Progress in the study of mercury methylation and demethylation in aquatic environments

Enhanced insight into the biogeochemical cycle of Hg in the Antarctic marine environment of Terra Nova Bay via isotopic analysis

Mercury (Hg) is a globally significant pollutant, which is particularly concerning due to its ability to undergo long-range atmospheric transport and its bioaccumulation and biomagnification in marine ecosystems, even in remote regions like Antarctica. This study explores the biogeochemical cycling of Hg in the marine coastal environment of Terra Nova Bay (Antarctica) by determining the total content of mercury (THg) and its isotopic composition in fish (Trematomus bernacchii), bivalve molluscs (Adamussium colbecki) and sediment samples, collected in 1996-1998 and 2021. Significantly lower THg concentrations are found in the organisms sampled in 2021 compared to those sampled in 1996-1998, with a concurrent shift toward higher δ202Hg (governed by mass-dependent isotope fractionation MDF) and lower Δ199Hg and Δ201Hg (governed by mass-independent isotope fractionation MIF) values. These results suggest changes in the exposure to Hg and the photochemical processes that the element and its species undergo, likely influenced by differences in the environmental conditions during the sampling periods, such as light exposure and ice cover. Sex-specific analysis of the 2021 fish samples further suggests differences in Hg accumulation and both MDF and MIF isotopic patterns between male and female specimens, emphasising a potential effect of sex on Hg exposure and dynamics. However, due to the limited number of individuals analyzed and the pooling of samples, this sex differentiation is still preliminary. Finally, the linear increase of Δ199Hg as a function of Δ201Hg during trophic transfer suggests MeHg bioaccumulation along the food chain. These findings provide valuable insights into the biogeochemical cycling of Hg in the Antarctic coastal marine environment and underscore the need for ongoing monitoring of Hg (including isotopic analysis) in this fragile ecosystem.

Distribution of mercury and methylmercury in ice-water-sediments in lakes during the freezing period under the influence of ice cover

The presence of lake ice cover alters the subglacial water environment, thereby influencing the migration and transformation of mercury (Hg) and methylmercury (MeHg) within the ice-water-sediment media of lakes. This study investigated the occurrence characteristics of mercury and methylmercury in various environmental compartments within lakes located at high latitudes in cold regions during the freezing period. To this end, Wuliangsuhai Lake, the largest freshwater lake situated at 40°N in China, was selected as the study site. The contents of mercury and methylmercury in lake ice were determined for the first time. The percentage of methylmercury (MeHg%) and ice-water partition coefficient were analyzed. The pollution situation and health risk were evaluated by single factor pollution index. The results show that the ice body and water body of Wuliangsuhai are not polluted by mercury and methylmercury, but some sampling points in the sediment are slightly polluted. The mercury content in sediment is negatively correlated with the ice thickness, and the methylmercury content in water is positively correlated with the methylmercury content in sediment, but negatively correlated with the ice thickness. The migration ability of methylmercury in ice-water system is stronger than that of mercury. The MeHg% of water in ice period is higher than that in non-freezing period, which is different from other lakes without ice sheet. The results show that in the dynamic equilibrium of methylation and demethylation in the high-latitude lake water, the methylation is higher in the ice period than in the non-freezing period due to the influence of light intensity, while the mercury in the non-freezing period is more susceptible to the demethylation.

System efficiency of methylmercury production, an important indicator for evaluating the long-term risk of mercury: From a case study in the Jiaozhou Bay to global coastal seas

With the implementation of the Minamata Convention, total Hg (THg) in coastal seas are expected to be reduced. However, methylmercury (MeHg) levels in aquatic environments depend not only on THg, but also the system efficiency of MeHg production (represented by MeHg/THg ratio in seawater) whose variations with time remain unclear. By choosing the Jiaozhou Bay (JZB) as a typical coastal system, combined with the published data from the global coastal systems, system efficiency of MeHg production in coastal seas were investigated. The mass budget of MeHg showed that the in situ production and degradation are the major source and sink of MeHg in the JZB. The relationships of MeHg in the seawater and fish with MeHg/THg in seawater indicate that system efficiency of MeHg production may control the risk of MeHg in coastal systems. The sulfate and nitrate in seawater, organic matter in sediment, biotic methylation, and transport of inorganic Hg from the seawater to sediment were identified to be critical parameters and processes for the MeHg/THg in global coastal seas. The findings of this study highlight the importance of monitoring the system efficiency of MeHg production and its controlling processes and parameters for evaluating the long-term risk of Hg.

