Ecology and biotechnology of selenium-respiring bacteria

First review on the selenium status in Germany covering the last 50 years and on the selenium content of selected food items

INTRODUCTION

Selenium is important for human health. However, the selenium status and selenium intake of the German population has not been recorded in a representative study so far.

MATERIAL AND METHODS

Thus, literature from the last 50 years was screened in a systematic way and the results of various studies were pulled together to shed light on the selenium status of the German population. Moreover, the selenium content of selected food items that were either found on the German market or grown in Germany was researched and evaluated.

RESULTS

Of 3542 articles identified, 37 studies met the inclusion criteria. These 37 studies comprised a total of 8,010 healthy adults living in Germany with a weighted arithmetic mean of 82 μg/l selenium in plasma or serum. The results will form a basis for interpreting upcoming results from national food consumption surveys. Furthermore, 363 selenium values for 199 food items were identified out of 20 data sources-published or analysed between 2002 and 2019. An estimation of the selenium intake of the German population will be possible with this data in future nutrition surveys.

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.

Biogenic Selenium Nanoparticles and Their Anticancer Effects Pertaining to Probiotic Bacteria—A Review

Selenium nanoparticles (SeNPs) can be produced by biogenic, physical, and chemical processes. The physical and chemical processes have hazardous effects. However, biogenic synthesis (by microorganisms) is an eco-friendly and economical technique that is non-toxic to human and animal health. The mechanism for biogenic SeNPs from microorganisms is still not well understood. Over the past two decades, extensive research has been conducted on the nutritional and therapeutic applications of biogenic SeNPs. The research revealed that biogenic SeNPs are considered novel competitors in the pharmaceutical and food industries, as they have been shown to be virtually non-toxic when used in medical practice and as dietary supplements and release only trace amounts of Se ions when ingested. Various pathogenic and probiotic/nonpathogenic bacteria are used for the biogenic synthesis of SeNPs. However, in the case of biosynthesis by pathogenic bacteria, extraction and purification techniques are required for further useful applications of these biogenic SeNPs. This review focuses on the applications of SeNPs (derived from probiotic/nonpathogenic organisms) as promising anticancer agents. This review describes that SeNPs derived from probiotic/nonpathogenic organisms are considered safe for human consumption. These biogenic SeNPs reduce oxidative stress in the human body and have also been shown to be effective against breast, prostate, lung, liver, and colon cancers. This review provides helpful information on the safe use of biogenic SeNPs and their economic importance for dietary and therapeutic purposes, especially as anticancer agents.

Selenium distribution and speciation in waters of pristine alpine lakes from central-western Pyrenees (France-Spain)

The speciation of both redox reactive and volatile selenium (Se) compounds, barely reported in pristine aquatic environments, has never been investigated in remote alpine lakes, considered as sensitive ecosystems to detect the effect of global change. This work presents an integrated investigation on Se distribution and speciation conducted in 20 high altitude pristine lakes from the central-western Pyrenees. Five seasonal sampling campaigns were carried out after snowmelt (June/July) and in early fall (October) for the period 2017-2019. Concentrations of total dissolved Se (TDSe) ranged from 7 to 78 ng L^-1, with selenate being ubiquitously observed in most cases (median of 61% of TDSe). Selenite was only occasionally detected up to 4 ng L^-1, therefore a fraction of TDSe was presumably in the forms of elemental Se(0) and/or selenides. Depth profiles obtained in different lakes showed the occurrence of such Se(-II, 0) pools in bottom hypoxic to anoxic waters. The production of volatile Se compounds presented a low median total concentration (TVSe) of 33 pg L^-1 (range 3-120 pg L^-1), mainly in the form of dimethylselenide in subsurface samples (median of 82% of TVSe). The Se concentration in lake waters was significantly correlated with the sulphate concentration (ρ = 0.93, p < 0.0001), demonstrating that it is influenced by erosion and dissolution of Se and S-enriched parent bedrocks. In addition, for Se depleted alpine lake-bedrock systems, long-range transport and wet atmospheric depositions represent a major source of Se for lake waters.

Thyroidal and Extrathyroidal Requirements for Iodine and Selenium: A Combined Evolutionary and (Patho)Physiological Approach

Iodide is an antioxidant, oxidant and thyroid hormone constituent. Selenoproteins are needed for triiodothyronine synthesis, its deactivation and iodine release. They also protect thyroidal and extrathyroidal tissues from hydrogen peroxide used in the 'peroxidase partner system'. This system produces thyroid hormone and reactive iodine in exocrine glands to kill microbes. Exocrine glands recycle iodine and with high urinary clearance require constant dietary supply, unlike the thyroid. Disbalanced iodine-selenium explains relations between thyroid autoimmune disease (TAD) and cancer of thyroid and exocrine organs, notably stomach, breast, and prostate. Seafood is iodine unconstrained, but selenium constrained. Terrestrial food contains little iodine while selenium ranges from highly deficient to highly toxic. Iodine vs. TAD is U-shaped, but only low selenium relates to TAD. Oxidative stress from low selenium, and infection from disbalanced iodine-selenium, may generate cancer of thyroid and exocrine glands. Traditional Japanese diet resembles our ancient seashore-based diet and relates to aforementioned diseases. Adequate iodine might be in the milligram range but is toxic at low selenium. Optimal selenoprotein-P at 105 µg selenium/day agrees with Japanese intakes. Selenium upper limit may remain at 300-400 µg/day. Seafood combines iodine, selenium and other critical nutrients. It brings us back to the seashore diet that made us what we currently still are.

Succession of function, assembly, and interaction of microbial community in sequencing biofilm batch reactors under selenite stress

The mechanism of interaction between selenite, a toxic substance, and the microbial community in wastewater is still not well understood. Herein, nine sequencing biofilm batch reactors were used to systematically investigate the response of the microbial community to the continuous selenite stress. The results showed that selenite affected the reactor performance and reduced the biofilm mass. Also, it increased the proportion of the living cells, and changed the protein and polysaccharide composition of the biofilm as well as cellular secretions. Selenite facilitated the removal of NO₃-N, according to water-quality and bioinformatics analyses. As such, the selenite was converted into selenium nanoparticles. α-diversity analysis further revealed that 20 μM selenite enhanced the microbial community resilience, while 200 μM selenite had the reverse effect. Community composition analysis showed that Variovorax, Rhizobium, and Simkania had positive correlations with selenite (P < 0.05). Functional prediction suggested that selenite changed the C, N, and S cycle functions. Furthermore, determinism dominated the community assembly process, and the deterministic proportion increased with the increase of selenite concentration. Network analysis showed that selenite improved the stability and positive correlation ratio of the overall microbial network, and accelerated the communication between microorganisms. However, when compared with the 20 μM selenite, the 200 μM selenite boosted the competition and parasitism/predation among microorganisms. Low-abundance genera played a key role in the network of selenite-reducing microbial community. In addition, under selenite stress, biofilm network exhibited better stability and faster information exchange than suspended network, and the positive association between biofilm and suspended microorganisms increased. All in all, this research sheds light on the interaction between selenite and microbial community, as well as provides crucial information on selenium-containing wastewater.

