Biotransformation Of Mercury Research Articles

Construction of protein-protein interaction network in sulfate-reducing bacteria: Unveiling of global response to Hg

Sulfate-reducing bacteria (SRB) play pivotal roles in the biotransformation of mercury (Hg). However, unrevealed global responses of SRB to Hg have restricted our understanding of details of Hg biotransformation processes. The absence of protein-protein interaction (PPI) network under Hg stimuli has been a bottleneck of proteomic analysis for molecular mechanisms of Hg transformation. This study constructed the first comprehensive PPI network of SRB in response to Hg, encompassing 67 connected nodes, 26 independent nodes, and 121 edges, covering 93% of differentially expressed proteins from both previous studies and this study. The network suggested that proteomic changes of SRB in response to Hg occurred globally, including microbial metabolism in diverse environments, carbon metabolism, nucleic acid metabolism and translation, nucleic acid repair, transport systems, nitrogen metabolism, and methyltransferase activity, partial of which could cover the known knowledge. Antibiotic resistance was the original response revealed by this network, providing insights into of Hg biotransformation mechanisms. This study firstly provided the foundational network for a comprehensive understanding of SRB's responses to Hg, convenient for exploration of potential targets for Hg biotransformation. Furthermore, the network indicated that Hg enhances the metabolic activities and modification pathways of SRB to maintain cellular activities, shedding light on the influences of Hg on the carbon, nitrogen, and sulfur cycles at the cellular level.

Effects of diet composition on gut microbiome and mercury biotransformation in the gobyfish

Mercury (Hg) biotransformation (methylation and demethylation) could play a key role in affecting the final Hg speciation and level in fish. The gut microbiota was identified to be involved in this process. Diet is recognized to greatly influence the gut microbiome, whereas the impact of food composition on Hg biotransformation in fish has yet to be addressed. The study investigated the biotransformation and bioaccumulation of Hg in the gobyfish (Mugilogobius chulae) under different food choices (natural prey and artificial food) and evaluated the role of gut microbiome in these processes. The results showed that different diet composition significantly affected the gut microbiome and subsequently resulted in varied phenomenon of Hg biotransformation within fish body. Significant demethylation (0.33 % d−1) was only observed in the natural prey (brine shrimp) treatment, whereas methylation was occurred extremely slowly (0.013 % d−1) only in the artificial food (commercial dry pellets) treatment. Furthermore, the growth of demethylators was also enhanced in the natural prey treatment, which contributed to the demethylation process in fish. Furthermore, the gut microbial structure of gobyfish was greatly altered by different diet composition. This study highlights the significance of food choices in the minimization of Hg contamination in aquaculture. Incorporating natural prey into fish diets could be a better choice to balance the fish production and control MeHg levels. CapsuleDiet composition greatly affects the gut microbial composition, and natural prey could help to reduce the potential of MeHg accumulation by fish.

Bacterial Metal-Scavengers Newly Isolated from Indonesian Gold Mine-Impacted Area: Bacillus altitudinis MIM12 as Novel Tools for Bio-Transformation of Mercury.

Selikat river, located in the north part of Bengkulu Province, Indonesia, has critical environmental and ecological issues of contamination by mercury due to artisanal small-scale gold mining (ASGM) activities. The present study focused on the identification and bioremediation efficiency of the mercury-resistant bacteria (MRB) isolated from ASGM-impacted areas in Lebong Tambang village, Bengkulu Province, and analyzed their merA gene function in transforming Hg2+ to Hg0. Thirty-four MRB isolates were isolated, and four out of the 34 isolates exhibited not only the highest degree of resistance to Hg (up to 200ppm) but also to cadmium (Cd), chromium (Cr), copper (Cu), and lead (Pb). Further analysis shows that all four selected isolates harbor a merA operon-encoded mercuric ion (Hg2+) reductase enzyme, with the Hg bioremediation efficiency varying from 71.60 to 91.30%. Additionally, the bioremediation efficiency for Cd, Cr, Cu, and Pb ranged from 54.36 to 98.37%. Among the 34, two isolates identified as Bacillus altitudinis possess effective and superior multi-metal degrading capacity up to 91.30% for Hg, 98.07% for Cu, and 54.36% for Cr. A pilot-scale study exhibited significant in situ bioremediation of Hg from gold mine tailings of 82.10 and 95.16% at 4- and 8-day intervals, respectively. Interestingly, translated nucleotide blast against bacteria and Bacilli merA sequence databases suggested that B. altitudinis harbor merA gene is the first case among Bacilli with the possibility exhibits a novel mechanism of bioremediation, considering our new finding. This study is the first to report the structural and functional Hg-resistant bacterial diversity of unexplored ASGM-impacted areas, emphasizing their biotechnological potential as novel tools for the biological transformation and adsorption of mercury and other toxic metals.

