Principal biogeochemical factors affecting the speciation and transport of mercury through the terrestrial environment

Aquatic and terrestrial organic matter in the diet of stream consumers: implications for mercury bioaccumulation

The relative contribution of aquatic vs. terrestrial organic matter to the diet of consumers in fluvial environments and its effects on bioaccumulation of contaminants such as mercury (Hg) remain poorly understood. We used stable isotopes of carbon and nitrogen in a gradient approach (consumer isotope ratio vs. periphyton isotope ratio) across temperate streams that range in their pH to assess consumer reliance on aquatic (periphyton) vs. terrestrial (riparian vegetation) organic matter, and whether Hg concentrations in fish and their prey were related to these energy sources. Taxa varied in their use of the two sources, with grazing mayflies (Heptageniidae), predatory stoneflies (Perlidae), one species of water strider (Metrobates hesperius), and the fish blacknose dace (Rhinichthys atratulus) showing strong connections to aquatic sources, while Aquarius remigis water striders and brook trout (Salvelinus fontinalis) showed a weak link to in-stream production. The aquatic food source for consumers, periphyton, had higher Hg concentrations in low-pH waters, and pH was a much better predictor of Hg in predatory invertebrates that relied mainly on this food source vs. those that used terrestrial C. These findings suggest that stream biota relying mainly on dietary inputs from the riparian zone will be partially insulated from the effects of water chemistry on Hg availability. This has implications for the development of a whole-system understanding of nutrient and material cycling in streams, the choice of taxa in contaminant monitoring studies, and in understanding the fate of Hg in stream food webs.

Investigation of factors affecting gaseous mercury concentrations in soils

The purpose of this study was to determine the effects of soil temperature, soil moisture, redox potential (Eh) and soil organic matter (SOM) on the total gaseous mercury (TGM) concentrations in background soils. Our measurements were made in a grass field and deciduous forest at the Piney Reservoir Ambient Air Monitoring Station (PRAAMS) in Garrett County, Maryland. Three plots in each area were sampled every third week from July 2009 to June 2010 at the Oe-A soil horizon interface, the A-E soil horizon interface, and 5 and 10 cm into the E soil horizon. The mean soil TGM concentration for all depths in the forest (2.3 ± 2.2 ng m(-3)) was significantly higher than the mean soil TGM concentration in the grass field (1.5 ± 1.9 ng m(-3)). Soil TGM at all depths was most strongly and consistently correlated to soil temperature. The soil TGM concentrations were highest and most variable at the forest Oe-A soil horizon interface (4.1 ± 2.0 ng m(-3)), ranging from 1.5 to 8.4 ng m(-3). This soil horizon interface had 11 to 26% more SOM and the soil Eh was 100 to 400 mV lower than the other soil depths. Our results suggest that soil temperature, soil Eh and SOM are significant factors affecting TGM concentrations in forest soils. Future studies of TGM dynamics in background soils may benefit from closely monitoring the organic soil horizon.

Mercury reduction and oxidation by reduced natural organic matter in anoxic environments

Natural organic matter (NOM)-mediated redox cycling of elemental mercury Hg(0) and mercuric Hg(II) is critically important in affecting inorganic mercury transformation and bioavailability. However, these processes are not well understood, particularly in anoxic water and sediments where NOM can be reduced and toxic methylmercury is formed. We show that under dark anoxic conditions reduced organic matter (NOM(re)) simultaneously reduces and oxidizes Hg via different reaction mechanisms. Reduction of Hg(II) is primarily caused by reduced quinones. However, Hg(0) oxidation is controlled by thiol functional groups via oxidative complexation, which is demonstrated by the oxidation of Hg(0) by low-molecular-weight thiol compounds, glutathione, and mercaptoacetic acid, under reducing conditions. Depending on the NOM source, oxidation state, and NOM:Hg ratio, NOM reduces Hg(II) at initial rates ranging from 0.4 to 5.5 h(-1), which are about 2 to 6 times higher than those observed for photochemical reduction of Hg(II) in open surface waters. However, rapid reduction of Hg(II) by NOM(re) can be offset by oxidation of Hg(0) with an estimated initial rate as high as 5.4 h(-1). This dual role of NOM(re) is expected to strongly influence the availability of reactive Hg and thus to have important implications for microbial uptake and methylation in anoxic environments.

