Phosphate-Suppressed Selenite Biotransformation by Escherichia coli

Novel mechanisms for selenite biotransformation and selenium nanoparticles biogenesis in Acinetobacter sp. SX5 isolated from seleniferous soil

The high biotoxicity of selenium (Se) has spurred research into its microbial biotransformation into less toxic Se nanoparticles (SeNPs). However, the molecular mechanisms underlying microbially driven selenite transformation remain largely unknown. In the present study, Acinetobacter sp. SX5, a bacterial strain with high Se reduction capacity, was isolated from soil. The biotransformation of selenite by SX5 and the molecular mechanisms underlying the formation of SeNPs were investigated. SX5 almost completely transformed 5.0 mM selenite into intracellular and extracellular spherical SeNPs within 48 h. Fourier-transform infrared spectroscopy indicated that lipids, proteins, and carbohydrates were present on the surface of these SeNPs. Transcriptomic data subsequently revealed the significant upregulation of genes related to redox homeostasis and arsenate, pyruvate, and butanoate metabolism pathways. Gene mutation/complementation analysis confirmed that arsenate reductase (arsC) and NAD(P)-dependent alcohol dehydrogenase (dhaT1) facilitated selenite reduction in vivo. In vitro assays found that arsC and dhaT1 catalyzed Se(IV) reduction with NADPH acting as co-factor. To the best of our knowledge, this study is the first to present evidence for the participation of arsC and dhaT1 in selenite reduction in vivo, providing important insights into the molecular mechanisms underlying the biotransformation of Se(IV) and the biogenesis of SeNPs using Se-reducing bacteria.

Potent and Selective Inhibition of Sulfate-Reducing Bacteria by Neutral Red

Sulfate-reducing bacteria (SRB) are anaerobic microorganisms that use sulfate as a terminal electron acceptor for the oxidation of organic compounds or H2. These organisms can cause a serious problem in, for example, the offshore oil industry, due to the production of sulfide. Thus, it is of fundamental and practical importance to identify potent and selective inhibitors of SRB. In this study, neutral red was identified as a previously unrecognized selective inhibitor of SRB, with several orders of magnitude higher potency than most commonly used industrial biocides and inorganic oxyanions. Neutral red remained a potent inhibitor of SRB growth under fermentative conditions and was tolerated by nitrate-reducing bacteria. After 30 days of exposure to 14.2 μM neutral red, the sulfidogenesis activity of SRB-enriched biomass was reduced by 98.3%, and the abundance of SRB populations declined from 25.5% to 0.76%. Transcriptomic analysis revealed that the inhibition of the central sulfate reduction pathway was implicated in the mechanism of neutral red toxicity against SRB growth. Furthermore, downregulation of two electron transport complexes (QmoABC and DsrMKJOP), ATP synthase, as well as cytoplasmic/periplasmic hydrogenase suggested the collapse of the proton gradient. These findings have implications for environmental control of SRB and may enhance economic benefits in industrial operations.

Genomic Insights into Selenate Reduction by Anaerobacillus Species

Selenium (Se), a potentially toxic trace element, undergoes complex biogeochemical cycling in the environment, largely driven by microbial activity. The reduction in selenate or selenite to elemental selenium is an environmentally beneficial process, as it decreases both Se toxicity and mobility. This reduction is catalyzed by enzymes encoded by various related genes. The link between Se reduction gene clusters and specific taxonomic groups is significant for elucidating the ecological roles and processes of Se reduction in diverse environments. In this study, a new species of Se-reducing microorganism belonging to the genus Anaerobacillus was isolated from a mining site. A comparative analysis of the growth characteristics reveals that Anaerobacillus species exhibit notable metabolic versatility, particularly in their fermentation abilities and utilization of diverse electron donors and acceptors. Genome analysis identified a diverse array of gene clusters associated with selenate uptake (sul, pst), selenate reduction (ser), and selenite reduction (hig, frd, trx, and bsh). Since selenate reduction is the first crucial step in Se reduction, genes linked to selenate reductase are the focus. The serA gene clusters analysis suggests that the serA gene is highly conserved across Anaerobacillus species. The surrounding genes of serA show significant variability in both presence and gene size. This evolutionary difference in coenzyme utilization and serA regulation suggests distinct survival strategies among Anaerobacillus species. This study offers insights into Se bio-transformations and the adaptive strategies of Se-reducing microorganisms.