Biogenic synthesis of selenium and tellurium nanoparticles by marine bacteria and their biological activity

Selenium (SeNPs) and tellurium nanoparticles (TeNPs) were synthesized by green technology using the three new bacterial marine isolates (strains PL 2476, AF 2469 and G 2451). Isolates were classified as Pseudoalteromonas shioyasakiensis according to 16S rRNA sequence analysis, morphological characteristics, and biochemical reactions. The bioreduction processes of isolates were studied in comparison with the previously described Alteromonas macleodii (strain 2328). All strains exhibited significant tolerance to selenite and tellurite up to 1000 µg/mL. A comparative analysis of the bioreduction processes of the isolates demonstrated that the strains have a high rate of reduction processes. Characterization of biogenic red SeNPs and black TeNPs using scanning electron microscopy (SEM), EDX analysis, Dynamic Light Scattering, and micro-Raman Spectroscopy revealed that all the isolates form stable spherical selenium and tellurium nanoparticles whose size as well as elemental composition depend on the producer strain. Nanoparticles of the smallest size (up to 100 nm) were observed only for strain PL 2476. Biogenic SeNPs and TeNPs were also characterized and tested for their antimicrobial, antifouling and cytotoxic activities. Significant antimicrobial activity was shown for nanoparticles at relatively high concentrations (500 and 1000 µg/mL), with the antimicrobial activity of TeNPs being more significant than SeNPs. In contrast, against cell cultures (breast cancer cells (SkBr3) and human dermal fibroblasts (HDF) SeNPs showed greater toxicity than tellurium nanoparticles. Studies have demonstrated the high antifouling effectiveness of selenium and tellurium nanoparticles when introduced into self-polishing coatings. According to the results obtained, the use of SeNPs and TeNPs as antifouling additives can reduce the concentration of leachable biocides used in coatings, reducing the pressure on the environment.

Retention of immobile Se(0) in flow-through aquifer column systems during bioreduction and oxic-remobilization

Selenium (Se) is a toxic contaminant with multiple anthropogenic sources, including ⁷⁹Se from nuclear fission. Se mobility in the geosphere is generally governed by its oxidation state, therefore understanding Se speciation under variable redox conditions is important for the safe management of Se contaminated sites. Here, we investigate Se behavior in sediment groundwater column systems. Experiments were conducted with environmentally relevant Se concentrations, using a range of groundwater compositions, and the impact of electron-donor (i.e., biostimulation) and groundwater sulfate addition was examined over a period of 170 days. X-Ray Absorption Spectroscopy and standard geochemical techniques were used to track changes in sediment associated Se concentration and speciation. Electron-donor amended systems with and without added sulfate retained up to 90% of added Se(VI)_(aq), with sediment associated Se speciation dominated by trigonal Se(0) and possibly trace Se(-II); no Se colloid formation was observed. The remobilization potential of the sediment associated Se species was then tested in reoxidation and seawater intrusion perturbation experiments. In all treatments, sediment associated Se (i.e., trigonal Se(0)) was largely resistant to remobilization over the timescale of the experiments (170 days). However, in the perturbation experiments, less Se was remobilized from sulfidic sediments, suggesting that previous sulfate-reducing conditions may buffer Se against remobilization and migration.

Bioremediation of selenium-contaminated soil using earthworm Eisenia fetida: Effects of gut bacteria in feces on the soil microbiome

Selenium (Se) contamination in the soil poses a food safety risk to humans. The present study was to investigate the role of earthworm Eisenia fetida in soil Se remediation. When exposed to selenite at 4 mg Se/kg, E. fetida efficiently concentrated Se in tissues (24.53 mg Se/kg dry weight), however, only accounting for a minor portion of the added Se. Microbial analysis shows 12 out of 15 functional genera became more abundant in the worm-inhabited soil when exposed to Se, suggesting E. fetida contributed to Se remediation mainly by introducing Se-reducing bacteria to the soil via feces, which were dominated by the genera Pseudomonas (∼62.65%) and Aeromonas (∼29.99%), whose abundance was also significantly boosted in the worm-inhabited soil. However, when isolated from worm feces at 200 mg Se/L, Pseudomonas strains only displayed a high tolerance to Se rather than removal capacity. In contrast, among 4 isolated Aeromonas strains, A. caviae rapidly removing 85.74% of the added selenite, mainly through accumulation (67.38%), while A. hydrophila and A. veronii were more effective at volatilizing Se (27.77% and 24.54%, respectively), and A. media performed best by reducing Se by ∼49.00% under anaerobic conditions. Overall, our findings have highlighted the importance of E. fetida as a key contributor of functional bacteria to the soil microbiome, building a strong foundation for the development of an earthworm-soil system for Se bioremediation.

Selenium nanoparticle rapidly synthesized by a novel highly selenite-tolerant strain Proteus penneri LAB-1

Microorganisms with high selenite-tolerant and efficient reduction ability of selenite have seldom been reported. In this study, a highly selenite-resistant strain (up to 500 mM), isolated from lateritic red soil, was identified as Proteus penneri LAB-1. Remarkably, isolate LAB-1 reduced nearly 2 mM of selenite within 18 h with the production of selenium nanoparticles (SeNPs) at the beginning of the exponential phase. Moreover, in vitro selenite reduction activities of strain LAB-1 were detected in the membrane protein fraction with or without NADPH/NADH as electron donors. Strain LAB-1 transported selenite to the membrane via nitrate transport protein. The selenite was reduced to SeNPs through the glutathione pathway and the catalysis of nitrate reductase, and the glutathione pathway played the decisive role. P. penneri LAB-1 could be a potential candidate for the selenite bioremediation and SeNPs synthesis.

•

A novel highly selenite-tolerant (up to 500mM) strain Proteus penneri LAB-1 was isolated

•

More than 93% of 2mM SeO₃²⁻ was reduced to Se⁰ by LAB-1 in 18 h

•

LAB-1 transports SeO₃²⁻ to its membrane by the nitrate transport protein

•

SeO₃²⁻ reduction takes place via glutathione pathway and catalysis of NR

Novel mechanisms of selenite reduction in Bacillus subtilis 168：Confirmation of multiple-pathway mediated remediation based on transcriptome analysis

Selenite biotransformation by microorganisms is an effective detoxification and assimilation process. Bacillus subtilis is a probiotic bacterium that can reduce Se(IV) to SeNPs under aerobic conditions. However, current knowledge on the molecular mechanisms of selenite reduction by B. subtilis remains limited. Here, the reduction of Se(IV) by probiotic bacterium Bacillus subtilis 168 was systematically analysed, and the molecular mechanisms of selenium nanoparticle (SeNPs) formation were characterised in detail. B. subtilis 168 reduced 5.0 mM selenite by nearly 40% in 24 h, and the produced SeNPs were spherical and localised intracellularly or extracellularly. FTIR (Fourier transform infrared) spectroscopy suggested the presence of proteins, lipids, and carbohydrates on the surface of the isolated SeNPs. Transcriptome data analysis revealed that the expression of genes associated with the proline metabolism, glutamate metabolism, and sulfite metabolism pathways was significantly up-regulated. Gene mutation and complementation revealed the ability of PutC, GabD, and CysJI to reduce selenite in vivo. In vitro experiments demonstrated that PutC, GabD, and CysJI could catalyse the reduction of Se(IV) under optimal conditions using NADPH as a cofactor. To the best of our knowledge, our study is the first to demonstrate the involvement of PutC and GabD in selenite reduction. Particularly, our findings demonstrated that the reduction of Se(IV) was mediated by multiple pathways both in vivo and in vitro. Our findings thus provide novel insights into the molecular mechanisms of Se(VI) reduction in aerobic bacteria.

