Microbial selenite reduction with organic carbon and electrode as sole electron donor by a bacterium isolated from domestic wastewater

Biochemical characterization, biosynthesis mechanism, and functional evaluation of selenium-enriched Aspergillus oryzae A02

The synthesis mechanisms and function evaluation of selenium(Se)-enriched microorganism remain relatively unexplored. This study unveils that total Se content within A. oryzae A02 mycelium soared to an impressive 8462 mg/kg DCW, surpassing Se-enriched yeast by 2-3 times. Selenium exists in two predominant forms within A. oryzae A02: selenoproteins (SeMet 32.1 %, SeCys 14.4 %) and selenium nanoparticles (SeNPs; 53.5 %). The extensive quantitative characterization of the elemental composition, surface morphology, and size of SeNPs on A. oryzae A02 mycelium significantly differs from those reported for other microorganisms. Comparative RNA-Seq analysis revealed the upregulation of functional genes implicated in selenium transformation, activating multiple potential pathways for selenium reduction. The assimilatory and dissimilatory reductions of Se oxyanions engaged numerous parallel and interconnected pathways, manifesting a harmonious equilibrium in overall Se biotransformation in A. oryzae A02. Furthermore, selenium-enriched A. oryzae A02 was observed to primarily upregulate peroxisome activity while downregulating estrogen 2-hydroxylase activity in mice hepatocytes, suggesting its potential in fortifying antioxidant physiological functions and upholding metabolic balance.

Electroactive Brevundimonas diminuta consortium mediated selenite bioreduction, biogenesis of selenium nanoparticles and bio-electricity generation

In this study, highly selenite-resistant strains belonging to Brevundimonas diminuta (OK287021, OK287022) genus were isolated from previously operated single chamber microbial fuel cell (SCMFC). The central composite design showed that the B. diminuta consortium could reduce selenite. Under optimum conditions, 15.38 Log CFU mL-1 microbial growth, 99.08% Se(IV) reduction, and 89.94% chemical oxygen demand (COD) removal were observed. Moreover, the UV-visible spectroscopy (UV) and Fourier transform infrared spectroscopy (FTIR) analyses confirmed the synthesis of elemental selenium nanoparticles (SeNPs). In addition, transmission electron microscopy (TEM) and scanning electron microscope (SEM) revealed the formation of SeNPs nano-spheres. Besides, the bioelectrochemical performance of B. diminuta in the SCMFC illustrated that the maximum power density was higher in the case of selenite SCMFCs than those of the sterile control SCMFCs. Additionally, the bioelectrochemical impedance spectroscopy and cyclic voltammetry characterization illustrated the production of definite extracellular redox mediators that might be involved in the electron transfer progression during the reduction of selenite. In conclusion, B. diminuta whose electrochemical activity has never previously been reported could be a suitable and robust biocatalyst for selenite bioreduction along with wastewater treatment, bioelectricity generation, and economical synthesis of SeNPs in MFCs.

Simultaneous selenite reduction and nitrogen removal using Paracoccus sp.: Reactor performance, microbial community, and mechanism

Selenium-containing wastewater has a high concentration of nitrogen compounds (ammonia nitrogen [NH4+-N]), leading to water pollution. Thus, the simultaneous reduction of selenium and removal of nitrogen compounds during wastewater treatment has become the top priority. However, the exogenous bacteria that can simultaneously reduce selenite and remove ammonia nitrogen and colonize in the wastewater treatment systems have not been reported. Additionally, the effects and the underlying mechanism of biofortification on the reduction and removal efficiency of the microorganisms remain unclear. In this study, we investigated the simultaneous selenite reduction and nitrogen removal efficiency of Paracoccus sp. (strain SSJ) isolated from selenium-contaminated soil and explored biofortification effects on the composition and structure of the microbial community. Using sequencing biofilm batch reactors (SBBRs), the structural and functional characteristics of the microbial community were systematically compared between the control (group A) and biofortified (group B) groups. Strain SSJ could simultaneously reduce 63.28% of selenite and remove 93.05% of NH4+-N within 24 h. Moreover, no accumulation of nitrate nitrogen (NO3--N) and nitrite nitrogen (NO2--N) was observed in the reaction process. The performance and stability of the SBBRs enhanced by strain SSJ were greatly improved. Illumina sequencing results showed that strain SSJ was surprisingly colonized, and Paracoccus was the predominant genus in group B (relative abundance: 13.93%). Moreover, PICRUSt2 analysis results suggested that the microbial community in group B demonstrated increased rates of ammonia nitrogen removal through ammonia assimilation and selenite reduction through sulfur metabolism and glutathione-mediated selenite reduction pathway. In summary, our findings shed light on the mechanism for simultaneous selenite reduction and nitrogen removal by biofortification and provide novel microbial resources for the treatment of selenite-containing wastewater.