Assessment of mercury methylation and methylmercury demethylation potentials in water and sediments along the Wabigoon River system

Monomethylmercury (MMHg) plays a crucial role in the accumulation of mercury (Hg) within aquatic food chains. Since ambient levels of methylmercury are governed by the balance of simultaneous methylation and demethylation processes, determining in situ methylation and demethylation rates is critically important to understand the dynamics of methylmercury in the environment. This is especially important in the Wabigoon River system in Ontario, Canada, which is severely contaminated with Hg by a chlor-alkali facility operating in the 1960s, and still exhibits some of the highest recorded fish mercury concentrations in Canada. This work used a simultaneous addition of isotope enriched Hg and MMHg tracers to ascertain Hg methylation and MMHg demethylation potentials. At the locations investigated for this study, the most favourable conditions for Hg methylation were found at the Hydroelectric dam, being able to transform 4.2 % and 4.4 % of added Hg in water and sediments per day, respectively, to MMHg. This could correspond to 1.9 ng/L and 29 ng/g of new MMHg being produced from current ambient Hg. Clay Lake, which is considered a sink for mercury and exhibiting a seasonal anoxic environment at its bottom waters, also demonstrated significant MMHg generation, being able to produce 2.7 ng/L and 13 ng/g of MMHg per day, respectively. Demethylation rates in sediments of riverbed and wetland locations showed an average half-life for methylmercury of 2.1 days, indicating a rapid turnover of MMHg in the Wabigoon River. However, significantly lower demethylation rates were also measured near the inflow of Clay Lake, where it took up to 144 days for MMHg to decrease by 50 %. Generally, most of the investigated locations downstream of the pollution source displayed the potential to generate methylmercury, which could be distributed throughout the Wabigoon River system and therefore require attention with respect to future remediation activities.

Determinations of methylmercury and mercury methylation/demethylation rate constants in environmental samples using isotope dilution/tracing methods by automatic ethylation-purge and trap-GC-ICP-MS

BACKGROUND

Mercury (Hg), especially methylmercury (MeHg) as a most toxic format of Hg in the environment, has been paid widely concern due to its high bioaccumulative capability and great risk to humans. Great efforts have been made to develop ethylation-purge and trap-gas chromatography-inductively coupled plasma mass spectrometry system for MeHg analysis and Hg biogeochemical cycling investigation. However, the generally manual operation limits the analytical efficiency, and the lack of applications in the real environmental samples restricts the future study. There is a great need for a rapid and accurate method to determine MeHg and Hg methylation/demethylation processes in environmental samples.

RESULTS

Herein, an automatic ethylation-purge and trap-GC-ICP-MS system based on isotope dilution method for MeHg analysis was developed. The results showed that the limit of detection of the developed method was 0.01 ng L-1, the MeHg can be analyzed within 6 min with a relative standard deviation of 4.3 %. The accuracy of this proposed method was verified by the satisfying recoveries of certified reference materials (99.0 ± 0.35 % in ECM-CC580, sediment; 98.0 ± 0.67 % in DORM-4, Fish protein). In addition, comparable concentrations of MeHg in natural water were measured using both of the developed and classical distillation methods. Subsequently, the developed method was adapted for measuring concentrations of MeHg in the water, sediment, and fish muscle collected from the coastal and freshwater systems. Finally, the photic demethylation and biotic methylation/demethylation rate constants in natural surface water and sediment were determined using isotope dilution/tracing methods by automatic ethylation-purge and trap-GC-ICP-MS.

SIGNIFICANCE AND NOVELTY

The developed automatic ethylation-purge and trap-GC-ICP-MS system is promising for accurate and convenient MeHg analysis and Hg biogeochemical cycling investigation in real environmental samples with isotope dilution and tracing methods.

