Progress toward clonable inorganic nanoparticles

Disruption of the cell division protein ftsK gene changes elemental selenium generation, selenite tolerance, and cell morphology in Rahnella aquatilis HX2

AIMS

Some studies have indicated that the alterations in cellular morphology induced by selenite [Se(Ⅳ)] may be attributed to its inhibitory effects on cell division. However, whether the genes associated with cell division are implicated in Se(Ⅳ) metabolism remains unclear.

METHODS AND RESULTS

The ftsK gene in Rahnella aquatilis HX2 was mutated with an in-frame deletion strategy. The ftsK mutation strongly reduced the tolerance to selenite [Se(Ⅳ)] and the production of red elemental selenium [Se(0)] in R. aquatilis HX2, and this effect could not be attributed solely to the inhibition of cell growth. Deleting the ftsK gene also resulted in a significant decrease in bacterial growth of R. aquatilis HX2 during both exponential and stationary phases. The deletion of ftsK inhibited cell division, resulting in the development of elongated filamentous cells. Furthermore, the loss-of-function of FtsK significantly impacted the expression of seven genes linked to cell division and Se(Ⅳ) metabolism by at least 2-fold, as unveiled by real-time quantitative PCR (RT-qPCR) under Se(Ⅳ) treatment.

CONCLUSIONS

These findings suggest that FtsK is associated with Se(Ⅳ) tolerance and Se(0) generation and is a key player in coordinating bacterial growth and cell morphology in R. aquatilis HX2.

Laboratory Evolution of Metalloid Reductase Substrate Recognition and Nanoparticle Product Size

Glutathione reductase-like metalloid reductase (GRLMR) is an enzyme that reduces selenodiglutathione (GS-Se-SG), forming zerovalent Se nanoparticles (SeNPs). Error-prone polymerase chain reaction was used to create a library of ∼10,000 GRLMR variants. The library was expressed in BL21Escherichia coli in liquid culture with 50 mM of SeO32- present, under the hypothesis that the enzyme variants with improved GS-Se-SG reduction kinetics would emerge. The selection resulted in a GRLMR variant with two mutations. One of the mutations (D-E) lacks an obvious functional role, whereas the other mutation is L-H within 5 Å of the enzyme active site. This mutation places a second H residue within 5 Å of an active site dicysteine. This GRLMR variant was characterized for NADPH-dependent reduction of GS-Se-SG, GSSG, SeO32-, SeO42-, GS-Te-SG, and TeO32-. The evolved enzyme demonstrated enhanced reduction of SeO32- and gained the ability to reduce SeO42-. This variant is named selenium reductase (SeR) because of its emergent broad activity for a wide variety of Se substrates, whereas the parent enzyme was specific for GS-Se-SG. This study overall suggests that new biosynthetic routes are possible for inorganic nanomaterials using laboratory-directed evolution methods.

Biological approaches for E-waste management: A green-go to boost circular economy

E-waste is a pressing situation on human due to its complex composition. Although E-waste on one hand has some toxic components but at the same time, it would be a promising business sector. Recycling of E-waste to mine-out valuable metals and other components has opened a chance of business and hence a way towards transformation of linear economy to circular one. Chemical, physical and traditional technologies are holding the position in E-waste recycling sector but sustainability with respect to cost and environmental issues is a major concern associated with these technologies. In order to overcome these gaps, lucrative, environment friendly and sustainable technologies need to be implied. Biological approaches could be a green and clean approach to handle E-waste through sustainable and cost-effective means by considering socio-economic and environmental aspects. This review elaborates biological approaches for E-waste management and advancements in expanse. The novelty covers the environmental and socio-economic impacts of E-waste, solution and further scope of biological approaches, further research and development need in this contour to come up with sustainable recycling process.

Genetic mechanisms for Se(VI) reduction and synthesis of trigonal 1-D nanostructures in Stenotrophomonas bentonitica: Perspectives in eco-friendly nanomaterial production and bioremediation

