Bioinformatic Analyses of Bacterial Mercury Ion (Hg2+) Transporters

Currently, there are five known types of mercury transporters in bacteria: MerC, MerE, MerF, MerH, and MerT. Their general function is to mediate mercuric ion uptake into the cell in preparation for reduction to Hg°. They are present in several bacterial phyla and comprise five distinct families. We have utilized standard statistical bioinformatic tools and the superfamily principle to show that they are related by common descent. After using programs such as Global Alignment Program and SSearch to establish homology, we aligned and analyzed their amino acid sequences to find a single well conserved motif. Although these proteins exhibit 2, 3, or 4 transmembrane helical segments (TMSs), TMSs 1 and 2 are common to all superfamily members. An ancestral sequence was determined, and reliable phylogenetic trees were constructed. The results support the conclusion of homology, establishing that these proteins belong to a single superfamily. This important discovery allows extrapolation of information about structure, function, and mechanism from one protein to all superfamily members to degrees inversely proportional to their phylogenetic distances.     

Mercury reactions in the human mouth with dental amalgams

This is a preliminary study of the reactions of mercury (Hg) in the human mouth with dental amalgams. It was conducted by analysing saliva samples from subjects with amalgam fillings and control subjects with no amalgams. Samples were collected both prior to and after cleaning the mouth. These samples were analyzed for elemental mercury (Hg⁰), inorganic mercury (Hg²⁺) and methylmercury (MeHg). We concluded that the concentrations after cleaning represented the systemic concentrations. Hg²⁺ and MeHg were found in all systemic samples from both subjects and controls, while Hg⁰ was found only in the samples from subjects with amalgams. In the control group, the concentrations found before and after cleaning the mouth were equivalent. In the amalgam group, concentrations of Hg²⁺ found before cleaning the mouth were 10 to 40 times higher than those found after cleaning, suggesting that the oxidation reaction of Hg° into Hg²⁺ takes place. For MeHg, a similar but less pronounced pattern as Hg²⁺ was found, supporting methylation in the mouth.     

Mercury speciation in contaminated soils by thermal release analysis

Thermal release analysis of mercury species in contaminated soils was performed by temperature controlled continuous heating of the samples in a furnace coupled to an Atomic Absorption Spectrophotometer (AAS). It was shown that this method allows the identification of different redox states of Hg-species through their characteristic releasing temperature ranges. The method was applied to Hg-contaminated samples from an inactive chlor-alkali production plant in former East Germany (GER), and from a gold mining area in Poconé, Mato Grosso, Brazil (BRA), as well as synthetic soil samples obtained by spiking pre-heated soil matrices (GER and BRA) with the following mercury species: Hg⁰, Hg₂Cl₂, HgCl₂, HgO and HgS. The samples GER, in general, frequently showed the presence of Hg²⁺ probably bound to humic substances, in the case of samples with higher total carbon content. Only in highly contaminated samples (>3000 ppm of mercury) was Hg⁰ the predominant species. The samples BRA more frequently showed the presence of mercury species in the lower oxidation states, i.e. Hg¹⁺ in combination with Hg⁰. The method allows observing changes in Hg-speciation in the samples with time, mainly changes among the oxidation states Hg⁰, Hg¹⁺ and Hg²⁺. The treated GER matrix showed a stronger tendency to oxidise Hf-species than the BRA treated matrix, in which only added Hg⁰ is partially oxidised to Hg²⁺ and Hg¹⁺. In contrast, the BRA matrix showed a pronounced tendency to reduce spiked Hg²⁺ to Hg¹⁺. This may be the reason for the presence of Hg¹⁺ in the majority of original BRA samples. The method appears to be very useful to study speciation of mercury and its dynamics. It can be used as a tool for monitoring mercury oxidation states and/or reactions of mercury in soils.     

