Isolation and characterization of bacteria from mercury contaminated sites in Rio Grande do Sul, Brazil, and assessment of methylmercury removal capability of a Pseudomonas putida V1 strain

Degradation of organic mercury in high salt environments by a marine aerobic bacterium Alteromonas macleodii KD01

Mercury (Hg), particularly organic mercury, poses a global concern due to its pronounced toxicity and bioaccumulation. Bioremediation of organic mercury in high-salt wastewater faces challenges due to the growth limitations imposed by elevated Cl- and Na+ concentrations on microorganisms. In this study, an isolated marine bacterium Alteromonas macleodii KD01 was demonstrated to degrade methylmercury (MeHg) efficiently in seawater and then was applied to degrade organic mercury (MeHg, ethylmercury, and thimerosal) in simulated high-salt wastewater. Results showed that A. macleodii KD01 can rapidly degrade organic mercury (within 20 min) even at high concentrations (>10 ng/mL), volatilizing a portion of Hg from the wastewater. Further analysis revealed an increased transcription of organomercury lyase (merB) with rising organic mercury concentrations during the exposure process, suggesting the involvement of mer operon (merA and merB). These findings highlight A. macleodii KD01 as a promising candidate for addressing organic mercury pollution in high-salt wastewater.

Genomic and transcriptomic characterization of methylmercury detoxification in a deep ocean Alteromonas mediterranea ISS312

Methylmercury (MeHg) is one of the most worrisome pollutants in marine systems. MeHg detoxification is mediated by merB and merA genes, responsible for the demethylation of MeHg and the reduction of inorganic mercury, respectively. Little is known about the biological capacity to detoxify this compound in marine environments, and even less the bacterial transcriptional changes during MeHg detoxification. This study provides the genomic and transcriptomic characterization of the deep ocean bacteria Alteromonas mediterranea ISS312 with capacity for MeHg degradation. Its genome sequence revealed four mer operons containing three merA gene and two merB gene copies, that could be horizontally transferred among distant related genomes by mobile genetic elements. The transcriptomic profiling in the presence of 5 μM MeHg showed that merA and merB genes are within the most expressed genes, although not all mer genes were equally transcribed. Besides, we aimed to identify functional orthologous genes that displayed expression profiles highly similar or identical to those genes within the mer operons, which could indicate they are under the same regulatory controls. We found contrasting expression profiles for each mer operon that were positively correlated with a wide array of functions mostly related to amino acid metabolism, but also to flagellar assembly or two component systems. Also, this study highlights that all merAB genes of the four operons were globally distributed across oceans layers with higher transcriptional activity in the mesopelagic deeper waters. Our study provides new insights about the transcriptional patterns related to the capacity of marine bacteria to detoxify MeHg, with important implications for the understanding of this process in marine ecosystems.

Unmasking the physiology of mercury detoxifying bacteria from polluted sediments

Marine sediments polluted from anthropogenic activities can be major reservoirs of toxic mercury species. Some microorganisms in these environments have the capacity to detoxify these pollutants, by using the mer operon. In this study, we characterized microbial cultures isolated from polluted marine sediments growing under diverse environmental conditions of salinity, oxygen availability and mercury tolerance. Specific growth rates and percentage of mercury removal were measured in batch cultures for a selection of isolates. A culture affiliated with Pseudomonas putida (MERCC_1942), which contained a mer operon as well as other genes related to metal resistances, was selected as the best candidate for mercury elimination. In order to optimize mercury detoxification conditions for strain MERCC_1942 in continuous culture, three different dilution rates were tested in bioreactors until the cultures achieved steady state, and they were subsequently exposed to a mercury spike; after 24 h, strain MERCC_1942 removed up to 76% of the total mercury. Moreover, when adapted to high growth rates in bioreactors, this strain exhibited the highest specific mercury detoxification rates. Finally, an immobilization protocol using the sol-gel technology was optimized. These results highlight that some sediment bacteria show capacity to detoxify mercury and could be used for bioremediation applications.