Selenate (Se(VI)) is one of the most soluble and toxic species of Se. Microbial Se(VI) reduction is an efficient tool for bioremediation strategies. However, this process is limited to a few microorganisms, and its molecular basis remains unknown. We present detailed Se(VI)-resistance mechanisms under 50 and 200 mM, in Stenotrophomonas bentonitica BII-R7, coupling enzymatic reduction of Se(VI) to formation of less toxic trigonal Se (t-Se). The results reveal a concentration-dependent response. Despite the lack of evidence of Se(VI)-reduction to Se(0) under 50 mM Se(VI), many genes were highly induced, indicating that Se(VI)-resistance could be based on intracellular reduction to Se(IV), mainly through molybdenum-dependent enzymes (e.g. respiratory nitrate reductase), and antioxidant activity by enzymes like glutathione peroxidase. Although exposure to 200 mM provoked a sharp drop in gene expression, a time-dependent process of reduction and formation of amorphous (a), monoclinic (m) and t-Se nanostructures was unravelled: a-Se nanospheres were initially synthesized intracellularly, which would transform into m-Se and finally into t-Se nanostructures during the following phases. This is the first work describing an intracellular Se(VI) reduction and biotransformation process to long-term stable and insoluble t-Se nanomaterials. These results expand the fundamental understanding of Se biogeochemical cycling, and the effectiveness of BII-R7 for bioremediation purposes.

Combined bioreduction and volatilization of SeVI by Stenotrophomonas bentonitica: Formation of trigonal selenium nanorods and methylated species

Nowadays, metal pollution due to the huge release of toxic elements to the environment has become one of the world's biggest problems. Bioremediation is a promising tool for reducing the mobility and toxicity of these contaminants (e.g. selenium), being an efficient, environmentally friendly, and inexpensive strategy. The present study describes the capacity of Stenotrophomonas bentonitica to biotransform Se^VI through enzymatic reduction and volatilization processes. HAADF-STEM analysis showed the bacterium to effectively reduce Se^VI (200 mM) into intra- and extracellular crystalline Se⁰ nanorods, made mainly of two different Se allotropes: monoclinic (m-Se) and trigonal (t-Se). XAS analysis appears to indicate a Se crystallization process based on the biotransformation of amorphous Se⁰ into stable t-Se nanorods. In addition, results from headspace analysis by gas chromatography-mass spectometry (GC-MS) revealed the formation of methylated volatile Se species such as DMSe (dimethyl selenide), DMDSe (dimethyl diselenide), and DMSeS (dimethyl selenenyl sulphide). The biotransformation pathways and tolerance are remarkably different from those reported with this bacterium in the presence of Se^IV. The formation of crystalline Se⁰ nanorods could have positive environmental implications (e.g. bioremediation) through the production of Se of lower toxicity and higher settleability with potential industrial applications.

Allotropy of selenium nanoparticles: Colourful transition, synthesis, and biotechnological applications

Elemental selenium (Se⁰ ) nanomaterials undergo allotropic transition from thermodynamically-unstable to more stable phases. This process is significantly different when Se⁰ nanoparticles (NPs) are produced via physico-chemical and biological pathways. While the allotropic transition of physico-chemically synthesized Se⁰ is fast (minutes to hours), the biogenic Se⁰ takes months to complete. The biopolymer layer covering biogenic Se⁰ NPs might be the main factor controlling this retardation, but this still remains an open question. Phylogenetically-diverse bacteria reduce selenium oxyanions to red amorphous Se⁰ allotrope, which has low market value. Then, red Se⁰ undergoes allotropic transition to trigonal (metallic grey) allotrope, the end product having important industrial applications (e.g. semiconductors, alloys). Is it not yet clear whether biogenic Se⁰ presents any biological function, or it is mainly a detoxification and respiratory by-product. The better understanding of this transition would benefit the recovery of Se⁰ NPs from secondary resources and its targeted utilization with respect to each allotropic stage. This review article presents and critically discusses the main physico-chemical methods and biosynthetic pathways of Se⁰ (bio)mineralization. In addition, the article proposes a conceptual model for the resource recovery potential of trigonal selenium nanomaterials in the context of circular economy.

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.

Delineation of cellular stages and identification of key proteins for reduction and biotransformation of Se(IV) by Stenotrophomonas bentonitica BII-R7