The alteration of gut microbiome community play an important role in mercury biotransformation in largemouth bass

Mercury (Hg) biotransformation is an important process that can affect the speciation and bioaccumulation of Hg in fish. The intestinal microbiota has been suggested to take part in this process. However, Hg biotransformation in fish is still unclear and the responses of gut microbiota to different Hg exposure scenarios have not been well addressed. The present study investigated the bioaccumulation and biotransformation of Hg in a freshwater fish (Micropterus salmoides) and characterized the gut microbiome community under dietary inorganic Hg (IHg) or methylmercury (MeHg) exposure, aiming to evaluate the effects of gut microbiome's activities on the internal-handling and fate of Hg in fish. Significant Hg methylation was observed in fish under IHg exposure, whereas there was no demethylation occurred in MeHg-treated fish. Both IHg and MeHg could significantly alter the community composition of gut microbiome. The administrated IHg in the food could enhance the growth of methylators, resulting in additional MeHg production in fish gut. However, abundance of demethylators was greatly decreased under either IHg or MeHg exposure, leading the demethylation process to be negligible. The results strongly suggested that the behaviors of gut bacterial community played an important role in the presence or absence of biotransformation processes. This study elucidated the importance of gut microbiome in Hg biotransformation process, and helped to develop a novel perspective to understand the Hg bioaccumulation of fish in realistic environment.

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.

An overview of the complications of acute and chronic mercury exposures. Past, present and future

Mercury is a chemical element that has been known since prehistoric times and has been used over time as a component of paints, in the treatment of various diseases, and later in various industrial compounds. Mercury enters the body transcutaneously, by inhaling or by ingesting food contaminated with mercury. The most common cause of mercury poisoning is eating seafood, but people can get mercury poisoning from industrial processing, thermometers, dental work, and old paints. The most vulnerable are people at occupational risk due to chronic exposure, as well as fetuses in the intrauterine period. Mercury poisoning produces extremely serious effects on the nervous, renal, and gastrointestinal systems. The nature and extent of mercury poisoning depend on factors such as the route of exposure, the rate of exposure, the distribution and biotransformation of mercury in the body, the chemical form of mercury or the mercury compound. Both mercury in the blood and the urine should not exceed 10 μg/L. The critical values of mercury in the blood are >150 μg/L and the lethal ones exceed 800 μg/L. New diagnostic methods allow the rapid and accurate identification of the concentration of mercury in biological products. Treatment is based on the use of chelating agents. However, the prognosis of mercury poisoning is extremely variable, due to the potentially irreversible nature of the lesions.

Modeling of the Passive Permeation of Mercury and Methylmercury Complexes Through a Bacterial Cytoplasmic Membrane.

Cellular uptake and export are important steps in the biotransformation of mercury (Hg) by microorganisms. However, the mechanisms of transport across biological membranes remain unclear. Membrane-bound transporters are known to be relevant, but passive permeation may also be involved. Inorganic HgII and methylmercury ([CH3HgII]+) are commonly complexed with thiolate ligands. Here, we have performed extensive molecular dynamics simulations of the passive permeation of HgII and [CH3HgII]+ complexes with thiolate ligands through a model bacterial cytoplasmic membrane. We find that the differences in free energy between the individual complexes in bulk water and at their most favorable position within the membrane are ∼2 kcal mol-1. We provide a detailed description of the molecular interactions that drive the membrane crossing process. Favorable interactions with carbonyl and tail groups of phospholipids stabilize Hg-containing solutes in the tail-head interface region of the membrane. The calculated permeability coefficients for the neutral compounds CH3S-HgII-SCH3 and CH3HgII-SCH3 are on the order of 10-5 cm s-1. We conclude that small, nonionized Hg-containing species can permeate readily through cytoplasmic membranes.

Cloning of merA Gene from Methylotenera Mobilis for Mercury Biotransformation.