Biotransformation of selenium in the mycelium of the fungus Phycomyces blakesleeanus

Biotransformation of toxic selenium ions to non-toxic species has been mainly focused on biofortification of microorganisms and production of selenium nanoparticles (SeNPs), while far less attention is paid to the mechanisms of transformation. In this study, we applied a combination of analytical techniques with the aim of characterizing the SeNPs themselves as well as monitoring the course of selenium transformation in the mycelium of the fungus Phycomyces blakesleeanus. Red coloration and pungent odor that appeared after only a few hours of incubation with 10 mM Se⁺⁴ indicate the formation of SeNPs and volatile methylated selenium compounds. SEM-EDS confirmed pure selenium NPs with an average diameter of 57 nm, which indicates potentially very good medical, optical, and photoelectric characteristics. XANES of mycelium revealed concentration-dependent mechanisms of reduction, where 0.5 mM Se⁺⁴ led to the predominant formation of Se-S-containing organic molecules, while 10 mM Se⁺⁴ induced production of biomethylated selenide (Se^-2) in the form of volatile dimethylselenide (DMSe) and selenium nanoparticles (SeNPs), with the SeNPs/DMSe ratio rising with incubation time. Several structural forms of elemental selenium, predominantly monoclinic Se₈ chains, together with trigonal Se polymer chain, Se₈ and Se₆ ring structures, were detected by Raman spectroscopy.

Label-free quantitative proteomic analysis of the mechanism of salt stress promoting selenium enrichment in Lactobacillus rhamnosus

Lactobacillus rhamnosus can metabolize selenite into organic selenium and Se⁰. In this paper, label-free quantitative proteomics was applied to explore the mechanism of salt stress promoting selenium enrichment of L.rhamnosus. 397 proteins were up-regulated and 147 proteins were down-regulated of selenium-enriched L.rhamnosus under salt stress. The differentially expressed proteins (DEPs) were mainly involved in metabolism, membrane transport and genetic information processing. The results of quantitative real-time PCR showed that gene opuA, metN, trxR and ldh of Se-enriched L.rhamnosus with salt stress were significantly up-regulated. However, the expression levels of gene luxS, groEL, dnaK and pgk were down-regulated. It was indicated that L.rhamnosus promoted part of amino acids combining with selenium into selenoamino acids with salt stress. Secondly, sodium chloride stimulated the expression of key enzymes involved in metabolism to provide energy for the process of Se-enrichment. In addition, NaCl induced the expression of enzymes and genes involved in the synthesis of selenoproteins. SIGNIFICANCE: It was indicated that L.rhamnosus promoted part of amino acids combining with selenium into selenoamino acids with salt stress. Secondly, sodium chloride stimulated the expression of key enzymes involved in metabolism to provide energy for the process of Se-enrichment. In addition, NaCl induced the expression of enzymes and genes involved in the synthesis of selenoproteins.

Changes in Alpine Soil Bacterial Communities With Altitude and Slopes at Mount Shergyla, Tibetan Plateau: Diversity, Structure, and Influencing Factors

The alpine ecosystem as one of the most representative terrestrial ecosystems has been highly concerned due to its susceptibility to anthropogenic impacts and climatic changes. However, the distribution pattern of alpine soil bacterial communities and related deterministic factors still remain to be explored. In this study, soils were collected from different altitudes and slope aspects of the Mount (Mt.) Shergyla, Tibetan Plateau, and were analyzed using 16S rRNA gene-based bioinformatics approaches. Acidobacteriota and Proteobacteria were identified consistently as the two predominant phyla in all soil samples, accounting for approximately 74% of the bacterial community. The alpha diversity of the soil bacterial community generally increased as the vegetation changed with the elevated altitude, but no significant differences in alpha diversity were observed between the two slopes. Beta diversity analysis of bacterial community showed that soil samples from the north slope were always differentiated obviously from the paired samples at the south slope with the same altitude. The whole network constituted by soil bacterial genera at the Mt. Shergyla was parsed into eight modules, and Elev-16S-573, Sericytochromatia, KD4-96, TK10, Pedomicrobium, and IMCC26256 genera were identified as the “hubs” in the largest module. The distance-based redundancy analysis (db-RDA) demonstrated that variations in soil bacterial community thereof with the altitude and slope aspects at the Mt. Shergyla were closely associated with environmental variables such as soil pH, soil water content, metal concentrations, etc. Our results suggest that environmental variables could serve as the deterministic factors for shaping the spatial pattern of soil bacterial community in the alpine ecosystems.

Natural selenium stress influences the changes of antibiotic resistome in seleniferous forest soils

BACKGROUND

Metal(loid)s can promote the spread and enrichment of antibiotic resistance genes (ARGs) in the environment through a co-selection effect. However, it remains unclear whether exposure of microorganisms to varying concentrations of selenium (Se), an essential but potentially deleterious metal(loid) to living organisms, can influence the migration and distribution of ARGs in forest soils.

RESULTS

Precisely 235 ARGs conferring resistance to seven classes of antibiotics were detected along a Se gradient (0.06-20.65 mg kg^-1) across 24 forest soils. (flor)/(chlor)/(am)phenicol resistance genes were the most abundant in all samples. The total abundance of ARGs first increased and then decreased with an elevated available Se content threshold of 0.034 mg kg^-1 (P = 2E-05). A structural equation model revealed that the dominant mechanism through which Se indirectly influences the vertical migration of ARGs is by regulating the abundance of the bacterial community. In addition, the methylation of Se (mediated by tehB) and the repairing of DNA damages (mediated by ruvB and recG) were the dominant mechanisms involved in Se resistance in the forest soils. The co-occurrence network analysis revealed a significant correlated cluster between Se-resistance genes, MGEs and ARGs, suggesting the co-transfer potential. Lelliottia amnigena YTB01 isolated from the soil was able to tolerate 50 μg mL^-1 ampicillin and 1000 mg kg^-1 sodium selenite, and harbored both Se resistant genes and ARGs in the genome.

CONCLUSIONS

Our study demonstrated that the spread and enrichment of ARGs are enhanced under moderate Se pressure but inhibited under severe Se pressure in the forest soil (threshold at 0.034 mg kg^-1 available Se content). The data generated in this pilot study points to the potential health risk associated with Se contamination and its associated influence on ARGs distribution in soil.

A Bibliometric Analysis of Research on Selenium in Drinking Water during the 1990-2021 Period: Treatment Options for Selenium Removal

A bibliometric analysis based on the Scopus database was carried out to summarize the global research related to selenium in drinking water from 1990 to 2021 and identify the quantitative characteristics of the research in this period. The results from the analysis revealed that the number of accumulated publications followed a quadratic growth, which confirmed the relevance this research topic is gaining during the last years. High research efforts have been invested to define safe selenium content in drinking water, since the insufficient or excessive intake of selenium and the corresponding effects on human health are only separated by a narrow margin. Some important research features of the four main technologies most frequently used to remove selenium from drinking water (coagulation, flocculation and precipitation followed by filtration; adsorption and ion exchange; membrane-based processes and biological treatments) were compiled in this work. Although the search of technological options to remove selenium from drinking water is less intensive than the search of solutions to reduce and eliminate the presence of other pollutants, adsorption was the alternative that has received the most attention according to the research trends during the studied period, followed by membrane technologies, while biological methods require further research efforts to promote their implementation.