Mercury Methylation Capacity and Removal of Hg Species from Aqueous Medium by Cyanobacteria

Most technologies used for decontamination presents good results for high concentrations, but limitations for lower ones. The desirable Hg concentration in the water is extremely low because of its toxicity. The aims of this study were to evaluate inorganic mercury (Hg²⁺) and methylmercury (CH₃Hg⁺) toxicity in Nostoc paludosum, to assess the potential of this cyanobacteria strain to remove these Hg species from aqueous medium and also to investigate Hg methylation by the cyanobacteria. CH₃Hg⁺ determination was performed by gas chromatography-pyrolysis-atomic fluorescence spectrometry in cultures exposed to a concentration of 20 μg L^?1 for 30 days. Both Hg species were removed from the supernatant, ranging from 73 to 96% of Hg²⁺ and from 73 to 95% of CH₃Hg⁺. Ultrastructural Hg²⁺ effects in the cyanobacteria cells investigated by transmission electron microscopy revealed higher production of glycogen, cyanophycin, and intrathylacoidal spaces than the control group. When Hg was added to the culture in the form of CH₃Hg⁺, a decrease corresponding to approximately 60% of the initial concentration due to Hg volatilization was observed. The production of CH₃Hg⁺ by the cyanobacteria was detected in concentrations near the limit of detection (0.0025%) of the bioaccumulated THg. This is an advantage for biotechnological decontamination applications, as CH₃Hg⁺ is a very toxic specie and can be bioaccumulated and biomagnified. The results demonstrated that cyanobacteria cells are an efficient alternative to retain and/or remove Hg at low concentrations and they constitute a potential tool for a “final cleaning” of contaminated waste water.     

Mercury in Small Freshwater Lakes: A Case Study: Lake Velenje, Slovenia

Lake Velenje is located in one of the most polluted regions ofSlovenia, the ?alek Valley. The major source of pollution is the coal-fired thermal power plant in ?o?tanj (?TPP). In the past, dumping of coal ash directly into Lake Velenje and drainage water from the ash disposal site resulted in unique chemical characteristics of the lake water, such as very high pH (10–12) and high concentrations of heavy metals. The introduction of a closed cycle ash transport system in 1995resulted in a very fast recovery of the lake water quality. The aim of our study was to establish the sources, fate and distribution of mercury in Lake Velenje. In order to establishrecent sources of mercury, total mercury and methylmercury concentrations were measured in various environmental samples(lake inflows, outflow, rainwater, sediments, etc.). Total mercury and methylmercury concentrations were measured at thesurface and at different depths to establish mercury cycling, its transport and chemical transformations in the lake. Generalwater quality parameters (such as pH, Eh, O₂, temperature,TDS, conductivity) were also determined. The results show that the major sources of mercury in Lake Velenje are lake inflows and wet deposition. Total mercury andmethylmercury concentrations in the water column are very low (total mercury: 0.2–2.7 ng L^-1; methylmercury: 20–86 pg L^-1) and can be compared to other non-contaminated freshwater lakes. These results suggest that the major form of mercury emitted from thermal power plant stacks is volatile Hg⁰, whichhas no or very little influence on the nearby surroundings, but rather is subject to long-range atmospheric transport.     

Mercury resistance among soil bacteria: Ecology and transferability of genes encoding resistance

A diverse population of bacteria, all with the common property of resistance to mercuric ions (Hg²⁺), was isolated from root-associated, and other soils. Each isolate was screened for its resistance to a variety of antibiotics and heavy metals as well as organomercurial compounds. Several isolates showed resistance values to individual antimicrobial agents that were significantly higher than the average resistance values with respect to those agents for the genus to which the isolate belonged. Of 76 distinct isolates tested, 4 were able to transfer the Hg^r phenotype to sensitive cells at environmental temperatures. “Mobilization” of putative Hg-resistant non-conjugative plasmids with the wide host range plasmid RPl resulted in the co-transfer of the RPl markers and resistance to Hg²⁺ in 9 cases. Pl transduetion showed that 7 of these transfers were due, at least in part. to the formation of RPl-Hg^r co-integrates. This suggests that translocatable DNA elements encoding resistance to Hg²⁺ may be partly responsible for the dissemination and maintenance of the Hg^r phenotype in microbial populations.     