Mercury (Hg) is one of the most toxic heavy metal and is extremely harmful for the environment. The permissible limit of mercury in industrial effluents is 0.001ppm, whereas there are various sites having very high levels of mercury contamination. In the present study, 10 different mercury (Hg) resistant bacterial strains were isolated from Ulhas Estuary, Mumbai (Hg concentration of 107ppm). All the strains were subsequently grown on higher concentration of mercuric chloride (HgCl2), one of the isolate (USP5) showed significant growth at high concentration of Hg (40ppm) and 16S rRNA gene sequencing revealed the identity of the bacterium as Methylotenera mobilis, (Accession no. KT714144). The mer operon was isolated and cloned in E.coli and checked for its ability to tolerate higher concentration of Hg. It has shown growth up to 70ppm of Hg, also presence of merA gene indicated its ability to detoxify Hg into less toxic volatile form. The atomic absorption spectrophotometry confirmed the ability of clone to efficiently detoxify 60-90% of the Hg (10-70ppm) within 48-72h. This clone can be used for effective volatilization of Hg from contaminated areas.

Evaluation of mercury biotransformation by heavy metal-tolerant Alcaligenes strain isolated from industrial sludge

Heavy metal pollution affects environment adversely and leads to severe implications for both flora and fauna. In the present work, bacterial strain JS-1 was isolated with tolerance for different metals such as mercury (Hg), lead (Pb), cadmium (Cd), nickel (Ni), arsenic (As), tin (Sn), selenium (Se), zinc (Zn), chromium (Cr) and copper (Cu). JS-1 showed a significant tolerance for mercuric chloride (up to 5,000 μg/g) along with an efficient metal uptake and transformation. Growth of JS-1 was marginally affected on exposure to high mercury concentration due to acclimatization of the culture towards mercury. No mercury was found in cell-free supernatant after 96 h of incubation with 500 μg/g and 1,000 μg/g of mercury as an active ingredient. Almost all the mercury was found associated with cell biomass as determined by hydride generation atomic absorption spectroscopy. Only 60 % of mercury was sequestered in bacterial biomass on exposure to 2,000 and 5,000 μg/g mercury. As a detoxification mechanism, nearly 5 % of sequestered mercury was volatilized by the selected isolate (JS-1). Further X-ray diffraction analysis of deposited silvery grey biomass confirmed biotransformation of sequestered mercuric ions into monovalent mercury (Hg2Cl2), a non-bioavailable form of mercury. Culture was characterized morphologically, physiologically and biochemically. 16S rRNA gene sequence of JS-1 revealed its phylogenetic relationship and 98 % homology with Alcaligenes faecalis, a Gram-negative rod-shaped bacterium.

Hydrologic controls on water chemistry and mercury biotransformation in a closed river system: The Carson River, Nevada

The Carson River flows in a closed basin system and the total flow of the river water decreases downstream due to both evaporation and consumptive uses. This river system is fed primarily by snow pack in the Sierra Nevada during the winter, which flows down gradient following melting in spring and summer. Water loss through evaporation in the Carson River results in a downstream buildup of conservative elements such as Cl − and certain oxyanion forming elements including Se, Mo and W, which are known to interfere with the transformation of Hg within the S cycle. In addition to these naturally occurring hydrologic processes and the resulting affects on water chemistry, the Carson River Basin has been historically impacted by Au and Ag mining that used Hg amalgamation techniques. Contamination of Hg in the Carson River system is now well documented and published Hg concentrations in different environmental compartments are extremely high. In this study, hydrologically driven changes in water chemistry of the river system and the resulting effects on Hg cycling were examined. Results show that periods of low water flow correspond to high water pH (up to 8.3), relatively high concentrations of oxyanion forming elements (e.g., As, Se, Mo and W), and low Hg methylation potential in sediment. In contrast, periods of high flow bring about dilution, which results in lower pH (∼7), lower concentrations of oxyanion forming elements, but higher Hg methylation potential. Overall, changes in flow regimes likely affect rates of methyl-Hg (MeHg) production through a combination of factors such as high pH, which favors MeHg demethylation, and the occurrence of relatively high concentrations of Group VI oxyanions that could interfere with microbial SO 4 reduction and MeHg production.