Improving selenium accumulation in broilers using Escherichia coli Nissle 1917 with surface-displayed selenite reductase SerV01

Selenium levels have a critical impact on livestock and poultry, and selenium nanoparticles (SeNPs) have shown significant efficiency in supplementation. This study identified a high-efficiency selenite reductase, SerV01, in Staphylococcus aureus LZ-01, which can convert Se₂O₃^2- to SeNPs. Subsequently, SerV01 was introduced into the intestines of the broilers using the surface display-engineered E. coli Nissle 1917 (EcN). The results showed that the engineered bacteria (EcN-IS) significantly increased the selenium content by 0.87 mg kg^-1, 0.52 mg kg^-1, and 6.10 mg L^-1 in the liver, breast muscle, and serum, respectively. With SeNPs + EcN-IS treatment, glutathione peroxidase and thioredoxin reductase levels reached 0.7536 ± 0.03176 U μL^-1 protein and 2.463 ± 0.1685 U μL^-1 protein, respectively. With the modified probiotics, the proportion of beneficial intestinal flora increased, with Lactobacillus and Propionibacterium accounting for 75.85% and 0.19%. This technology provides a novel idea to facilitate the exploitation of selenium in broiler diets and improve antioxidant capability.

Treatment technologies for selenium contaminated water: A critical review

Selenium is an indispensable trace element for humans and other organisms; however, excessive selenium in water can jeopardize the aquatic environment. Investigations on the biogeochemical cycle of selenium have shown that anthropogenic activities such as mining, refinery, and coal combustion mainly contribute to aquatic selenium pollution, imposing tremendous risks on ecosystems and human beings. Various technologies thus have been developed recently to treat selenium contaminated water to reduce its environmental impacts. This work provides a critical review on the applications, characteristics, and latest developments of current treatment technologies for selenium polluted water. It first outlines the present status of the characteristics, sources, and toxicity of selenium in water. Selenium treatment technologies are then classified into three categories: 1) physicochemical separation including membrane filtration, adsorption, coagulation/precipitation, 2) redox decontamination including chemical reduction and catalysis, and 3) biological transformation including microbial treatment and constructed wetland. Details of these methods including their overall efficiencies, applicability, advantages and drawbacks, and latest developments are systematically analyzed and compared. Although all these methods are promising in treating selenium in water, further studies are still needed to develop sustainable strategies based on existing and new technologies. Perspectives on future research directions are laid out at the end.

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.

Selenium (Se) recovery for technological applications from environmental matrices based on biotic and abiotic mechanisms

Selenium (Se) is an essential element with application in manufacturing from food to medical industries. Water contamination by Se is of concern due to anthropogenic activities. Recently, Se remediation has received increasing attention. Hence, different types of remediation techniques are listed in this work, and their potential for Se recovery is evaluated. Sorption, co-precipitation, coagulation and precipitation are effective for low-cost Se removal. In photocatalytic, zero-valent iron and electrochemical systems, the above mechanisms occur with reduction as an immobilization and detoxification process. In combination with magnetic separation, the above techniques are promising for Se recovery. Biological Se oxyanions reduction has been widely recognized as a cost-effective method for Se remediation, simultaneously generating biosynthetic Se nanoparticles (BioSeNPs). Increasing the extracellular production of BioSeNPs and controlling their morphology will benefit its recovery. However, the mechanism of the microbial production of BioSeNPs is not well understood. Se containing products from both microbial reduction and abiotic methods need to be refined to obtain pure Se. Eco-friendly and cost-effective Se refinery methods need to be developed. Overall, this review offers insight into the necessity of shifting attention from Se remediation to Se recovery.

Anaerobic RSH-dependent tellurite reduction contributes to Escherichia coli tolerance against tellurite

BACKGROUND

Tellurium is a rare metalloid that exerts high toxicity on cells, especially on bacteria, partly due to reactive oxygen species (ROS) generation. Moreover, it has also been observed that tellurite can target free cell thiols groups (RSH) (i.e. reduced glutathione (GSH)), enhancing the cellular redox imbalance. Additionally, in vitro experiments have suggested that several enzymes can reduce tellurite (IV) to its elemental form (0); where RSH present on their active sites may be responsible for the process. Nevertheless, the mechanisms implemented by bacteria for tellurite reduction and its role in resistance have not been evaluated in vivo.

RESULTS

This work shows that tellurite reduction to elemental tellurium is increased under anaerobic conditions in E. coli cells. The in vivo tellurite reduction is related to the intracellular concentration of total RSH, in the presence and absence of oxygen. This metabolization of tellurite directly contributes to the resistance of the bacteria to the oxyanion.

CONCLUSIONS

We demonstrated that in vivo tellurite reduction is related to the intracellular thiol concentration, i.e. large availability of cellular RSH groups, results in a more significant reduction of tellurite. Furthermore, we observed that, when the bacterium exhibits less resistance to the oxyanion, a decreased tellurite reduction was seen, affecting the growth fitness. Together, these results let us propose that tellurite reduction and the intracellular RSH content are related to the oxyanion bacterial resistance, this tripartite mechanism in an oxygen-independent anaerobic process.

Selenite Reduction by Proteus sp. YS02: New Insights Revealed by Comparative Transcriptomics and Antibacterial Effectiveness of the Biogenic Se0 Nanoparticles

Biotransformation of selenite by microorganisms is an effective detoxification (in cases of dissimilatory reduction, e.g., to Se⁰) and assimilation process (when Se is assimilated by cells). However, the current knowledge of the molecular mechanism of selenite reduction remains limited. In this study, a selenite-resistant bacterium was isolated and identified as Proteus sp. YS02. Strain YS02 reduced 93.2% of 5.0 mM selenite to selenium nanoparticles (SeNPs) within 24 h, and the produced SeNPs were spherical and localized intracellularly or extracellularly, with an average dimension of 140 ± 43 nm. The morphology and composition of the isolated and purified SeNPs were characterized using dynamic light scattering (DLS), scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) spectrometry, and Fourier transform infrared (FTIR) spectroscopy. FTIR spectroscopy indicated the presence of proteins, polysaccharides, and lipids on the surface of the isolated SeNPs. Furthermore, the SeNPs showed excellent antimicrobial activity against several Gram-positive and Gram-negative pathogenic bacteria. Comparative transcriptome analysis was performed to elucidate the selenite reduction mechanism and biosynthesis of SeNPs. It is revealed that 197 genes were significantly upregulated, and 276 genes were significantly downregulated under selenite treatment. Gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that genes associated with ABC transporters, sulfur metabolism, pentose phosphate pathway (PPP), and pyruvate dehydrogenase were significantly enhanced, indicating selenite is reduced by sulfite reductase with PPP and pyruvate dehydrogenase supplying reducing equivalents and energy. This work suggests numerous genes are involved in the response to selenite stress, providing new insights into the molecular mechanisms of selenite bioreduction with the formation of SeNPs.

A New Strategy for Microbial Taxonomic Identification through Micro-Biosynthetic Gold Nanoparticles and Machine Learning

Microorganisms can serve as biological factories for the synthesis of inorganic nanomaterials that can become useful as nanocatalysts, energy-harvesting-storage components, antibacterial agents, and biomedical materials. Herein, the development of biosynthesis of inorganic nanomaterials into a simple, stable, and accurate strategy for distinguishing microorganisms from multiple classification levels (i.e., kingdom, order, genus, and species) without gene amplification, biochemical testing, or target recognition is reported. Gold nanoparticles (AuNPs) biosynthesized by different microorganisms differ in color of the solution, and their features can be characterized, including the particle size, the surface plasmon resonance (SPR) spectrum, and the surface potential. The inter-relation between the features of micro-biosynthetic AuNPs and the classification of microorganisms are exploited at different levels through machine learning to establish a taxonomic model. This model agrees well with traditional classification methods that offers a new strategy for microbial taxonomic identification. The underlying mechanism of this strategy is related to the biomolecules produced by different microorganisms including glucose, glutathione, and nicotinamide adenine dinucleotide phosphate-dependent reductase that regulate the features of micro-biosynthetic AuNPs. This work broadens the application of biosynthesis of inorganic materials through micro-biosynthetic AuNPs and machine learning, which holds great promise as a tool for biomedical research.