Multifaceted synergistic facilitation mechanism of conductive polymers in promoting selenite bioreduction and biological detoxification

Here, polypyrrole (PPY) was first used to the bioreduction of toxic selenite, while the acceleration effect and mechanism were explored. Experiment results suggested that PPY could enhance the selenite bioreduction from 0.42 to 1.04 mg/(L·h). The tests of electrochemical analysis and cytochrome c (cyt-c) content confirmed that PPY promoted the intracellular/intracellular electron transfer of Shewanella oneidensis·MR-1 in selenite bioreduction process. The enhancement of metabolic activity by PPY contributed to biological detoxification, which was manifested in the increased extracellular polymeric substances (EPS), adenosine triphosphate (ATP), electron transfer system activity (ETSA), membrane permeability and enzyme activity. Transcriptome analysis of DEGs, KEGG pathway enrichment and GO functional classification verified that the environmental adaptability of Shewanella oneidensis·MR-1 was enhanced with the addition of PPY. The transmission electron microscopy (TEM) images indicated that PPY promoted the biosynthesis of selenium nanoparticles (SeNPs), which was beneficial to reduce cell damage. Combined with the above results, a multifaceted synergistic facilitation mechanism based on "conductive cross-linking network" was elaborated from electron transfer, microbial metabolism and environmental adaptability. This study shed light the effect of conductive polymers (CPs) on selenite bioreduction and provided new insights into the bioremediation of toxic pollutants.

Selenium in soil-plant system: Transport, detoxification and bioremediation

Selenium (Se) is an essential micronutrient for humans and a beneficial element for plants. However, high Se doses always exhibit hazardous effects. Recently, Se toxicity in plant-soil system has received increasing attention. This review will summarize (1) Se concentration in soils and its sources, (2) Se bioavailability in soils and influencing factors, (3) mechanisms on Se uptake and translocation in plants, (4) toxicity and detoxification of Se in plants and (5) strategies to remediate Se pollution. High Se concentration mainly results from wastewater discharge and industrial waste dumping. Selenate (Se [VI]) and selenite (Se [IV]) are the two primary forms absorbed by plants. Soil conditions such as pH, redox potential, organic matter and microorganisms will influence Se bioavailability. In plants, excessive Se will interfere with element uptake, depress photosynthetic pigment biosynthesis, generate oxidative damages and cause genotoxicity. Plants employ a series of strategies to detoxify Se, such as activating antioxidant defense systems and sequestrating excessive Se in the vacuole. In order to alleviate Se toxicity to plants, some strategies can be applied, including phytoremediation, OM remediation, microbial remediation, adsorption technique, chemical reduction technology and exogenous substances (such as Methyl jasmonate, Nitric oxide and Melatonin). This review is expected to expand the knowledge of Se toxicity/detoxicity in soil-plant system and offer valuable insights into soils Se pollution remediation strategies.

Selenite bioreduction by a consortium of halophilic/halotolerant bacteria and/or yeasts in saline media

Selenium oxyanions are released into environments by natural and anthropogenic activities and are present in agricultural and glass manufacturing wastewater in several locations worldwide. Excessive amounts of this metalloid have adverse effects on the health of living organisms. Halophilic and halotolerant microorganisms were selected for selenium oxyanions remediation due to presence of significant amount of salt in selenium-containing wastewater. Effects of aeration, carbon sources, competitive electron acceptors, and reductase inhibitors were investigated on SeO₃^2- bio-removal. Additionally, NO₃^--containing wastewater were exploited to investigate SeO₃^2- remediation in synthetic agricultural effluents. The results showed that the SeO₃^2- removal extent is maximum in aerobic conditions with succinate as a carbon source. SO₄^2- and PO₄^3- do not significantly interfere with SeO₃^2- reduction, while WO₄^2- and TeO₃^2- decrease the SeO₃^2- removal percentage (up to 35 and 37%, respectively). Furthermore, NO₃^- had an adverse effect on SeO₃^2- biotransformation by our consortia. All consortia reduced SeO₃^2- in synthetic agricultural wastewaters with a 45-53% removal within 120 h. This study suggests that consortia of halophilic/halotolerant bacteria and yeasts could be applied to treat SeO₃^2--contaminated drainage water. In addition, sulphates, and phosphates do not interfere with selenite bioreduction by these consortia, which makes them suitable candidates for the bioremediation of selenium-containing wastewater.

