Detoxification of mercury and organomercurials by nitrogen-fixing soil bacteria

Minimal inhibitory concentration values of HgCl₂ and 5 organomercurials were determined against 24 mercury-resistant N₂-fixing soil bacteria previously isolated from soil and identified in our laboratory. These bacterial strains also displayed multiple antibiotic resistant properties. Typical growth pattern of a highly mercury-resistantBeijerinckia sp (KDr₂) was studied in liquid broth supplemented with toxic levels of mercury compounds. Four bacterial strains were selected for determining their ability to volatilize mercury and their Hg-volatilizing capacity was different. Cell-free extracts prepared from overnight mercury-induced cells catalyzed Hg²⁺-induced NADPH oxidation. Specific activities of Hg²⁺-reductase which is capable of catalyzing conversion of Hg²⁺ →Hg(o) of 10 Hg-resistant bacterial strains are also reported.     

Development of a transgenic tobacco plant for phytoremediation of methylmercury pollution

To develop the potential of plant for phytoremediation of methylmercury pollution, a genetically engineered tobacco plant that coexpresses organomercurial lyase (MerB) with the ppk-specified polyphosphate (polyP) and merT-encoding mercury transporter was constructed by integrating a bacterial merB gene into ppk/merT-transgenic tobacco. A large number of independent transgenic tobaccos was obtained, in some of which the merB gene was stably integrated in the plant genome and substantially translated to the expected MerB enzyme in the transgenic tobacco. The ppk/merT/merB-transgenic tobacco callus showed more resistance to methylmercury (CH₃Hg⁺) and accumulated more mercury from CH₃Hg⁺-containing medium than the ppk/merT-transgenic and wild-type progenitors. These results suggest that the MerB enzyme encoded by merB degraded the incorporated CH₃Hg⁺ to Hg²⁺, which then accumulated as a less toxic Hg-polyP complex in the tobacco cells. Phytoremediation of CH₃Hg⁺ and Hg²⁺ in the environment with this engineered ppk/merT/merB-transgenic plant, which prevents the release mercury vapor (Hg⁰) into the atmosphere in addition to generating potentially recyclable mercury-rich plant residues, is believed to be more acceptable to the public than other competing technologies, including phytovolatilization.     

Volatilization of mercury byThiobacillus ferrooxidans

Thiobacillus ferrooxidans became significantly more tolerant to mercury stress after culturing in media of increasing mercury(II) concentrations. When mercuric chloride was added to the growth medium, the resistant organisms were found to volatilize elemental mercury (Hg⁰).T. ferrooxidans may be an important factor in the natural mercury cycle, since the environments whereT. ferrooxidans is found typically contain elevated levels of heavy metals, including mercury.     

Influence of Mercury on the Green Alga Haematococcus lacustris. Inhibition Effects and Recovery of Impact

The influence of mercury ions on germination of the resting cells (aplanospores) and on cell division, cell structure, phototaxis, and photosynthesis during the flagellate stage of Hoemotococcus lacustris was investigated. Aplanospores possess a higher tolerance against mercury ions than flagellates. The reason could be seen in the thicker wall of the resting cells which possibly provide a detoxifying effect by immobilisation of Hg²⁺. This is confirmed by a normal phototactic orientation of flagellates formed from Hg ²⁺-influenced aplanospores. In contrast, a direct addition of Hg²⁺ (0.1 to 1 /*M) to the flagellate stage induced an immediate loss of the flagellates to react phototactically, but it was interesting that the inhibition was overcome with time. Obviously, these Hg²⁺ concentrations influence only the sensory transduction chain, whereas the energetic background is not injured because the velocity of movement and the percentage of motile cells were scarcely affected. This is supported by the high level of the photochemical efficiency of photosystem II, which remains unchanged at 1 uM Hg²⁺. Recovery of photosynthesis from inhibition by 10/iM Hg²⁺ suggests a connection between Hg²⁺ influence and metabolism of the D, protein in the reaction centre of photosystem II. The Hg²⁺ effect was reversed with time in light, but not in darkness, and streptomycin, an inhibitor of chloroplast protein synthesis, prevented recovery. In flagellates, showing no reactivation, exposure to 10uA/l Hg²⁺ caused cell swelling, a loss of the flagella, and a disorganisation in structure of the chloroplast and of nucleus.     