The widespread use of selenium (Se) in technological applications (e.g., solar cells and electronic devices) has led to an accumulation of this metalloid in the environment to toxic levels. The newly described bacterial strain Stenotrophomonas bentonitica BII-R7 has been demonstrated to reduce mobile Se(IV) to Se(0)-nanoparticles (Se(0)NPs) and volatile species. Amorphous Se-nanospheres are reported to aggregate to form crystalline nanostructures and trigonal selenium. We investigated the molecular mechanisms underlying the biotransformation of Se(IV) to less toxic forms using differential shotgun proteomics analysis of S. bentonitica BII-R7 grown with or without sodium selenite for three different time-points. Results showed an increase in the abundance of several proteins involved in Se(IV) reduction and stabilization of Se(0)NPs, such as glutathione reductase, in bacteria grown with Se(IV), in addition to many proteins with transport functions, including RND (resistance-nodulation-division) systems, possibly facilitating Se uptake. Notably proteins involved in oxidative stress defense (e.g., catalase/peroxidase HPI) were also induced by Se exposure. Electron microscopy analyses confirmed the biotransformation of amorphous nanospheres to trigonal Se. Overall, our results highlight the potential of S. bentonitica in reducing the bioavailability of Se, which provides a basis both for the development of bioremediation strategies and the eco-friendly synthesis of biotechnological nanomaterials.

Antimicrobial Activity of Se-Nanoparticles from Bacterial Biotransformation

Selenium nanoparticles (SeNPs) are gaining importance in the food and medical fields due to their antibacterial properties. The microbial inhibition of these kinds of particles has been tested in a wide range of Gram (+) and Gram (−) pathogenic bacteria. When SeNPs are synthesized by biological methods, they are called biogenic SeNPs, which have a negative charge caused by their interaction between surface and capping layer (bioorganic material), producing their high stability. This review is focused on SeNPs synthesis by bacteria and summarizes the main factors that influence their main characteristics: shape, size and surface charge, considering the bacteria growth conditions for their synthesis. The different mechanisms of antimicrobial activity are revised, and this review describes several biosynthesis hypotheses that have been proposed due to the fact that the biological mechanism of SeNP synthesis is not fully known.

Selenium respiration in anaerobic bacteria: Does energy generation pay off?

Selenium (Se) respiration in bacteria was revealed for the first time at the end of 1980s. Although thermodynamically-favorable, energy-dense and documented in phylogenetically-diverse bacteria, this metabolic process appears to be accompanied by a number of challenges and numerous unanswered questions. Selenium oxyanions, SeO₄^2- and SeO₃^2-, are reduced to elemental Se (Se⁰) through anaerobic respiration, the end product being solid and displaying a considerable size (up to 500 nm) at the bacterial scale. Compared to other electron acceptors used in anaerobic respiration (e.g. N, S, Fe, Mn, and As), Se is one of the few elements whose end product is solid. Furthermore, unlike other known bacterial intracellular accumulations such as volutin (inorganic polyphosphate), S⁰, glycogen or magnetite, Se⁰ has not been shown to play a nutritional or ecological role for its host. In the context of anaerobic respiration of Se oxyanions, biogenic Se⁰ appears to be a by-product, a waste that needs proper handling, and this raises the question of the evolutionary implications of this process. Why would bacteria use a respiratory substrate that is useful, in the first place, and then highly detrimental? Interestingly, in certain artificial ecosystems (e.g. upflow bioreactors) Se⁰ might help bacterial cells to increase their density and buoyancy and thus avoid biomass wash-out, ensuring survival. This review article provides an in-depth analysis of selenium respiration (model selenium respiring bacteria, thermodynamics, respiratory enzymes, and genetic determinants), complemented by an extensive discussion about the evolutionary implications and the properties of biogenic Se⁰ using published and original/unpublished results.

Identification of a TeO32- reductase/mycothione reductase from Rhodococcus erythropolis PR4

A Rhodococcus erythropolis bacterium that tolerates normally lethal concentrations of Fe(II), Cu(II), AsO32-, SeO32-, TeO32-, Cd(II) and Zn(II) was identified from an environmental isolate. In characterizing the molecular basis for metal tolerance, a mycothione reductase (Mtr) with remarkable selectivity for TeO32- reduction over SeO32- was identified. In equimolar concentrations of TeO32- and SeO32-, the enzymatic product contains a 7-fold excess of Te. This selectivity is remarkable because the standard reduction potential of SeO32- is 0.20 V more favorable for reduction than TeO32. Selectivity of the enzyme for TeO32- decreases with increasing assay pH. Homology modeling of the enzyme identifies four aromatic residues near the active site, including two histidine residues, that are not present in a related SeO32- preferring reductase. On the basis of more favorable π-interactions for Te than for Se and the pH dependence of the selectivity, the Te-selectivity is attributed in part to these aromatic residues. The resulting Te0 enzymatic product resembles Te nanowires.