The different effects of molybdate on Hg(II) bio-methylation in aerobic and anaerobic bacteria

In nature, methylmercury (MeHg) is primarily generated through microbial metabolism, and the ability of bacteria to methylate Hg(II) depends on both bacterial properties and environmental factors. It is widely known that, as a metabolic analog, molybdate can inhibit the sulfate reduction process and affect the growth and methylation of sulfate-reducing bacteria (SRB). However, after it enters the cell, molybdate can be involved in various intracellular metabolic pathways as a molybdenum cofactor; whether fluctuations in its concentration affect the growth and methylation of aerobic mercury methylating strains remains unknown. To address this gap, aerobic γ-Proteobacteria strains Raoultella terrigena TGRB3 (B3) and Pseudomonas putida TGRB4 (B4), as well as an obligate anaerobic δ-Proteobacteria strain of the SRB Desulfomicrobium escambiense CGMCC 1.3481 (DE), were used as experimental strains. The growth and methylation ability of each strain were analyzed under conditions of 500 ng·L-1 Hg(II), 0 and 21% of oxygen, and 0, 0.25, 0.50, and 1 mM of MoO4 2-. In addition, in order to explore the metabolic specificity of aerobic strains, transcriptomic data of the facultative mercury-methylated strain B3 were further analyzed in an aerobic mercuric environment. The results indicated that: (a) molybdate significantly inhibited the growth of DE, while B3 and B4 exhibited normal growth. (b) Under anaerobic conditions, in DE, the MeHg content decreased significantly with increasing molybdate concentration, while in B3, MeHg production was unaffected. Furthermore, under aerobic conditions, the MeHg productions of B3 and B4 were not influenced by the molybdate concentration. (c) The transcriptomic analysis showed several genes that were annotated as members of the molybdenum oxidoreductase family of B3 and that exhibited significant differential expression. These findings suggest that the differential expression of molybdenum-binding proteins might be related to their involvement in energy metabolism pathways that utilize nitrate and dimethyl sulfoxide as electron acceptors. Aerobic bacteria, such as B3 and B4, might possess distinct Hg(II) biotransformation pathways from anaerobic SRB, rendering their growth and biomethylation abilities unaffected by molybdate.

Methylmercury photodegradation in paddy water: An overlooked process mitigating methylmercury risks

Photodegradation is critical to reduce the potent neurotoxic methylmercury (MeHg) in water and its subsequent accumulation along food chains. However, this process has been largely ignored in rice paddies, which are hotspots of MeHg production and receive about a quarter of the world's developed freshwater resources. Here, we reported that significant MeHg photodegradation, primarily mediated by hydroxyl radicals, occurs in the overlying water during rice growth. By incorporating field-measured light interception into a rice paddy biogeochemistry model, as well as photodegradation rates obtained from 42 paddy soils stretching ∼3500 km across China, we estimated that photodegradation reduced MeHg concentrations in paddy water and rice by 82 % and 11 %, respectively. Without photodegradation, paddy water could be a significant MeHg source for downstream ecosystems, with an annual export of 178 - 856 kg MeHg to downstream waters in China, the largest rice producer. These findings suggest that photodegradation in paddy water is critical for preventing greater quantities of MeHg entering human food webs.

Development and optimization of a triple quadrupole ICP-MS-based system for the quantification of pre-concentrated gaseous elementary mercury

Understanding the redox reactions and transformation rates of mercury (Hg) species in the environment is important for predicting future gaseous elemental Hg (Hg0) levels and assessing the impacts of anthropogenic Hg0 emissions on human health. Stable Hg isotope tracers are a promising tool for estimating Hg0 production rates; however, traditional analytical approaches for quantifying Hg0, such as atomic fluorescence spectroscopy or atomic absorption spectrometry, cannot differentiate between Hg isotopes, and alternative approaches, such as inductively coupled plasma mass spectrometry (ICP-MS) with a typical aqueous sample introductory system, have relatively higher detection limit of Hg. Here, we developed and evaluated a custom-made thermal desorption unit coupled directly to a triple quadrupole ICP-MS (ICP-QQQ) for the quantification of Hg0 pre-concentrated on Au traps. The performance of the system was validated with measurements of a Hg standard gas and of Hg0 generated from aqueous Hg standards. Using our system, we were able to detect ultra-trace amounts of Hg0 and obtain precise Hg isotope measurements with an analytical error of ≤ 3.5%. Calibration curves with superb linearity (r2 > 0.999) were obtained for the Hg concentration range of 0-300 pg. The method detection limit was approximately 0.01-0.03 pg of Hg. Using the latest ICP-QQQ instrument (Agilent 8900; Agilent Technologies Ltd.) was far superior to using a previous model (Agilent 8800), with the Agilent 8900 showing approximately five times higher sensitivity than the Agilent 8800 as well as the ability to precisely and simultaneously analyze up to five Hg isotopes by time-resolved analysis.