Study of mercury resistance and Fourier transform infrared spectroscopy-based metabolic profiling of a potent Bacillus tropicus strain from forest soil

Mercury (Hg) is a highly toxic heavy metal and Hg-resistant indigenous bacterial isolates may offer a green and cost-effective bioremediation strategy to counter Hg contamination. In this study, a potent Hg-resistant bacterium was isolated from the forest soil of a bird sanctuary. Identification using matrix-assisted laser desorption ionization-time of flight mass spectrometry depicted the isolate as a strain of Bacillus tropicus, validated by morphological, biochemical, and molecular studies. The isolate demonstrated biological Hg removal efficiency and capacity of 50.67% and 19.76 mg g-1 , respectively. The plasmid borne resistance determinant, merA, encoding mercuric reductase, was detected in the bacterium endowing it with effective Hg volatilization and resistance capability. A Fourier-transform infrared spectroscopic comparative metabolic profiling revealed the involvement of various functional groups like -COOH, -CH2 , -OH, PO4 - and so on, resulting in differential spectral patterns of the bacterium both in control and Hg-exposed situations. A temporal variance in metabolic signature was also observed during the early and mid-log phase of growth in the presence of Hg. The bacterium described in this study is the first indigenous Hg-resistant strain isolated from the Uttar Dinajpur region, which could be further explored and exploited as a potent bioresource for Hg remediation.

Characterization on nicotine degradation and research on heavy metal resistance of a strain Pseudomonas sp. NBB

The remediation of polluted sites containing multiple contaminants like nicotine and heavy metals poses significant challenges, due to detrimental effects like cell death. In this study, we isolated a new strain Pseudomonas sp. NBB capable of efficiently degrading nicotine even in high level of heavy metals. It degraded nicotine through pyrrolidine pathway and displayed minimum inhibitory concentrations of 2 mM for barium, copper, and lead, and 5 mM for manganese. In the presence of 2 mM Ba2+ or Pb2+, 3 g L-1 nicotine could be completely degraded within 24 h. Moreover, under 0.5 mM Cu2+ or 5 mM Mn2+ stress, 24.13% and 72.56% of nicotine degradation were achieved in 60 h, respectively. Strain NBB tolerances metal stress by various strategies, including morphological changes, up-regulation of macromolecule transporters, cellular response to DNA damage, and down-regulation of ABC transporters. Notably, among the 153 up-regulated genes, cds_821 was identified as manganese exporter (MneA) after gene disruption and recovery experiments. This study presents a novel strain capable of efficiently degrading nicotine and displaying remarkable resistance to heavy metals. The findings of this research provide valuable insights into the potential application of nicotine bioremediation in heavy metal-contaminated areas.

Shoot-root signal circuit: Phytoremediation of heavy metal contaminated soil

High concentrations of heavy metals in the environment will cause serious harm to ecosystems and human health. It is urgent to develop effective methods to control soil heavy metal pollution. Phytoremediation has advantages and potential for soil heavy metal pollution control. However, the current hyperaccumulators have the disadvantages of poor environmental adaptability, single enrichment species and small biomass. Based on the concept of modularity, synthetic biology makes it possible to design a wide range of organisms. In this paper, a comprehensive strategy of "microbial biosensor detection - phytoremediation - heavy metal recovery" for soil heavy metal pollution control was proposed, and the required steps were modified by using synthetic biology methods. This paper summarizes the new experimental methods that promote the discovery of synthetic biological elements and the construction of circuits, and combs the methods of producing transgenic plants to facilitate the transformation of constructed synthetic biological vectors. Finally, the problems that should be paid more attention to in the remediation of soil heavy metal pollution based on synthetic biology were discussed.

Toward efficient bioremediation of methylmercury in sediment using merB overexpressed Escherichia coli

Sediment is the primary hotspot for microbial production of toxic and bio-accumulative methylmercury (MeHg). Common remediation strategies such as sediment dredging and capping can be too expensive and cannot degrade MeHg efficiently. Here, we constructed an Escherichia coli strain overexpressing merB gene (DH5α J23106) and assessed the effectiveness of this recombinant strain in degradation of MeHg in culture medium and sediment. DH5α J23106 can efficiently degrade MeHg (with initial concentration from 0.01 to 50 ng/mL) to more than 81.6% in a culture medium under anoxic and oxic conditions. Enriched isotope addition (¹⁹⁹HgCl₂) revealed that this recombinant strain can degrade 78.6% of newly produced Me¹⁹⁹Hg in actual sediment, however the biodegradation decreased to 36.3% for intrinsic MeHg. Degradation of spiked MeHg after aging in anoxic and oxic sediments further demonstrated DH5α J23106 can efficiently degrade newly produced MeHg and the degradation decreased with aging significantly, especially for oxic sediment. Eight sediments were further assessed for the biodegradation of aged MeHg by DH5α J23106 under oxic conditions, with degradation ratios ranging from 9.0% to 66.9%. When combined with (NH₄)₂S₂O₃ leaching, the degradation of MeHg increased by 15.8-38.8% in on-site and off-site modes through enhanced MeHg bioavailability in some of these sediments. Thus, this recombinant strain DH5α J23106 can degrade MeHg efficiently and have the potential for remediating bioavailable MeHg in contaminated sediments.

