Bioremediation potential of a highly mercury resistant bacterial strain Sphingobium SA2 isolated from contaminated soil

Bacillus sp. strain MRS-1: A potential candidate for uranyl biosorption from uranyl polluted sites

The uranyl tolerance of a metal-resistant Bacillus sp. strain MRS-1, was determined in this current study. This was done due to a rise in anthropogenic activities, such as the production of uranium-based nuclear energy, which contributes to environmental degradation and poses risks to ecosystems and human health. The purpose of the research was to find effective strategies for uranium removal to minimize the contamination. In this paper, the biosorption of uranyl was investigated by batch tests. Bacteria could continue to multiply up to 350 ppm uranyl concentrations, however this growth was suppressed at 400 ppm, that generally accepted as the minimum concentration for bacterial growth inhibition. The optimal conditions for uranyl biosorption were pH 7, 20 °C, and a contact duration of 30 min with living bacteria. According to the findings of an investigation that used isotherm and kinetics models (Langmuir, Freundlich and pseudo second order), Bacillus sp. strain MRS-1 biosorption seemed to be dependent on monolayer adsorption as well as certain functional groups that had a strong affinity for uranyl confirmed by Fourier Transform Infrared Spectroscopy (FTIR) analysis. The shifts/sharping of peaks (1081-3304 cm-1) were prominent in treated samples compared to control one. These functional groups could be hydroxyl, amino, and carboxyl. Our findings showed that Bacillus sp. strain MRS-1 has an elevated uranyl biosorption ability, with 24.5 mg/g being achieved. This indicates its potential as a powerful biosorbent for dealing with uranium contamination in drinking water sources and represents a breakthrough in the cleanup of contaminated ecosystems.

Complex interactions between diverse mobile genetic elements drive the evolution of metal-resistant bacterial genomes

In this study, we compared the genomes of three metal-resistant bacteria isolated from mercury-contaminated soil. We identified diverse and novel MGEs with evidence of multiple LGT events shaping their genomic structure and heavy metal resistance. Among the three metal-resistant strains, Sphingobium sp SA2 and Sphingopyxis sp SE2 were resistant to multiple metals including mercury, cadmium, copper, zinc and lead. Pseudoxanthomonas sp SE1 showed resistance to mercury only. Whole genome sequencing by Illumina and Oxford Nanopore technologies was undertaken to obtain comprehensive genomic data. The Sphingobium and Sphingopyxis strains contained multiple chromosomes and plasmids, whereas the Pseudoxanthomonas strain contained one circular chromosome. Consistent with their metal resistance profiles, the strains of Sphingobium and Sphingopyxis contained a higher quantity of diverse metal resistance genes across their chromosomes and plasmids compared to the single-metal resistant Pseudoxanthomonas SE1. In all three strains, metal resistance genes were principally associated with various novel MGEs including genomic islands (GIs), integrative conjugative elements (ICEs), transposons, insertion sequences (IS), recombinase in trio (RIT) elements and group II introns, indicating their importance in facilitating metal resistance adaptation in a contaminated environment. In the Pseudoxanthomonas strain, metal resistance regions were largely situated on a GI. The chromosomes of the strains of Sphingobium and Sphingopyxis contained multiple metal resistance regions, which were likely acquired by several GIs, ICEs, numerous IS elements, several Tn3 family transposons and RIT elements. Two of the plasmids of Sphingobium were impacted by Tn3 family transposons and ISs likely integrating metal resistance genes. The two plasmids of Sphingopyxis harboured transposons, IS elements, an RIT element and a group II intron. This study provides a comprehensive annotation of complex genomic regions of metal resistance associated with novel MGEs. It highlights the critical importance of LGT in the evolution of metal resistance of bacteria in contaminated environments.

Mercury remediation potential of mercury-resistant strain Rheinheimera metallidurans sp. nov. isolated from a municipal waste dumping site

A novel mercury-resistant bacterium, designated strain DCL_24^T, was isolated from the legacy waste at the Daddu Majra dumping site in Chandigarh, India. It showed resistance up to 300 µM of inorganic mercury (mercuric chloride). The isolate was found to be a Gram-negative, facultative anaerobic, motile, and rod-shaped bacterium that can grow at 4 - 30 °C (optimum 25 °C), pH 6.0 - 12.0 (optimum 7.0), and 0 - 4.0 % (w/v) NaCl (optimum 0.5 - 2.0 %). The 16 S rRNA gene-based phylogenetic analysis showed that DCL_ 24 ^T shared a 97.53 % similarity with itsºlosest type strain Rheinheimera muenzenbergensis E-49^T. Insilico DNA-DNA hybridization and average nucleotide identity values were found to be 18.60 % and 73.77 %, respectively, between the genomes of DCL_24^T and R. muenzenbergensis E-49^T. The strain DCL_24^T has 44.33 DNA G+C content (mol %). Based on the phenotypic, chemotaxonomic, and genotypic data, the strain DCL_24^T represents a novel species within the genus Rheinheimera, for which the name Rheinheimera metallidurans sp. nov is proposed. The type strain is DCL_24^T (MTCC13203^T = NBRC115780^T = JCM 35551 ^T). The isolate was found to volatilize and remove mercury efficiently, as demonstrated by X-ray film and dithizone-based colorimetric methods. Around 92 % of mercury removal was observed within 48 h. The mercury-resistant determinant mer operon consisting of merA, encoding the mercuric reductase enzyme, and transport and regulatory genes (merT, merP, merD, and merR) were found in the isolate. Relative expression analysis of merA at increasing concentrations of HgCl₂ was confirmed by quantitative real-time PCR. These data indicate the merA-mediated reduction of toxic Hg²⁺ into a non-toxic volatile Hg⁰. The phytotoxicity assay performed using Arabidopsis thaliana seeds further demonstrated the mercury toxicity reduction potential of DCL_24^T. The study shows that this novel isolate, DCL_24^T, is an interesting candidate for mercury bioremediation. However, further studies are required to assess the bioremediation efficacy of the strain under the harsh environmental conditions prevailing in polluted sites.

River sediment microbial community composition and function impacted by thallium spill

Thallium (Tl) is widely used in various industries, which increases the risk of leakage into the environment. Since Tl is highly toxic, it can do a great harm to human health and ecosystem. In order to explore the response of freshwater sediment microorganisms to sudden Tl spill, metagenomic technique was used to elucidate the changes of microbial community composition and functional genes in river sediments. Tl pollution could have profound impacts on microbial community composition and function. Proteobacteria remained the dominance in contaminated szediments, indicating that it had a strong resistance to Tl contamination, and Cyanobacteria also showed a certain resistance. Tl pollution also had a certain screening effect on resistance genes and affected the abundance of resistance genes. Metal resistance genes (MRGs) and antibiotic resistance genes (ARGs) were enriched at the site near the spill site, where Tl concentration was relatively low among polluted sites. When Tl concentration was higher, the screening effect was not obvious and the resistance genes even became lower. Moreover, there was a significant correlation between MRGs and ARGs. In addition, co-occurrence network analysis showed that Sphingopyxis had the most links with resistance genes, indicating that it was the biggest potential host of resistance genes. This study provided new insight towards the shifts in the composition and function of microbial communities after sudden serious Tl contamination.

Exopolysaccharide-producing bacteria enhanced Pb immobilization and influenced the microbiome composition in rhizosphere soil of pakchoi (Brassica chinensis L.)