Mercury distribution across 14 U.S. Forests. Part I: spatial patterns of concentrations in biomass, litter, and soils

Results from a systematic investigation of mercury (Hg) concentrations across 14 forest sites in the United States show highest concentrations in litter layers, strongly enriched in Hg compared to aboveground tissues and indicative of substantial postdepositional sorption of Hg. Soil Hg concentrations were lower than in litter, with highest concentrations in surface soils. Aboveground tissues showed no detectable spatial patterns, likely due to 17 different tree species present across sites. Litter and soil Hg concentrations positively correlated with carbon (C), latitude, precipitation, and clay (in soil), which together explained up to 94% of concentration variability. We observed strong latitudinal increases in Hg in soils and litter, in contrast to inverse latitudinal gradients of atmospheric deposition measures. Soil and litter Hg concentrations were closely linked to C contents, consistent with well-known associations between organic matter and Hg, and we propose that C also shapes distribution of Hg in forests at continental scales. The consistent link between C and Hg distribution may reflect a long-term legacy whereby old, C-rich soil and litter layers sequester atmospheric Hg depositions over long time periods. Based on a multiregression model, we present a distribution map of Hg concentrations in surface soils of the United States.

Tracing and quantifying anthropogenic mercury sources in soils of northern france using isotopic signatures

The mercury (Hg) isotopic composition was investigated in topsoils from two case studies in north of France. The Hg isotope composition was first determined in agricultural topsoils contaminated by a close by Pb-Zn smelter. The Hg isotopic composition was also measured in topsoils from an urban area in northeastern France (Metz). In both cases, no significant mass independent isotope fractionation could be found in the soils. However, the soil isotopic composition (δ(202)Hg) was enriched in the heavier isotopes as the Hg concentration increased in the soils. A linear relationship between the δ(202)Hg in soils and 1/[Hg] indicated a mixing between a contamination source and the Hg derived from the geogenic background soils. Such findings demonstrate that the contamination signature was preserved in the soils and that the deposition of anthropogenic Hg was predominant compared to reactions leading to isotope fractionation such as biotic and abiotic reduction of Hg(II) and resulting in Hg mobility or evasion from the soils. It was therefore possible, for the first time in the case of Hg, to evaluate the contribution of the contamination source relative to the background Hg source in urban topsoils using relative isotope abundances.

Mercury uptake and translocation in Impatiens walleriana plants grown in the contaminated soil from Oak Ridge

Mercury (Hg) contaminated soils from Oak Ridge, Tennessee were investigated for phytoavailability of mercury as measured by degree of Hg translocation in aboveground biomass of Impatiens walleriana plants grown in the soils. After 90 days of incubation, results revealed a higher concentration of total Hg in the leaves than in the flowers or the stems. Plants that were grown in the soils with higher Hg concentrations showed significantly higher Hg uptake and translocation in the aboveground plant-biomass, and the correlation with the initial soil-Hg was significant for the leaves and the stems in the plants that were tested. On an average, only 4.06 microg of Hg could be found in the above ground plant biomass of all the plants, compared to an average 3673.50 microg of initial total Hg concentrations in these soils. Statistical analysis revealed a greater affinity of Hg for the soil carbon, which supported the finding of this study on low soil Hg bioavailability.