Biogenic selenium nanoparticles with antifungal activity against the wood-rotting fungus Oligoporus pelliculosus

Selenium nanoparticles (SeNPs) have antimicrobial and antifungal activity. SeNPs using Se resistant bacteria is a low cost and eco-friendly technology. Fungal contamination of wood during drying is one of the main causes of economic losses in the wood industry. The bacterium Delftia sp. 5 resistance to Se and its ability to produce SeNPs able to inhibit the growth of the wood brown-rotting fungus Oligoporus pelliculosus was analyzed. The strain showed an optimal SeNPs production when selenite concentration was 160 mg L ^-1. The SeNPs were spherical with an average size 192.33 ± 8.6 nm and a zeta potential of -41.4 ± 1.3 nm. The SeNPs produced by Delftia sp. 5 (33.6 ± 0.1 mg L ^-1 Se) inhibited the growth of O. pelliculosus in agar plates and in Nothofagus pumilio (Lenga) wood samples. Delftia sp. 5 SeNPs could be used for embedding lenga wood prior to drying for preventing the growth of the deteriorating fungi O. pelliculosus.

Insights into Increasing Selenate Reductase Enzyme Activity in the Presence of Nitrogen-Doped Graphite Electrodes for Selenium Effluent Treatment

The weathering of selenium-rich rocks or anthropogenic activities such as mining or smelting can release selenium into the environment, posing a significant environmental risk. The increased monitoring and enforcement of selenium regulations have resulted in protocols to efficiently measure and treat selenium in water and effluent water. The principal aqueous forms of inorganic selenium are selenite (Se(IV)) and selenate (Se(VI)). Selenate, due to its oxy-anionic nature, high mobility, and lack of affinity to conventional adsorbents, is typically more difficult to treat and remove. Thus, it is proposed to remove selenate from water by first reducing it to selenite and then to insoluble elemental selenium, a form that has low toxicity. A naturally occurring selenate reductase enzyme from Thauera selenatis was previously shown to specifically reduce selenate to selenite. To exploit this functionality, recombinant enzyme technologies were used to produce a cell-free, enriched Thauera selenatis selenate reductase heterotrimeric enzyme complex (TsSer-αβγ). The addition of the recombinant enzyme complex to effluent water was found to successfully reduce the selenate. Interestingly, upon adding nitrogen-doped graphite electrodes to the reaction, the selenate-reducing activity significantly increased. Overall, these findings highlight a new, potentially sustainable solution to the reduction of selenate in water and effluent water.

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.

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.

A novel permeable reactive biobarrier for ortho-nitrochlorobenzene pollution control in groundwater: Experimental evaluation and kinetic modelling

Three novel permeable reactive barrier (PRB) materials composed of Cu/Fe with 0.24% and 0.43% (w/w) Cu loadings or Fe⁰ supported on wheat straw were prepared (termed materials E, F and G). These materials exhibited excellent pollutant removal efficiency and physical stability as well as the ongoing release of organic carbon and iron. Column experiments showed that materials E, F and G removed almost 100% of ortho-nitrochlorobenzene (o-NCB) from water. The rates of iron release from the E and F columns exceeded those from column G but this had no significant effect on o-NCB removal. The bacteria that degraded o-NCB in E and F were also different from those in G. The levels of these bacteria in the columns were higher than those in the initial materials, with the highest level in column E. The simultaneous reduction and microbial degradation of o-NCB was observed, with the latter being dominant. A kinetic model was established to simulate the dynamic interactions and accurately predicted the experimental results. Organic carbon from the wheat straw supported the majority of the biomass in each column, which was essential for the bioremediation process. The findings of this study suggest an economically viable approach to mitigating o-NCB pollution.