Harnessing biosynthesized selenium nanoparticles for recruitment of beneficial soil microbes to plant roots

Root exudates can benefit plant growth and health by reshaping the rhizosphere microbiome. Whether nanoparticles biosynthesized by rhizosphere microbes play a similar role in plant microbiome manipulation remains enigmatic. Herein, we collect elemental selenium nanoparticles (SeNPs) from selenobacteria associated with maize roots. In vitro and soil assays show that the SeNPs enhanced plant performance by recruiting plant growth-promoting bacteria (e.g., Bacillus) in a dose-dependent manner. Multiomic profilings unravel a cross-kingdom-signaling cascade that mediates efficient biosynthesis of SeNPs by selenobacteria. Specifically, maize roots perceive histamine signaling from Bacillus spp., which stimulates the plant to produce p-coumarate via root exudation. The rpoS gene in selenobacteria (e.g., Pseudomonas sp. ZY71) responds to p-coumarate signaling and positively regulates the biosynthesis of SeNPs. This study demonstrates a novel mechanism for recruiting host-beneficial soil microbes by microbially synthesized nanoparticles and unlocks promising possibilities for plant microbiome manipulation.

Metabolic engineering of Escherichia coli for seleno-methylselenocysteine production

Selenium (Se) is an essential trace element for life. Seleno-methylselenocysteine (SeMCys) can serve as a Se supplement with anticarcinogenic activity and can improve cognitive deficits. We engineered Escherichia coli for microbial production of SeMCys. The genes involved in the synthesis of SeMCys were divided into three modules-the selenocysteine (SeCys) synthesis, methyl donor synthesis and SMT modules-and expressed in plasmids with different copy numbers. The higher copy number of the SeCys synthesis module facilitated SeMCys production. The major routes for SeCys degradation were then modified. Deletion of the cysteine desulfurase gene csdA or sufS improved SeMCys production the most, and the strain that knocked out both genes doubled SeMCys production. The addition of serine in the mid-logarithmic growth phase significantly improved SeMCys synthesis. When the serine synthetic pathway was enhanced, SeMCys production increased by 12.5 %. Fed-batch culture for sodium selenite supplementation in the early stationary phase improved SeMCys production to 3.715 mg/L. This is the first report of the metabolic engineering of E. coli for the production of SeMCys and provide information on Se metabolism.

Amplified selenite toxicity in methanogenic archaea mediated by cysteine

The challenge of understanding the interaction between trace elements and microbial life is critical for assessing environmental and ecological impacts. Nevertheless, cysteine (Cys), a low molecular weight thiol substance prevalent in the ecosystem, is able to influence the fate of certain trace elements, which increases the complexity of the interaction between trace elements and microorganisms. Therefore, we chose Cys, selenite and the model methanogenic archaeon Methanosarcina acetivorans C2A as research targets, and comprehensively explored the intricate role of Cys in modulating the biological effects of selenite on M. acetivorans C2A in terms of population growth, methane production and oxidative stress. Our results demonstrate that Cys significantly exacerbates the inhibitory effects of selenite on growth and methane production in M. acetivorans C2A. This increased toxicity is linked to heightened membrane permeability and oxidative stress, with a marked upregulation in reactive oxygen species and changes in NADPH levels. Transcriptomic analysis reveals alterations in genes associated with transmembrane transport and methanogenesis. Intriguingly, we also observed a potential interaction between selenite and phosphate transmembrane transporters, suggesting a novel pathway for selenite entry into cells. These findings highlight the complex interplay between trace elements and microbial processes, with significant implications for understanding environmental risks and developing remediation strategies.