Impact of the state-of-the-art of flue gas cleaning on mercury species emissions from coal-fired steam generators

When balancing the element mercury (Hg) two coal-fired power plant units — one with slag tap boilers (ST, 2 × 220 MW) and one with a dry bottom boiler (DB, 475 MW) were compared. Both systems are provided with electrostatic precipitators (ESP), nitrogen oxides removal (DeNO_x) and flue gas desulfurization (FGD) systems. The Hg in the flue gas is predominantly in gas phase. Only 15 % of the Hg introduced by the coal leaves the unit with the bottom or fly ash. Depending on the operating mode, 30 to 40 % of the Hg is separated in the FGD systems. The overall separation rate for the total system ranges between 45 to 55 %, the residue is emitted in the form of gaseous Hg species. At full load, the Hg concentration in the cleaned gas is less than 6 μg/m³. In the flue gas path of another dry bottom boiler (DB1, 480 MW) the concentrations of the gaseous species of bivalent mercury (Hg²⁺), elemental mercury (Hg⁰), and total mercury content (Σ Hg) were determined. The sum of the concentrations of Hg²⁺ and Hg⁰ is in agreement with the measurement of Σ Hg. Directly downstream of the boiler Hg²⁺ dominates with 77 %, while Hg⁰ amounts to 23 %. In the high-dust DeNO_x system Hg⁰ is oxidized almost completely to Hg²⁺ (96 %). Air heater and electrostatic precipitator do not influence the Hg species concentrations. The FGD system eliminates approximately 80 % of the Hg²⁺. At the same time the quantity of Hg⁰ increases by the factor 10. In the cleaned gas Hg⁰ dominates with 76 % as compared to Hg²⁺ with 24 %. At full load the concentration of Σ Hg in the cleaned gas is also below 6 μg/m³.     

Exposure to mercury in Canada: A multimedia analysis

A thorough assessment of Canadian exposure to mercury (Hg) has not been undertaken since 1970. A multimedia approach was used to update estimates of Hg exposure to members of the general population in Canada, based on currently available information. Adult Canadians have an estimated intake of all Hg species via all routes of 7.7μg/day (0.11μg/kg body weight/day), which equated to an absorbed dose of 5.3μg/day (0.076μg/kg bw/day). Fish consumption accounts for much of this exposure (27% of intake, 40% of absorbed dose), in the form of methylmercury. However, dental amalgam appears to account for a greater proportion of total Hg exposure than fish consumption. Exposure from amalgam was estimated for intake and absorbed doses (of Hg⁰) at 2.81 and 2.25μg/day, respectively. This represents 36% of total Hg intake and 42% of absorbed dose. Hg²⁺ arises principally from foods other than fish. Intake of Hg²⁺ by adults was determined to be 1.82μg/day and absorbed dose only 0.18μg/day. Exposures for four other age groups of the population were also evaluated.     

Enhanced Tolerance to Mercury in a Streptomycin-Resistant Strain of Euglena gracilis

Toxicity of Hg²⁺ was determined in two strains of Euglena gracilis var. bacillaris: the wild-type (B) and a streptomycin-resistant strain (Sm^r). Cells were cultured under 12-h dark/12-h light regime or under continuous darkness. In the dark/light cultures, cellular growth was severely diminished in the B strain by Hg²⁺, whereas only a slight decrease was attained in the Sm^r strain; Hg²⁺ also affected the photosynthetic and respiratory activities of the B strain, but not those of Sm^r strain. Under continuous darkness, cellular growth of both strains was lower than under dark/light cycles, but it was inhibited by Hg²⁺ to a much lesser extent. Cell culture by 5 days under continuous dark or by 8 days of dark/light cycles resulted in a higher intracellular content of mercury in B strain than in Sm^r strain. In contrast, in both culture conditions, the fraction of mercury removed from medium by B strain was lower than that attained by Sm^r strain, whereas the ability to bio-transform (reduce) Hg²⁺ was two times higher in the mutant strain. The results suggested that Sm^r strain cells acquired an ability to remove Hg²⁺ from the medium, which was not associated to accumulation and which conferred protection against mercury.     