Highly mercury-resistant strains from different Colombian Amazon ecosystems affected by artisanal gold mining activities

Two sites of the Colombian Amazon region with different levels of human intervention and mercury pollution were selected for the collection of samples of river and lake water, sediments, and associated forest soils. The Tarapacá region, affected mainly by barrage mining, showed low mercury concentrations, whilst in the Taraira region, affected by underground mining, there were several points with high mercury pollution levels. A collection of 72 bacterial and 10 yeast strains with different levels of mercury resistance was isolated and characterized. Most of the highly resistant bacterial strains (MIC > 40 mg L⁻¹ HgCl₂) were isolated from soil and sediment samples and belonged to either Pseudomonas (60%) or Bacillus (20%). Most of highly resistant bacterial strains were positive for the presence of the merA gene, suggesting an active mercury resistance mechanism. This was confirmed in the two most resistant strains, Pseudomonas sp. TP30 and Burkholderia contaminans TR100 (MIC = 64 and 71 mg L⁻¹ HgCl₂, respectively), which in the presence of increasing mercury concentrations expressed the merA gene at increasing levels, concomitant with a significant mercury reduction activity. Analysis of the MerA sequences present in the different isolates suggested a high gene conservation within the taxonomic groups but also several horizontal gene transfer events between taxonomically distant genera. We also observed a positive correspondence between the presence of the merA gene and the number of antibiotics to which the strains were resistant to. The most resistant strains are good candidates for future applications in the bioremediation of mercury-contaminated sites in the Amazon.

Key points

• Amazon sediments affected by underground gold mining have higher Hg levels.

• Highly Hg-resistant isolates belonged to Pseudomonas and Bacillus genera.

• TR100 and TP30 strains showed remediation potential to be used in the Amazon region.

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.

Bioremediation: An effective approach of mercury removal from the aqueous solutions

Mercury (Hg(II)) is the 16th rarest element present in the earth's crust. Due to rapid industrialization and urban expansions, the mercury concentration has been elevated in the environment. Hg(II) contamination in the aqueous environment has become a great challenge for human beings. The main source of Hg(II) in the aqueous phase is untreated effluent industries (such as the paper industry). Hg(II) is non-biodegradable in nature and even its trace amount in an aqueous environment can pose chronic threats among the humans (damage to the central nervous system, respiratory system, and cardiovascular system, mutation of DNA), animals, and aquatic creatures. Therefore, the removal of mercury from aqueous solutions is an urgent need of the modern era. The conventional techniques such as ion exchange, precipitation, membrane filtrations are costly and also generate byproducts in the environment. Bioremediation is a sustainable, environmentally sound, and cost-effective technique to remove Hg(II) from the aqueous solutions. In this process, naturally occurring microorganisms are utilized to remove the Hg(II) from the aqueous solutions. Lentinus edodes, U. lactuca, and Typha domingensis are found to have great potential to remove mercury from water ranged from ~100 mg g^-1 to 337 mg g^-1.

The Mercury-Tolerant Microbiota of the Zooplankton Daphnia Aids in Host Survival and Maintains Fecundity under Mercury Stress

Many aquatic organisms can thrive in polluted environments by having the genetic capability to withstand suboptimal conditions. However, the contributions of microbiomes under these stressful environments are poorly understood. We investigated whether a mercury-tolerant microbiota can extend its phenotype to its host by ameliorating host survival and fecundity under mercury-stress. We isolated microbiota members from various clones of Daphnia magna, screened for the mercury-biotransforming merA gene, and determined their mercury tolerance levels. We then introduced the mercury-tolerant microbiota, Pseudomonas-10, to axenic D. magna and quantified its merA gene expression, mercury reduction capability, and measured its impact on host survival and fecundity. The expression of the merA gene was up-regulated in Pseudomonas-10, both in isolation and in host-association with mercury exposure. Pseudomonas-10 is also capable of significantly reducing mercury concentration in the medium. Notably, mercury-exposed daphnids containing only Pseudomonas-10 exhibited higher survival and fecundity than mercury-exposed daphnids supplemented with parental microbiome. Our study showed that zooplankton, such as Daphnia, naturally harbor microbiome members that are eco-responsive and tolerant to mercury exposure and can aid in host survival and maintain host fecundity in a mercury-contaminated environment. This study further demonstrates that under stressful environmental conditions, the fitness of the host can depend on the genotype and the phenotype of its microbiome.