Removal of inorganic mercury and methylmercury from surface waters following coagulation of dissolved organic matter with metal-based salts

The presence of inorganic mercury (IHg) and methylmercury (MeHg) in surface waters is a health concern worldwide. This study assessed the removal potential use of metal-based coagulants as a means to remove both dissolved IHg and MeHg from natural waters and provides information regarding the importance of Hg associations with the dissolved organic matter (DOM) fraction and metal hydroxides. Previous research indicated coagulants were not effective at removing Hg from solution; however these studies used high concentrations of Hg and did not reflect naturally occurring concentrations of Hg. In this study, water collected from an agricultural drain in the Sacramento-San Joaquin Delta was filtered to isolate the dissolved organic matter (DOM) fraction. The DOM was then treated with a range of coagulant doses to determine the efficacy of removing all forms of Hg from solution. Three industrial-grade coagulants were tested: ferric chloride, ferric sulfate, and polyaluminum chloride. Coagulation removed up to 85% of DOM from solution. In the absence of DOM, all three coagulants released IHg into solution, however in the presence of DOM the coagulants removed up to 97% of IHg and 80% of MeHg. Results suggest that the removal of Hg is mediated by DOM-coagulant interactions. There was a preferential association of IHg with the more aromatic, higher molecular weight fraction of DOM but no such relationship was found for MeHg. This study offers new fundamental insights regarding large-scale removal of Hg at environmentally relevant regarding large-scale removal of Hg at environmentally relevant concentrations.

Empirical models for estimating mercury flux from soils

Multiple parameters have been suggested to influence the exchange of mercury (Hg) between the atmosphere and soils. However, models applied for estimating soil Hg flux are simple and do not consider the potential synergistic and antagonist relationships between factors controlling the exchange. This study applied a two-level factorial experimental design in a gas exchange chamber (GEC) to investigate the individual and combined effects of three environmental factors (temperature, light, and soil moisture) on soil Hg flux. It was shown that individually irradiation, soil moisture, and air temperature all significantly enhance Hg evasive flux (by 90-140%). Synergistic effects (20-30% of additional flux enhancement) were observed for all two-factor interactions, with air temperature/soil moisture and air temperature/irradiation being the most significant. Results from the factorial experiments suggest that a model incorporating the second-order interactions can appropriately explain the flux response to the changes of the studied factors. Based on the factorial experiment results and using the flux data for twelve soil materials measured with a dynamic flux chamber (DFC) at various temperatures, soil moisture contents, solar radiation exposures, and soil Hg contents, two empirical models for estimating Hg flux from soils were developed. Model verification with ambient flux data not used to develop the models suggested that the models were capable of estimating dry soil Hg flux with a high degree of predictability (r ∼ 0.9).

Behavior of mercury in a soil-plant system as affected by inoculation with the arbuscular mycorrhizal fungus Glomus mosseae

Effects of inoculation with the arbuscular mycorrhizal (AM) fungus Glomus mosseae on the behavior of Hg in soil-plant system were investigated using an artificially contaminated soil at the concentrations of 0, 1.0, 2.0, and 4.0 mg Hg kg(-1). Mercury accumulation was lower in mycorrhizal roots than in nonmycorrhizal roots when Hg was added at the rates of 2.0 and 4.0 mg kg(-1), while no obvious difference in shoot Hg concentration was found between mycorrhizal and nonmycorrhizal treatments. Mycorrhizal inoculation significantly decreased the total and extractable Hg concentrations in soil as well as the ratio of extractable to total Hg in soil. Equilibration sorption of Hg by soil was investigated, and the results indicated that mycorrhizal treatment enhanced Hg sorption on soil. The uptake of Hg was lower by mycorrhizal roots than by nonmycorrhizal roots. These experiments provide further evidence for the role of mycorrhizal inoculation in increasing immobilization of Hg in soil and reducing the uptake of Hg by roots. Calculation on mass balance of Hg in soil suggests the presence of Hg loss from soil presumably through evaporation, and AM inoculation enhanced Hg evaporation. This was evidenced by a chamber study to detect the Hg evaporated from soil.