Lead (Pb) contamination of planting soils is increasingly serious, leading to harmful effects on soil microflora and food safety. Exopolysaccharides (EPSs) are carbohydrate polymers produced and secreted by microorganisms, which are efficient biosorbent materials and has been widely used in wastewater treatment to remove heavy metals. However, the effects and underlying mechanism of EPS-producing marine bacteria on soil metal immobilization, plant growth and health remain unclear. The potential of Pseudoalteromonas agarivorans Hao 2018, a high EPS-producing marine bacterium, to produce EPS in soil filtrate, immobilize Pb, and inhibit its uptake by pakchoi (Brassica chinensis L.) was studied in this work. The effects of strain Hao 2018 on the biomass, quality, and rhizospheric soil bacterial community of pakchoi in Pb-contaminated soil were further investigated. The results showed that Hao 2018 reduced the Pb concentration in soil filtrate (16%–75%), and its EPS production increased in the presence of Pb2+. When compared to the control, Hao 2018 remarkably enhanced pakchoi biomass (10.3%–14.3%), decreased Pb content in edible tissues (14.5%–39.2%) and roots (41.3%–41.9%), and reduced the available Pb content (34.8%–38.1%) in the Pb-contaminated soil. Inoculation with Hao 2018 raised the pH of the soil, the activity of several enzymes (alkaline phosphatase, urease, and dehydrogenase), the nitrogen content (NH4+-N and NO3−-N), and the pakchoi quality (Vc and soluble protein content), while also raising the relative abundance of bacteria that promote plant growth and immobilize metals, such as Streptomyces and Sphingomonas. In conclusion, Hao 2018 reduced the available Pb in soil and pakchoi Pb absorption by increasing the pH and activity of multiple enzymes and regulating microbiome composition in rhizospheric soil.

Gaseous mercury evasion from bare and grass-covered soils contaminated by mining and ore roasting (Isonzo River alluvial plain, Northeastern Italy)

High amounts of mercury (Hg) can be released into the atmosphere from soil surfaces of legacy contaminated areas as gaseous elemental mercury (Hg⁰). The alluvial plain of the Isonzo River (NE Italy) suffered widespread Hg contamination due to the re-distribution of Hg-enriched material discharged by historical cinnabar mining at the Idrija mine (Slovenia), but an assessment of Hg⁰ releases from the soils of this area is still lacking. In this work, Hg⁰ fluxes at the soil-air interface were evaluated using a non-steady state flux chamber coupled with a real-time Hg⁰ analyser at 6 sites within the Isonzo River plain. Measurements were performed in summer, autumn, and winter both on bare and grass-covered soil plots at regular time intervals during the diurnal period. Moreover, topsoils were analysed for organic matter content and Hg total concentration and speciation. Overall, Hg⁰ fluxes tracked the incident UV radiation during the sampling periods with daily averages significantly higher in summer (62.4 ± 14.5-800.2 ± 178.8 ng m^-2 h^-1) than autumn (15.2 ± 4.7-280.8 ± 75.6 ng m^-2 h^-1) and winter (16.9 ± 7.9-187.8 ± 62.7 ng m^-2 h^-1) due to higher irradiation and temperature, which favoured Hg reduction reactions. In summer and autumn significant correlations were observed between Hg⁰ fluxes and soil Hg content (78-95% cinnabar), whereas this relationship was not observed in winter likely due to relatively low emissions found in morning measurements in all sites coupled with low temperatures. Finally, vegetation cover effectively reduced Hg⁰ releases in summer (∼9-68%) and autumn (∼41-78%), whereas the difference between fluxes from vegetated and bare soils was not evident during winter dormancy due to scarce soil shading. These results suggest the opportunity of more extended spatial monitoring of Hg⁰ fluxes particularly in the croplands covering most of the Isonzo River alluvial plain and where bare soils are frequently disturbed by agricultural practices and directly exposed to radiation.

Applying fulvic acid for sediment metals remediation: Mechanism, factors, and prospect

Fulvic acid (FA) has been shown to play a decisive role in controlling the environmental geochemical behavior of metals. As a green and natural microbial metabolite, FA is widely used in environmental remediation because of its good adsorption complexation and redox ability. This paper introduces the reaction mechanism and properties of FA with metals, and reviews the progress of research on the remediation of metal pollutant by FA through physicochemical remediation and bioremediation. FA can control the biotoxicity and migration ability of some metals, such as Pb, Cr, Hg, Cd, and As, through adsorption complexation and redox reactions. The concentration, molecular weight, and source are the main factors that determine the remediation ability of FA. In addition, the ambient pH, temperature, metal ion concentrations, and competing components in sediment environments have significant effects on the extent and rate of a reaction between metals and FA during the remediation process. Finally, we summarize the challenges that this promising environmental remediation tool may face. The research directions of FA in the field of metals ecological remediation are also prospected. This review can provide new ideas and directions for the research of remediation of metals contaminants in sediments.

Poly-γ-glutamic acid promoted maize root development by affecting auxin signaling pathway and the abundance and diversity of rhizosphere microbial community

BACKGROUND

The root systems of higher plants play an important role in plant growth and development. In our present study, it was found that poly-γ-glutamic acid (γ-PGA), an environmentally friendly biomacromolecule, significantly improved root development in maize.

RESULTS

After treatment with γ-PGA for 7 days, the fresh weight of maize roots was significantly increased and the differences between γ-PGA treated group and control group were mainly caused by the number (higher by 71.87% compared to the control) and length of lateral roots. RNAseq and RT-PCR analyses showed that γ-PGA treatment upregulated the expression of genes related to the synthesis of auxins and auxin signal in maize roots. In addition, γ-PGA promoted the accumulation of plant growth-promoting bacteria, such as Azospirillum, Azohydromonas, Ramlibacter, and Sphingobium (Proteobacteria), Streptomyces (Actinobacteria), Parasegetibacter (Bacteroidetes), and Gemmatimonas (Gemmatimonadetes) in rhizosphere soil and the secretion of auxins. The results of this study deepened our understanding of the effects and mechanism of γ-PGA on maize root development, and as well as highlighted the possibility of using γ-PGA to improve crop growth and soil environment.

CONCLUSIONS

γ-PGA promotes early growth and development of maize roots by inducing the secretion and accumulation of auxin in roots and in rhizosphere soil, and increasing the abundance of plant growth promoting bacteria.

The trafficking of HgII by alleviating its toxicity via Citrobacter sp. IITISM25 in batch and pilot-scale investigation

The study aims to see how effective the Citrobacter species strain is in removing Hg^II under stressful conditions. For this, a response surface methodology was chosen to optimized pH, temperature, and biomass for effective biotransformation of Hg^II. The optimized value for pH, temperature, and biomass were 6.5, 30 °C, and 2 mg/l with 89% Hg^II removal potential. TEM-EDX showed accumulated mercury onto the bacterial surface. Pot study was conducted to check the potentiality of this strain in alleviating the toxicity in Solanum lycopersicum L. under different concentrations of mercury. The enhancement in antioxidative enzymes, as well as mercury accumulation, was observed in test plants inoculated with IITISM25. Obtained result showed a greater accumulation of mercury in the root system than that of the shoot system due to poor translocation. Moreover, mercury reductase enzyme synthesis was also boosted by the addition of β-mercaptoethanol and L-cysteine. The optimized condition for maximum enzyme synthesis was at pH 7.5 and temperature 30 °C with Km = 48.07 μmol and Vmax = 9.75 μmol/min. Thus, we can say that Citrobacter species strain IITISM25 can be effectively applied in remediation of Hg^II stress condition as well as promotion of Solanum lycopersicum L growth under stress conditions as a promising host.