Development of Highly Selective and Efficient Prototype Sensor for Potential Application in Environmental Mercury Pollution Monitoring

Mercury (Hg) is an environmental pollutant which is detrimental to the health of living beings due to the toxicity in its all oxidation states. To control mercury pollution development of low cost, efficient and highly sensitive prototype mercury sensor remains a challenge. In the present work, we have proposed a low-cost prototype device based on silver nanoparticle-impregnated poly(vinyle alcohol) (PVA-Ag-NPs) nanocomposite thin film for mercury detection. The thin film, fabricated through a facile protocol, is shown to be a fast, efficient, and selective sensor for Hg²⁺ in aqueous medium with a detection limit of 10 ppb. We have utilized the aggregation and amalgamation of Ag-NPs with Hg²⁺ to develop the low-cost, highly efficient and feasible prototype mercury sensor. In the presence of Hg²⁺, the yellowish thin film turned into colourless due to the loss of intense surface plasmon resonance (SPR) absorption band of the silver nanoparticles (Ag-NPs) through aggregation and amalgamation with mercury. The developed sensor has high selectivity for Hg²⁺ ions over a wide range of other competing heavy metal ions, generally present in water of natural sources. The sensor response is found to be linear over the Hg²⁺ ion concentration regime from 10 ppb to 5 ppm. The developed sensor has shown to determine a trace Hg²⁺ ions in real water samples. Finally, using the proposed technique, we have developed a simple and inexpensive prototype device for monitoring in field environmental mercury pollution.

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Graphical Abstract ?

     

Immobilization of Mercury Species under the Influence of Typical Organic Compounds in Waste Gases by the Circulating Fluidized Bed

Hearth furnace coke (HOK), a special type of lignite (brown-coal)coke produced in a manufacturing process called `hearth furnace process', and portlandite (Ca(OH)₂) particles were placed ina circulating fluidized bed reactor. Defined model waste gases containing HCl/SO₂/Hg⁰/Hg²⁺ and organic compounds were injected into the reactor to investigate the adsorption of different mercury species. Elemental mercuryreacted immediately with HCl to form HgCl₂, but for Hg²⁺ further investigations had to be made, because todaygas cleaning plants still have problems in observing national limits for the mercury output. The temperature, the mercury content of the gas, and its content of acid compounds as majorinfluences in cleaning exhaust gases were varied without relevantpositive effects on the adsorption rate which decreased from nearly 100% in the first five minutes to unsatisfactory20% some minutes later. The mercury load on the HOK particles onlyshowed a value of 60 μg Hg g^-1 HOK. Then organic compounds (guide pollutants such as 1-chlorobutane, monochlorobenzene, toluene, and naphthalene, which are alwayspresent in incineration plant exhaust gases) were added to thegas stream and the results improved significantly. The mercuryload on the HOK particles varied depending on the chemical nature of the organic compound and amounted to 300 μg Hg g^-1 HOK maximum. The measured values of mercury in the clean gas stream fell below 10 μg m^-3. The characterization of the dynamic behavior of mercury in hot, acid waste gases and the analytical identification of mercurysticking to HOK under the influence of organic compounds leadto new methods for improving the effectiveness and performanceof gas cleaning plants.     

Catalytic effect of various metal ions on the methylation of mercury in the presence of humic substances

Methylation of Hg²⁺ (Hg(NO₃)₂) in the presence of fulvic acid (FA) and various metal ions has been studied. The concentrations of Hg²⁺ and FA ranged from 5 to 20 mg L^?1 and 171 to 285 mg L^?1 DOC, respectively. The pH range was 3 to 6.5. FA was isolated from an acid brown-water lake by XAD-8 polymeric adsorbent. Methylmercury production in the dark during 2 to 4 days incubation at 30 °C increased with increasing concentrations of Hg²⁺ ion and FA as well as with additions of metal ions (5 to 10 × 10^?5 mole L^?1 The observed catalytic activity of metal ions followed the order Fe³⁺ (Fe²⁺) > Cu²⁺ ≈ Mn²⁺, > Al³⁺. The production of methylmercury had a pH-optimum around 4 to 4.5 at the conditions tested.     