Phosphate-Induced Acidic Microenvironment for Improved Contaminant Removal during FeS Oxygenation

The coexistence of mackinawite (FeS) and phosphate is widely observed in natural systems. However, the underlying mechanism regarding how phosphate influences the environmental behavior of FeS, especially during the FeS oxygenation in aquatic systems, remains in its fancy. This study for the first time reported that the presence of phosphate, even at a low concentration of 0.3 mM, significantly promoted the FeS-mediated O2 activation and thus the pollutant degradation. The enhancement was attributed to a substantial increase in the generation of •OH, as evidenced by the electron paramagnetic resonance tests and the identification of the probing products. A combination of experiments and theoretical calculations revealed that phosphate adsorbed onto the FeS surface via a monodentate mononuclear configuration, establishing an acidic microenvironment on the FeS surface. Such acidic microenvironment not only increased the utilization efficiency of Fe(II) toward H2O2 generation (i.e., O 2 + 2 H + + 2 F e ( I I ) → H 2 O 2 + 2 F e ( I I I ) ), but also prevented the subsequent side reaction of H2O2 self-decomposition (i.e., H 2 O 2 + O H - → H O 2 - + H 2 O ). The results highlight the beneficial role of commonly encountered phosphate in FeS-based systems, which has profound implications for the degradation of waterborne contaminants.

The actinomycete Kitasatospora sp. SeTe27, subjected to adaptive laboratory evolution (ALE) in the presence of selenite, varies its cellular morphology, redox stability, and tolerance to the toxic oxyanion

The effects of oxyanions selenite (SeO32-) in soils are of high concern in ecotoxicology and microbiology as they can react with mineral particles and microorganisms. This study investigated the evolution of the actinomycete Kitasatospora sp. SeTe27 in response to selenite. To this aim, we used the Adaptive Laboratory Evolution (ALE) technique, an experimental approach that mimics natural evolution and enhances microbial fitness for specific growth conditions. The original strain (wild type; WT) isolated from uncontaminated soil gave us a unique model system as it has never encountered the oxidative damage generated by the prooxidant nature of selenite. The WT strain exhibited a good basal level of selenite tolerance, although its growth and oxyanion removal capacity were limited compared to other environmental isolates. Based on these premises, the WT and the ALE strains, the latter isolated at the end of the laboratory evolution procedure, were compared. While both bacterial strains had similar fatty acid profiles, only WT cells exhibited hyphae aggregation and extensively produced membrane-like vesicles when grown in the presence of selenite (challenged conditions). Conversely, ALE selenite-grown cells showed morphological adaptation responses similar to the WT strain under unchallenged conditions, demonstrating the ALE strain improved resilience against selenite toxicity. Whole-genome sequencing revealed specific missense mutations in genes associated with anion transport and primary and secondary metabolisms in the ALE variant. These results were interpreted to show that some energy-demanding processes are attenuated in the ALE strain, prioritizing selenite bioprocessing to guarantee cell survival in the presence of selenite. The present study indicates some crucial points for adapting Kitasatospora sp. SeTe27 to selenite oxidative stress to best deal with selenium pollution. Moreover, the importance of exploring non-conventional bacterial genera, like Kitasatospora, for biotechnological applications is emphasized.

Selenium volatilization in plants, microalgae, and microorganisms

The augmented prevalence of Se (Se) pollution can be attributed to various human activities, such as mining, coal combustion, oil extraction and refining, and agricultural irrigation. Although Se is vital for animals, humans, and microorganisms, excessive concentrations of this element can give rise to potential hazards. Consequently, numerous approaches have been devised to mitigate Se pollution, encompassing physicochemical techniques and bioremediation. The recognition of Se volatilization as a potential strategy for mitigating Se pollution in contaminated environments is underscored in this review. This study delves into the volatilization mechanisms in various organisms, including plants, microalgae, and microorganisms. By assessing the efficacy of Se removal and identifying the rate-limiting steps associated with volatilization, this paper provides insightful recommendations for Se mitigation. Constructed wetlands are a cost-effective and environmentally friendly alternative in the treatment of Se volatilization. The fate, behavior, bioavailability, and toxicity of Se within complex environmental systems are comprehensively reviewed. This knowledge forms the basis for developing management plans that aimed at mitigating Se contamination in wetlands and protecting the associated ecosystems.