Gaseous elemental mercury emissions and CO(2) respiration rates in terrestrial soils under controlled aerobic and anaerobic laboratory conditions

Mercury (Hg) levels in terrestrial soils are linked to the presence of organic carbon (C). Carbon pools are highly dynamic and subject to mineralization processes, but little is known about the fate of Hg during decomposition. This study evaluated relationships between gaseous Hg emissions from soils and carbon dioxide (CO(2)) respiration under controlled laboratory conditions to assess potential losses of Hg to the atmosphere during C mineralization. Results showed a linear correlation (r(2)=0.49) between Hg and CO(2) emissions in 41 soil samples, an effect unlikely to be caused by temperature, radiation, different Hg contents, or soil moisture. Stoichiometric comparisons of Hg/C ratios of emissions and underlying soil substrates suggest that 3% of soil Hg was subject to evasion. Even minute emissions of Hg upon mineralization, however, may be important on a global scale given the large Hg pools sequestered in terrestrial soils and C stocks. We induced changes in CO(2) respiration rates and observed Hg flux responses, including inducement of anaerobic conditions by changing chamber air supply from N(2)/O(2) (80% and 20%, respectively) to pure N(2). Unexpectedly, Hg emissions almost quadrupled after O(2) deprivation while oxidative mineralization (i.e., CO(2) emissions) was greatly reduced. This Hg flux response to anaerobic conditions was lacking when repeated with sterilized soils, possibly due to involvement of microbial reduction of Hg(2+) by anaerobes or indirect abiotic effects such as alterations in soil redox conditions. This study provides experimental evidence that Hg volatilization, and possibly Hg(2+) reduction, is related to O(2) availability in soils from two Sierra Nevada forests. If this result is confirmed in soils from other areas, the implication is that Hg volatilization from terrestrial soils is partially controlled by soil aeration and that low soil O(2) levels and possibly low soil redox potentials lead to increased Hg volatilization from soils.

Mercury distribution and exchanges between the Amazon River and connected floodplain lakes

This work presents the distribution and the partition of mercury (Hg) in the Curuai floodplain lakes along the Amazon River. The maximum Total Filtered Hg (T-FHg) concentrations in the floodplain lakes (28 to 52 pmol L(-1)) coincide with the maximum T-FHg concentrations of the Amazon River and are measured during the flooding period. The lowest T-FHg values (3 to 5 pmol L(-1)) are observed during the flood peak of the mainstream, during the rainy season, when waters are diluted by the local rainfall. In this system, Hg is mainly transported in the particulate phase, confirmed by elevated values of the Hg partition coefficient (4.77<K(d) (Hg)<5.83 Lkg(-1)). The highest Total Particulate Hg (T-PHg) concentrations (47-478 pmol L(-1)) in the lakes are measured during the dry season when they are isolated from the mainstream. This enrichment is due to the elevated TSS content associated to the re-suspension of the bottom sediments by the wind action and the bioturbation in shallow water lakes. In the flooded system, the lakes show different geochemical characteristics that control the Hg distribution and partition. In the white water (WW) lakes, characterized by oxidative neutral waters and highest TSS contents (till 2041 mg L(-1)), the T-PHg is associated to the particulate organic matter mainly during the dry season, while the T-FHg and T-FMn concentrations are correlated. In the black water (BW) lakes that show reductive pH conditions and lowest TSS load (2 to 5 2 mg L(-1)), P-iron and T-PHg display a positive relationship whereas the redox conditions favor the desorption of Hg from the particulate to the filtered phase. The mercury mass budget estimated in this study confirms that the Curuai floodplain system acts as a particulate mercury trap, with a net storage of particulate Hg of 150 kg PHg year(-1).