A coumarin‐based fluorescent probe for Hg2+ and its application in living cells and zebrafish

Mercury (Hg) is a heavy metal with high toxicity and easy migration; it can be enriched through the food chain, and cause serious threats to the natural environment and human health. So, the development of a method that can be used to detect mercury ions (Hg²⁺) in the environment, in cells, and in organisms is very important. Here, a new 7‐hydroxycoumarin‐derived carbonothioate‐based probe ( CC‐Hg ) was designed and synthesized for detection of Hg²⁺. After addition of Hg²⁺, a large fluorescence enhancement was observed due to the formation of 7‐hydroxyl, which reinforced the intramolecular charge transfer process. The CC‐Hg probe had good water solubility and selectivity. Moreover, the probe was able to detect Hg²⁺ quantitatively over the concentration range 0–2 μM and with a detection limit of 7.9 nM. Importantly, we successfully applied the probe to detect Hg²⁺ in water samples, in living cells, and in zebrafish. The experimental results demonstrated its potential value in practical applications.     

Phytoextraction of HG by parsley (Petroselinum crispum) and its growth responses

The effect of mercury (Hg) on the growth and survival of parsley (Petroselinum crispum) was explored at various treatments. The plants were grown in pots having Hoagland's solution to which various Hg treatments were applied and placed under greenhouse conditions. The treatments were: no metal applied (control) and six doses of Hg as mercuric chloride for 15 days. Linear trend of Hg accumulation was noted in roots, stems, and leaves with increasing Hg treatments. The maximum Hg concentration in root, stem and leaf was 8.92, 8.27, and 7.88 at Hg treatments of 25 mg l^–1, respectively. On the whole, Hg accumulation in different plant parts was in the following order: leaves > stem > roots. Linear trend was also observed for Bioaccumulation Factor (BF) and Translocation Factor (TF) with increasing Hg concentrations in the growth medium. The highest respective BF_Hg and TF_Hg values were 9.32 and 2.02 for the Hg treatments of 25 and 50 mg l^–1. In spite of the reduced growth in the presence of Hg, the plant has phytoremediation potential. It is recommended that parsley should not be cultivated in Hg contaminated sites in order to avoid dietary toxicity.     

Light-Induced Hg Volatilization and O2 Evolution in Chlorella and the Effect of DCMU and Methylamine

The rate of volatilization of Hg²⁺ as metallic Hg is accelerated by illumination of Chlorella cells. In the presence of the uncoupler methylamine the rate of volatilization in the light is greatly but transiently increased. DCMU (3-(3,4-dichlorophenyl)-1,1-dimethyl urea) prevented the light response. In the presence of Hg²⁺, O₂ evolution by the cells was not completely inhibited by DCMU. Hg²⁺ appears to prevent DCMU reaching its binding site. Light seems to increase the amount of or leakage from the cells of a metabolite capable of reducing Hg²⁺ to Hg°.     

A Michaelis–Menten type equation for describing methylmercury dependence on inorganic mercury in aquatic sediments

Methylation of mercury (Hg) is the crucial process that controls Hg biomagnification along the aquatic food chains. Aquatic sediments are of particular interest because they constitute an essential reservoir where inorganic divalent Hg (Hg^II) is methylated. Methylmercury (MeHg) concentrations in sediments mainly result from the balance between methylation and demethylation reactions, two opposite natural processes primarily mediated by aquatic microorganisms. Thus, Hg availability and the activity of methylating microbial communities control the MeHg abundance in sediments. Consistently, some studies have reported a significant positive correlation between MeHg and Hg^II or total Hg (Hg_T), taken as a proxy for Hg^II, in aquatic sediments using enzyme-catalyzed methylation/demethylation mechanisms. By compiling 1,442 published and unpublished Hg_T–MeHg couples from lacustrine, riverine, estuarine and marine sediments covering various environmental conditions, from deep pristine abyssal to heavily contaminated riverine sediments, we show that a Michaelis–Menten type relationship is an appropriate model to relate the two parameters: MeHg = aHg_T/(K _m  + Hg_T), with a = 0.277 ± 0.011 and K _m  = 188 ± 15 (R ² = 0.70, p < 0.001). From K _m variations, which depend on the various encountered environmental conditions, it appears that MeHg formation and accumulation are favoured in marine sediments compared to freshwater ones, and under oxic/suboxic conditions compared to anoxic ones, with redox potential and organic matter lability being the governing factors.     