The metagenomic landscape of xenobiotics biodegradation in mangrove sediments

Metagenomics is a powerful approach to study microorganisms present in any given environment and their potential to maintain and improve ecosystem health without the need of cultivating these microorganisms in the laboratory. In this study, we combined a cultivation-independent metagenomics approach with functional assays to identify the detoxification potential of microbial genes evaluating their potential to contribute to xenobiotics resistance in oil-impacted mangrove sediments. A metagenomic fosmid library containing 12,960 clones from highly contaminated mangrove sediment was used in this study. For assessment of metal resistance, clones were grown in culture medium with increasing concentrations of mercury. The analyses metagenomic library sequences revealed the presence of genes related to heavy metals and antibiotics resistance in the oil-impacted mangrove microbiome. The taxonomic profiling of these sequences suggests that at the genus level, Geobacter was the most abundant genus in our dataset. A functional screening assessment of the metagenomic library successfully detected 24 potential heavy metal tolerant clones, six of which were capable of growing with increased concentrations of mercury. The genetic characterization of selected clones allowed the detection of genes related to detoxification processes, such as chromate transport protein ChrA, haloacid dehalogenase-like hydrolase, lipopolysaccharide transport system, and 3-oxoacyl-[acyl-carrier-protein] reductase. Clones were capable of growing in medium containing increased concentrations of metals and antibiotics, but none manifested strong mercury removal from culture medium characteristic of mercuric reductase activity. These results suggest that resistance to xenobiotic stress varies greatly and that additional studies to elucidate the potential of metal biotransformation need to be carried out with the goal of improving bioremediation application.

Characterization of mercury-reducing potential bacteria isolated from Keputih non-active sanitary landfill leachate, Surabaya, Indonesia under different saline conditions

The objectives of this research were to identify the capability of bacteria isolated from a non-active sanitary landfill to remove mercury under different saline conditions and to understand the removal kinetics. The mercury concentrations used in the minimum inhibitory concentration (MIC) test were 0, 5, 10, 15 and 20 mg/L. The capability of one selected bacterium from the MIC test to remove mercury under different saline conditions (0, 10, 20, and 30‰) was also tested. Five indigenous bacteria were isolated from the Keputih non-active sanitary landfill, Surabaya, Indonesia. The MICs of mercury for FA-1, FA-2, FA-3, FA-4, and FA-5 were 5, 10, 5, 5, and 5 mg/L, respectively. Based on biochemical characterization, FA-2 was identified as Pseudomonas aeruginosa. The isolate of P. aeruginosa was capable of removing Hg under different saline conditions. The optimum saline condition for P. aeruginosa to remove Hg was 10‰, with a removal percentage of 99.7 ± 0.18% following pseudo-second-order kinetics (R² = 0.9939) with k₂ = 2.0059 mg substrate/g adsorbent/hr. Hence, isolated P. aeruginosa showed potential for the bioremediation of mercury-contaminated areas.

Reducing methylmercury accumulation in fish using Escherichia coli with surface-displayed methylmercury-binding peptides

Seafood consumption is widely considered as the primary route for human exposure to the neurotoxin methylmercury (MeHg) that is produced by certain anaerobic microorganisms and can bioaccumulate to high concentration levels in natural aquatic food webs. In this study, a novel methylmercury-binding peptide with seven amino acids was displayed on the cell surfaces of Escherichia coli strain W-1, which was isolated from fish feces and fused with ice nucleation protein. These cells exhibited high affinity and selectivity toward methylmercury. They efficiently removed more than 96% of 12 μM methylmercury, and accumulation of methylmercury in the engineered strain was four times higher than that in the wild type. Transmission electron microscopy confirmed methylmercury accumulation on cell membranes. Carassius auratus was fed by engineered bacteria, which showed a decrease in methylmercury concentration in muscles of about 36.3 ± 0.7%; whereas an increase in methylmercury concentration was observed in the feces (36.7 ± 0.8%) in comparison to the control group. The engineered strain in the gut captured methylmercury and prevented it's absorption by muscles, while some bacteria with methylmercury were excreted in the feces. The surface-engineered E. coli effectively protected fish from methylmercury contamination.

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.

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.