Phosphate-solubilizing bacteria: Their agroecological function and optimistic application for enhancing agro-productivity

Phosphorus (P) is a limiting nutrient in the soil-plant nutrient cycling. Although the exogenous application of chemical P fertilizers can satisfy crop P requirements during critical growth phases. While excessive P fertilizers use results in low phosphorus acquisition efficiency (PAE), it has serious environmental consequences and hastens the depletion of P mineral reserves. Phosphate-solubilizing bacteria (PSB) have the potential to make insoluble phosphate available to plants through solubilization and mineralization, increasing crop yields while maintaining environmental sustainability. Existing reviews mainly focus on the beneficial effects of PSB on crop performance and related mechanisms, while few of them elucidate the action mechanisms of PSB in soil-microbe-plant interactions for crop cultivation with high yield efficiency. Hence, this study provides a comprehensive review of the physicochemical and molecular mechanisms (e.g., root exudates, extracellular polysaccharides, organic acids, phosphatases, and phosphate-specific transport systems) of PSB to facilitate the P cycle in the soil-plant systems. Further, the potential of commercial applications of PSB (e.g., genetic engineering, seed priming and coating) are also discussed in order to highlight their contribution to sustainable agriculture. Finally, existing challenges and future prospects in agricultural applications are proposed. In conclusion, we firmly believe that PSB represent a highly significant biotechnological tool for enhancing agricultural productivity and offers a wide range of extensive potential applications.

Anaerobic hydrolysis of recalcitrant tetramethylammonium from semiconductor wastewater: Performance and mechanisms

The treatment of tetramethylammonium hydroxide (TMAH)-bearing wastewater, generated in the electronic and semiconductor industries, raises significant concerns due to the neurotoxic, recalcitrant, and bio-inhibiting effects of TMAH. In this study, we proposed the use of an anaerobic hydrolysis bioreactor (AHBR) for TMAH removal, achieving a high removal efficiency of approximately 85%, which greatly surpassed the performance of widely-used advanced oxidation processes (AOPs). Density functional theory calculations indicated that the unexpectedly poor efficiency (5.8-8.0%) of selected AOPs can be attributed to the electrostatic repulsion between oxidants and the tightly bound electrons of TMAH. Metagenomic analyses of the AHBR revealed that Proteobacteria and Euryarchaeota played a dominant role in the transformation of TMAH through processes such as methyl transfer, methanogenesis, and acetyl-coenzyme A synthesis, utilizing methyl-tetrahydromethanopterin as a substrate. Moreover, several potential functional genes (e.g., mprF, basS, bcrB, sugE) related to TMAH resistance have been identified. Molecular docking studies between five selected proteins and tetramethylammonium further provided evidence supporting the roles of these potential functional genes. This study demonstrates the superiority of AHBR as a pretreatment technology compared to several widely-researched AOPs, paving the way for the proper design of treatment processes to abate TMAH in semiconductor wastewater.

Aggregation and stability of selenium nanoparticles: Complex roles of surface coating, electrolytes and natural organic matter

The application of selenium nanoparticles (SeNPs) as nanofertilizers may lead to the release of SeNPs into aquatic systems. However, the environmental behavior of SeNPs is rarely studied. In this study, using alginate-coated SeNPs (Alg-SeNPs) and polyvinyl alcohol-coated SeNPs (PVA-SeNPs) as models, we systematically investigated the aggregation and stability of SeNPs under various water conditions. PVA-SeNPs were highly stable in mono- and polyvalent electrolytes, probably due to the strong steric hindrance of the capping agent. Alg-SeNPs only suffered from a limited increase in size, even at 2500 mmol/L NaCl and 200 mmol/L MgCl₂, while they underwent apparent aggregation in CaCl₂ and LaCl₃ solutions. The binding of Ca²⁺ and La³⁺ with the guluronic acid part in alginate induced the formation of cross-linking aggregates. Natural organic matter enhanced the stability of Alg-SeNPs in monovalent electrolytes, while accelerated the attachment of Alg-SeNPs in polyvalent electrolytes, due to the cation bridge effects. The long-term stability of SeNPs in natural water showed that the aggregation sizes of Alg-SeNPs and PVA-SeNPs increased to several hundreds of nanometers or above 10 µm after 30 days, implying that SeNPs may be suspended in the water column or further settle down, depending on the surrounding water chemistry. The study may contribute to the deep insight into the fate and mobility of SeNPs in the aquatic environment. The varying fate of SeNPs in different natural waters also suggests that the risks of SeNPs to organisms living in diverse depths in the aquatic compartment should be concerned.