Review of characteristics of mercury speciation and mobility from areas of mercury mining in semi-arid environments

The speciation of mercury—including most phase minerals, secondary phases, gaseous and aqueous species—is very important for evaluating the environmental impact and mobilization of this contaminant. Mining activities produce mercury mine waste, which includes several types of material (mainly mine waste and calcines) with varying mercury content and speciation depending on the ore deposit and processing technology. The main phase minerals are cinnabar, metacinnabar, metallic Hg⁰, corderoite, livingstonite, calomel and schuetteite. The aqueous mobilization of mercury is controlled by complex formation, pH-Eh conditions, the primary phase mineral of mercury, and organic-matter and iron oxyhydroxide content. The possibility of colloidal transport of mercury from mine waste is influenced by the atmospheric emission of metallic Hg⁰ and the leaching of waste by episodic high-intensity precipitations. In these climatic conditions, mercury can be mobilized to pore water, surface water or groundwater by the dissolution of metallic Hg⁰ and cinnabar in acidic conditions, and by the colloidal transport. The presence of Hg-soluble phases (chlorides and oxychlorides) may enhance the mobilization of mercury. In the semi-arid conditions of the Valle del Azogue (SE Spain) the atmospheric emissions of the Hg⁰ present in calcines and mine waste may be significant and the dissolution of Hg⁰ and metal-sulfate salts during episodic runoff events may explain the mobilization of Hg, Fe, Pb, Zn and other heavy metals.     

Mercury Tolerance of Thermophilic <Emphasis Type="Italic">Bacillus</Emphasis> sp. and <Emphasis Type="Italic">Ureibacillus</Emphasis> sp

Although resistance of microorganisms to Hg(II) salts has been widely investigated and resistant strains have been reported from many eubacterial genera, there are few reports of mercuric ion resistance in extremophilic microorganisms. Moderately thermophilic mercury resistant bacteria were selected by growth at 62 °C on Luria agar containing HgCl₂. Sequence analysis of 16S rRNA genes of two isolates showed the closest matches to be with Bacillus pallidus and Ureibacillus thermosphaericus. Minimum inhibitory concentration (MIC) values for HgCl₂ were 80 μg/ml and 30 μg/ml for these isolates, respectively, compared to 10 μg/ml for B. pallidus H12 DSM3670, a mercury-sensitive control. The best-characterised mercury-resistant Bacillus strain, B. cereus RC607, had an MIC of 60 μg/ml. The new isolates had negligible mercuric reductase activity but removed Hg from the medium by the formation of a black precipitate, identified as HgS by X-ray powder diffraction analysis. No volatile H₂S was detected in the headspace of cultures in the absence or presence of Hg²⁺, and it is suggested that a new mechanism of Hg tolerance, based on the production of non-volatile thiol species, may have potential for decontamination of solutions containing Hg²⁺ without production of toxic volatile H₂S.     

A highly selective colorimetric and long‐wavelength fluorescent probe for the detection of Hg2+

Currently, the fluorescent probe is an important method for detecting heavy metal ions, especially mercury ion (Hg²⁺), which is harmful to the health of humans and the environment due to its toxicity and extensive use. In this paper, we designed and synthesized a colorimetric and long‐wavelength fluorescent probe Hg‐P with high sensitivity and excellent selectivity, which could detect Hg²⁺ by the changes of visual color, fluorescence and absorption spectroscopy. With the addition of Hg²⁺ to probe Hg‐P solution, its color changed from yellow to pink, and showed a 171 nm red‐shifted absorption spectrum. Probe Hg‐P was used in real water and soil solution samples to detect Hg²⁺, and the result is satisfactory. Therefore, this new probe shows great value and application in detecting Hg²⁺ in the environment.     

Mercury mobilization by chemical and microbial iron oxide reduction in soils of French Guyana

Iron oxy(hydr)oxides (oxides) are important mercury sinks in tropical oxisols and the geochemistry of these two elements are thus closely entwined. We hypothesized that bacterial Fe-oxide reduction in anoxic conditions could be a significant mechanism for mobilizing associated Hg. Iron oxide and mercury solubilisation in presence of two chemical reducers (ascorbate and dithionite, dissolving amorphous and amorphous plus well crystallized Fe-oxides, respectively) was compared to their solubilisation in presence of autochthonous ferri-reducing bacteria. This work was carried out on two soil profiles from a small catchment basin in French Guyana, an oxisol (O) from a well drained slope and a water-saturated hydromorphic soil (H). The chemical reductions showed that in the oxisol 20 and 48% of total Hg (Hg_T) was associated to amorphous and well crystallized iron oxides, respectively. However, in the hydromorphic soil, no Hg seemed to be associated to amorphous iron oxides while the well crystallized fraction contained less than 9% of Hg_T. Chemical Fe-oxide reduction showed that Hg solubility was correlated to Fe reduction in the oxisol, demonstrating a relationship between the geochemistry of these two metals. During bacterial growth, while bacterial iron reduction solubilised up to 3.2 mg Fe g^?1 soil in the oxisol sample, Hg_T remained unchanged. No mercury was detected in the culture medium either. However, chemical analysis showed a decrease of the amounts of Hg associated to amorphous and well crystallized Fe-oxides after 14 days of incubation, underlining the potential for iron-reducing bacteria to modify mercury distribution in soil.     