Mercury species in formerly contaminated soils and released soil gases

Total mercury (T-Hg), elemental mercury (Hg⁰), methylmercury (MeHg⁺), phenylmercury (PhHg⁺), and gaseous elemental mercury (GEM) species were determined in soils formerly contaminated by different processes from two sites in the Czech Republic. Analytical methods involved atomic absorption spectrometry (AAS) using a single-purpose Advanced Mercury Analyser AMA-254 and radiochemical neutron activation analysis (RNAA) for T-Hg determination, a thermal desorption method was used for Hg⁰ determination, gas chromatography coupled with atomic fluorescence spectrometry (GC-AFS) was employed for assay of MeHg⁺ and PhHg⁺, while GEM measurement was carried out using a portable Zeeman-AAS device Lumex RA-915⁺. The first sampling site was in the surroundings of a former PhHgCl-based fungicide processing plant next to Příbram (central Bohemia). Although the use of Hg-based fungicides as seed mordant have been banned, and their production stopped at the end of 1980's, highly elevated Hg contents in soil are still observed in the vicinity of the former plant, reaching T-Hg values >13mgkg^-1. The second sampling site was an abandoned mining area named Jedová hora Hill near Hořovice (central Bohemia), where cinnabar (HgS) was occasionally mined as by-product of Fe ores hematite and siderite. Mining activities have been stopped here in 1857. Very high contents of T-Hg are still found at this site, up to 144mgkg^-1. In most cases we found a statistically significant correlation between T-Hg and Hg⁰ values regardless of the pollution source. On the contrary, insignificant correlation was observed neither between T-Hg and GEM values, nor between GEM and Hg⁰. Concentrations of the investigated organomercury species were above a limit of detection (LOD) only in the most contaminated samples, where their levels were about two to three orders of magnitude lower compared to those of T-Hg.

Geographic distribution of cadmium and its interaction with the microbial community in the Longjiang River: risk evaluation after a shocking pollution accident

A shocking Longjiang River cadmium pollution accident occurred in 2012, the effects of which on microbial communities remain unclear. Alkaline precipitation technology was applied for remediation, but concerns rose about the stability of this technology. To understand the geographic distribution of cadmium and its correlation with microbes, in this study, 39 water samples and 39 sludge samples from this river and 2 soil samples from the nearby farmland were collected for chemical and microbial analyses. The Cd concentrations of all water samples were lower than 0.005 mg/L and reached the quality standards for Chinese surface water. A ranking of sludge samples based on Cd contents showed sewage outfall > dosing sites > farmland, all of which were higher than the quality standard for soil. Alkaline precipitation technology was effective for Cd precipitation. Cd was unstable; it was constantly dissolving and being released from the sludge. The Cd content of each phase was mainly influenced by the total Cd content. Over 40,000 effective sequences were detected in each sample, and a total of 59,833 OTUs and 1,273 genera were found using Illumina MiSeq sequencing. Two phyla and 39 genera were notably positively correlated with the Cd distribution, while the cases of 10 phyla and 6 genera were the opposite.

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).

Methylmercury degradation by Pseudomonas putida V1

Environmental contamination of mercury (Hg) has caused public health concerns with focuses on the neurotoxic substance methylmercury, due to its bioaccumulation and biomagnification in food chains. The goals of the present study were to examine: (i) the transformation of methylmercury, thimerosal, phenylmercuric acetate and mercuric chloride by cultures of Pseudomonas putida V1, (ii) the presence of the genes merA and merB in P. putida V1, and (iii) the degradation pathways of methylmercury by P. putida V1. Strain V1 cultures readily degraded methylmercury, thimerosal, phenylmercury acetate, and reduced mercuric chloride into gaseous Hg(0). However, the Hg transformation in LB broth by P. putida V1 was influenced by the type of Hg compounds. The merA gene was detected in P. putida V1, on the other hand, the merB gene was not detected. The sequencing of this gene, showed high similarity (100%) to the mercuric reductase gene of other Pseudomonas spp. Furthermore, tests using radioactive (14)C-methylmercury indicated an uncommon release of (14)CO2 concomitant with the production of Hg(0). The results of the present work suggest that P. putida V1 has the potential to remove methylmercury from contaminated sites. More studies are warranted to determine the mechanism of removal of methylmercury by P. putida V1.

Removal of mercury by adsorption: a review

Due to natural and production activities, mercury contamination has become one of the major environmental problems over the world. Mercury contamination is a serious threat to human health. Among the existing technologies available for mercury pollution control, the adsorption process can get excellent separation effects and has been further studied. This review is attempted to cover a wide range of adsorbents that were developed for the removal of mercury from the year 2011. Various adsorbents, including the latest adsorbents, are presented along with highlighting and discussing the key advancements on their preparation, modification technologies, and strategies. By comparing their adsorption capacities, it is evident from the literature survey that some adsorbents have shown excellent potential for the removal of mercury. However, there is still a need to develop novel, efficient adsorbents with low cost, high stability, and easy production and manufacture for practical utility.