Biodetoxification mercury by using a marine bacterium Marinomonas sp. RS3 and its merA gene expression under mercury stress

Mercury (Hg) pollution in water has been a problem for the ecosystem and human health, thus eco-friendly remediation methods are gaining traction around the world. In this study, a bacterial strain designated as RS3 isolated from the Red Sea (Saudi Arabia) has shown tolerance to more than 250 mg/L of Hg²⁺ on minimum inhibitory studies. The isolate RS3 was identified as Marinomonas sp., (Accession No: OK271312) using 16s rRNA sequencing. Tracing the growth curve for the RS3 showed that maximum growth attained at 72 h and only 10% reduction than the control medium for 50 mg/L HgCl₂ supplemented seawater medium, which continued to reduce as 21% to 60 with the increment of HgCl₂ from 100 to 350 mg/L. The Hg²⁺ removal potential of RS3 is observed to be 78% at 50 mg/L HgCl₂/72 h, which is significantly altered with the addition of carbon source such as glucose (84.5%) > fructose (79.8%) > control (78%) > citrate (73.4%) > acetate (60.2%) > maltose (54.7%). Box-Behnken design (BBD) well proposed a model with R² value of 0.8922, which predict a utmost Hg²⁺ removal of 89.5% by RS2 at favorable conditions (pH-7; NaC 1% and glucose 5%) at 72 h. Mercuric reductase enzyme encoded merA gene expression was found to be high in RS3 isolates cultivated in 100 mg/L of HgCl₂ in comparison with other variables. Thus the seawater isolate Marinomonas sp. RS3 expressed a significant tolerance and removal potential towards the Hg²⁺, which would make it is a noteworthy applicant for effective mercury remediation practices.

Mechanisms controlling the transformation of and resistance to mercury(II) for a plant-associated Pseudomonas sp. strain, AN-B15

Bioremediation using mercury (Hg)-volatilizing and immobilizing bacteria is an eco-friendly and cost-effective strategy for Hg-polluted farmland. However, the mechanisms controlling the transformation of and resistance to Hg(II) by these bacteria remain unknown. In this study, a plant-associated Pseudomonas sp. strain, AN-B15 was isolated and determined to effectively remove Hg(II) under both nutrient-poor and nutrient-rich conditions via volatilization by transforming Hg(II) to Hg(0) and immobilization by transforming Hg(II) to mercury sulfide and Hg-sulfhydryl. Genome and transcriptome analyses revealed that the molecular mechanisms involved in Hg(II) resistance in AN-B15 were a collaborative process involving multiple metabolic systems at the transcriptional level. Under Hg(II) stress, AN-B15 upregulated genes involved in the mer operon and producing the reducing power to rapidly volatilize Hg(II), thereby decreasing its toxicity. Hydroponic culture experiments also revealed that inoculation with strain AN-B15 alleviated Hg-induced toxicity and reduced the uptake of Hg(II) in the roots of wheat seedlings, as explained by the volatilization and immobilization of Hg(II) and plant growth-promoting traits of AN-B15. Overall, the results from the in vitro assays provided vital information that are essential for understanding the mechanism of Hg(II) resistance in plant-associated bacteria, which can also be applied for the bioremediation of Hg-contamination in future.

Phytoremediation of Cadmium Polluted Soils: Current Status and Approaches for Enhancing

Cadmium (Cd) is a heavy metal present in atmosphere, rocks, sediments, and soils without a known role in plants. It is relatively mobile and can easily enter from soil into groundwater and contaminate the food chain. Its presence in food in excess amounts may cause severe conditions in humans, therefore prevention of cadmium entering the food chain and its removal from contaminated soils are important steps in preserving public health. In the last several years, several approaches for Cd remediation have been proposed, such as the use of soil amendments or biological systems for reduction of Cd contamination. One of the approaches is phytoremediation, which involves the use of plants for soil clean-up. In this review we summarized current data on the use of different plants in phytoremediation of Cd as well as information about different approaches which have been used to enhance phytoremediation. This includes data on the increasing metal bioavailability in the soil, plant biomass, and plant accumulation capacity as well as seed priming as a promising novel approach for phytoremediation enhancing.

Deciphering the bacterial microbiome in response to long-term mercury contaminated soil

Hg contaminated soils are of concern due to the toxic effects on soil microbes. Currently, the adaptation of bacterial community to long-term Hg contamination remains largely unknown. Here, we assessed the effects of Hg contaminated soils on the bacterial communities under controlled conditions using 16S rRNA gene amplicon sequencing. The results showed that the bacterial α-diversity and richness were significant positively correlated with total Hg (p < 0.05). Land-use type, pH, EC, TK, and nitrate-N played important roles in shaping the bacterial communities. Long-term Hg-contaminated soils can be divided into three types based on land use types: slag type, farmland type, and mining area type. The dominant phyla include Proteobacteria, Actinobacteriota, Acidobacteriota, Chloroflexi, and Firmicutes. The dominant genera identified were Pseudomonas, Gaiella, Sphingomonas, Bacillus, Arthrobacter, Nocardioides. Network analysis showed that dominant taxa had non-random co-occurrence patterns and module 1 had an important role in responding Hg stress. Keystone genera identified were Bauldia, Phycicoccus, Sphingomonas, Gaiella, Nitrospira. The above results further our understanding of the adaptation of the bacterial community in long-term Hg-contaminated soil. This study has important guiding significance for the use of bacterial consortia to remediate Hg-contaminated soil.

Responses of bacterial taxonomic attributes to mercury species in rhizosphere paddy soil under natural sulphur-rich biochar amendment

Biochar and sulphur (S) are important factors regulating the level, speciation and transformation of mercury (Hg), leading to alterations in the assemblage of the soil microbial community. However, variations in the taxonomic attributes of the rhizosphere soil bacterial community arising from the Hg speciation in paddy soil, amended with natural S-rich biochar (NSBC) derived from the pyrolysis of S-rich oilseed rape straw, remain unclear. Herein, a rice pot experiment was conducted. Hg-polluted paddy soils were amended with NSBC and low-S biochar (LSBC) to evaluate the role of Hg chemical form affected by NSBC in regulating the taxonomic attributes of rhizosphere soil, including microbial abundance, composition, and ecological clusters within the co-occurrence network of microbial communities. Results showed that microbial abundance was higher in soils with lower Hg levels, and mean increases of 149 observed operational taxonomic units (OTUs) and 238 predicted OTUs (Chao 1) were observed, with a 1 mg kg^-1 decrease in the total Hg (T-Hg) content. Among the 13 predictor variables, the T-Hg content was the strongest and most consistent predictor of the bacterial taxonomic attributes. This finding may be attributed to the fact that the drastic reduction in T-Hg and Hg bioavailability induced by NSBC results in the decrease of Hg stress on the soil microbiome. Moreover, NSBC amendment shifted the ecological clusters toward the amelioration of Hg pollution.

Bioengineered Microbes for Soil Health Restoration - Present Status and Future

According to the United Nations Environment Programme (UNEP), soil health is declining over the decades and it has an adverse impact on human health and food security. Hence, soil health restoration is a need of the hour. It is known that microorganisms play a vital role in remediation of soil pollutants like heavy metals, pesticides, hydrocarbons, etc. However, the indigenous microbes have a limited capacity to degrade these pollutants and it will be a slow process. Genetically modified organisms (GMOs) can catalyze the degradation process as their altered metabolic pathways lead to hypersecretions of various biomolecules that favor the bioremediation process. This review provides an overview on the application of bioengineered microorganisms for the restoration of soil health by degradation of various pollutants. It also sheds light on the challenges of using GMOs in environmental application as their introduction may affect the normal microbial community in soil. Since soil health also refers to the potential of native organisms to survive, the possible changes in the native microbial community with the introduction of GMOs are also discussed. Finally, the future prospects of using bioengineered microorganisms in environmental engineering applications to make the soil fertile and healthy have been deciphered. With the alarming rates of soil health loss, the treatment of soil and soil health restoration need to be fastened to a greater pace and the combinatorial efforts unifying GMOs, plant growth-promoting rhizobacteria, and other soil amendments will provide an effective solution to soil heath restoration ten years ahead.