Evaluation of two in vitro protocols for determination of mercury bioaccessibility: influence of mercury fractionation and soil properties

Soil ingestion by children can be a significant exposure pathway to mercury (Hg). Unfortunately, no reliable in vivo results for Hg oral bioavailability determination in soils have been published. In vitro extractions enable the assessment of metals' bioaccessibility, which is an estimate of oral bioavailability. Therefore, the goal of this study was to evaluate two in vitro protocols (CDM (Camp Dresser and Mc Kee Inc.) and IVG (In Vitro Gastrointestinal)) for determination of Hg bioaccessibility in soils using pure Hg-compounds, reference materials (CRM 025-050 and ERM-CC580), and field-collected soils with elevated Hg concentrations. The influence of Hg fractionation and soil properties on Hg bioaccessibility was also investigated. In the field-collected soils, IVG bioaccessibility ranged from 1.5 to 7.5%, and was always below 3.15%, using the CDM method. Mercury bioaccessibility in CRM 025-050 was 61.5 and 34.7%, using IVG and CDM protocols, respectively, whereas Hg bioaccessibility was much lower in the certified sediment sample ERM-CC580 (<7%). Overall, the CDM protocol resulted in lower Hg bioaccessibility values. The water-soluble and exchangeable Hg fraction was highly correlated with gastrointestinal bioaccessibility (r=0.99, p<0.001 for both methods) and this fraction could be a potentially good indicator of Hg bioaccessibility. Because the IVG method is less time-consuming than the CDM protocol and includes organic physiological components which seem to increase Hg bioaccessibility, it might be preferred for determination of Hg bioaccessibility.

Reduction of Hg(II) to Hg(0) by magnetite

Mercury (Hg) is a highly toxic element, and its contamination of groundwater presents a significant threat to terrestrial ecosystems. Understanding the geochemical processes that mediate mercury transformations in the subsurface is necessary to predict its fate and transport. In this study, we investigated the redox transformation of mercuric Hg (Hg[II]) in the presence of the Fe(II)/Fe(III) mixed valence iron oxide mineral magnetite. Kinetic and spectroscopic experiments were performed to elucidate reaction rates and mechanisms. The experimental data demonstrated that reaction of Hg(II) with magnetite resulted in the loss of Hg(II) and the formation of volatile elemental Hg (Hg[0]). Kinetic experiments showed that Hg(II) reduction occurred within minutes, with reaction rates increasing with increasing magnetite surface area (0.5 to 2 m2/L) and solution pH (4.8 to 6.7), and decreasing with increasing chloride concentration (10(-6) to 10(-2) mol/L). Mössbauer spectroscopic analysis of reacted magnetite samples revealed a decrease in Fe(II) content, corresponding to the oxidation of Fe(II) to Fe(III) in the magnetite structure. X-ray photoelectron spectroscopy detected the presence of Hg(II) on magnetite surfaces, implying that adsorption is involved in the electron transfer process. These results suggest that Hg(II) reaction with solid-phase Fe(II) is a kinetically favorable pathway for Hg(II) reduction in magnetite-hearing environmental systems.

Mercury adsorption-desorption and transport in soils

Kinetic sorption and column miscible displacement transport experiments were performed to quantify the extent of retention/release and the mobility of mercury in different soils. Results indicated that adsorption of mercury was rapid and highly nonlinear with sorption capacities having the following sequence: Sharkey clay > Olivier loam > Windsor sand. Mercury adsorption by all soils was strongly irreversible where the amounts released or desorbed were often less than 1% of that applied. Moreover, the removal of soil organic matter resulted in a decrease of mercury adsorption in all soils. Adsorption was described with limited success using a nonlinear (Freundlich) model. Results from the transport experiments indicated that the mobility of mercury was highly retarded, with extremely low concentrations of mercury in column effluents. Furthermore, mercury breakthrough curves exhibited erratic patterns with ill-distinguished peaks. Therefore, mercury is best regarded as strongly retained and highly "immobile" in the soils investigated. This is most likely due to highly stable complex formation (irreversible forms) and strong binding to high-affinity sites. In a column packed with reference sand material, a symmetric breakthrough curve was obtained where the recovery of mercury in the leachate was only 17.3% of that applied. Mercury retention by the reference sand was likely due to adsorption by quartz and metal-oxides.