Speeding up selenite bioremediation using the highly selenite-tolerant strain Providencia rettgeri HF16-A novel mechanism of selenite reduction based on proteomic analysis

Selenite in the environment is extremely biotoxic, thus, the biotransformation of selenite into selenium nanoparticles (SeNPs) by microorganisms is gaining increasing interest. However, the relatively low selenite tolerance and slow processing by known microorganisms limit its application. In this study, a highly selenite-resistant strain (up to 800 mM) was isolated from coalmine soil and identified as Providencia rettgeri HF16. Remarkably, 5 mM selenite was entirely transformed by this strain within 24 h, and SeNPs were detected as early as 2 h of incubation, which is a more rapid conversion than that described for other microorganisms. The SeNPs were spherical in shape with diameters ranging from 120 nm to 295 nm, depending on the incubation time. Moreover, in vitro selenite-reduction activity was detected in the cytoplasmic protein fraction with NADPH or NADH serving as electron donors. Proteomics analysis and key enzyme activity tests revealed the presence of a sulfite reductase-mediated selenite reduction pathway. To our knowledge, this is the first report to identify the involvement of sulfite reductase in selenite reduction under physiological conditions. P. rettgeri HF16 could be a suitable and robust biocatalyst for the bioremediation of selenite, and would accelerate the efficient and economical synthesis of selenium nanoparticles.

DNA origami signposts for identifying proteins on cell membranes by electron cryotomography

Electron cryotomography (cryoET), an electron cryomicroscopy (cryoEM) modality, has changed our understanding of biological function by revealing the native molecular details of membranes, viruses, and cells. However, identification of individual molecules within tomograms from cryoET is challenging because of sample crowding and low signal-to-noise ratios. Here, we present a tagging strategy for cryoET that precisely identifies individual protein complexes in tomograms without relying on metal clusters. Our method makes use of DNA origami to produce “molecular signposts” that target molecules of interest, here via fluorescent fusion proteins, providing a platform generally applicable to biological surfaces. We demonstrate the specificity of signpost origami tags (SPOTs) in vitro as well as their suitability for cryoET of membrane vesicles, enveloped viruses, and the exterior of intact mammalian cells.

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Asymmetric DNA signpost origami tags (SPOTs) precisely localize proteins

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SPOTs identify specific proteins in electron cryomicroscopy

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SPOTs have a high contrast “sign” and functionalized “post” base for targeting

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SPOTs recognize fluorescent fusion proteins on vesicles, viruses, and cell surfaces

Metalloid Reductase Activity Modified by a Fused Se0 Binding Peptide

A selenium nanoparticle binding peptide was isolated from a phage display library and genetically fused to a metalloid reductase that reduces selenite (SeO₃^2-) to a Se⁰ nanoparticle (SeNP) form. The fusion of the Se binding peptide to the metalloid reductase regulates the size of the resulting SeNP to ∼35 nm average diameter, where without the peptide, SeNPs grow to micron sized polydisperse precipitates. The SeNP product remains associated with the enzyme/peptide fusion. The Se binding peptide fusion to the enzyme increases the enzyme's SeO₃^2- reductase activity. Size control of particles was diminished if the Se binding peptide was only added exogenously to the reaction mixture. The enzyme-peptide construct shows preference for binding smaller SeNPs. The peptide-SeNP interaction is attributed to His based ligation that results in a peptide conformational change on the basis of Raman spectroscopy.

Selenium biofortification in the 21st century: status and challenges for healthy human nutrition

BACKGROUND

Selenium (Se) is an essential element for mammals and its deficiency in the diet is a global problem. Plants accumulate Se and thus represent a major source of Se to consumers. Agronomic biofortification intends to enrich crops with Se in order to secure its adequate supply by people.

SCOPE

The goal of this review is to report the present knowledge of the distribution and processes of Se in soil and at the plant-soil interface, and of Se behaviour inside the plant in terms of biofortification. It aims to unravel the Se metabolic pathways that affect the nutritional value of edible plant products, various Se biofortification strategies in challenging environments, as well as the impact of Se-enriched food on human health.

CONCLUSIONS

Agronomic biofortification and breeding are prevalent strategies for battling Se deficiency. Future research addresses nanosized Se biofortification, crop enrichment with multiple micronutrients, microbial-integrated agronomic biofortification, and optimization of Se biofortification in adverse conditions. Biofortified food of superior nutritional quality may be created, enriched with healthy Se-compounds, as well as several other valuable phytochemicals. Whether such a food source might be used as nutritional intervention for recently emerged coronavirus infections is a relevant question that deserves investigation.