Characterization of a mercury tolerant strain of Staphylococcus arlettae from Darjeeling hills with an account of its antibiotic resistance pattern and metabolome

Mercury (Hg) is a ubiquitous heavy metal grouped with the top ten most toxic pollutants affecting both human and environmental health. Consequently, mercury contamination due to anthropogenic interference has become a rising global concern. The bacterial strain MTD10A was isolated from soil samples collected over the Darjeeling hills. Heavy metal tolerance study conducted exhibited considerable tolerance to mercury by this bacterial isolate at unprecedented concentrations of up to 0.1 mg/mL of HgCl₂. Biochemical characterization and molecular identification via 16S rRNA sequencing identified this highly tolerant bacteria as a strain of a Coagulase Negative Staphylococcus arlettae. This study also maps the resistance pattern of MTD10A against clinically relevant antibiotics and contains a broad assessment of the metabolomic profile of the bacteria achieved via GC-MS. Tolerance of MTD10A to such excessive levels of mercury shown in our study suggests the possibility of a promising candidate for bioremediation in heavily mercury contaminated areas.

The synergy of mercury biosorption through Brevundimonas sp. IITISM22: Kinetics, isotherm, and thermodynamic modeling

This research experiment was conducted to investigate the potential of Brevundimonas species IITISM22 to remove mercury by using live biomass of bacterial cells at 298, 308, and 318 K. Characterization of bio-sorbent was done by FT-IR and SEM-EDX. The prime functional groups accountable for binding Hg were OH, -NH₂, -CH, -SH and -COO. The deformed bacterial structure was seen after Hg adsorption over the bacterial cell. Influences of different experimental factors, such as pH, temperature, contact time, Hg concentration, and biomass dose was examined. IITISM22 exhibited the highest Hg absorption at pH 6.5, contact time of 4 h, and showed an increased adsorption capacity while increasing the concentration of Hg. Kinetics were recommended by pseudo-second-order for adsorption process and isotherm was adequately defined by the Linear Langmuir isotherm model (K_L) = 1.4, 1.2, 0.9 mg/l; (R_L) = 0.020, 0.015, 0.013, respectively than Freundlich isotherm model. The Activation energy (E_a) of biosorption calculated were (131.10 KJ/mole) by using Arrhenius equation, and the thermodynamic parameters were ΔG⸰ (-41.03, -16.33, -16.12 KJ/mol), ΔH⸰ (-36.87 KJ/mol) and ΔS⸰ (-194.03 J/mol), respectively. These findings suggest that the removal process was based on chemisorption and the biosorption was exothermic. The result of the current experiment indicated that the IITISM22 could be an authentic biosorbent for Hg detoxification.

Analytical Methodologies for Agrometallomics: A Critical Review

Agrometallomics, as an independent interdiscipline, is first defined and described in this review. Metallic elements widely exist in agricultural plants, animals and edible fungi, seed, fertilizer, pesticide, feedstuff, as well as the agricultural environment and ecology, and even functional and pathogenic microorganisms. So, the agrometallome plays a vital role in molecular and organismic mechanisms like environmetallomics, metabolomics, proteomics, lipidomics, glycomics, immunomics, genomics, etc. To further reveal the inner and mutual mechanism of the agrometallome, comprehensive and systematic methodologies for the analysis of beneficial and toxic metals are indispensable to investigate elemental existence, concentration, distribution, speciation, and forms in agricultural lives and media. Based on agrometallomics, this review summarizes and discusses the advanced technical progress and future perspectives of metallic analytical approaches, which are categorized into ultrasensitive and high-throughput analysis, elemental speciation and state analysis, and spatial- and microanalysis. Furthermore, the progress of agrometallomic innovativeness greatly depends on the innovative development of modern metallic analysis approaches including, but not limited to, high sensitivity, elemental coverage, and anti-interference; high-resolution isotopic analysis; solid sampling and nondestructive analysis; metal chemical species and metal forms, associated molecular clusters, and macromolecular complexes analysis; and metal-related particles or metal within the microsize and even single cell or subcellular analysis.

Characterization of Bacillus nealsonii strain KBH10 capable of reducing aqueous mercury in laboratory-scale reactor

The environmental release of mercury is continuously increasing with high degree of mobility, transformation and amplified toxicity. Improving remediation strategies is becoming increasingly important to achieve more stringent environmental safety standards. This study develops a laboratory-scale reactor for bioremediation of aqueous mercury using a biofilm-producing bacterial strain, KBH10, isolated from mercury-polluted soil. The strain was found resistant to 80 mg/L of HgCl₂ and identified as Bacillus nealsonii via 16S rRNA gene sequence analysis. The strain KBH10 was characterized for optimum growth parameters and its mercury biotransformation potential was validated through mercuric reductase assay. A packed-bed column bioreactor was designed for biofilm-mediated mercury removal from artificially contaminated water and residual mercury was estimated. Strain KBH10 could grow at a range of temperature (20-50 °C) and pH (6.0-9.0) with optimum temperature established at 30 °C and pH 7.0. The optimum mercuric reductase activity (77.8 ± 1.7 U/mg) was reported at 30 °C and was stable at a temperature range of 20-50 °C. The residual mercury analysis of artificially contaminated water indicated 60.6 ± 1.5% reduction in mercury content within 5 h of exposure. This regenerative process of biofilm-mediated mercury removal in a packed-bed column bioreactor can provide new insight into its potential use in mercury bioremediation.

Kinetic modelling of the uranium biosorption by Deinococcus radiodurans biofilm

Increasing number of reports on uranium contamination in groundwater bodies is a growing concern. Deinococcus radiodurans biofilm-based U(VI) bioremediation has great potential to provide solution. This study focuses on the kinetic modelling of uranium biosorption by D. radiodurans biofilm biomass and identification of the functional groups involved in the sequestration process. The effect of temperature, pH and amount of biofilm dry mass were studied using two uranyl ion concentrations (100 and 1000 mg/L). D. radiodurans dry biomass showed good affinity for uranyl ion adsorption. The kinetic experiments revealed that the biosorption process was spontaneous and exothermic in nature. The modelling of kinetic adsorption data revealed that U(VI) sorption by D. radiodurans biofilm biomass follows a pseudo-second-order reaction. Mechanism of U(VI) sorption was suggested to follow an intra-particle diffusion model, which includes covalent bonding between U(VI) and functional groups present on the surface of biofilm biomass, and diffusional barrier acts as a rate limiting step. External mass transfer was the rate-limiting step as evident from Boyd and Elovich plot. Chemical modifications in surface functional groups of biofilm biomass, confirmed the involvement of carboxyl, phosphate, and hydroxyl groups in uranium binding as a significant loss in U(VI) sorption capacity was recorded in these chemically modified biomasses. XRD data indicated the formation of metal deposits, predominantly as uranyl phosphates.