Influence of mercury speciation and fractionation on bioaccessibility in soils

Ingestion of contaminated soils by children during hand-to-mouth activities can be a significant exposure pathway to toxic chemicals. Bioaccessibility, which corresponds to the fraction of an ingested contaminant dissolved in the gastrointestinal tract and potentially available for absorption, can be determined by in vitro extractions and gives a conservative value of relative oral bioavailability. The goal of this study was to investigate the validity of the CDM in vitro extraction protocol, developed by Camp Dresser and Mc Kee, by assessing the influence of soil properties and Hg fractionation on bioaccessibility. Mercury bioaccessibility was determined in two pure mercury compounds, two reference materials (a soil and a sediment), and three field-collected contaminated soils. Soils and reference materials were characterized and a sequential extraction procedure was applied to the samples. Bioaccessibility of HgCl(2) was 99.8% in the gastric phase and 88.6% in the intestinal phase, whereas bioaccessibility of HgS was lower than 0.01%. In field-collected soils A, B, M, and, in ERM-CC580, mercury bioaccessibility was lower than 3.2% (below detection). In contrast, CRM 025-050 had a high Hg bioaccessibility (44.3% for gastric phase and 34.7% for intestinal phase). Gastric and intestinal bioaccessibility values were positively correlated with sulfate content in soils (r = 0.99, p < 0.001, for both gastric and intestinal bioaccessibility). In field-collected soils and ERM-CC580, the residual fraction represented near 100% of the mercury recovered, with less than 2% of mercury being in the water-soluble (F1) and CaCl(2)-exchangeable (F2) fractions. In contrast, 46% of mercury in the reference material CRM 025-050 was extracted in the CaCl(2)-exchangeable fraction. Results of the sequential extractions were in agreement with bioaccessibility values, with the sum of the water-soluble and CaCl(2)-exchangeable fractions (F1 + F2) highly correlated with intestinal bioaccessibility values (r = 0.99, p < 0.001). Hence, the sequential extraction procedure used in this study could be a simple means to help validate mercury bioaccessibility.

The role of natural purified humic acids in modifying mercury accessibility in water and soil

Contamination of soils with mercury can be a serious problem. It can be mobilized or stabilized by humic substances (HS) containing binding sites with reduced sulfur that can have different binding capacities for CH(3)Hg(+) and for Hg(2+). In this work we investigated the influence of different humic acids (HAs, extracted from lignite, compost, and forest soil) on mercury mobility and availability, both in a model solution and in soil samples from a mercury-polluted region. The technique of diffusive gradients in thin-films (DGT), which is capable of measuring: (i) free metal in solution; (ii) dissociated metal complexes previously mobilized by HA; (iii) mobilized metal-HA complexes that liberate metals by dissociation or by exchange reaction between the metal-HA complexes and the chelating groups on the resin-gel, was used in solutions and soils. The DGT measurements in solution, together with ultrafiltration, allowed estimation of the lability of Hg-HA complexes. Ultrafiltration results were also compared with predictions made by the windermere humic-aqueous model (WHAM). According to both these different approaches, Hg(2+) resulted nearly 100% complexed by HAs, whereas results from ultrafiltration showed that 32 to 72% of the CH(3)Hg(+) was bound to the HAs, with higher values for compost and lower values for forest and Aldrich HA. The DGT-measured mercury in soils was below 0.20 microg L(-1), irrespective of the extent of the contamination. Addition of HA increased the concentration of DGT-measured mercury in soil solution up to 100-fold in the contaminated soil and up to 30-fold in the control soil. The level of the increase also depended on the HA. The smallest increase (about 10 times) was found for lignite HA in both control and contaminated soils. The addition of forest HA gave the largest increases in DGT-measured mercury, in particular for the contaminated soil. Overall, the results demonstrated that DGT can be used for estimating the lability of mercury complexes in solution and for verifying enhanced mercury mobility when HA is added to contaminated soils.