Elemental Sulfur Formation by Sulfuricurvum kujiense Is Mediated by Extracellular Organic Compounds

Elemental sulfur [S(0)] is a central and ecologically important intermediate in the sulfur cycle, which can be used by a wide diversity of microorganisms that gain energy from its oxidation, reduction, or disproportionation. S(0) is formed by oxidation of reduced sulfur species, which can be chemically or microbially mediated. A variety of sulfur-oxidizing bacteria can biomineralize S(0), either intracellularly or extracellularly. The details and mechanisms of extracellular S(0) formation by bacteria have been in particular understudied so far. An important question in this respect is how extracellular S(0) minerals can be formed and remain stable in the environment outside of their thermodynamic stability domain. It was recently discovered that S(0) minerals could be formed and stabilized by oxidizing sulfide in the presence of dissolved organic compounds, a process called S(0) organomineralization. S(0) particles formed through this mechanism possess specific signatures such as morphologies that differ from that of their inorganically precipitated counterparts, encapsulation within an organic envelope, and metastable crystal structures (presence of the monoclinic β- and γ-S₈ allotropes). Here, we investigated S(0) formation by the chemolithoautotrophic sulfur-oxidizing and nitrate-reducing bacterium Sulfuricurvum kujiense (Epsilonproteobacteria). We performed a thorough characterization of the S(0) minerals produced extracellularly in cultures of this microorganism, and showed that they present all the specific signatures (morphology, association with organics, and crystal structures) of organomineralized S(0). Using "spent medium" experiments, we furthermore demonstrated that soluble extracellular compounds produced by S. kujiense are necessary to form and stabilize S(0) minerals outside of the cells. This study provides the first experimental evidence of the importance of organomineralization in microbial S(0) formation. The prevalence of organomineralization in extracellular S(0) precipitation by other sulfur bacteria remains to be investigated, and the biological role of this mechanism is still unclear. However, we propose that sulfur-oxidizing bacteria could use soluble organics to stabilize stores of bioavailable S(0) outside the cells.

Enzyme-Catalyzed in situ Synthesis of Temporally and Spatially Distinct CdSe Quantum Dots in Biological Backgrounds

The cellular machinery of metal metabolism is capable of making a wide range of inorganic nanoparticles and quantum dots. Individual enzymes from these metabolic pathways are being identified with metal reducing activity, and some have been isolated for in situ particle formation and labeling. We previously identified a glutathione reductase like metalloid reductase (GRLMR) from Pseudomonas Moravenis stanleyae with a high affinity for the bioavailable selenium thiolate selenodiglutatione, and exhibiting NADPH-dependent reduction of selenodiglutathione to Se(0); initiating the growth of pure selenium metal nanoparticles. In this study, we demonstrate that the GRLMR enzyme can further reduce selenium to a Se(2-) oxidative state, which is capable of nucleating with Cd(2+) to rapidly form CdSe quantum dots. We show that GRLMR can outcompete background sources of cellular selenium reduction (such as glutathione) and can control the kinetics of quantum dot formation in complex media. The resulting particles are smaller diameter, with a distinguishingly shifted emission spectra and superior FWHM. This study indicates that there is great potential for using GRLMR to study and design enzymes capable of controlled biosynthesis of nanoparticles and quantum dots; paving the way for cellularly assembled nanoparticle-biosensors and reporters.

Metalloid Reductase of Pseudomonas moravenis Stanleyae Conveys Nanoparticle Mediated Metalloid Tolerance

A glutathione reductase (GSHR)-like enzyme in Pseudomonas moraviensis stanleyae was previously implicated as underlying the bacterium's remarkable SeO3 2- tolerance. Herein, this enzyme is sequenced, recombinantly expressed, and fully characterized. The enzyme is highly adapted for selenodiglutathione substrates (K m = 336 μM) compared to oxidized glutathione (K m = 8.22 mM). The recombinant expression of this enzyme in the laboratory strains of Escherichia coli conveys a 10-fold increase in IC90 for SeO3 2-. Moreover, selenium nanoparticles are observed when the enzyme is overexpressed in the cells exposed to SeO3 2-, but not in the corresponding no-enzyme controls. The analyses of the structural homology models of the enzyme reveal changes in the parts of the enzyme associated with product release, which may underlie the Se substrate specialization. Combined, the observations of adaptation to Se reduction over oxidized glutathione reduction as well as the portability of this nanoparticle-mediated SeO3 2- tolerance into other cell lines suggest that the P. moraviensis GSHR may be better described as a GSHR-like metalloid reductase.