MeHg production in eutrophic lakes: Focusing on the roles of algal organic matter and iron-sulfur-phosphorus dynamics

The mechanisms by which eutrophication affects methylmercury (MeHg) production have not been comprehensively summarized, which hinders accurately predicting the MeHg risk in eutrophic lakes. In this review, we first discussed the effects of eutrophication on biogeochemical cycle of mercury (Hg). Special attentions were paid to the roles of algal organic matter (AOM) and iron (Fe)-sulfur (S)-phosphorus (P) dynamics in MeHg production. Finally, the suggestions for risk control of MeHg in eutrophic lakes were proposed. AOM can affect in situ Hg methylation by stimulating the abundance and activities of Hg methylating microorganisms and regulating Hg bioavailability, which are dependent on bacteria-strain and algae species, the molecular weight and composition of AOM as well as environmental conditions (e.g., light). Fe-S-P dynamics under eutrophication including sulfate reduction, FeS formation and P release could also play crucial but complicated roles in MeHg production, in which AOM may participate through influencing the dissolution and aggregation processes, structural order and surface properties of HgS nanoparticles (HgS_NP). Future studies should pay more attention to the dynamics of AOM in responses to the changing environmental conditions (e.g., light penetration and redox fluctuations) and how such variations will subsequently affect MeHg production. The effects of Fe-S-P dynamics on MeHg production under eutrophication also deserve further investigations, especially the interactions between AOM and HgS_NP. Remediation strategies with lower disturbance, greater stability and less cost like the technology of interfacial O₂ nanobubbles are urgent to be explored. This review will deepen our understanding of the mechanisms of MeHg production in eutrophic lakes and provide theoretical guidance for its risk control.

Impacts of anthropogenic discharge on distribution, mass budget, and long-term risk of total mercury in coastal region: A case study of the Jiaozhou Bay, China

Mercury (Hg) as a toxic pollutant in marine systems have been paid more attention. The Jiaozhou Bay (JZB) is located at the western coast of the Yellow Sea surrounded by Qingdao city, a developed city in China. >10 rivers and several sewage treatment plants carry lots of Hg input it, increasing the environmental risks JZB facing. However, there is still a lack of knowledge on its cycling in the JZB, limiting sound understanding of Hg fate in coastal regions. To address these needs, four cruises were conducted in different seasons, to investigate distribution, influencing factors, and mass budget of total Hg (THg) in the JZB. Higher THg concentrations were determined in seawater (22.8 ± 13.9 ng L^-1) and sediment (148 ± 107 ng g^-1), indicating serious Hg pollution in the JZB. Temperature, salinity in water and enrich factor (EF) in sediment were identified to be possible environmental factors influencing THg distribution in the JZB. Mass budget of THg showed that anthropogenic discharge (river, wastewater input, and atmospheric deposition) was dominant source of THg in the JZB. The results of statistical analyses and mass budget of THg also indicated that anthropogenic discharge plays important roles in long-term risk of THg in the JZB. These results suggested that anthropogenic discharge comprehensively affects distribution, mass budget, and long-term risk of THg in coastal systems. The outcomes highlighted that regular investigations of Hg cycling should be conducted to assess Hg pollution in coastal ecosystems. Our study also shed new light on control of long-term risk posed by Hg in marine systems according to investigations of Hg cycling and link between Hg contamination and other pollutant (e.g., nutrient).

Influence of oxygen, UV light and reactive dissolved organic matter on the photodemethylation and photoreduction of monomethylmercury in model freshwater