Transcriptomic analyses reveal the pathways associated with the volatilization and resistance of mercury(II) in the fungus Lecythophora sp. DC-F1

The biotic enzymatic reduction of mercury II [Hg(II)] to elemental Hg [Hg(0)] is an important pathway for Hg detoxification in natural ecosystems. However, the mechanisms of Hg(II) volatilization and resistance in fungi have not been understood completely. In the present study, we investigated the mechanisms of Hg(II) volatilization and resistance in the fungus Lecythophora sp. DC-F1. Hg(II) volatilization occurred during the investigation via the reduction of Hg(II) to Hg(0) in DC-F1. Comparative transcriptome analyses of DC-F1 revealed 3439 differentially expressed genes under 10 mg/L Hg(II) stress, among which 2770 were up-regulated and 669 were down-regulated. Functional enrichment analyses of genes and pathways further suggested that the Hg(II) resistance of DC-F1 is a multisystem collaborative process with three important transcriptional responses to Hg(II) stress: a mer-mediated Hg detoxification system, a thiol compound metabolism, and a cell reactive oxygen species stress response system. The phylogenetic analysis of merA protein homologs suggests that the Hg(II) reduction by merA is widely distributed in fungi. Overall, this study provides evidence for the reduction of Hg(II) to Hg(0) in fungi via the mer-mediated Hg detoxification system and offers a comprehensive explanation for its role within Hg biogeochemical cycling. These findings offer a strong theoretical basis for the application of fungi in the bioremediation of Hg-contaminated envionments.

Bioremediation by Cupriavidus metallidurans Strain MSR33 of Mercury-Polluted Agricultural Soil in a Rotary Drum Bioreactor and Its Effects on Nitrogen Cycle Microorganisms

Nitrogen cycle microorganisms are essential in agricultural soils and may be affected by mercury pollution. The aims of this study are to evaluate the bioremediation of mercury-polluted agricultural soil using Cupriavidus metallidurans MSR33 in a rotary drum bioreactor (RDB) and to characterize the effects of mercury pollution and bioremediation on nitrogen cycle microorganisms. An agricultural soil was contaminated with mercury (II) (20-30 ppm) and subjected to bioremediation using strain MSR33 in a custom-made RDB. The effects of mercury and bioremediation on nitrogen cycle microorganisms were studied by qPCR. Bioremediation in the RDB removed 82% mercury. MSR33 cell concentrations, thioglycolate, and mercury concentrations influence mercury removal. Mercury pollution strongly decreased nitrogen-fixing and nitrifying bacterial communities in agricultural soils. Notably, after soil bioremediation process nitrogen-fixing and nitrifying bacteria significantly increased. Diverse mercury-tolerant strains were isolated from the bioremediated soil. The isolates Glutamicibacter sp. SB1a, Brevundimonas sp. SB3b, and Ochrobactrum sp. SB4b possessed the merG gene associated with the plasmid pTP6, suggesting the horizontal transfer of this plasmid to native gram-positive and gram-negative bacteria. Bioremediation by strain MSR33 in an RDB is an attractive and innovative technology for the clean-up of mercury-polluted agricultural soils and the recovery of nitrogen cycle microbial communities.

Poly-γ-glutamic acid-producing bacteria reduced Cd uptake and effected the rhizosphere microbial communities of lettuce

Poly-γ-glutamic acid (γ-PGA) could efficiently stabilize heavy metals in the environment. This study characterized the effects of two plant growth-promoting and γ-PGA-producing bacteria Bacillus subtilis W7 and Bacillus amyloliquefaciens W25 on Cd immobilization and γ-PGA production in soil filtrate and on the biomass and Cd uptake by lettuce in Cd-contaminated soil, the impact of these strains on the rhizosphere soil bacterial community was also evaluated. The strains reduced Cd concentration (16-75 %) in soil filtrate and strain W25 had a higher ability of producing γ-PGA and immobilizing Cd than strain W7. Compared with the control, the strains significantly increased the biomass (41-85 %) and reduced Cd uptake (19-41 %) by lettuce, reduced available Cd content (25-37 %) and increased the relative abundance of γ-PGA-producing bacteria (24-30 %) in Cd-contaminated soil, among which the effects of strain W25 were better than that of strain W7. Besides, these isolates also increased soil pH value, urease activity and the relative abundance of plant growth-promoting and metal-immobilizing bacteria such as Sphingomonas and Bacillus. In summary, the two strains reduced soil available Cd and lettuce Cd uptake by increasing the pH value, urease activity and the abundance of γ-PGA-producing bacteria, and regulating bacterial community structure in rhizosphere soil.

The bioremediation potentials and mercury(II)-resistant mechanisms of a novel fungus Penicillium spp. DC-F11 isolated from contaminated soil

Bioremediation of Hg-contaminated soil using microbe-based strategies is a promising and efficient method as it is inexpensive and not harmful to the environment. In this study, a novel Hg(II)-volatilizing fungus Penicillium spp., DC-F11 was isolated and showed bioremediation potential for reducing Hg(II) phytotoxicity, total Hg, and exchangeable Hg in Hg(II)-polluted soil. Subsequently, the mechanisms of Hg(II) volatilization and resistance involved were investigated using multiple complementary techniques. The fungal cells could detoxify Hg(II) by extracellular sequestration via adsorption and precipitation. Moreover, a comparative transcriptome analysis uncovered the primary intracellular adaptive responses of the DC-F11 to Hg(II) stress, including mer-mediated detoxification system, thiol compound metabolism, and oxidative stress defense and damage repair metabolism. These results showed that the resistance of DC-F11 to Hg(II) was generally a multisystem collaborative process. Here, we report, for the first time, that the mer-mediated detoxification system was responsible for Hg(II) volatilization in fungus. These findings provide a better understanding of the mechanisms involved in Hg(II) volatilization and resistance that occur in fungi and also provide a strong theoretical basis for the future application of fungi in the bioremediation of Hg-polluted environments.

Bioremediation of toxic heavy metals (THMs) contaminated sites: concepts, applications and challenges

Heavy metal contamination is a global issue, where the prevalent contaminants are arsenic (As), cadmium (Cd), chromium (Cr)(VI), mercury (Hg), and lead (Pb). More often, they are collectively known as "most problematic heavy metals" and "toxic heavy metals" (THMs). Their treatment through a variety of biological processes is one of the prime interests in remediation studies, where heavy metal-microbe interaction approaches receive high interest for their cost effective and ecofriendly solutions. In this review, we provide an up to date information on different microbial processes (bioremediation) for the removal of THMs. For the same, emphasis is put on oxidation-reduction, biomineralization, bioprecipitation, bioleaching, biosurfactant technology, biovolatilization, biosorption, bioaccumulation, and microbe-assisted phytoremediation with their selective advantages and disadvantages. Further, the literature briefly discusses about the various setups of cleaning processes of THMs in environment under ex situ and in situ applications. Lately, the study sheds light on the manipulation of microorganisms through genetic engineering and nanotechnology for their advanced treatment approaches.

Mercury detoxification by absorption, mercuric ion reductase, and exopolysaccharides: a comprehensive study

Mercury (Hg), the environmental toxicant, is present in the soil, water, and air as it is substantially distributed throughout the environment. Being extremely toxic even at low concentration, its remediation is utterly important. Therefore, it is necessary to detoxify the contaminant within the acceptable limits before threatening the environment. Although various conventional methods are being used, irrespective of high cost, it produces intermediate toxic by-product too. Biological methods are eco-friendly, clean, greener, and safer for the remediation of heavy metals corresponding to the conventional remediation due to their economic and high-tech constraints. Bioremediation is now being used for Hg (II) removal, which involves biosorption and bioaccumulation mechanisms or both, also mercuric ion reductase, exopolysaccharide play significant role in detoxification of mercury by acting a potential instrument for the remediation of heavy metals. In this review paper, we shed light on problems caused by mercury pollution, mercury cycle, and its global scenario and detoxification approaches by biological methods and result found in the literature.