Recent advances in mercury speciation analysis with focus on spectrometric methods and enriched stable isotope applications

This paper discusses some recent advances in spectrometric methods and approaches for mercury speciation analysis of environmental samples with focus on isotope dilution techniques for determination of mercury species' concentrations in gaseous samples and reaction rates in soils and sediments. Such analytical data is important inter alia in fundamental research on mercury biogeochemistry and for risk assessments of mercury-contaminated soils and sediments and for designing effective remedial actions. The paper describes how the use of enriched stable isotope tracers in mercury speciation analysis can improve the traceability and accuracy of results, facilitate rational method developments, and be useful for studying biogeochemical processes, i.e. rate of reactions and fluxes, of mercury species. In particular the possibilities to study and correct for unwanted species transformation reactions during sample treatment and to study "natural" transformations of species in environmental samples, or micro- and mesocosm ecosystems, during incubations are highlighted. Important considerations to generate relevant data in isotope tracer experiments as well as reliability and quality assurance of mercury speciation analysis in general are also discussed.

Novel reduction of mercury (II) by mercury-sensitive dissimilatory metal reducing bacteria

The dissimilatory metal reducing bacterium (DMRB) Shewanella oneidensis MR-1 reduces ionic mercury (Hg[II]) to elemental mercury (Hg[0]) by an activity not related to the MerA mercuric reductase. In S. oneidensis, this activity is constitutive and effective at Hg(II) concentrations too low to induce mer operon functions. Reduction of Hg(II) by MR-1 required the presence of electron donors and electron acceptors. Reduction occurred with oxygen or fumarate, but had the highest rate when ferric oxyhydroxide was used as a terminal electron acceptor. Geobacter sulfurreducens PCA and Geobacter metallireducens GS-15 reduced Hg(II) to Hg(0) with activity comparable to MR-1; however, neither the DMRB Anaeromyxobacter dehalogenans 2CP-C nor the nitrate reducer Pseudomonas stutzeri OX-1 reduced Hg(II) during growth. This discovery of constitutive mercury reduction among anaerobes has implications to the mobilization of mercury and production of methylmercury in anoxic environments.

Biotransformation of Hg(II) by cyanobacteria

The biotransformation of Hg(II) by cyanobacteria was investigated under aerobic and pH-controlled culture conditions. Mercury was supplied as HgCl(2) in amounts emulating those found under heavily impacted environmental conditions where bioremediation would be appropriate. The analytical procedures used to measure mercury within the culture solution, including that in the cyanobacterial cells, used reduction under both acid and alkaline conditions in the presence of SnCl(2). Acid reduction detected free Hg(II) ions and its complexes, whereas alkaline reduction revealed that meta-cinnabar (beta-HgS) constituted the major biotransformed and cellularly associated mercury pool. This was true for all investigated species of cyanobacteria: Limnothrix planctonica (Lemm.), Synechococcus leopoldiensis (Racib.) Komarek, and Phormidium limnetica (Lemm.). From the outset of mercury exposure, there was rapid synthesis of beta-HgS and Hg(0); however, the production rate for the latter decreased quickly. Inhibitory studies using dimethylfumarate and iodoacetamide to modify intra- and extracellular thiols, respectively, revealed that the former thiol pool was required for the conversion of Hg(II) into beta-HgS. In addition, increasing the temperature enhanced the amount of beta-HgS produced, with a concomitant decrease in Hg(0) volatilization. These findings suggest that in the environment, cyanobacteria at the air-water interface could act to convert substantial amounts of Hg(II) into beta-HgS. Furthermore, the efficiency of conversion into beta-HgS by cyanobacteria may lead to the development of applications in the bioremediation of mercury.