Electrified bioreactors: the next power-up for biometallurgical wastewater treatment

Over the past decades, biological treatment of metallurgical wastewaters has become commonplace. Passive systems require intensive land use due to their slow treatment rates, do not recover embedded resources and are poorly controllable. Active systems however require the addition of chemicals, increasing operational costs and possibly negatively affecting safety and the environment. Electrification of biological systems can reduce the use of chemicals, operational costs, surface footprint and environmental impact when compared to passive and active technologies whilst increasing the recovery of resources and the extraction of products. Electrification of low rate applications has resulted in the development of bioelectrochemical systems (BES), but electrification of high rate systems has been lagging behind due to the limited mass transfer, electron transfer and biomass density in BES. We postulate that for high rate applications, the electrification of bioreactors, for example, through the use of electrolyzers, may herald a new generation of electrified biological systems (EBS). In this review, we evaluate the latest trends in the field of biometallurgical and microbial-electrochemical wastewater treatment and discuss the advantages and challenges of these existing treatment technologies. We advocate for future research to focus on the development of electrified bioreactors, exploring the boundaries and limitations of these systems, and their validity upon treating industrial wastewaters.

Sequential removal of selenate, nitrate and sulfate and recovery of elemental selenium in a multi-stage bioreactor process with redox potential feedback control

Bioreduction can facilitate oxyanions removal from wastewater. However, simultaneously removing selenate, nitrate and sulfate and recovering high-purity elemental selenium (Se⁰) from wastewater by a single system is difficult and may lead to carcinogenic selenium monosulfide (SeS) formation. To solve this issue, a two-stage biological fluidized bed (FBR) process with ethanol dosing based on oxidation-reduction potential (ORP) feedback control was developed in this study. FBR1 performance was first evaluated at various ORP setpoints (between -520 and -360 mV vs. Ag/AgCl) and elevated sulfate concentration. Subsequently, ethanol-fed FBR2 was used to reduce sulfate from FBR1 effluent, followed by an aerated sulfide oxidation reactor (SOR). At - 520 mV≤ ORPs≤ -480 mV, FBR1 removed 100 ± 0.1% nitrate and 99.7 ± 0.3% selenate without sulfate reduction. At ORPs ≥ -440 mV, selenate reduction was incomplete, whereas nitrate removal remained stable. Se⁰ recovery efficiency from FBR1 effluent was 37.5% with 71% Se purity. FBR2 converted 86% of the remaining sulfate in FBR1 effluent to hydrogen sulfide, but the over-oxidation of dissolved sulfide in SOR decreased the overall sulfate removal efficiency to ~46.3%. Overall, the two-stage FBR process with ORP feedback dosing of ethanol was effective for sequentially removing selenate, nitrate and sulfate and recovering Se⁰ from wastewater.

An economical strategy towards the managing of selenium pollution from contaminated water: A current state-of-the-art review

During the last few decades, contamination of selenium (Se) in groundwater has turned out to be a major environmental concern to provide safe drinking water. The content of selenium in such contaminated water might range from 400 to 700 μg/L, where bringing it down to a safe level of 40 μg/L for municipal water supply employing appropriate methodologies is a major challenge for the global researcher communities. The current review focuses mostly on the governing selenium remediation technologies such as coagulation-flocculation, electrocoagulation, bioremediation, membrane-based approaches, adsorption, electro-kinetics, chemical precipitation, and reduction methods. This study emphasizes on the development of a variety of low-cost adsorbents and metal oxides for the selenium decontamination from groundwater as a cutting-edge technology development along with their applicability, and environmental concerns. Moreover, after the removal, the recovery methodologies using appropriate materials are analyzed which is the need of the hour for the reutilization of selenium in different processing industries for the generation of high valued products. From the literature survey, it has been found that hematite modified magnetic nanoparticles (MNP) efficiently adsorb Se (IV) (25.0 mg/g) from contaminated groundwater. MNP@hematite reduced Se (IV) concentration from 100 g/L to 10 g/L in 10 min at pH 4-9 using a dosage of 1 g/L. In 15 min, the magnetic adsorbent can be recycled and regenerated using a 10 mM NaOH solution. The adsorption and desorption efficiencies were over 97% and 82% for five consecutive cycles, respectively. To encourage the notion towards scale-up, a techno-economic evaluation with possible environmentally sensitive policy analysis has been introduced in this article to introspect the aspects of sustainability. This type of assessment is anticipated to be extremely encouraging to convey crucial recommendations to the scientific communities in order to produce high efficiency selenium elimination and further recovery from contaminated groundwater.

Selenium Valence-to-Core X-ray Emission Spectroscopy and Kβ HERFD X-ray Absorption Spectroscopy as Complementary Probes of Chemical and Electronic Structure

Selenium X-ray absorption spectroscopy (XAS) has found widespread use in investigations of Se-containing materials, geochemical processes, and biologically active sites. In contrast to sulfur Kβ X-ray emission spectroscopy (XES), which has been found to contain electronic and structural information complementary to S XAS, Se Kβ XES remains comparatively underexplored. Herein, we present the first Se Valence-to-Core (VtC) XES studies of reduced Se-containing compounds and FeSe dimers. Se VtC XES is found to be sensitive to changes in covalent Se bonding interactions (Se–Se/Se–C/Se–H bonding) while being relatively insensitive to changes in Fe oxidation states as selenide bridges in FeSe dimers ([Fe₂Se₂]²⁺ vs [Fe₂Se₂]⁺). In contrast, Se Kβ HERFD XAS is demonstrated to be quite sensitive to changes in the Fe oxidation state with Se Kβ HERFD XAS demonstrating experimental resolution equivalent to Kα HERFD XAS. Additionally, computational studies reveal both Se VtC XES and XAS to be sensitive to selenium protonation in FeSe complexes.

Selenium is a trace element that plays vital roles in biological and geochemical cycles, energy storage, photovoltaics, and nanomaterials. Herein, selenium Valence-to-Core X-ray emission spectroscopy is explored as a new method of probing the chemical and electronic structure in selenium-containing compounds, demonstrating sensitivity to selenium bonding interactions. When paired with high-resolution Se X-ray absorption spectroscopy (HERFD XAS), these two methods have the potential to reveal greater insight into protonation and redox changes of Se-substituted FeS clusters.

Linking microbial community composition to farming pattern in selenium-enriched region: Potential role of microorganisms on Se geochemistry

Selenium (Se) is an essential micronutrient for lives. Indigenous microbial communities play an important role on Se geochemistry in soils. In this study, the microbial community composition and functions of 53 soil samples were investigated using high-throughput sequencing. Samples were divided into 3 groups with different farming types based on the measured geochemical parameters and microbial functional structures. Results indicated that putative Se related bacteria Bacillus, Dyella, Paenibacillus, Burkholderia and Brevibacillus were dominant in dryland plantation soils which were characterized with higher available Se and low contents of H₂O, total organic carbon (TOC), NH₄⁺ and NO₂^-. In contrast, the putative denitrifier Pseudomonas dominated in flooded paddy soils with higher TOC, NO₃^- and organic Se, whereas genera Rhizobium, Nitrosospira, and Geobacter preferred woodland soils with higher oxidation-reduction potential (ORP), pH, NH₄⁺ and Fe. Farming patterns resulted in distinct geochemical parameters including moisture, pH, ORP, TOC, and contents of soluble Fe, NO₂^- and NH₄⁺, shaping the microbial communities, which in turn affected Se forms in soils. This study provides a valuable insight into understanding of Se biogeochemistry in soils and prospective strategy for Se-rich agriculture production.