Effects of Mercury II on Cupriavidus metallidurans Strain MSR33 during Mercury Bioremediation under Aerobic and Anaerobic Conditions

Mercury is a toxic element that harms organisms and disturbs biogeochemical cycles. Mercury bioremediation is based on the reduction of Hg (II) to Hg (0) by mercury-resistant bacteria. Cupriavidus metallidurans MSR33 possesses a broad-spectrum mercury resistance. This study aims to establish the effects of mercury on growth, oxygen uptake, and mercury removal parameters by C. metallidurans MSR33 in aqueous solution during aerobic and anaerobic mercury bioremediation. A new culture medium (GBC) was designed. The effects of mercury (II) (20 ppm) on growth parameters, oxygen uptake, and mercury removal were evaluated in GBC medium in a bioreactor (3 L) under aerobiosis. The anaerobic kinetics of mercury removal was evaluated by nitrogen replacement during mercury bioremediation in a bioreactor. Strain MSR33 reached a growth rate of µ = 0.43 h−1 in the bioreactor. Mercury inhibited oxygen uptake and bacterial growth; however, this inhibition was reversed after 5 h. Strain MSR33 was able to reduce Hg (II) under aerobic and anaerobic conditions, reaching, at 24 h, a metal removal of 97% and 71%, respectively. Therefore, oxygen was crucial for efficient mercury removal by this bacterium. Strain MSR33 was capable of tolerating the toxic effects of mercury (II) during aerobic bioremediation and recovered its metabolic activity.

Characterization of a Linuron-Specific Amidohydrolase from the Newly Isolated Bacterium Sphingobium sp. Strain SMB

The phenylurea herbicide linuron is globally used and has caused considerable concern because it leads to environmental pollution. In this study, a highly efficient linuron-transforming strain Sphingobium sp. SMB was isolated, and a gene (lahB) responsible for the hydrolysis of linuron to 3,4-dichloroaniline and N,O-dimethylhydroxylamine was cloned from the genome of strain SMB. The lahB gene encodes an amidohydrolase, which shares 20-53% identity with other biochemically characterized amidohydrolases, except for the newly reported linuron hydrolase Phh (75%). The optimal conditions for the hydrolysis of linuron by LahB were determined to be pH 7.0 and 30 °C, and the K_m value of LahB for linuron was 37.3 ± 1.2 μM. Although LahB and Phh shared relatively high identity, LahB exhibited a narrow substrate spectrum (specific for linuron) compared to Phh (active for linuron, diuron, chlortoluron, etc.). Sequence analysis and site-directed mutagenesis revealed that Ala261 of Phh was the key amino acid residue affecting the substrate specificity. Our study provides a new amidohydrolase for the specific hydrolysis of linuron.

Remediation of mercury contaminated soil, water, and air: A review of emerging materials and innovative technologies

Mercury contamination in soil, water and air is associated with potential toxicity to humans and ecosystems. Industrial activities such as coal combustion have led to increased mercury (Hg) concentrations in different environmental media. This review critically evaluates recent developments in technological approaches for the remediation of Hg contaminated soil, water and air, with a focus on emerging materials and innovative technologies. Extensive research on various nanomaterials, such as carbon nanotubes (CNTs), nanosheets and magnetic nanocomposites, for mercury removal are investigated. This paper also examines other emerging materials and their characteristics, including graphene, biochar, metal organic frameworks (MOFs), covalent organic frameworks (COFs), layered double hydroxides (LDHs) as well as other materials such as clay minerals and manganese oxides. Based on approaches including adsorption/desorption, oxidation/reduction and stabilization/containment, the performances of innovative technologies with the aid of these materials were examined. In addition, technologies involving organisms, such as phytoremediation, algae-based mercury removal, microbial reduction and constructed wetlands, were also reviewed, and the role of organisms, especially microorganisms, in these techniques are illustrated.

Bioremediation of Hg-contaminated soil by combining a novel Hg-volatilizing Lecythophora sp. fungus, DC-F1, with biochar: Performance and the response of soil fungal community

Reducing Hg contamination in soil using eco-friendly approaches has attracted increasing attention in recent years. In this study, a novel multi-metal-resistant Hg-volatilizing fungus belonging to Lecythophora sp., DC-F1, was isolated from multi-metal-polluted mining-area soil, and its performance in reducing Hg bioavailability in soil when used in combination with biochar was investigated. The isolate displayed a minimum inhibitory concentration of 84.5mg·L^-1 for Hg(II) and volatilized >86% of Hg(II) from LB liquid medium with an initial concentration of 7.0mg·L^-1 within 16h. Hg(II) contents in soils and grown lettuce shoots decreased by 13.3-26.1% and 49.5-67.7%, respectively, with DC-F1 and/or biochar addition compared with a control over 56days of incubation. Moreover, treatment with both bioagents achieved the lowest Hg content in lettuce shoots. Hg presence and DC-F1 addition significantly decreased the number of fungal ITS gene copies in soils. High-throughput sequencing showed that the soil fungal community compositions were more largely influenced by DC-F1 addition than by biochar addition, with the proportion of Mortierella increasing and those of Penicillium and Thielavia decreasing with DC-F1 addition. Developing the coupling of Lecythophora sp. DC-F1 with biochar into a feasible approach for the recovery of Hg-contaminated soils is promising.

Potential of mercury-tolerant bacteria for bio-uptake of mercury leached from discarded fluorescent lamps

In this study, ten bacterial strains were found to be mercury resistant after their isolation from Qatari coastal sediments. Tolerance was found to be up to 100-150 ppm for five strains. Those strains had optimum growth conditions at salinity level of 10 ppm NaCl and pH 7-8. Starting from a concentration 7.9 ppm of mercury extracted from fluorescent lamps and after 6 days of incubation at 37 °C, two isolated strains HA6 (Bacillus spp.) and HA9 (Acinetobacter sp.) showed 96.7% and 98.9% of mercury bio-uptake efficiency, respectively. Other strains were capable of removing more than 60% of extracted mercury.

Characterization of an Hg(II)-volatilizing Pseudomonas sp. strain, DC-B1, and its potential for soil remediation when combined with biochar amendment

Hg contamination is a critical environmental problem, and its remediation using cost-effective and environmentally friendly methods is highly desirable. In this study, a multi-metal-resistant bacterium showing strong Hg(II) volatilization ability, Pseudomonas sp. DC-B1, was isolated from heavy metal-contaminated soils. DC-B1 volatilized 81.1%, 79.2% and 74.3% of the initial Hg2+ from culture solutions with initial Hg2+ concentrations of 5.1, 10.4, and 15.7 mg/L, respectively, within 24 h. Microcosm experiments were performed to investigate the remediation of Hg(II)-spiked soils inoculated with DC-B1 coupled with sawdust biochar amendment. The efficiency of Hg removal from two types of soil samples with different properties and an initial Hg(II) content of approximately 100 mg/kg was enhanced 5.7-13.1% by bio-augmentation with inoculation of the bacterial strain DC-B1, 5.4-10.7% by amendment of 4% (w/w) biochar, and 10.7-23.2% by the combination of DC-B1 and biochar amendments over an incubation period of 24 d over the efficiency in the control treatment under flooded conditions. Longer root lengths were observed in lettuce grown in the treated soils than in lettuce from the control soil, confirming the bioremediation efficacy of the two bioagents for soil Hg contamination.