Mercury Content of Illinois Soils

For a survey of Illinois soils, 101 cores had been collected and analyzed to determine the current and background elemental compositions of Illinois soils. Mercury and other elements were determined in six samples per core, including a surface sample from each core. The mean mercury content in the surface samples was 33 ± 20 g/kg soil, and the background content was 20 ± 9 g/kg. The most probable sources of mercury in these soils were the parent material, and wet and dry deposition of Hg⁰ and Hg²⁺ derived from coal-burning power plants, other industrial plants, and medical and municipal waste incinerators. Mercury-bearing sewage sludge or other fertilizers applied to agricultural fields could have been the local sources of mercury. Although the mercury content correlated with organic carbon content or clay content in individual cores, when all the data were considered, there was no strong correlation between mercury and either the organic carbon or the clay-size content.     

Studies on the Persistence of a Commercial Formulation of Chlorpyrifos on an Agricultural Soil from Provincia de Buenos Aires,República Argentina

Mercury and cadmium are priority hazardous substances. Some titanosilicates have been tested for the removal of Cd²⁺ and Hg²⁺ from single solutions by ion exchange. In this work, the competition between both contaminants for the exchanger binding sites of titanosilicate Engelhard titanosilicate material number 4 (ETS-4) was studied by performing batch experiments with aqueous solutions containing the two counter ions. The results evidenced the large capacity of ETS-4 and shown that the cadmium(II) diffusivity through the sorbent is higher than that of mercury(II). Furthermore, the ETS-4 exhibited higher kinetic and equilibrium selectivities for Cd²⁺, which attained values in the ranges 8.9–12.5 and 7.9–12.8, respectively. With respect to modelling, the pseudo-second-order equation described successfully the competitive removal of Cd²⁺ and Hg²⁺.     

Translocation of 203Hg labelled HgCl2 and CH3HgCl in an iron-humus podzol studied by radio-analytical techniques

Since increased Hg-concentrations in fish in lakes and rivers in northern Europe, northern parts of the U.S.A. and Canada were found, environmental Hg research has focused intensively on the factors determining leaching of mercury from soil into water systems. This article presents the results of a leaching experiment with undisturbed soil columns treated with HgCI₂ and CH₃HgCl using radio-analytical techniques. The columns were irrigated with rain of different acidity, rain volumes and irrigation intensities. The leaching of mercury was traced by detecting the vertical distribution of ²⁰³Hg in the soil profiles. Advantages and disadvantages of radioanalytical scanning techniques are discussed. The results of Hg leaching in the soil columns indicate a considerably stronger leaching of monomethyl mercury compared to inorganic mercury. Leaching of the two Hg-species is ruled by competition of H⁺ induced soil-Hg desorption with DOM-Hg complex formation; both being affected by rain acidity. Rain intensity had no visible effect on leaching of Hg²⁺ and CH₃Hg⁺. An extended rain duration increased the leaching of CH₃Hg⁺.     

Mercury Partitioning in Surface Sediments of the Upper St. Lawrence River (Canada): Evidence of the Importance of the Sulphur Chemistry

An intensive survey of mercury speciation was performed at a site on the Upper St. Lawrence River near Cornwall, Ontario, Canada with a history mercury contamination in sediments. Surface sediments were collected every 1.50 h. Total mercury (Hg_total), methylmercury (MeHg), organic carbon, inorganic and organic sulphur were determined in the solid fraction. Dissolved Hg_total, MeHg and dissolved organic carbon (DOC) were measured in pore waters. Concentrations of Hg_total in the upper layers (first 5 cm) were high, ranging from 1.42 to 25.8 nmol g^?1 in solids and from 125 to 449 pM in pore waters. MeHg levels were also high, ranging from 4.34 to 34.1 pmol g^?1 in solids and from 40 to 96 pM in pore waters. This amounts to up to 1.4% of Hg_total present as MeHg in solids and 64% in pore waters. A daily pattern for Hg_total was observed in the solid fraction. The MeHg distribution in solids and pore waters was not correlated with Hg_total or DOC_, suggesting that the concentrations of MeHg are probably more influenced by the relative rates of methylation/demethylation reactions in the sediment–water interface. Acid volatile sulphide levels and DOC were inversely correlated with organic sulphur (S_org) levels suggesting that both parameters are involved in the rapid production of S_org. A positive correlation was also observed between Hg_total and S_org in solids (R?=?0.87, p?<?0.01) illustrating the importance of organic sulphur in the retention and distribution of Hg in the solid fraction of the sediments. The results suggest that variations of Hg_total concentrations in Upper St. Lawrence River surface sediments were strongly influenced by the formation/deposition/retention of organic sulphur compounds in the sediment–water interface.     