The occurrence, transformation and control of selenium in coal-fired power plants: Status quo and development

As a trace element, selenium can cause serious harm to organisms when the concentration is too high. Coal-fired power plants are the main source of man-made selenium emissions. How to control the selenium pollution of coal-fired power plants to realize the renewable selenium and the sustainability of coal has not attracted enough attention from the whole world. This paper outlines the conversion and occurrence of selenium in coal-fired power plants. A small part of the selenium produced by combustion can be removed by selective catalytic reduction (SCR) and electrostatic precipitator (ESP) after the gas phase undergoes physical condensation and chemical adsorption to combine with the particulate matter in the flue gas.Because the chemical precipitation method has poor selenium removal effect, the remaining part enters the flue gas desulfurization absorption tower and can be enriched in the desulfurization slurry. The occurrence situation and conversion pathway of selenium in desulfurization slurry are introduced subsequently, the research progress of selenium removal from wet desulfurization wastewater is reviewed from three aspects: physics, biology and chemistry. We believe that the coupling application of oxidation-reduction potential (ORP) and pH can optimize selenium removal in the desulfurization system by improving the oxidation control. As a technology for wet desulfurization system to treat selenium pollution, it has a good development prospect in near future.Implications: Selenium is a trace element present in coal. It is not only of great significance to the life activities of organisms, but also a kind of rare resource. As the most important source of man-made emissions, coal-fired power plants will cause waste of selenium resources and selenium pollution in the surrounding environment. In this study, the occurrence, conversion and control of selenium in coal-fired power plants were systematically sorted out and analyzed. It is helpful for scholars to study the selenium transformation process more deeply. It is of great significance for policy formulation of recommended control technologies and emission limits. It is of great value for the formulation of recommended control technology and the in-depth study of the selenium transformation process.

The beneficial and hazardous effects of selenium on the health of the soil-plant-human system: An overview

Selenium (Se), which can be both hazardous and beneficial to plants, animals and humans, plays a pivotal role in regulating soil-plant-human ecosystem functions. The biogeochemical behavior of Se and its environmental impact on the soil-plant-human system has received broad attention in the last decades. This review provides a comprehensive understanding of Se biogeochemistry in the soil-plant-human system. The speciation, transformation, bioavailability as well as the beneficial and hazardous effects of Se in the soil-plant-human system are summarized. Several important aspects in Se in the soil-plant-human system are detailed mentioned, including (1) strategies for biofortification in Se-deficient areas and phytoremediation of soil Se in seleniferous areas; (2) factors affecting Se uptake and transport by plants; (3) metabolic pathways of Se in the human body; (4) the interactions between Se and other trace elements in plant and animals, in particular, the detoxification of heavy metals by Se. Important research hotspots of Se biogeochemistry are outlined, including (1) the coupling of soil microbial activity and the Se biogeochemical cycle; (2) the molecular mechanism of Se metabolic in plants and animals; and (3) the application of Se isotopes as a biogeochemical tracer in research. This review provides up-to-date knowledge and guidelines on Se biogeochemistry research.

Aerobic granular sludge for efficient biotransformation of chalcogen SeIV and TeIV oxyanions: Biological nutrient removal and biogenesis of Se0 and Te0 nanostructures

Simultaneous removal of selenite (Se^IV), tellurite (Te^IV) and nutrients by aerobic granular sludge (AGS) was investigated. A sequencing batch reactor (SBR) was operated with increasing Se^IV and Te^IV (up to 500 µM each) for 205 days to evaluate metalloid oxyanion and nutrient removal. AGS efficiently removed Se^IV and Te^IV by readily converting them to biomass associated forms. The total Se and Te removal efficiencies were higher at 98% and 99%, respectively. Formation of biomass-associated Se⁰ and Te⁰ was confirmed by XRD, Raman spectroscopy and SEM-EDX. Feeding of Se^IV and Te^IV elicited inhibitory action on ammonium removal initially, nonetheless removal performance was recovered during the subsequent cycles. Ammonium, total nitrogen and phosphorus removals were stabilized at 85%, 80% and 75%, respectively, at 500 µM of Se^IV and Te^IV. Sequencing of 16S rRNA gene confirmed enrichment of known Se^IV and Te^IV reducing bacteria in the granules. qPCR and removal kinetics supported ammonia removal via nitritation-denitritation. This work demonstrates functional capabilities of AGS for effectively removing toxic Se^IV and Te^IV oxyanions apart from performing simultaneous COD, nitrogen and phosphorus removal. Efficient biological nutrient removal in the presence of toxic Se^IV and Te^IV concentrations, suggests robustness of AGS and its resilience to toxic contaminants.

A critical review on sulfur reduction of aqueous selenite: Mechanisms and applications

Selenite, which is extremely toxic at high concentrations, can easily be enriched in natural aquatic environments due to human activities, which causes great harm to ecosystems. Sulfur reduction can effectively reduce soluble selenite in large quantities to nontoxic solid elemental selenium, which plays a significant role in controlling the toxicity and cycle of selenium. In view of the bright prospects of the sulfur reduction reaction of selenite, this review comprehensively summarizes the continuous development in the sulfidation of selenite. First, the geochemical characteristics of aqueous selenium in different sulfur systems involving species distribution and various phase types at Eh-pH conditions were summarized. Second, sulfur reductions of selenite with chemical sulfide in natural water environments, sulfur reductase and extracellular polymer substances containing thiol groups in sulfate-reducing bacteria have been reviewed to further understand the corresponding mechanisms, rates and influencing factors. Furthermore, applications of sulfur reduction of selenium, including removal of selenium, enrichment of selenium, synthesis of selenoproteins and prevention of leakage of selenium, were also summarized. Finally, this review identified future research needs for the sulfidation of selenite for environmental applications.

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.

Selective photocatalytic reduction of selenate over TiO2 in the presence of nitrate and sulfate in mine-impacted water

Selenium contamination is a critical global issue across numerous industries. Industrial waters such as mine-impacted water (MIW) can contain toxic levels of selenate, in addition to varying concentrations of many different dissolved species from the underlying strata, such as sulfate, carbonate, nitrate, organic matter, and many dissolved metals. The removal of selenate from MIW is desired, due to selenate's acute and chronic toxicity in aquatic ecosystems at elevated concentrations. However, due to the complexity of the water matrix and the presence of many other dissolved constituents, this is often very challenging. In this study, we present for the first time the reduction of selenate in a real industrial wastewater, namely MIW, and reveal a significant advantage of photocatalytic reduction; the ability to selectively reduce selenate from >500 μg L^-1 to <2 μg L^-1 in the presence of the more energetically favourable electron acceptor, nitrate (250× molar concentration of selenate) and high concentrations of sulfate (1,940× molar concentration of selenate). The presence and impacts of sulfate, chloride, carbonate, and nitrate on the competitive adsorption and reduction of selenate on TiO₂ are thoroughly investigated for the first time, using formic acid as an electron hole scavenger. The electron transfer mechanism proposed follows TiO₂ conduction band electrons are responsible for the reduction of selenate to elemental Se (Se⁰) and both carbon dioxide radicals (CO₂^·-) and Se conduction band electrons are responsible for the further reduction of Se⁰ to hydrogen selenide (H₂Se).