Mercury-resistant plasmids in bacteria from a mercury and antimony deposit area

Summary Most bacterial cells (Pseudomonas, Acinetobacter) obtained from the soil at the Khaidarkan mercury and antimony mine (Kirghiz USSR) contain R plasmids with mercury (HgCl₂) resistance determinants. The plasmids have a large molecular mass (about 100 MD, though smaller ones also occur), and at least some of them are transmissive. Many of the Hg^r bacteria also display an elevated antimony (SbCl₃) resistance, though this trait was not shown to be plasmid-dependent. There are practically no Hg^r plasmids in bacteria taken from the soil at different distances from the mine: the saturation of bacteria with Hg^r plasmids is maintained by selective pressure only in the area with a high enough toxin concentration.In the same mercury and antimony deposit area Hg^r plasmids were also found in Escherichia coli isolates from the gut of Mus musculus mice and Bufo viridis toads. At least some of the bacterial plasmids obtained from animals also carry antibiotic-resistance determinants (Tc^r, Cm^r, Sm^r). These plasmids are also transmissive. They display internal instability and lose their resistance determinants after a conjugation transfer to other E. coli trains.     

Diel variability of mercury phase and species distributions in the Florida Everglades

Preliminary studies of mercury (Hg) cycling in the Everglades revealed that dissolved gaseous mercury (DGM), total mercury (Hg_T), and reactive mercury (Hg_R) show reproducible, diel trends. Peak water-column DGM concentrations were observed on or about noon, with a 3 to 7 fold increase over night-time concentrations. Production of DGM appears to cease during dark periods, with nearly constant water column concentrations that were at or near saturation with respect to the overlying air. A simple mass balance shows that the flux of Hg to the atmosphere from diel DGM production and evasion represents about 10% of the annual input from atmospheric deposition. Production of DGM is likely the result of an indirect photolysis reaction that involves the production of reductive species and/or reduction by electron transfer. Diel variability in Hg_T and Hg_R appears to be controlled by two factors: inputs from rainfall and photolytic sorption/desorption processes. A possible mechanism involves photolysis of chromophores on the surface of a solid substrate (e.g., the periphyton mat) giving rise to destabilization of sorbed mercury and net desorption during daylight. At night, the sorption reactions predominate and the water-column Hg_T decreases. Methylmercury (MeHg) also showed diel trends in concentration but were not clearly linked to the solar cycle or rainfall at the study site.     

Inorganic mercury (Hg2+) transport through lipid bilayer membranes

Summary Diffusion of inorganic mercury (Hg²⁺) through planar lipid bilayer membranes was studied as a function of chloride concentration and pH. Membranes were made from egg lecithin plus cholesterol in tetradecane. Tracer (²⁰³Hg) flux and conductance measurements were used to estimate the permeabilities to ionic and nonionic forms of Hg. At pH 7.0 and [Cl^–] ranging from 10–1000mm, only the dichloride complex of mercury (HgCl₂) crosses the membrane at a significant rate. However, several other Hg complexes (HgOHCl, HgCl ₃ ^– and HgCl ₄ ^2– ) contribute to diffusion through the aqueous unstirred layer adjacent to the membrane. The relation between the total mercury flux (J _Hg), Hg concentrations, and permeabilities is: 1/J _Hg=1/P ^ul[Hg ^t ]+1/P ^m [HgCl₂], where [Hg ^t ] is the total concentration of all forms of Hg,P ^ul is the unstirred layer permeability, andP ^m is the membrane permeability to HgCl₂. By fitting this equation to the data we find thatP ^m =1.3×10^–2 cm sec^–1. At Cl^– concentrations ranging from 1–100mm, diffusion of Hg ^t through the unstirred layer is rate limiting. At Cl^– concentrations ranging from 500–1000mm, the membrane permeability to HgCl₂ becomes rate limiting because HgCl₂ comprises only about 1% of the total Hg. Under all conditions, chemical reactions among Hg²⁺, Cl^– and/or OH^– near the membrane surface play an important role in the transport process. Other important metals, e.g., Zn²⁺, Cd²⁺, Ag⁺ and CH₃Hg⁺, form neutral chloride complexes under physiological conditions. Thus, it is likely that chloride can facilitate the diffusion of a variety of metals through lipid bilayer and biological membranes.     