Cell Surface Display of MerR on Saccharomyces cerevisiae for Biosorption of Mercury

The metalloregulatory protein MerR which plays important roles in mer operon system exhibits high affinity and selectivity toward mercury (II) (Hg²⁺). In order to improve the adsorption ability of Saccharomyces cerevisiae for Hg²⁺, MerR was displayed on the surface of S. cerevisiae for the first time with an α-agglutinin-based display system in this study. The merR gene was synthesized after being optimized and added restriction endonuclease sites EcoR I and Mlu I. The display of MerR was indirectly confirmed by the enhanced adsorption ability of S. cerevisiae for Hg²⁺ and colony PCR. The hydride generation atomic absorption spectrometry was applied to measure the Hg²⁺ content in water. The engineered yeast strain not only showed higher tolerance to Hg, but also their adsorption ability was much higher than that of origin and control strains. The engineered yeast could adsorb Hg²⁺ under a wide range of pH levels, and it could also adsorb Hg²⁺ effectively with Cd²⁺ and Cu²⁺ coexistence. Furthermore, the engineered yeast strain could adsorb ultra-trace Hg²⁺ effectively. The results above showed that the surface-engineered yeast strain could adsorb Hg²⁺ under complex environmental conditions and could be used for the biosorption and bioremediation of environmental Hg contaminants.

Development of a two-stage biotransformation system for mercury-contaminated soil remediation

Utilization of bacterial volatilization can be problematic to remediate mercury (Hg)-contaminated soils because most of the Hg in soils is bound to soil particles. The objective of this study was to develop a two-stage system (chemical extraction followed by microbial reduction) for Hg-contaminated soil remediation. The tasks were to (1) select the extraction reagents for Hg extraction, (2) assess the effects of extraction reagents on the growth of Hg-reducing bacterial strains, and (3) evaluate the effectiveness of Ca²⁺ and Mg²⁺ addition on merA gene (Hg reductase) induction. Bacterial inhibition was observed with the addition of 0.1 M ethylenediaminetetraacetic acid or citric acid. Up to 65% of Hg was biotransformed (Hg concentration = 69 mg/kg) from the soils after a 24 h extraction using 0.5 M ammonium thiosulfate. Ca²⁺ and Mg²⁺ were selected because they have the same electric charge as Hg and the studied groundwater contained high concentrations of Ca²⁺ and Mg²⁺. Results showed that the addition of 200 mg/L Ca²⁺ or 650 mg/L Mg²⁺ could reach effective merA induction. In the two-stage experiment, 120 mg/kg Hg-contaminated soils were extracted with 2 rounds of extraction processes for 10 h using 0.5 M ammonium thiosulfate. Approximately 77% of Hg was extracted from the soils after the first-step extraction process. Up to 81% of Hg²⁺ was transformed from the washing solution via the biotransformation processes with Enterobacter cloacae addition and Ca²⁺ and Mg²⁺ supplementation. The two-stage remedial system has the potential to be developed into a practical technology to remediate Hg-contaminated sites.

Structural and functional characterization of mercuric reductase from Lysinibacillus sphaericus strain G1

In response to the widespread presence of inorganic Hg in the environment, bacteria have evolved resistance systems with mercuric reductase (MerA) as the key enzyme. MerA enzymes have still not been well characterized from gram positive bacteria. Current study reports physico-chemical, kinetic and structural characterization of MerA from a multiple heavy metal resistant strain of Lysinibacillus sphaericus, and discusses its implications in bioremediation application. The enzyme was homodimeric with subunit molecular weight of about 60 kDa. The K_m and V_max were found to be 32 µM of HgCl₂ and 18 units/mg respectively. The enzyme activity was enhanced by β-mercaptoethanol and NaCl up to concentrations of 500 µM and 100 mM respectively, followed by inhibition at higher concentrations. The enzyme showed maximum activity in the pH range of 7-7.5 and temperature range of 25-50 °C, with melting temperature of 67 °C. Cu²⁺ exhibited pronounced inhibition of the enzyme with mixed inhibition pattern. The enzyme contained FAD as the prosthetic group and used NADPH as the preferred electron donor, but it showed slight activity with NADH as well. Structural characterization was carried out by circular dichroism spectrophotometry and X-ray crystallography. X-ray confirmed the homodimeric structure of enzyme and gave an insight on the residues involved in catalytic binding. In conclusion, the investigated enzyme showed higher catalytic efficiency, temperature stability and salt tolerance as compared to MerA enzymes from other mesophiles. Therefore, it is proposed to be a promising candidate for Hg²⁺ bioremediation.

Transcriptomic analysis of nickel exposure in Sphingobium sp. ba1 cells using RNA-seq

Nickel acts as cofactor for a number of enzymes of many bacteria species. Its homeostasis is ensured by proteins working as ion efflux or accumulation systems. These mechanisms are also generally adopted to counteract life-threatening high extra-cellular Ni²⁺ concentrations. Little is known regarding nickel tolerance in the genus Sphingobium. We studied the response of the novel Sphingobium sp. ba1 strain, able to adapt to high Ni²⁺ concentrations. Differential gene expression in cells cultured in 10 mM Ni²⁺, investigated by RNA-seq analysis, identified 118 differentially expressed genes. Among the 90 up-regulated genes, a cluster including genes coding for nickel and other metal ion efflux systems (similar to either cnrCBA, nccCBA or cznABC) and for a NreB-like permease was found. Comparative analyses among thirty genomes of Sphingobium species show that this cluster is conserved only in two cases, while in the other genomes it is partially present or even absent. The differential expression of genes encoding proteins which could also work as Ni²⁺-accumulators (HupE/UreJ-like protein, NreA and components of TonB-associated transport and copper-homeostasis systems) was also detected. The identification of Sphingobium sp. ba1 strain adaptive mechanisms to nickel ions, can foster its possible use for biodegradation of poly-aromatic compounds in metal-rich environments.

Cloning, Expression, Isotope Labeling, and Purification of Transmembrane Protein MerF from Mercury Resistant Enterobacter sp. AZ-15 for NMR Studies

Mercury resistant (HgR) Enterobacter sp. AZ-15 was isolated from heavy metal polluted industrial wastewater samples near to districts Kasur and Sheikhupura, Pakistan. 16S rDNA ribotyping and phylogentic analysis showed 98% homology with already reported Enterobacter species. The merF gene encoding transmembrane protein-MerF was amplified from genomic DNA and ligated into pET31b+ vector using restriction endonucleases, SphI and XhoI. The genetic codons of merF gene encoding cysteine residues were mutated into codons, translating into serine residues by site-directed mutagenesis. Ketosteroid isomerase (KSI), a fusion tag which is present in pET31b+ vector, was used in the expression of merFm gene. KSI was used to drive the target peptide (MerFm) into inclusion bodies so that the peptide yield and purity were increased. The stable plasmid pET31b+:merFm was transformed into C43(DE3) E.coli cells. The high expression of uniformly 15N isotopically labeled-MerFm protein was induced with 1 mM IPTG. The purification of 15N-MerFm recombinant protein by Ni-NTA and size exclusion chromatography involved an unfolding/refolding procedure. The two-dimensional HSQC NMR spectra of MerFm protein showed the purity and correct number of resonances for each amide. 1H-15N HSQC NMR experiment also confirmed that no modification of the tryptophan residue occurred during cyanogen bromide cleavage. A small scale reservoir of Luria Bertani (LB) medium supplemented with 20 μg/ml of HgCl2 showed 90% detoxification of Hg by Enterobacter sp. AZ-15. The accumulation of Hg on the cell surface of this strain was visualized by scanning electron microscopy (SEM) which confirmed its potential use in Hg-bioremediation.