Sources and trends of environmental mercury emissions in Asia

This paper focuses on environmental mercury emissions in Asia and elaborates its probable trend in the future and associated implications given the anticipated socioeconomic outlook and other macro-environmental factors. Among the various regions, Asia has become the largest contributor of anthropogenic atmospheric Hg, responsible for over half of the global emission. In the next few decades, a significant increase in anthropogenic Hg emissions in Asia is likely owing to rapid economic and industrial development, unless drastic measures are taken. In particular, the dominance of Asia in some Hg-emitting industries, such as coal combustion, steel production and gold mining, provokes a serious environmental concern over their potential contributions of incidental Hg in the region. Moreover, the increasing prevalence of electrical and electronic manufacturing industry as a user and a contributor of Hg in Asia is also worrying. Specifically, disposal of obsolete electrical and electronic wastes represents a phenomenon increasingly encountered in Asia. In addition to escalating anthropogenic Hg emissions in Asia, associated environmental and health implications may also exacerbate in the region for the probable effects of a unique combination of climatic (e.g. subtropical climate), environmental (e.g. acid rain) and socioeconomic factors (e.g. high population density). Hence, much effort is still needed to understand the role of Asia in global Hg cycle and associated environmental and health effects in the region.

Detoxification of selenite and mercury by reduction and mutual protection in the assimilation of both elements by Pseudomonas fluorescens

A study on the assimilation and detoxification of selenium and mercury and on the interaction between these two elements was conducted on Pseudomonas fluorescens. P. fluorescens was able to convert separately both elements to their elemental forms, which are less toxic and biologically less available. To study the converting mechanism of selenite to elemental Se, cells were grown in the presence of various selenite concentrations and several parameters such as extracellular protein concentrations, pH, carbohydrate concentrations, isocitrate dehydrogenase (ICDH) and malic enzyme were monitored. Transmission electron microscopy (TEM) and various analytical methods were applied to confirm the interaction between selenium and cell. The former appeared as a red precipitate localized predominantly in the consumed culture medium. P. fluorescens also resisted to the toxic effect of mercury by converting Hg2+ to the volatile and less toxic form Hg0. Mercury reductase was likely responsible for the conversion of Hg2+ to Hg0. More importantly, the interaction between mercury and selenium was also studied. The presence of selenite significantly reduced the accumulation of mercury in P. fluorescens. It was also interesting to note that mercury appeared to behave as a protecting agent against selenium intoxication as the bioaccumulation of Se was also inhibited by this metal. The formation of Se-Hg complexes could explain this mutual protective effect. No precipitate of elemental Se could be detected when Hg was present in the cultures.

Mercury analysis of acid- and alkaline-reduced biological samples: identification of meta-cinnabar as the major biotransformed compound in algae

The biotransformation of Hg(II) in pH-controlled and aerated algal cultures was investigated. Previous researchers have observed losses in Hg detection in vitro with the addition of cysteine under acid reduction conditions in the presence of SnCl2. They proposed that this was the effect of Hg-thiol complexing. The present study found that cysteine-Hg, protein and nonprotein thiol chelates, and nucleoside chelates of Hg were all fully detectable under acid reduction conditions without previous digestion. Furthermore, organic (R-Hg) mercury compounds could not be detected under either the acid or alkaline reduction conditions, and only beta-HgS was detected under alkaline and not under acid SnCl2 reduction conditions. The blue-green alga Limnothrix planctonica biotransformed the bulk of Hg(II) applied as HgCl2 into a form with the analytical properties of beta-HgS. Similar results were obtained for the eukaryotic alga Selenastrum minutum. No evidence for the synthesis of organomercurials such as CH3Hg+ was obtained from analysis of either airstream or biomass samples under the aerobic conditions of the study. An analytical procedure that involved both acid and alkaline reduction was developed. It provides the first selective method for the determination of beta-HgS in biological samples. Under aerobic conditions, Hg(II) is biotransformed mainly into beta-HgS (meta-cinnabar), and this occurs in both prokaryotic and eukaryotic algae. This has important implications with respect to identification of mercury species and cycling in aquatic habitats.