Biotransformation of mercury in pH-stat cultures of eukaryotic freshwater algae

Eukaryotic algae were studied to determine their ability to biotransform Hg(II) under aerated and pH controlled conditions. All algae converted Hg(II) into beta-HgS and Hg(0) to varying degrees. When Hg(II) was administered as HgCl(2) to the algae, biotransformation by species of Chlorophyceae (Selenastrum minutum and Chlorella fusca var. fusca) was initiated with beta-HgS synthesis (K (1/2) of hours) and concomitant Hg degrees evolution occurred in the first hour. Hg degrees synthesis was impeded by the formation of beta-HgS and this inhibition was released in C. fusca var. fusca when cellular thiols were oxidized by the addition of dimethylfumarate (DMF). The diatom, Navicula pelliculosa (Bacillariophyceae), converted a substantially greater proportion of the applied Hg(II) into Hg(0), whereas the thermophilic alga, Galdieria sulphuraria (Cyanidiophyceae), rapidly biotransformed as much as 90% of applied Hg(II) into beta-HgS (K (1/2) approximately 20 min). This thermophile was also able to generate Hg(0) even after all exogenously applied HgCl(2) had been biotransformed. The results suggest that beta-HgS may be the major dietary mercurial for grazers of contaminated eukaryotic algae.

Methylmercury, Amalgams, and Children’s Health: Björnberg et al. Respond

We acknowledge the points raised by Guzzi et al. regarding our recent publication on the transport of methylmercury and inorganic mercury to the fetus and breast-fed infant (Bjornberg et al. 2005). The issue is whether the methylation of inorganic mercury from dental amalgam is of sufficient size to significantly contribute to the exposure to organic mercury. Demethylation of methylmercury into inorganic mercury is the key step in the excretion process of methylmercury. This process occurs through microbial activity within the intestine. To a limited extent demethylation may also take place in the blood (Berglund et al. 2005). The kinetics of mercury in the human body may also include methylation of inorganic mercury, but the present knowledge of this process is rather limited. Based on findings from in vitro studies (Heintze et al. 1983; Lyttle et al. 1993), Guzzi et al. postulate that organic mercury in saliva is due to bacterial transformation in the oral cavity. It is of course of toxicologic interest to further investigate the biotransformation of mercury in both directions. Inorganic mercury has been shown to accumulate in exocrine glands, and saliva is also one excretion pathway for inorganic mercury (Joselow 1968). It should be pointed out that the saliva samples used by Leistevuo et al. (2001), to which Guzzi et al. refer, consist of paraffin-stimulated whole saliva. Therefore it is not possible to ascertain to what extent the sample reflects excreted mercury from the central circulation (which could originate from both inorganic mercury and methylmercury exposure) or mercury derived directly from the fillings in the oral cavity. In the study by Leistevuo et al. (2001), 15–18% of total mercury in saliva (5–12.5 nmol/L) was organic in a group of subjects with amalgam fillings. These subjects had, on average, 22 amalgam-filled surfaces (range, 2–51). In the non-amalgam group, the organic mercury was 2–5 nmol/L. As calculated by Guzzi et al., the subjects with amalgam would ingest about 2–3 μg/day of methylmercury derived from oral bacteria biomethylation of inorganic mercury. Our study group of pregnant women (Bjornberg et al. 2005) was exposed to low levels of both methylmercury and inorganic mercury, as reflected in the low concentrations found in blood. They consumed small amounts of fish and had few amalgam fillings, on average five amalgam-filled surfaces (range, 0–24). Therefore, the exposure to methylmercury possible originating from bacterial methylation of inorganic mercury in the oral cavity is far lower than that reported by Leistevuo et al. (2001). It should also be pointed out that a meal of fish (200 g) containing 500 μg/kg methylmercury would result in the ingestion of 100 μg methylmercury. Also, consumption of fish with more moderate levels (50 μg/kg) would give rise to significant exposure (10 μg methylmercury). Even though a small exposure to methylmercury may occur from bacterial methylation of inorganic mercury in the oral cavity, this exposure would be far lower than methylmercury exposure via fish consumption.