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

A mercury resistant bacterial strain, SA2, was isolated from soil contaminated with mercury. The 16S rRNA gene sequence of this isolate showed 99% sequence similarity to the genera Sphingobium and Sphingomonas of α-proteobacteria group. However, the isolate formed a distinct phyletic line with the genus Sphingobium suggesting the strain belongs to Sphingobium sp. Toxicity studies indicated resistance to high levels of mercury with estimated EC50 values 4.5 mg L(-1) and 44.15 mg L(-1) and MIC values 5.1 mg L(-1) and 48.48 mg L(-1) in minimal and rich media, respectively. The strain SA2 was able to volatilize mercury by producing mercuric reductase enzyme which makes it potential candidate for remediating mercury. ICP-QQQ-MS analysis of Hg supplemented culture solutions confirmed that almost 79% mercury in the culture suspension was volatilized in 6 h. A very small amount of mercury was observed to accumulate in cell pellets which was also evident according to ESEM-EDX analysis. The mercuric reductase gene merA was amplified and sequenced. The deduced amino acid sequence demonstrated sequence homology with α-proteobacteria and Ascomycota group.

Effects of the soil microbial community on mobile proportions and speciation of mercury (Hg) in contaminated soil

The precise characterization of the behavior of individual microorganisms in the presence of increased mercury contents in soil is necessary for better elucidation of the fate of mercury in the soil environment. In our investigation, resistant bacterial strains isolated from two mercury contaminated soils, represented by Paenibacillus alginolyticus, Burkholderia glathei, Burkholderia sp., and Pseudomonas sp., were used. Two differently contaminated soils (0.5 and 7 mg kg(-1) total mercury) were chosen. Preliminary soil analysis showed the presence of methylmercury and phenylmercury with the higher soil mercury level. Modified rhizobox experiments were performed to assess the ability of mercury accumulating strains to deplete the mobile and mobilizable mercury portions in the soil by modification; microbial agar cultures were used rather than the plant root zone. A sequential extraction procedure was performed to release the following mercury fractions: water soluble, extracted in acidic conditions, bound to humic substances, elemental, and bound to complexes, HgS and residual. Inductively coupled plasma mass spectrometry (ICP-MS) and a single-purpose atomic absorption spectrometer (AMA-254) were applied for mercury determination in the samples and extracts. Gas chromatography coupled to atomic fluorescence spectrometry (GC-AFS) was used for the determination of organomercury compounds. The analysis of the microbial community at the end of the experiment showed a 42% abundance of Paenibacillus sp. followed by Acetivibrio sp., Brevibacillus sp., Cohnella sp., Lysinibacillus sp., and Clostridium sp. not exceeding 2% abundance. The results suggest importance of Paenibacillus sp. in Hg transformation processes. This genus should be tested for potential bioremediation use in further research.

Pattern of multiresistant to antimicrobials and heavy metal tolerance in bacteria isolated from sewage sludge samples from a composting process at a recycling plant in southern Brazil

The composting process is a viable alternative for the recycling of household organic waste and sewage sludge generated during wastewater treatment. However, this technique can select microorganisms resistant to antimicrobials and heavy metals as a result of excess chemicals present in compost windrow. This study evaluates the antimicrobial multiresistant and tolerance to heavy metals in bacteria isolated from the composting process with sewage sludge. Fourteen antimicrobials were used in 344 strains for the resistance profile and four heavy metals (chromium, copper, zinc, and lead) for the minimum biocide concentration assay. The strains used were from the sewage sludge sample (beginning of the process) and the compost sample (end of the process). Strains with higher antimicrobial and heavy metal profile were identified by 16S rRNA gene sequencing. The results showed a multiresistant profile in 48 % of the strains, with the highest percentage of strains resistant to nitrofurantoin (65 %) and β-lactams (58 %). The strains isolated from the sewage sludge and the end of the composting process were more tolerant to copper, with a lethal dose of approximately 900 mg L(-1) for about 50 % of the strains. The genera that showed the highest multiresistant profile and increased tolerance to the metals tested were Pseudomonas and Ochrobactrum. The results of this study may contribute to future research and the revision and regulation of legislation on sewage sludge reuse in soils.