Biological selenite removal and recovery of selenium nanoparticles by haloalkaliphilic bacteria isolated from the Nakdong River

Microbial selenite reduction has increasingly attracted attention from the scientific community because it allows the separation of toxic Se from waste sources with the concurrent recovery of Se nanoparticles, a multifunctional material in nanotechnology industries. In this study, four selenite-reducing bacteria, isolated from a river water sample, were found to reduce selenite by > 85% within 3 d of incubation, at ambient temperature. Among them, strain NDSe-7, belonging to genus Lysinibacillus, can reduce selenite and produce Se nanospheres in alkaline conditions, up to pH 10.0, and in salinity of up to 7.0%. This strain can reduce 80 mg/L of selenite to elemental Se within 24 h at pH 6.0-8.0, at a temperature of 30-40 °C, and salinity of 0.1-3.5%. Strain NDSe-7 exhibited potential for use in Se removal and recovery from industrial saline wastewater with high alkalinity. This study indicates that extremophilic microorganisms for environmental remediation can be found in a conventional environment.

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

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

Remediation of selenium-contaminated soil through combined use of earthworm Eisenia fetida and organic materials

Selenium (Se)-polluted soils pose serious threats to terrestrial ecosystems through food chains. This study evaluated the use of earthworm Eisenia fetida and organic materials for Se remediation. The greater mortality (6.7%) and weight loss (28.6%) were observed for earthworms exposed to selenate than selenite at 20 mg Se Kg^-1 over 21 d, while selenate was taken up 2.5-fold faster than selenite. Compared with peat moss, Se accumulation increased by 119% in selenite-exposed earthworms supplied with cow manure. Earthworm activity caused greater reduction in selenite (17.4%), with little impact on selenate-treated soil. X-ray absorption spectroscopy speciation analysis shows 87-91% of tissue Se was transformed to organo-Se, i.e., SeMet and SeCys, in earthworms exposed to either selenite or selenate, posing great risks to their predators. The study also found selenium increased bacterial diversity in earthworm casts, while greater relative abundances (~37.57%) of functional genera were obtained for selenite. Over 24 h, two bacteria strains, Bacillus cereus and Aeromonas encheleia, isolated from casts, rapidly reduced selenite by ~94%, compared to ~25% for selenate. Elemental Se was present only in strains (~27%), casts (~11%) and worm-inhabited soil (~2.7%) of selenite treatments, suggesting earthworm gut microbiota could buffer earthworms and other soil fauna from selenite toxicity.

Selenium volatilization from tundra soils in maritime Antarctica

Maritime Antarctica harbors a large number of penguins and seals that provide considerable input of selenium (Se) originating as guano into terrestrial ecosystems. Subsequent Se emissions via biomethylation and volatilization from these sources of Se have not been studied. Here, penguin colony soils (PCS) and adjacent tundra marsh soils (TMS), seal colony soils (SCS) and adjacent tundra soils (STS), and normal upland tundra soils (NTS) were collected in maritime Antarctica. For the first time, Se volatilization and speciation were investigated in these soils through incubation experiments using chemo-trapping method. The Se contents in PCS, SCS, STS and TMS were highly enriched compared with NTS, with organic matter-bound Se accounting for 70%-80%. Laboratory incubations yielded the greatest Se volatilization rates (VR_Se) in PCS (0.20 ± 0.01 μg kg^-1 d^-1), followed by SCS (0.14 ± 0.01 μg kg^-1 d^-1) at low temperature (4 °C). Soil frozen-thawing induced 1-4 fold increase in VR_Se, and the VR_Se continuously increased until the soils fully thawed. The VR_Se showed a significant positive correlation (R² = 0.96, p < 0.01) with soil temperature. Methylated Se species were dominated by dimethylselenide (DMSe) in PCS and dimethyldiselenide (DMDSe) in SCS. Our results imply that the combination of climate warming, frozen-thawing processes, and high-Se inputs from sea animals will significantly increase tundra soil Se volatilization in maritime Antarctica. High VR_Se from penguin colony soils, and significantly elevated Se levels in the mosses close to penguin colony, suggest that volatilization of Se from penguin colony soils play an important role in the mobilization and regional biogeochemical cycling of Se in maritime Antarctica.