Uptake of Selenite by Rahnella aquatilis HX2 Involves the Aquaporin AqpZ and Na+/H+ Antiporter NhaA

Microbial transformation of selenite [Se(IV)] to elemental selenium nanoparticles (SeNPs) is known to be an important process for removing toxic soluble selenium (Se) oxyanions and recovery of Se from the environment as valuable nanoparticles. However, the mechanism of selenite uptake by microorganisms, the first step through which Se exerts its cellular function, remains not well studied. In this study, the effects of selenite concentration, time, pH, metabolic inhibitors, and anionic analogues on selenite uptake in Rahnella aquatilis HX2 were investigated. Selenite uptake by R. aquatilis HX2 was concentration- and time-dependent, and its transport activity was significantly dependent on pH. In addition, selenite uptake in R. aquatilis HX2 was significantly inhibited by the aquaporin inhibitor AgNO₃ and sulfite (SO₃^2-), and partially inhibited by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 2,4-dinitrophenol (2,4-DNP) treatments. Three mutants with in-frame deletions of aqpZ, glpF, and nhaA genes were constructed. The transport assay showed that the water channel protein AqpZ, and not GlpF, was a key channel of selenite uptake by R. aquatilis HX2, and sulfite and selenite had a common uptake pathway. In addition, the Na⁺/H⁺ antiporter NhaA is also involved in selenite uptake in R. aquatilis HX2.

AQDS Activates Extracellular Synergistic Biodetoxification of Copper and Selenite via Altering the Coordination Environment of Outer-Membrane Proteins

The biotransformation of heavy metals in the environment is usually affected by co-existing pollutants like selenium (Se), which may lower the ecotoxicity of heavy metals, but the underlying mechanisms remain unclear. Here, we shed light on the pathways of copper (Cu²⁺) and selenite (SeO₃^2-) synergistic biodetoxification by Shewanella oneidensis MR-1 and illustrate how such processes are affected by anthraquinone-2,6-disulfonate (AQDS), an analogue of humic substances. We observed the formation of copper selenide nanoparticles (Cu_2-xSe) from synergistic detoxification of Cu²⁺ and SeO₃^2- in the periplasm. Interestingly, adding AQDS triggered a fundamental transition from periplasmic to extracellular reaction, enabling 14.7-fold faster Cu²⁺ biodetoxification (via mediated electron transfer) and 11.4-fold faster SeO₃^2- detoxification (via direct electron transfer). This is mainly attributed to the slightly raised redox potential of the heme center of AQDS-coordinated outer-membrane proteins that accelerates electron efflux from the cells. Our work offers a fundamental understanding of the synergistic detoxification of heavy metals and Se in a complicated environmental matrix and unveils an unexpected role of AQDS beyond electron mediation, which may guide the development of more efficient environmental remediation and resource recovery biotechnologies.