Glacial Ice Age Shapes Microbiome Composition in a Receding Southern European Glacier

Glaciers and their microbiomes are exceptional witnesses of the environmental conditions from remote times. Climate change is threatening mountain glaciers, and especially those found in southern Europe, such as the Monte Perdido Glacier (northern Spain, Central Pyrenees). This study focuses on the reconstruction of the history of microbial communities over time. The microorganisms that inhabit the Monte Perdido Glacier were identified using high-throughput sequencing, and the microbial communities were compared along an altitudinal transect covering most of the preserved ice sequence in the glacier. The results showed that the glacial ice age gradient did shape the diversity of microbial populations, which presented large differences throughout the last 2000 years. Variations in microbial community diversity were influenced by glacial conditions over time (nutrient concentration, chemical composition, and ice age). Some groups were exclusively identified in the oldest samples as the bacterial phyla Fusobacteria and Calditrichaeota, or the eukaryotic class Rhodophyceae. Among groups only found in modern samples, the green sulfur bacteria (phylum Chlorobi) stood out, as well as the bacterial phylum Gemmatimonadetes and the eukaryotic class Tubulinea. A patent impact of human contamination was also observed on the glacier microbiome. The oldest samples, corresponding to the Roman Empire times, were influenced by the beginning of mining exploitation in the Pyrenean area, with the presence of metal-tolerant microorganisms. The most recent samples comprise 600-year-old ancient ice in which current communities are living.

Combined foliar and soil selenium fertilizer improves selenium transport and the diversity of rhizosphere bacterial community in oats

Agronomic selenium (Se) biofortification of grain crops is considered the best method for increasing human Se intake, which may help people alleviate Se-deficiency. To investigate the efficiency of agronomic Se biofortification of oat, four Se fertilizer application treatments were tested: topsoil (T), foliar (S), the combination of T and S (TS), and control without Se application (CK). Compared with CK, TS significantly increased the 1000-grain weight, grain yield, Se contents in all parts of oats, contents of soil available N, K, and organic matter by 18%, 8.70%, 19.7-60.2%, 6.00%, 8.02%, and 17.95%, respectively. Leaves, roots, and ears had the highest conversion rate of exogenous Se in S (644.63%), T (416.00%), and TS (273.20%), respectively. TS also increased the activities of soil urease, alkaline phosphatase, and sucrose and the diversity of soil bacterial communities. TS and T increased the relative abundance of bacteria involved in the decomposition of organic matter, such as Actinobacteria, Gemmatimonadetes, Chloroflexi, and Bacteroidetes positively correlated with soil nutrients and enzyme activities, and reduced Proteobacteria and Firmicutes negatively correlated with them, Granulicella, Bacillus, Raoultella, Lactococcus, Klebsiella, and Pseudomonas. Furthermore, TS significantly increased the relative abundance of Planctomycetes, Chlorobi, Nitrospinae, Nitrospirae, Aciditeromonas, Gemmatimonas, Geobacter, and Thiobacter. T significantly increased the abundance of Lysobacter, Holophaga, Candidatus-Koribacter, Povalibacter, and Pyrinomonas. S did not significantly change the bacterial communities. Thus, a combined foliar and soil Se fertilizer proved conducive for achieving higher yield, grain Se content, and improving Se transport, the diversity of rhizosphere bacterial community, and bacterial functions in oats.

The potential of Pseudomonas for bioremediation of oxyanions

Non-metal, metal and metalloid oxyanions occur naturally in minerals and rocks of the Earth's crust and are mostly found in low concentrations or confined in specific regions of the planet. However, anthropogenic activities including urban development, mining, agriculture, industrial activities and new technologies have increased the release of oxyanions to the environment, which threatens the sustainability of natural ecosystems, in turn affecting human development. For these reasons, the implementation of new methods that could allow not only the remediation of oxyanion contaminants but also the recovery of valuable elements from oxyanions of the environment is imperative. From this perspective, the use of microorganisms emerges as a strategy complementary to physical, mechanical and chemical methods. In this review, we discuss the opportunities that the Pseudomonas genus offers for the bioremediation of oxyanions, which is derived from its specialized central metabolism and the high number of oxidoreductases present in the genomes of these bacteria. Finally, we review the current knowledge on the transport and metabolism of specific oxyanions in Pseudomonas species. We consider that the Pseudomonas genus is an excellent starting point for the development of biotechnological approaches for the upcycling of oxyanions into added-value metal and metalloid byproducts.

Multiple exposures to high concentrations of selenate significantly improve selenate tolerability, red elemental selenium (Se0) and selenoprotein biosynthesis in Herbaspirillum camelliae WT00C

Herbaspirillum camelliae WT00C is a gram-negative endophyte isolated from the tea plant. It has an intact selenate metabolism pathway but poor selenate tolerability. In this study, microbiological properties of the strain WT00C were examined and compared with other three strains CT00C, NCT00C and NT00C, which were obtained respectively from four, six and eight rounds of 24-h exposures to 200 mM selenate. The selenate tolerability and the ability to generate red elemental selenium (Se0) and selenoproteins in H. camelliae WT00C has significantly improved by the forced evolution via 4-6 rounds of multiple exposures a high concentration of selenate. The original strain WT00C grew in 200 mM selenate with the lag phase of 12 h and 400 mM selenate with the lag phase of 60 h, whereas the strains CT00C and NCT00C grew in 800 mM selenate and showed a relatively short lag phase when they grew in 50-400 mM selenate. Besides selenate tolerance, the strains CT00C and NCT00C significantly improved the biosynthesis of red elemental selenium (Se⁰) and selenoproteins. Two strains exhibited more than 30% selenium conversion efficiency and 40% selenoprotein biosynthesis, compared to the original strain WT00C. These characteristics of the strains CT00C and NCT00C make them applicable in pharmaceuticals and feed industries. The strain NT00C obtained from eight rounds of 24-h exposures to 200 mM selenate was unable to grow in ≥ 400 mM selenate. Its selenium conversion efficiency and selenoprotein biosynthesis were similar to the strain WT00C, indicating that too many exposures may cause gene inactivation of some critical enzymes involving selenate metabolism and antioxidative stress. In addition, bacterial cells underwent obviously physiological and morphological changes, including gene activity, cell enlargement and surface-roughness alterations during the process of multiple exposures to high concentrations of selenate.

The role of available phosphorous in vanadate decontamination by soil indigenous microbial consortia

Indigenous microbial consortia are closely associated with soil inherent components including nutrients and minerals. Although indigenous microbial consortia present great prospects for bioremediation of vanadate [V(V)] contaminated soil, influences of some key components, such as available phosphorus (AP), on V(V) biodetoxification are poorly understood. In this study, surface soils sampled from five representative vanadium smelter sites were employed as inocula without pretreatment. V(V) removal efficiency ranged from 81.7 ± 1.4% to 99.5 ± 0.2% in batch experiment, and the maximum V(V) removal rates were positively correlated with AP contents. Long-term V(V) removal was achieved under fluctuant hydrodynamic and hydrochemical conditions in column experiment. Geobacter and Bacillus, which were found in both original soils and bioreactors, catalytically reduced V(V) to insoluble tetravalent vanadium. Phosphate-solubilizing bacterium affiliated to Gemmatimonadaceae were also identified abundantly. Microbial functional characterization indicated the enrichment of phosphate ABC transporter, which could accelerate V(V) transfer into intercellular space for efficient reduction due to the structural similarity of V(V) and phosphate. This study reveals the critical role of AP in microbial V(V) decontamination and provides promising strategy for in situ bioremediation of V(V) polluted soil.