Cathodic selenium recovery in bioelectrochemical system: Regulatory influence on anodic electrogenic activity

Metal(loid)s are used in various industrial activities and widely spread across the environmental settings in various forms and concentrations. Extended releases of metal(loid)s above the regulatory levels cause environmental and health hazards disturbing the ecological balance. Innovative processes for treating the metal(loid)-contaminated sites and recovery of metal(loid)s from disposed waste streams employing biotechnological routes provide a sustainable way forward. Conventional metal recovery technologies demand high energy and/or resource inputs, which are either uneconomic or unsustainable. Microbial electrochemical systems are promising for removal and recovery of metal(loid)s from metal(loid)-laden wastewaters. In this communication, a bioelectrochemical system (BES) was designed and operated with selenium (Se) oxyanion at varied concentrations as terminal electron acceptor (TEA) for reduction of selenite (Se⁴⁺) to elemental selenium (Se⁰) in the abiotic cathode chamber. The influence of varied concentrations of Se⁴⁺ towards Se⁰ recovery at the cathode was also evaluated for its regulatory role on the electrometabolism of anode-respiring bacteria. This study observed 26.4% Se⁰ recovery (cathode; selenite removal efficiency: 73.6%) along with organic substrate degradation of 74% (anode). With increase in the initial selenite concentration, there was a proportional increase in the dehydrogenase activity. Bioelectrochemical characterization depicted increased anodic electrogenic performance with the influence of varied Se⁴⁺ concentrations as TEA and resulted in a maximum power density of 0.034 W/m². The selenite reduction (cathode) was evaluated through spectroscopic, compositional and structural analysis. X-ray diffraction and Raman spectroscopy showed the amorphous nature, while Energy Dispersive X-ray spectroscopy confirmed precipitates of the deposited Se⁰ recovered from the cathode chamber. Scanning electron microscopic images clearly depicted the Se⁰ depositions (spherical shaped; sized approximately 200 nm in diameter) on the electrode and cathode chamber. This study showed the potential of BES in converting soluble Se⁴⁺ to insoluble Se⁰ at the abiotic cathode for metal recovery.

Developments in the study and applications of bacterial transformations of selenium species

Microbial bio-transformations of the essential trace element selenium are now recognized to occur among a wide variety of microorganisms. These transformations are used to convert this element into its assimilated form of selenocysteine, which is at the active center of a number of key enzymes, and to produce selenium nanoparticles, quantum dots, metal selenides, and methylated selenium species that are indispensable for biotechnological and bioremediation applications. The focus of this review is to present the state-of-the-art of all aspects of the investigations into the bacterial transformations of selenium species, and to consider the characterization and biotechnological uses of these transformations and their products.

Prediction of selenium uptake by pak choi in several agricultural soils based on diffusive gradients in thin-films technique and single extraction

The accurate assessment of soil selenium (Se) bioavailability is crucial for Se biofortification in Se-deficient areas and risk assessment in selenosis areas. However, a universally accepted approach to evaluate Se bioavailability in soil is currently lacking. This research investigated Se bioavailability in six soils treated with selenite (Se(IV)) or selenate (Se(VI)) by comparing diffusive gradients in thin-films (DGT) technique and chemical extraction methods through pot experiments. A bioindicator method was used to evaluate Se concentrations in pak choi and compare the results with the Se concentration measured by other methods. Results showed that chemical extraction methods presented different extraction efficiencies for available Se over a range of soil types, and the same extraction method had various extraction efficiencies for different Se species in the same soil. DGT measured Se concentrations (C_DGT-Se) for Se(VI) treatment were 2.3-34.1 times of those for Se(IV) treatment. KH₂PO₄-K₂HPO₄ and AB-DTPA extractable Se could predict the bioavailability of soil Se, but they were disturbed by soil properties. HAc extraction was unsuitable for evaluating Se bioavailability in different Se(IV)-treated soils. By contrast, DGT technique was preferable for predicting plant uptake of Se(IV) over chemical extraction methods. Although DGT technique was independent of soil properties, KH₂PO₄-K₂HPO₄ extraction provided the best fitting regression equation for Se(VI) when it was dependent on soil organic matter. Thus, KH₂PO₄-K₂HPO₄ extraction may be preferred to assess Se(VI) bioavailability in different soil types on a large scale.

Microbial antimonate reduction with a solid-state electrode as the sole electron donor: A novel approach for antimony bioremediation

The anaerobic antimonate [Sb(V)] reduction with a solid-state electrode serving as the sole electron donor was demonstrated by employing a bioelectrochemical system. The highest Sb(V) reduction efficiency was observed at the biocathode potential of -0.7 V versus standard hydrogen electrode using a cathode potential range from -0.5 V to -1.1 V. The scanning electron microscopy and energy dispersive X-ray spectroscopy indicated that both amorphous and crystallized Sb₂O₃ were formed as products of Sb(V) reduction. The irreversible recovery of bioelectrochemical Sb(V), when the cathode potential deviated from the optimal potential, was explained through the alteration in microbial communities, which was further elucidated by the next-generation sequencing of 16S rRNA gene amplicons. Chryseobacterium koreense and Stenotrophomonas nitritireducens were the dominant species of microbial consortia at Sb(V)-reducing biocathodes. This study revealed a novel option for bioremediation of Sb at underground contaminated sites, where the delivery of organic electron donors is limited or ineffective.