The biotransformation of mercury in pH-stat cultures of microfungi

The biotransformation of mercury was monitored in Hymenoscyphus ericae (Read) Korf & Kernan, Neocosmospora vasinfecta E. F. Sm., and Verticillium terrestre (Link) Lindau following the exposure of these fungi to environmentally relevant doses of HgII(HgCl2) in aerated pH-controlled cultures. Mercury applied at 120 and 300 μg·L–1quickly associated with cells of N. vasinfecta. Within 3 h, approximately 80% of the remaining Hg in both treatments had been converted to β-HgS, and at 8 h, 15% and 53% of the original doses had been converted into volatile Hg0. Hymenoscyphus ericae was less efficient at converting HgIIto β-HgS, but it volatilized larger amounts of Hg0. A mercury-tolerant isolate of V. terrestre withstood levels of Hg in the parts per million. When exposed to 270, 1000, or 2000 μg·L–1HgCl2, 93% ± 3% of the amounts remaining in cultures of V. terrestre after 2 h was β-HgS. At 2 mg·L–1, 72% of the Hg was reduced to Hg0in 2 h. Volatilization accounts for the tolerance of V. terrestre to high doses of Hg, whereas the conversion of Hg to β-HgS was the major detoxifying mechanism at lower doses. The formation of β-HgS occurs preferentially with volatilization operating as the default pathway, when the former biotransformation mechanism is saturated.

Analysis of genetic susceptibility to mercury contamination evaluated through molecular biomarkers in at-risk Amazon Amerindian populations

We investigated Individual differences in susceptibility to methylmercury (MeHg) contamination and its relationship with polymorphisms of the detoxifying enzyme glutathione S-transferase (GST). In Brazil, some Amerindian tribes from the Amazon region have an increased level of mercury in their hair. Samples of hair and blood were taken from inhabitants of two villages in the Kayabi and Munduruku Amerindian communities to investigate mercury levels in association with genetic polymorphism of GSTs. Other molecular biological markers were also studied, such as hemoglobin, haptoglobin and glucose 6-phosphate dehydrogenase (G-6-PDH). Higher levels of mercury contamination were found in the Kayabi villagers, who had a null genotype (GSTM1 0/0, also denominated GSTM1 null) frequency of 26%, than in the Munduruku villagers, for which the null genotype frequency was 0%. Individuals with the GSTM1 null phenotype had higher concentrations of mercury in their hair than individuals with GSTM1+/+ phenotypes (F = 21.51, p < 0.0001). No association with other markers studied was observed. This study suggests that GSTM1 may be involved in the biotransformation of mercury in humans.

Mercury speciation in blood and brain tissue from monkeys. Interlaboratory comparison of Magos' method with other spectroscopic methods, using alkylation and gas chromatography separation as well as RNAA in combination with West��'s extraction methods

In a long term ongoing project on brain mercury biotransformation and clearance in monkeys, Magos' method is used for speciation of mercury. This old method has been compared with a number of new methods used for the same purpose. Results are presented from two interlaboratory control studies for total, inorganic and organic mercury in blood and brain tissues from monkeys.

Biotransformation of mercury by bacteria isolated from a river collecting cinnabar mine waters

One hundred six strains of aerobic bacteria were isolated from the Fiora River which drains an area of cinnabar deposits in southern Tuscany, Italy. Thirty-seven of the strains grew on an agar medium containing 10μg/ml Hg (as HgCl2) with all of these strains producing elemental mercury. Seven of the 37 strains also degraded methylmercury. None of 106 sensitive and resistant strains produced detectable monomethylmercury although 15 strains produced a benzene-soluble mercury species. Two strains of alkylmercury (methyl-, ethyl- and phenylmercury) degrading bacteria were tested for the ability to degrade several other analogous organometals and organic compounds, but no activity was detected toward these compounds. Mercury methylation is not a mechanism of Hg resistance in aerobic bacteria from this environment. Growth of bacteria on the agar medium containing 10μg/ml HgCl2 was diagnostic for Hg detoxification based on reduction.

Plasmid-controlled mercury biotransformation by Clostridium cochlearium T-2.

A strain of Clostridium cochlearium having methylmercury-decomposing ability was isolated. The ability was cured by the treatment with acridine dye and recovered by the conjugation of the cured strain with the parent strain. The cured strain then showed the activity to methylate mercuric ion as previously reported (M. Yamada and K. Tonomura, J. Ferment. Technol. 50:159-166, 1971). These results and the agarose gel electrophoretic pattern of the deoxyribonucleic acids from the lysates indicate a possible role of plasmids in controlling the mercury biotransformation of the two opposite directions in a single bacterial strain: methylation in the absence of the plasmid and demethylation in the presence of it. A possible mechanism for mercury resistance involving hydrogen sulfide is discussed.