Selenium Nanoparticle Synthesized by Proteus mirabilis YC801: An Efficacious Pathway for Selenite Biotransformation and Detoxification

Selenite is extremely biotoxic, and as a result of this, exploitation of microorganisms able to reduce selenite to non-toxic elemental selenium (Se⁰) has attracted great interest. In this study, a bacterial strain exhibiting extreme tolerance to selenite (up to 100 mM) was isolated from the gut of adult Monochamus alternatus and identified as Proteus mirabilis YC801. This strain demonstrated efficient transformation of selenite into red selenium nanoparticles (SeNPs) by reducing nearly 100% of 1.0 and 5.0 mM selenite within 42 and 48 h, respectively. Electron microscopy and energy dispersive X-ray analysis demonstrated that the SeNPs were spherical and primarily localized extracellularly, with an average hydrodynamic diameter of 178.3 ± 11.5 nm. In vitro selenite reduction activity assays and real-time PCR indicated that thioredoxin reductase and similar proteins present in the cytoplasm were likely to be involved in selenite reduction, and that NADPH or NADH served as electron donors. Finally, Fourier-transform infrared spectral analysis confirmed the presence of protein and lipid residues on the surfaces of SeNPs. This is the first report on the capability of P. mirabilis to reduce selenite to SeNPs. P. mirabilis YC801 might provide an eco-friendly approach to bioremediate selenium-contaminated soil/water, as well as a bacterial catalyst for the biogenesis of SeNPs.

Superoxide generated from the glutathione-mediated reduction of selenite damages the iron-sulfur cluster of chloroplastic ferredoxin

Selenium assimilation in plants is facilitated by several enzymes that participate in the transport and assimilation of sulfate. Manipulation of genes that function in sulfur metabolism dramatically affects selenium toxicity and accumulation. However, it has been proposed that selenite is not reduced by sulfite reductase. Instead, selenite can be non-enzymatically reduced by glutathione, generating selenodiglutathione and superoxide. The damaging effects of superoxide on iron-sulfur clusters in cytosolic and mitochondrial proteins are well known. However, it is unknown if superoxide damages chloroplastic iron-sulfur proteins. The goals of this study were twofold: to determine whether decreased activity of sulfite reductase impacts selenium tolerance in Arabidopsis, and to determine if superoxide generated from the glutathione-mediated reduction of selenite damages the iron-sulfur cluster of ferredoxin. Our data demonstrate that knockdown of sulfite reductase in Arabidopsis does not affect selenite tolerance or selenium accumulation. Additionally, we provide in vitro evidence that the non-enzymatic reduction of selenite damages the iron-sulfur cluster of ferredoxin, a plastidial protein that is an essential component of the photosynthetic light reactions. Damage to ferredoxin's iron-sulfur cluster was associated with formation of apo-ferredoxin and impaired activity. We conclude that if superoxide damages iron-sulfur clusters of ferredoxin in planta, then it might contribute to photosynthetic impairment often associated with abiotic stress, including toxic levels of selenium.

Removal of colloidal biogenic selenium from wastewater

Biogenic selenium, Se(0), has colloidal properties and thus poses solid-liquid separation problems, such as poor settling and membrane fouling. The separation of Se(0) from the bulk liquid was assessed by centrifugation, filtration, and coagulation-flocculation. Se(0) particles produced by an anaerobic granular sludge are normally distributed, ranging from 50 nm to 250 nm, with an average size of 166±29 nm and a polydispersity index of 0.18. Due to its nanosize range and protein coating-associated negative zeta potential (-15 mV to -23 mV) between pH 2 and 12, biogenic Se(0) exhibits colloidal properties, hampering its removal from suspension. Centrifugation at different centrifugal speeds achieved 22±3% (1500 rpm), 73±2% (3000 rpm) and 91±2% (4500 rpm) removal. Separation by filtration through 0.45 μm filters resulted in 87±1% Se(0) removal. Ferric chloride and aluminum sulfate were used as coagulants in coagulation-flocculation experiments. Aluminum sulfate achieved the highest turbidity removal (92±2%) at a dose of 10(-3) M, whereas ferric chloride achieved a maximum turbidity removal efficiency of only 43±4% at 2.7×10(-4) M. Charge repression plays a minor role in particle neutralization. The sediment volume resulting from Al2(SO3)4 treatment is three times larger than that produced by FeCl3.