Microbial selenite reduction coupled to anaerobic oxidation of methane

Denitrifying anaerobic methane oxidation (DAMO) is the process of coupling the anaerobic oxidation of methane (AOM) with denitrification, which plays an important part in controlling the flow of methane in anoxic niches. In this study, we explored the feasibility of microbial selenite reduction using methane by DAMO culture. Isotopic ¹³CH₄ and long-term experiments showed that selenite reduction was coupled to methane oxidation, and selenite was ultimately reduced to Se (0) by the analyses of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The introduction of nitrate, the original electron acceptor in the DAMO culture, inhibited selenite reduction. Meanwhile, the microbial community of DAMO culture was significantly changed when the electron acceptor was changed from nitrate to selenite after long-term selenite reduction. High-throughput 16S rRNA gene sequencing indicated that Methylococcus (26%) became the predominant microbe performing selenite reduction and methane oxidation and the possible pathways of AOM accompanied with selenite reduction were proposed. This study revealed more potential relation during the biogeochemical cycle of carbon, nitrogen, and selenium.

Biological treatment of selenate-containing saline wastewater by activated sludge under oxygen-limiting conditions

Selenium often coincides with high salinity in certain industrial wastewaters, which can be a limitation in the practical application of biological treatment. However, there are no studies on the biological treatment of selenate-containing saline wastewater. A sequencing batch reactor inoculated with activated sludge was applied to treat selenate in the presence of 3% (w/v) NaCl. Start-up of the sequencing batch reactor with a 7-day cycle duration and excessive acetate as the sole carbon source succeeded in removing above 98% and 72% soluble and solid selenium, respectively, under oxygen-limiting conditions. Further selenium removal experiments with a shorter cycle duration of 3 days and a stepwise decrease of acetate addition achieved soluble and total selenium removal efficiencies in most batches above 96% and 80%, respectively. Mass balance analysis revealed that selenate was converted into elemental selenium, most of which was accumulated in the sludge. Microscopic analyses also found that elemental selenium particles were primarily present as approximately 2 μm large rods, with some extremely large particles above 10 μm. Although the bacterial populations responsible for selenium removal, especially selenate reduction, could not be identified by microbial community analysis, this study reported for the first time that selenate could be biologically treated in the presence of considerable salinity, offering implications for the practical treatment of selenium in certain industrial wastewaters.

Kinetics of microbial selenite reduction by novel bacteria isolated from activated sludge

A total of three bacteria isolated from activated sludge of a wastewater treatment plant were found to reduce selenite to elemental selenium nanoparticles as both amorphous nanospheres and monoclinic nanocrystals. The three isolated strains, which are potential candidates for bioremediation of selenite-contaminated water sources, were designated as Citrobacter sp. NVK-2, Providencia sp. NVK-2A, and Citrobacter sp. NVK-6 based on 16S rRNA sequencing. Despite belonging to the same genus, the kinetics of selenite reduction by strain NVK-2 (V_max = 58.82 μM h^-1, K_m = 3737.12 μM) completely differed from that of strain NVK-6 (V_max = 19.23 μM h^-1, K_m = 1300.17 μM). The selenite reduction rate by strain NVK-2A (V_max = 9.26 μM h^-1, K_m = 3044.73 μM) was the slowest among the investigated microorganisms. The microbial selenite reduction rates according to various organic sources indicated that simple organic sources such as acetate and lactate were better than more complex organic sources such as propionate, butyrate, and glucose for selenite removal. Interestingly, the selenite reduction rate was significantly enhanced when the organic source was strategically divided into small portions and consecutively supplied to the culture.