Biomolecular Insights into Extracellular Pollutant Reduction Pathways of Geobacter sulfurreducens Using a Base Editor System

Geobacter species are critically involved in elemental biogeochemical cycling and environmental bioremediation processes via extracellular electron transfer (EET), but the underlying biomolecular mechanisms remain elusive due to lack of effective analytical tools to explore into complicated EET networks. Here, a simple and highly efficient cytosine base editor was developed for engineering of the slow-growing Geobacter sulfurreducens (a doubling time of 5 h with acetate as the electron donor and fumarate as the electron acceptor). A single-plasmid cytosine base editor (pYYDT-BE) was constructed in G. sulfurreducens by fusing cytosine deaminase, Cas9 nickase, and a uracil glycosylase inhibitor. This system enabled single-locus editing at 100% efficiency and showed obvious preference at the cytosines in a TC, AC, or CC context than in a GC context. Gene inactivation tests confirmed that it could effectively edit 87.7-93.4% genes of the entire genome in nine model Geobacter species. With the aid of this base editor to construct a series of G. sulfurreducens mutants, we unveiled important roles of both pili and outer membrane c-type cytochromes in long-range EET, thereby providing important evidence to clarify the long-term controversy surrounding their specific roles. Furthermore, we find that pili were also involved in the extracellular reduction of uranium and clarified the key roles of the ExtHIJKL conduit complex and outer membrane c-type cytochromes in the selenite reduction process. This work developed an effective base editor tool for the genetic modification of Geobacter species and provided new insights into the EET network, which lay a basis for a better understanding and engineering of these microbes to favor environmental applications.

Selenium Metabolism and Selenoproteins in Prokaryotes: A Bioinformatics Perspective

Selenium (Se) is an important trace element that mainly occurs in the form of selenocysteine in selected proteins. In prokaryotes, Se is also required for the synthesis of selenouridine and Se-containing cofactor. A large number of selenoprotein families have been identified in diverse prokaryotic organisms, most of which are thought to be involved in various redox reactions. In the last decade or two, computational prediction of selenoprotein genes and comparative genomics of Se metabolic pathways and selenoproteomes have arisen, providing new insights into the metabolism and function of Se and their evolutionary trends in bacteria and archaea. This review aims to offer an overview of recent advances in bioinformatics analysis of Se utilization in prokaryotes. We describe current computational strategies for the identification of selenoprotein genes and generate the most comprehensive list of prokaryotic selenoproteins reported to date. Furthermore, we highlight the latest research progress in comparative genomics and metagenomics of Se utilization in prokaryotes, which demonstrates the divergent and dynamic evolutionary patterns of different Se metabolic pathways, selenoprotein families, and selenoproteomes in sequenced organisms and environmental samples. Overall, bioinformatics analyses of Se utilization, function, and evolution may contribute to a systematic understanding of how this micronutrient is used in nature.

Tellurite and Selenite: how can these two oxyanions be chemically different yet so similar in the way they are transformed to their metal forms by bacteria?

This opinion review explores the microbiology of tellurite, TeO32- and selenite, SeO32- oxyanions, two similar Group 16 chalcogen elements, but with slightly different physicochemical properties that lead to intriguing biological differences. Selenium, Se, is a required trace element compared to tellurium, Te, which is not. Here, the challenges around understanding the uptake transport mechanisms of these anions, as reflected in the model organisms used by different groups, are described. This leads to a discussion around how these oxyanions are subsequently reduced to nanomaterials, which mechanistically, has controversies between ideas around the molecule chemistry, chemical reactions involving reduced glutathione and reactive oxygen species (ROS) production along with the bioenergetics at the membrane versus the cytoplasm. Of particular interest is the linkage of glutathione and thioredoxin chemistry from the cytoplasm through the membrane electron transport chain (ETC) system/quinones to the periplasm. Throughout the opinion review we identify open and unanswered questions about the microbial physiology under selenite and tellurite exposure. Thus, demonstrating how far we have come, yet the exciting research directions that are still possible. The review is written in a conversational manner from three long-term researchers in the field, through which to play homage to the late Professor Claudio Vásquez.

Microbial reduction and resistance to selenium: Mechanisms, applications and prospects

Selenium is an essential trace element for humans, animals and microorganisms. Microbial transformations, in particular, selenium dissimilatory reduction and bioremediation applications have received increasing attention in recent years. This review focuses on multiple Se-reducing pathways under anaerobic and aerobic conditions, and the phylogenetic clustering of selenium reducing enzymes that are involved in these processes. It is emphasized that a selenium reductase may have more than one metabolic function, meanwhile, there are several Se(VI) and/or Se(IV) reduction pathways in a bacterial strain. It is noted that Se(IV)-reducing efficiency is inconsistent with Se(IV) resistance in bacteria. Moreover, we discussed the links of selenium transformations to biogeochemical cycling of other elements, roles of Se-reducing bacteria in soil, plant and digestion system, and the possibility of using functional genes involved in Se transformation as biomarker in different environments. In addition, we point out the gaps and perspectives both on Se transformation mechanisms and applications in terms of bioremediation, Se fortification or dietary supplementation.