The key factors which affect the abiotic photodemethylation process of monomethylmercury (MMHg) in the freshwaters has remained unclear. Hence, this work aimed to better elucidate the abiotic photodemethylation pathway in a model freshwater. Anoxic and oxic conditions were implemented to investigate the simultaneous photodemethylation to Hg(II) and photoreduction to Hg(0). MMHg freshwater solution was irradiated through exposure to three wavelength ranges of full light (280-800 nm), without short UVB (305-800 nm), and visible light (400-800 nm). The kinetic experiments were performed following dissolved and gaseous Hg species concentrations (i.e., MMHg, iHg(II), Hg(0)). A comparison between two methods of post-irradiation purging and continuous-irradiation purging confirmed MMHg photodecomposition to Hg(0) is mainly induced by a first photodemethylation step to iHg(II) followed by a photoreduction step to Hg(0). Photodemethylation under full light extent normalized to absorbed radiation energy showed a higher rate constant in anoxic conditions at 18.0 ± 2.2 kJ^-1 compared to oxic conditions at 4.5 ± 0.4 kJ^-1. Moreover, photoreduction also increased up to four-fold under anoxic conditions. Normalized and wavelength-specific photodemethylation (K_pd) and photoreduction (K_pr) rate constants were also calculated for natural sunlight conditions to evaluate the role of each wavelength range. The relative ratio in wavelength-specific K_PAR: K_{long UVB+ UVA}: K _{short UVB} showed higher dependence on UV light for photoreduction at least ten-fold compared to photodemethylation, regardless of redox conditions. Both results using Reactive Oxygen Species (ROS) scavenging methods and Volatile Organic Compounds (VOC) measurements revealed the occurrence and production of low molecular weight (LMW) organic compounds that are as photoreactive intermediates responsible for MMHg photodemethylation and iHg(II) photoreduction in the dominant pathway. This study also supports the role of dissolved oxygen as an inhibitor for the photodemethylation pathways driven by LMW photosensitizers.

Diverse Methylmercury (MeHg) Producers and Degraders Inhabit Acid Mine Drainage Sediments, but Few Taxa Correlate with MeHg Accumulation

Methylmercury (MeHg) is a notorious neurotoxin, and its production and degradation in the environment are mainly driven by microorganisms. A variety of microbial MeHg producers carrying the gene pair hgcAB and degraders carrying the merB gene have been separately reported in recent studies. However, surprisingly little attention has been paid to the simultaneous investigation of the diversities of microbial MeHg producers and degraders in a given habitat, and no studies have been performed to explore to what extent these two contrasting microbial groups correlate with MeHg accumulation in the habitat of interest. Here, we collected 86 acid mine drainage (AMD) sediments from an area spanning approximately 500,000 km² in southern China and profiled the sediment-borne putative MeHg producers and degraders using genome-resolved metagenomics. 46 metagenome-assembled genomes (MAGs) containing hgcAB and 93 MAGs containing merB were obtained, including those from various taxa without previously known MeHg-metabolizing microorganisms. These diverse MeHg-metabolizing MAGs were formed largely via multiple independent horizontal gene transfer (HGT) events. The putative MeHg producers from Deltaproteobacteria and Firmicutes as well as MeHg degraders from Acidithiobacillia were closely correlated with MeHg accumulation in the sediments. Furthermore, these three taxa, in combination with two abiotic factors, explained over 60% of the variance in MeHg accumulation. Most of the members of these taxa were characterized by their metabolic potential for nitrogen fixation and copper tolerance. Overall, these findings improve our understanding of the ecology of MeHg-metabolizing microorganisms and likely have implications for the development of management strategies for the reduction of MeHg accumulation in the AMD sediments. IMPORTANCE Microorganisms are the main drivers of MeHg production and degradation in the environment. However, little attention has been paid to the simultaneous investigation of the diversities of microbial MeHg producers and degraders in a given habitat. We used genome-resolved metagenomics to reveal the vast phylogenetic and metabolic diversities of putative MeHg producers and degraders in AMD sediments. Our results show that the diversity of MeHg-metabolizing microorganisms (particularly MeHg degraders) in AMD sediments is much higher than was previously recognized. Via multiple linear regression analysis, we identified both microbial and abiotic factors affecting MeHg accumulation in AMD sediments. Despite their great diversity, only a few taxa of MeHg-metabolizing microorganisms were closely correlated with MeHg accumulation. This work underscores the importance of using genome-resolved metagenomics to survey MeHg-metabolizing microorganisms and provides a framework for the illumination of the microbial basis of MeHg accumulation via the characterization of physicochemical properties, MeHg-metabolizing microorganisms, and the correlations between them.