Hexavalent Chromium Removal and Prokaryotic Community Analysis in Glass Column Reactor Packed with Aspen Wood as Solid Organic Substrate

Microbial hexavalent chromium (Cr(VI)) reduction is a promising method for Cr(VI)-laden wastewater treatment. However, the soluble organic substrate required for heterotrophic microbial Cr(VI) reduction necessitates constant supervision, and an excessive supply of soluble organic substrate can result in deterioration of the quality of the effluent. In this study, we evaluated aspen wood, a low-cost lignocellulose biomass, as a solid organic substrate for heterotrophic Cr(VI) reduction. A laboratory-scale aspen wood-packed glass column reactor inoculated with activated sludge was operated for 148 days for evaluation. Following reactor operation, an effective average dissolved Cr(VI) removal rate of 0.75 mg L^-1 h^-1 was confirmed under an average dissolved Cr(VI) loading rate of 0.90 mg L^-1 h^-1. Subsequently, 16S ribosomal ribonucleic acid gene amplicon sequencing analysis revealed that the dominant prokaryotic operational taxonomic units detected in the reactor were associated with prokaryotic lineages with the capacity for lignocellulose biodegradation, Cr compound resistance, and Cr(VI) reduction. Proteobacteria and Chloroflexi were two major prokaryotic phyla in the reactor. Our data indicate that aspen wood is an effective solid organic substrate for the development of simplified, effective, and low-cost microbial Cr(VI)-removing reactors.

Materials interacting with inorganic selenium from the perspective of electrochemical sensing

Inorganic selenium, the most common form of harmful selenium in the environment, can be determined using electrochemical sensors, which are compact, fast, reliable and easy-to-operate devices. Despite progress in this area, there is still significant room for developing high-performance selenium electrochemical sensors. To achieve this, one should take into account (i) the electrochemical process that selenium undergoes on the electrode; (ii) the valence state of selenium species in the sample and (iii) modification of the sensor surface by a material with high affinity to selenium. The goal of this review is to provide a knowledge base for these issues. After the Introduction section, mechanisms and principles of the electrochemical reduction of selenium are introduced, followed by a section introducing the modification of electrodes with materials interacting with selenium and a section dedicated to speciation methods, including the reduction of non-detectable Se(VI) to detectable Se(IV). In the following sections, the main types of materials (metallic, polymers, hybrid (nano)materials…) interacting with inorganic selenium (mostly absorbents) are reviewed to show the diversity of properties that may be endowed to sensors if the materials were to be used for the modification of electrodes. These features for the main material categories are outlined in the conclusion section, where it is stated that the engineered polymers may be the most promising modifiers.

Selenium-oxidizing Agrobacterium sp. T3F4 steadily colonizes in soil promoting selenium uptake by pak choi (Brassica campestris)

Selenium (Se) deficiency in soil is linked to its low content in edible crops, resulting in adverse impacts on the health of 15% of the global population. The crop mainly absorbs oxidized selenate and selenite from soil, then converts them into organic Se. However, the role of Se-oxidizing bacteria in soil Se oxidation, Se bioavailability and Se absorption into plants remains unclear. The strain Agrobacterium sp. T3F4, isolated from seleniferous soil, was able to oxidize elemental Se into selenite under pure culture conditions. The green fluorescent protein (gfp)-gene-marked strain (T3F4-GFP) and elemental Se or selenite (5 mg·kg^-1) were added to pak choi (Brassica campestris ssp. chinensis) pot cultures. Observation of the fluorescence and viable counting indicated that GFP-expressing bacterial cells steadily colonized the soil in the pots and the leaves of the pak choi, reaching up to 6.6 × 10⁶ and 2.0 × 10⁵ CFU g^-1 at 21 days post cultivation, respectively. Moreover, the total Se content (mostly organic Se) was significantly increased in the pak choi under T3F4 inoculated pot culture, with elemental Se(0) being oxidized into Se(IV), and soil Se(IV) being dissolved before being absorbed by the crop. After strain T3F4 was inoculated, no significant differences in microbial diversity were observed in the soils and roots, whereas the abundance of Rhizobium spp. was significantly increased. To our knowledge, this is the first time that Se-oxidizing Agrobacterium sp. T3F4 has been found to steadily colonize soil and plant tissues, and that its addition to soil increases the absorption of Se in plants. This study provides a potential strategy for Se biofortification.

Se transformation and removal by a cattail litter treatment system inoculated with sulfur-based denitrification sludge: Role of the microbial community composition under various temperature and aeration conditions

With a narrow margin between deficiency and toxicity, rising levels of selenium (Se) are threatening aquatic ecosystems. To investigate the role of microorganisms in Se bioremediation, a cattail litter system inoculated with the sulfur-based denitrification sludge was conducted. The results show the litter, as a carrier and nutrient source for bacteria, efficiently removed Se by ~ 97.0% during a 12-d treatment with water circulating. As the major removal pathways, immobilization rates of selenite were ~ 2.9-fold higher than selenate, and the volatilization, contributing to ~ 87.7% of the total Se removal, was significantly correlated with temperature (positively) and oxidation-reduction potential (ORP; negatively). Using X-ray absorption spectroscopy to speciate litter-borne Se, more Se⁰ formed without aeration due to abundant Se-reducing bacteria, among which Azospira and Azospirillum were highly related to the removal of both Se oxyanions, while Desulfovibrio, Azoarcus, Sulfurospirillum, Thauera, Geobacter, Clostridium, and Pediococcus were the major contributors to selenate removal. Overall, our study suggests microbial Se metabolism in the litter system was significantly affected by temperature and ORP, which could be manipulated to enhance Se removal efficiency and the transformation of selenate/selenite into low toxic Se⁰ and volatile Se, reducing risks posed by the residual Se in the system.

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.

Biosynthesis of Selenium Nanoparticles (via Bacillus subtilis BSN313), and Their Isolation, Characterization, and Bioactivities

Among the trace elements, selenium (Se) has great demand as a health supplement. Compared to its other forms, selenium nanoparticles have minor toxicity, superior reactivity, and excellent bioavailability. The present study was conducted to produce selenium nanoparticles (SeNPs) via a biosynthetic approach using probiotic Bacillus subtilis BSN313 in an economical and easy manner. The BSN313 exhibited a gradual increase in Se reduction and production of SeNPs up to 5-200 µg/mL of its environmental Se. However, the capability was decreased beyond that concentration. The capacity for extracellular SeNP production was evidenced by the emergence of red color, then confirmed by a microscopic approach. Produced SeNPs were purified, freeze-dried, and subsequently characterized systematically using UV-Vis spectroscopy, FTIR, Zetasizer, SEM-EDS, and TEM techniques. SEM-EDS analysis proved the presence of selenium as the foremost constituent of SeNPs. With an average particle size of 530 nm, SeNPs were shown to have a -26.9 (mV) zeta potential and -2.11 µm cm/Vs electrophoretic mobility in water. SeNPs produced during both the 24 and 48 h incubation periods showed good antioxidant activity in terms of DPPH and ABST scavenging action at a concentration of 150 µg/mL with no significant differences (p > 0.05). Moreover, 200 µg/mL of SeNPs showed antibacterial reactivity against Escherichia coli ATCC 8739, Staphylococcus aureus ATCC 9027, and Pseudomonas aeruginosa ATCC 25923. In the future, this work will be helpful to produce biogenic SeNPs using probiotic Bacillus subtilis BSN313 as biofactories, with the potential for safe use in biomedical and nutritional applications.