Selenium Nanoparticles as an Innovative Selenium Fertilizer Exert Less Disturbance to Soil Microorganisms

Selenium (Se) is an essential trace element in the human body. Se-enriched agricultural products, obtained by applying Se fertilizer, are important sources of Se supplement. However, Se fertilizer may cause a series of environmental problems. This study investigated the transformation of exogenous selenium nanoparticles (SeNPs) and selenite (SeO₃ ^2-) in soil and explored their effects on soil microbial community and typical microorganisms. SeNPs exhibited a slow-release effect in soil, which promoted the growth of soil microorganisms and enriched soil probiotics. SeO₃ ^2- was converted to a stable and low toxic state in soil, increasing persistent free radicals and decreasing microbial abundance and diversity. The influences of SeNPs and SeO₃ ^2- on two typical soil microorganisms (Bacillus sp. and Escherichia coli) were also evaluated, and SeNPs were more difficult to enter into microorganisms directly, with lower toxicity and higher safety. These results indicated that SeNPs were a more environment-friendly Se additive for agriculture applications. This work provides useful information for better understanding the environmental fate and behavior of Se fertilizer in the soil.

Selenite bioreduction and biosynthesis of selenium nanoparticles by Bacillus paramycoides SP3 isolated from coal mine overburden leachate

A native strain of Bacillus paramycoides isolated from the leachate of coal mine overburden rocks was investigated for its potential to produce selenium nanoparticles (SeNPs) by biogenic reduction of selenite, one of the most toxic forms of selenium. 16S rDNA sequencing was used to identify the bacterial strain (SP3). The SeNPs were characterized using spectroscopic (UV-Vis absorbance, dynamic light scattering, X-ray diffraction, and Raman), surface charge measurement (zeta potential), and ultramicroscopic (FESEM, EDX, FETEM) analyses. SP3 exhibited extremely high selenite tolerance (1000 mM) and reduced 10 mM selenite under 72 h to produce spherical monodisperse SeNPs with an average size of 149.1 ± 29 nm. FTIR analyses indicated exopolysaccharides coating the surface of SeNPs, which imparted a charge of -29.9 mV (zeta potential). The XRD and Raman spectra revealed the SeNPs to be amorphous. Furthermore, biochemical assays and microscopic studies suggest that selenite was reduced by membrane reductases. This study reports, for the first time, the reduction of selenite and biosynthesis of SeNPs by B. paramycoides, a recently discovered bacterium. The results suggest that B. paramycoides SP3 could be exploited for eco-friendly removal of selenite from contaminated sites with the concomitant biosynthesis of SeNPs.

Artificial-regulated synthesis of nanocrystals in live cells

Live cells, as reservoirs of biochemical reactions, can serve as amazing integrated chemical plants where precursor formation, nucleation and growth of nanocrystals, and functional assembly can be carried out accurately following an artificial program. It is crucial but challenging to deliberately direct intracellular pathways to synthesize desired nanocrystals that cannot be produced naturally in cells, because the relevant reactions exist in different spatiotemporal dimensions and will never encounter spontaneously. This article summarizes progress in the introduction of inorganic functional nanocrystals into live cells via the ‘artificial-regulated space–time-coupled live-cell synthesis’ strategy. We also describe ingenious bio-applications of the nanocrystal–cell systems, and quasi-biosynthesis strategies expanded from live-cell synthesis. Artificial-regulated live-cell synthesis—which involves the interdisciplinary application of biology, chemistry, nanoscience and medicine—will enable researchers to better exploit the unanticipated potentialities of live cells and open up new directions in synthetic biology.