Distribution of mercury species in soil from a mercury-contaminated site

The distribution of mercury species was determined in soil from a site with Hg contamination. Mercury contamination was primarily confined to the top 40 cm of soil, and the concentration of total Hg ranged from 0.5 to 3000 µg Hg g^?1. Of total Hg present, we determined that 91% was inorganic, 0.01% organic (as methyl Hg), and 6% elemental Hg. Furthermore, of total inorganic Hg present, 85% was in the insoluble mercuric sulfide form. Thus, of total Hg present in soil at this contaminated site, 91% was in the relatively insoluble HgS and Hg⁰ forms.     

An experimental study of two potential methylation agents of mercury in the atmosphere: CH3I and DMS

Experimental results from a study of the gas and aqueous phase reactions of elemental mercury (Hg⁰) with methyl iodide (CH₃I) and dimethyl sulfide (DMS) are presented. In aqueous phase experiments with CH₃I we found no observable increase in methyl mercury (MeHg). A small formation of MeHg, however, was observed in some (but not all) gas phase experiments in sunlight. A loss of Hg⁰ and a simultaneous formation of oxidized mercury (Hg(II)) was also observed in these experiments. No reaction, neither methylation or oxidation, was found between Hg⁰ and DMS under any conditions investigated. These experiments suggest that a simple homogeneous gas or aqueous phase methylation of Hg⁰ by DMS or CH₃I in the atmosphere cannot account for the significant levels of MeHg observed in precipitation.     

Rapid Determination of Mercury in Plant and Soil Samples Using Inductively Coupled Plasma Atomic Emission Spectroscopy, a Comparative Study

The objectives of this study were to simplify sample preparation and validate mercury detection in soil and plant samples using inductively coupled plasma atomic emission spectroscopy (ICP-AES). A set of mercury contaminated and mercury free soil and plant samples were digested and analyzed by ICP-AES, inductively coupled plasma mass spectrometry (ICP-MS), and cold vapor atomic absorption spectroscopy (CVAAS). Results show that mercury measurements in soil and plant samples using ICP-AES were in agreement with those analyzed using ICP-MS and CVAAS. The concentrations of mercury in soils and plant tissues determined by ICP-AES were 92.2% and 90.5% of those determined by CVAAS and ICP-MS, respectively. Digestion of soil samples with 4 M HNO₃ and direct measurement by ICP-AES without reduction of Hg²⁺ to Hg⁰ gave a reasonable and acceptable recovery (92%) for determining Hg in soils. We conclude that ICP-AES with optimized conditions (addition of gold chloride, extension of washing time, linear working range, and selection of wavelength – 194 nm) resulted in reliable detection of mercury in environmental samples.     

Evidence of microbial control of Hg0emissions from uncontaminated terrestrial soils

It is known that in wetland soils and soils contaminated with mercury (Hg), direct biotic reduction of Hg²⁺ to Hg⁰ leads to Hg⁰ emissions to the atmosphere. In terrestrial soils, numerous factors have been reported that control Hg⁰ emissions, but it is still unclear if biotic processes are also important. In this study, microbiological activity of Cambisol monoliths from a subalpine grassland with Hg concentrations of approx. 100 ng g^–1 were manipulated in laboratory incubation experiments. Elemental Hg emissions were recorded together with CO₂ emission rates as proxy for microbiological respiration. Emissions of Hg⁰ increased from approx. 5 ng m^–2 h^–1 up to 130 ng m^–2 h^–1 with stimulated biological activity (glucose addition, increase in temperature) and decreased with inhibited activity (chloroform fumigation, autoclaving, drying). Similar patterns with evasion rates of >90 ng m^–2 h^–1 were observed after dried soils were remoistened again. Our results indicated that processes leading to Hg⁰ emissions from uncontaminated terrestrial soils are at least partly controlled by biotic processes. However, it is still uncertain if Hg⁰ emission is caused directly by biotic reduction of Hg²⁺ or indirectly by abiotic reduction, induced by products of microbiological degradation, e.g., humic acids.