Reduced sulphur sources favour HgII reduction during anoxygenic photosynthesis by Heliobacteria

The consumption of rice has become a global food safety issue because rice paddies support the production of high levels of the potent neurotoxin, methylmercury. The production of methylmercury is carried out by chemotrophic anaerobes that rely on a diversity of terminal electron acceptors, namely sulphate. Sulphur can be a limiting nutrient in rice paddies, and sulphate amendments are often used to stimulate crop production, which can increase methylmercury production. Mercury (Hg) redox cycling can affect Hg methylation by controlling the delivery of inorganic Hg substrates to methylators in anoxic habitats. Whereas sulphur is recognized as a key substrate controlling methylmercury production, the controls sulphur exerts on other microbe‐mediated Hg transformations remain poorly understood. To explore the potential coupling between sulphur assimilation and anaerobic Hg^II reduction to Hg⁰, we studied Heliobacillus mobilis, a mesophilic anoxygenic phototroph representative from the Heliobacteriacea family originally isolated from a rice paddy. Here, we tested whether the redox state of the sulphur sources available to H. mobilis would affect its ability to reduce Hg^II. By comparing Hg⁰ production over a redox gradient of sulphur sources, we demonstrate that phototrophic Hg^II reduction is favoured in the presence of reduced sulphur sources such as thiosulphate and cysteine. We also show that cysteine exerts dynamic control on Hg cycling by affecting not only Hg's bioavailability but also its abiotic photoreduction under low light conditions. Specifically, in the absence of cells we show that organic matter (as yeast extract) and cysteine are both required for photoreduction to occur. This study offers insights into how one of the most primitive forms of photosynthesis affects Hg redox transformations and frames Heliobacteria as key players in Hg cycling within paddy soils, forming a basis for management strategies to mitigate Hg accumulation in rice.     

Engineering expression of bacterial polyphosphate kinase in tobacco for mercury remediation

To develop the potential of plants to sequester and accumulate mercurials from the contaminated sites, we engineered a tobacco (Nicotiana tabacum) plant to express a bacterial ppk gene, encoding polyphosphate kinase (PPK), under control of a plant promoter. The designated plant expression plasmid pPKT116 that contains the entire coding region of ppk was used for Agrobacterium-mediated gene transfer into tobacco plants. A large number of independent transgenic tobacco plants were obtained, in some of which the ppk gene was stably integrated in the plant genome and substantially translated to the expected PPK protein in the transgenic tobacco. The presence of Hg²⁺ did not cause considerable morphological abnormalities in the transgenic tobacco, which grew, flowered, and set seed similarly to the wild-type tobacco on the medium containing normally toxic levels of Hg²⁺. The ppk-transgenic tobacco showed more resistance to Hg²⁺ and accumulated more mercury than its wild-type progenitors. These results suggest that ppk-specified polyphosphate has abilities to reduce mercury toxicity, probably via chelation mechanism, and also to accumulate mercury in the transgenic tobacco. Based on the results obtained in the present study, the expression of ppk gene in transgenic tobacco plants might provide a means for phytoremediation of mercury pollution.     

Biosorption of inorganic and methyl mercury by a biosorbent from Aspergillus niger

A biosorbent prepared by alkaline extraction of Aspergillus niger biomass was evaluated for its potential to remove mercury species – inorganic (Hg²⁺) and methyl mercury (CH₃Hg⁺) – from aqueous solutions. Batch experiments were carried out to determine the pH and time profile of sorption for both species in the pH range 2–7. The Hg²⁺ exhibited more rapid sorption and higher capacity than the CH₃Hg⁺. Further, removal of both mercury species from spiked ground water samples was efficient and not influenced by other ions. Sorption studies with esterified biosorbent indicated loss of binding of both mercury species (>80%), which was regained when the ester groups were removed by alkaline hydrolysis, suggesting the involvement of carboxyl groups in binding. Further, no interconversion of sorbed species occurred on the biomass. The biosorbent was reusable up to six cycles without serious loss of binding capacity. Our results suggest that the biosorbent from Aspergillus niger can be used for removal of mercury and methyl mercury ions from polluted aqueous effluents.     

A new proposal regarding the transport mechanism of mercury in biological membranes

The interactions of mercury (Hg²⁺) with biological membranes have been investigated. The experimental results indicate that Hg²⁺ induces a rapid alkalinization in energized Lysosomes from rat liver. The interpretation of the process is that the mercury enters the Lysosomes as a Hg(OH)₂ electroneutral compound, thus inducing alkalinization in the matrix.