Two selenium tolerant Lysinibacillus sp. strains are capable of reducing selenite to elemental Se efficiently under aerobic conditions

Microbes play important roles in the transport and transformation of selenium (Se) in the environment, thereby influencing plant resistance to Se and Se accumulation in plant. The objectives are to characterize the bacteria with high Se tolerance and reduction capacity and explore the significance of microbial origins on their Se tolerance, reduction rate and efficiency. Two bacterial strains were isolated from a naturally occurred Se-rich soil at tea orchard in southern Anhui Province, China. The reduction kinetics of selenite was investigated and the reducing product was characterized using scanning electron microscopy and transmission electron microscopy-energy dispersive spectroscopy. The bacteria were identified as Lysinibacillus xylanilyticus and Lysinibacillus macrolides, respectively, using morphological, physiological and molecular methods. The results showed that the minimal inhibitory concentrations (MICs) of selenite for L. xylanilyticus and L. macrolides were 120 and 220 mmol/L, respectively, while MICs of selenate for L. xylanilyticus and L. macrolides were 800 and 700 mmol/L, respectively. Both strains aerobically reduced selenite with an initial concentration of 1.0 mmol/L to elemental Se nanoparticles (SeNPs) completely within 36 hr. Biogenic SeNPs were observed both inside and outside the cells suggesting either an intra- or extracellular reduction process. Our study implied that the microbes from Se-rich environments were more tolerant to Se and generally quicker and more efficient than those from Se-free habitats in the reduction of Se oxyanions. The bacterial strains with high Se reduction capacity and the biological synthesized SeNPs would have potential applications in agriculture, food, environment and medicine.

Selenite Reduction and the Biogenesis of Selenium Nanoparticles by Alcaligenesfaecalis Se03 Isolated from the Gut of Monochamus alternatus (Coleoptera: Cerambycidae)

In this study, a bacterial strain exhibiting high selenite (Na₂SeO₃) tolerance and reduction capacity was isolated from the gut of Monochamus alternatus larvae and identified as Alcaligenes faecalis Se03. The isolate exhibited extreme tolerance to selenite (up to 120 mM) when grown aerobically. In the liquid culture medium, it was capable of reducing nearly 100% of 1.0 and 5.0 mM Na₂SeO₃ within 24 and 42 h, respectively, leading to the formation of selenium nanoparticles (SeNPs). Electron microscopy and energy dispersive X-ray analysis demonstrated that A. faecalis Se03 produced spherical electron-dense SeNPs with an average hydrodynamic diameter of 273.8 ± 16.9 nm, localized mainly in the extracellular space. In vitro selenite reduction activity and real-time PCR indicated that proteins such as sulfite reductase and thioredoxin reductase present in the cytoplasm were likely to be involved in selenite reduction and the SeNPs synthesis process in the presence of NADPH or NADH as electron donors. Finally, using Fourier-transform infrared spectrometry, protein and lipid residues were detected on the surface of the biogenic SeNPs. Based on these observations, A. faecalis Se03 has the potential to be an eco-friendly candidate for the bioremediation of selenium-contaminated soil/water and a bacterial catalyst for the biogenesis of SeNPs.

Characterization of diversified Sb(V)-reducing bacterial communities by various organic or inorganic electron donors

This study aims to enrich Sb(V)-reducing bacterial communities from Sb-contaminated soils using various electron donors for bioremediation of Sb-contaminated sites and recovery of Sb from wastewater. When the organic electron donors were used, Sb(V) reduction rates were 2-24 times faster but electron recoveries were 24-59% lower compared to the culture using inorganic electron donor. The morphological crystallizations of the antimony-reduced precipitates were completely different depending on the electron donor. Different microbial populations were enriched with various electron donors but most commonly, only Proteobacteria and Firmicutes phyla were enriched from a diversified soil microbial community. Geobacter sp. seemed to be an important bacterium in organic electron donors-fed cultures whereas an unclassified Rhodocyclaceae was dominant in inorganic electron donor-fed cultures. The results indicated that organic electron donors especially sugar groups were preferable options to obtain rapid Sb(V)-reduction whereas inorganic electron donor like H₂ was better option to achieve high electron recovery.

Opportunities for groundwater microbial electro-remediation

Groundwater pollution is a serious worldwide concern. Aromatic compounds, chlorinated hydrocarbons, metals and nutrients among others can be widely found in different aquifers all over the world. However, there is a lack of sustainable technologies able to treat these kinds of compounds. Microbial electro‐remediation, by the means of microbial electrochemical technologies (MET), can become a promising alternative in the near future. MET can be applied for groundwater treatment in situ or ex situ, as well as for monitoring the chemical state or the microbiological activity. This document reviews the current knowledge achieved on microbial electro‐remediation of groundwater and its applications.