Metal resistance mechanisms in Gram-negative bacteria and their potential to remove Hg in the presence of other metals

Contamination of the environment by heavy metals has been increasing in recent years due to industrial activities. Thus research involving microorganisms capable of surviving in multi-contaminated environments is extremely important. The objectives of the present study were to evaluate the removal of mercury alone and in the presence of cadmium, nickel and lead by four mercury-resistant microorganisms; estimate the removal of Cd, Ni and Pb; understand the mechanisms involved (reduction, siderophores, biofilms, biosorption and bioaccumulation) in the metal resistance of the isolate Pseudomonas sp. B50D; and determine the capacity of Pseudomonas sp. B50D in removing Hg, Cd, Ni and Pb from an industrial effluent. It was shown that the four isolates evaluated were capable of removing from 62% to 95% of mercury from a culture medium with no addition of other metals. The isolate Pseudomonas sp. B50D showed the best performance in the removal of mercury when evaluated concomitantly with other metals. This isolate was capable of removing 75% of Hg in the presence of Cd and 91% in the presence of Ni and Pb. With respect to the other metals it removed 60%, 15% and 85% of Cd, Ni and Pb, respectively. In tests with effluent, the isolate Pseudomonas sp. B50D removed 85% of Hg but did not remove the other metals. This isolate presented reduction, biosorption, biofilm production and siderophore production as its metal resistance mechanisms. Pseudomonas sp. B50D was thus a candidate with potential for application in the bioremediation of effluents with complex metal contaminations.

Bio-augmentation and nutrient amendment decrease concentration of mercury in contaminated soil

Four mercury (Hg) contaminated soils with different pH (7.6, 8.5, 4.2 and 7.02) and total organic carbon contents (2.1, 2.2, 4 and 0.9%) were subjected to bioremediation utilizing a Hg volatilizing bacterial strain Sphingobium SA2 and nutrient amendment. In a field with ~280mg/kgHg, 60% of Hg was removed by bio-augmentation in 7days, and the removal was improved when nutrients were added. Whereas in artificially spiked soils, with ~100mg/kgHg, removal due to bio-augmentation was 33 to 48% in 14days. In the field contaminated soil, nutrient amendment alone without bio-augmentation removed 50% of Hg in 28days. Nutrient amendment also had an impact on Hg remediation in the spiked soils, but the best results were obtained when the strain and nutrients both were applied. The development of longer root lengths from lettuce and cucumber seeds grown in the remediated soils confirmed that the soil quality improved after bioremediation. This study clearly demonstrates the potential of Hg-reducing bacteria in remediation of Hg-contaminated soils. However, it is desirable to trap the volatilized Hg for enhanced bioremediation.

Mercury remediation potential of a mercury resistant strain Sphingopyxis sp. SE2 isolated from contaminated soil

A mercury resistant bacterial strain SE2 was isolated from contaminated soil. The 16s rRNA gene sequencing confirms the strain as Sphingopyxis belongs to the Sphingomonadaceae family of the α-Proteobacteria group. The isolate showed high resistance to mercury with estimated concentrations of Hg that caused 50% reduction in growth (EC₅₀) of 5.97 and 6.22mg/L and minimum inhibitory concentrations (MICs) of 32.19 and 34.95mg/L in minimal and rich media, respectively. The qualitative detection of volatilized mercury and the presence of mercuric reductase enzyme proved that the strain SE2 can potentially remediate mercury. ICP-QQQ-MS analysis of the remaining mercury in experimental broths indicated that a maximum of 44% mercury was volatilized within 6hr by live SE2 culture. Furthermore a small quantity (23%) of mercury was accumulated in live cell pellets. While no volatilization was caused by dead cells, sorption of mercury was confirmed. The mercuric reductase gene merA was amplified and sequenced. Homology was observed among the amino acid sequences of mercuric reductase enzyme of different organisms from α-Proteobacteria and ascomycota groups.

Screening of mercury-resistant and indole-3-acetic acid producing bacterial-consortium for growth promotion of Cicer arietinum L

Mercury resistant (Hg^R ) bacteria were screened from industrial effluents and effluents-polluted rhizosphere soils near to districts Kasur and Sheikhupura, Pakistan. Out of 60 isolates, three bacterial strains, Bacillus sp. AZ-1, Bacillus cereus AZ-2, and Enterobacter cloacae AZ-3 showed Hg-resistance as 20 μg ml^-1 of HgCl₂ and indole-3-acetic acid (IAA) production as 8-38 μg ml^-1 . Biochemical and molecular characterization of selected bacteria was confirmed by 16S ribotyping. Mercury resistant genes merA, merB, and merE of mer operon in Bacillus spp. were checked by PCR amplification. The merE gene involved in the transportation of elemental mercury (Hg⁰ ) via cell membrane was first time cloned into pHLV vector and transformed in C43(DE3) Escherichia coli cells. The recombinant plasmid (pHLMerE) was expressed and purified by nickel (Ni⁺² ) affinity chromatography. Chromatographic techniques viz. thin layer chromatography (TLC), high performance liquid chromatography (HPLC), and Gas chromatography-mass spectrometry (GC-MS) confirmed the presence of Indole-3-acetic acid (IAA) in supernatant of selected bacteria. The strain E. cloacae AZ-3 detoxified 88% of mercury (Hg⁺² ) from industrial effluent (p < 0.05) after immobilization in Na-alginate beads. Finally, Hg-resistant and IAA producing bacterial consortium of two strains, Bacillus sp. AZ-1 and E. cloacae AZ-3, inoculated in mercury amended soil with 20 μg ml^-1 HgCl₂ resulted 80, 22, 64, 116, 50, 75, 30, and 100% increase as compared to control plants in seed germination, shoot and root length, shoot and root fresh weight, number of pods per plant, number of seeds and weight of seeds, respectively, of chickpea (Cicer arietinum L.) in pot experiments (p < 0.05).

Heavy metal capture by autochthonous yeasts from a volcanic influenced environment of Patagonia

Heavy metals at elevated concentrations are a major threat to agricultural and human health. Typically, human activities tend to release these metals to the environment in aqueous solutions, generating high levels of pollution due to the mobility of the heavy metals. The aim of the present work was to assess heavy metal tolerance in yeasts isolated from Río Agrio - Lake Caviahue volcanic acidic aquatic environment and to evaluate the capacity of selected strains to capture metals in acidic culture media conditions. The ability of three yeast species, Cryptococcus agrionensis, Cryptococcus sp. 2, and Coniochaeta fodinicola, to tolerate and capture metals in live cultures has been evaluated. These three yeast species showed high tolerance to low pH and elevated concentrations of metals, thus implying their autochthonous status. Minimal inhibitory concentration (MIC) for growth obtained for these isolates showed elevated tolerance to the six heavy metals evaluated and were significantly higher than those registered for other microorganisms. C. agrionensis was able to capture 15.80 mg (g biomass)^-1 of Cu²⁺ (MIC: 0.22 g L^-1 ), Cryptococcus sp. 2 was able to capture 36.25 and 65.28 mg (g biomass)^-1 of Ni²⁺ and Zn²⁺ , respectively (MIC: 0.56 and 1.68, respectively), and C. fodinicola was able to capture 67.11 mg (g biomass)^-1 of Zn²⁺ (MIC: 3.75). This work reported the ability of yeasts to capture metals in acidic conditions for the first time. We hope that it represents the step-stone for future researches in the ability and metabolism of yeasts form acidic aquatic environment related to metal tolerance and capture.