type phases, and high-proportion of microbial sequences akin to "Nitrosopumilus maritimus", a mesophilic Thaumarchaeota strain capable of chemoautotrophic growth on hydrothermal ammonia and CO₂. Our findings highlight that acidic shallow-submarine hydrothermal vents nourish marine ecosystems in which nitrifying Archaea are important and suggest ferrihydrite-type Fe³⁺-(hydrated)-oxyhydroxides in associated low-temperature iron mats are formed by anaerobic Fe²⁺-oxidation, dependent on microbially produced nitrate.

A facile bacterial assisted electrochemical self-assembly of polypyrrole micro-pillars: towards underwater low adhesive superoleophobicity

By taking advantage of bacterial extracellular electron transfer behavior, a facile method was developed to fabricate oriented polypyrrole micro-pillars (PPy-MP) with nanoscale surface roughness. Microbes acted as a living conductive template on which PPy was in situ polymerized. The as-prepared PPy-MP exhibit the distinctive underwater low adhesive superoleophobicity which is attributable to the unique hierarchical micro/nano-structures and the high surface energy by doping with inorganic small anions.

Recent advances in the synthesis of inorganic nano/microstructures using microbial biotemplates and their applications

Microbial biotemplates for synthesizing inorganic nanostructures of defined morphology and size.

Microbiological reduction of Sb(V) in anoxic freshwater sediments

Microbiological reduction of millimolar concentrations of Sb(V) to Sb(III) was observed in anoxic sediments from two freshwater settings: (1) a Sb- and As-contaminated mine site (Stibnite Mine) in central Idaho and 2) an uncontaminated suburban lake (Searsville Lake) in the San Francisco Bay Area. Rates of Sb(V) reduction in anoxic sediment microcosms and enrichment cultures were enhanced by amendment with lactate or acetate as electron donors but not by H2, and no reduction occurred in sterilized controls. Addition of 2-(14)C-acetate to Stibnite Mine microcosms resulted in the production of (14)CO2 coupled to Sb(V) reduction, suggesting that this process proceeds by a dissimilatory respiratory pathway in those sediments. Antimony(V) reduction in Searsville Lake sediments was not coupled to acetate mineralization and may be associated with Sb-resistance. The microcosms and enrichment cultures also reduced sulfate, and the precipitation of insoluble Sb(III)-sulfide complexes was a major sink for reduced Sb. The reduction of Sb(V) by Stibnite Mine sediments was inhibited by As(V), suggesting that As(V) is a preferred electron acceptor for the indigenous community. These findings indicate a novel pathway for anaerobic microbiological respiration and suggest that communities capable of reducing high concentrations of Sb(V) commonly occur naturally in the environment.

Promoted reduction of tellurite and formation of extracellular tellurium nanorods by concerted reaction between iron and Shewanella oneidensis MR-1

The reduction of tellurite (Te(IV)) by dissimilatory metal reducing bacterium, Shewanella oneidensis MR-1, was promoted in the presence of Fe(III) in comparison with Te(IV) bioreduction in the absence of Fe(III). Electron microscopic analyses revealed that iron promoted Te(IV) reduction led to form exclusively extracellular crystalline Te(0) nanorods, as compared to the mostly intracellular formation of Te(0) nanorods in the absence of Fe(III). The Te K-edge X-ray absorption spectrometric analyses demonstrated that S. oneidensis MR-1 in the presence of Fe(III) reduced Te(IV) to less harmful metallic Te(0) nanorods through the precipitation of tellurite (Te(IV)Ox) complex by the bacterial respiration of Fe(III) to Fe(II) under anaerobic conditions. However, Fe(II) ion itself was only able to precipitate the solid tellurite (Te(IV)Ox) complex from the Te(IV) solution, which was not further reduced to Te(0). The results clearly indicated that bacterial S. oneidensis MR-1 plays important roles in the reduction and crystallization of Te(0) nanorods by as yet undetermined biochemical mechanisms. As compared to the slow bacterial Te(IV) reduction in the absence of Fe(III), the rapid reduction of Te(IV) to Te(0) by the concerted biogeochemical reaction between Fe(II) and S. oneidensis MR-1 could be applied for the sequestration and detoxification of Te(IV) in the environments as well as for the preparation of extracellular Te(0) nanorod structures.

Differential arsenic mobilization from As-bearing ferrihydrite by iron-respiring Shewanella strains with different arsenic-reducing activities

Arsenic immobilization and release in the environment is significantly influenced by bacterial oxidation and reduction of arsenic and arsenic-bearing minerals. In this study, we tested three iron-reducing bacteria, Shewanella oneidensis MR-1, Shewanella sp. HN-41, and Shewanella putrefaciens 200, which have diverse arsenate-reducing activities with regard to reduction of an As-bearing ferrihydrite slurry. In the cultures of S. oneidensis MR-1 and Shewanella sp. HN-41, which are not capable of respiratory reduction of As(V) to As(III), arsenic was maintained predominantly in its pentavalent form, existing in particulate poorly crystalline As-bearing ferrihydrite and formed small quantities of a stable ferrous arsenate [Fe3(AsO4)2] precipitate. However, in the culture of the As(V) reducer, S. putrefaciens 200, As(V) was reduced to As(III) and a small fraction of As-bearing ferrihydrite was transformed into ribbon-shaped siderite that subsequently re-released arsenic into the liquid phase. Our results indicated that release of arsenic and formation of diverse secondary nanoscale Fe-As minerals are specifically closely related to the arsenic-reducing abilities of different bacteria. Therefore, bacterial arsenic reduction appears to significantly influence As mobilization in soils, minerals, and other Fe-rich environments.

Precipitation of alacranite (As8S9) by a novel As(V)-respiring anaerobe strain MPA-C3

Strain MPA-C3 was isolated by incubating arsenic-bearing sediments under anaerobic, mesophilic conditions in minimal media with acetate as the sole source of energy and carbon, and As(V) as the sole electron acceptor. Following growth and the respiratory reduction of As(V) to As(III), a yellow precipitate formed in active cultures, while no precipitate was observed in autoclaved controls, or in uninoculated media supplemented with As(III). The precipitate was identified by X-ray diffraction as alacranite, As8 S9 , a mineral previously only identified in hydrothermal environments. Sequencing of the 16S rRNA gene indicated that strain MPA-C3 is a member of the Deferribacteres family, with relatively low (90%) identity to Denitrovibrio acetiphilus DSM 12809. The arsenate respiratory reductase gene, arrA, was sequenced, showing high homology to the arrA gene of Desulfitobacterium halfniense. In addition to As(V), strain MPA-C3 utilizes NO3(-), Se(VI), Se(IV), fumarate and Fe(III) as electron acceptors, and acetate, pyruvate, fructose and benzoate as sources of carbon and energy. Analysis of a draft genome sequence revealed multiple pathways for respiration and carbon utilization. The results of this work demonstrate that alacranite, a mineral previously thought to be formed only chemically under hydrothermal conditions, is precipitated under mesophilic conditions by the metabolically versatile strain MPA-C3.