Evaluating the influence of seasonal stratification on mercury methylation rates in the water column and sediment in a contaminated section of a western U.S.A. reservoir

Mercury methylation frequently occurs at the active oxic/anoxic boundary between the sediment bed and water column of lakes and reservoirs. Previous studies suggest that the predominant mercury methylation zone moves to the water column during periods of stratification and that high potential methylation rates (K_m) in sediment require oxygenated overlying water. However, simultaneous measurements of methylmercury (MeHg) production in both the sediment and water column remain limited. Understanding the relative importance of sediment versus water column methylation and the impact of seasonal stratification on these processes has important implications for managing MeHg production. This study measured K_m and potential demethylation rates (K_dm) using stable isotope tracers of unfiltered inorganic mercury and MeHg in sediments and water of the littoral and profundal zones of a shallow branch of the Nacimiento Reservoir in California's central coastal range. Field sampling was conducted once during winter (well-mixed/oxygenated conditions) and once during late summer (thermally stratified/anoxic conditions). The results showed very high ambient MeHg concentrations in hypolimnetic waters (up to 7.5 ng L^-1; 79% MeHg/total Hg). During late summer, littoral sediments had higher K_m (0.024 day^-1) compared to profundal sediments (0.013 day^-1). Anoxic water column K_m were of similar magnitude to K_m in the sediment (0.03 day^-1). Following turnover, profundal sediment K_m did not change significantly, but water column K_m became insignificant. Summer and winter sediment K_dm were higher in profundal (2.35, 3.54 day^-1, respectively) compared to the littoral sediments (0.52, 2.56 day^-1, respectively). When modelled, K_m in the water column could account for approximately 40% of the hypolimnetic MeHg. Our modelling results show that the remaining MeHg in the hypolimnion could originate from the profundal sediment. While further study is needed, these results suggest that addressing methylation in the water column and profundal sediment are of equal importance to any remediation strategy.

Role of light in methylmercury photodegradation: From irradiation to absorption in the presence of organic ligands

Photochemical degradation acts as the principal sink for methylmercury (MeHg) in surface water, which is regulated by light and solution matrix (especially the presence of dissolved organic matter, DOM). The spectral composition of light irradiation and the light absorption properties of reaction media (often exerted by DOM) are important in MeHg photodegradation, which has not yet been clearly resolved. Aiming to understand the role of light in MeHg photodegradation from the perspectives of both light irradiation and absorption, we investigated the photodegradation of MeHg under different simulated sunlight sources, with and without DOM model compounds of different molecular structures. The results show that the photodegradation of MeHg under different sunlight irradiation yields distinct first-order date constant, mainly due to the slight difference in UVB composition. The degradation of MeHg without DOM under a light source with high intensity in the UV region and no MeHg degradation under the UV-filtered light even in the presence of DOM showed the importance of UV lights in MeHg photodegradation. The use of ultrapure water as a reaction medium may be subject to MeHg loss through vessel adsorption, not just photolysis. Additionally, this work found that the type and position of coexisting substituents on aromatic thiols play a critical role in improving the photodegradation of MeHg, following amino > hydroxyl > carboxyl, para > meta > ortho. Based on the characterization of ultraviolet-visible absorption spectra and our previous work, it was concluded that the presence of DOM could induce red-shift in light absorption and reduce the electronic transition energy of the CHg bond, facilitating MeHg photodegradation. The structures of DOM affect the light absorption properties, which are related to MeHg photodegradation.

Mercury isotope fractionation during methylmercury transport and transformation: A review focusing on analytical method, fractionation characteristics, and its application

Methylmercury (MeHg), a potent neurotoxin, can be formed, migrated and transformed in environmental compartments, accompanying with unique mass-dependent and mass-independent fractionation of mercury (Hg). These Hg isotope fractionation signals have great potential to probe the transformation and transport of MeHg in aquatic environments. However, the majority of studies to date have focused on total Hg isotopic composition, with less attention to the isotopic fractionation of MeHg due to technical difficulties in analysis, which severely hinders the understanding of MeHg isotopic fractionation and its applications. This review a) evaluates the reported analytical methods for Hg isotopic composition of MeHg, including online and offline measurement techniques; b) summarizes the extent and characteristics of Hg isotopic fractionation during MeHg transport and transformation, focusing on methylation, demethylation, trophic transfer and internal metabolism; and c) briefly discusses several applications of MeHg isotopic fractionation signatures in estimating the extent of photodemethylation, tracing the source of Hg species, and diagnosing reaction mechanisms. Additionally, the existing problems and future directions in MeHg isotope fractionation are highlighted to improve the analytical protocol for Hg isotope fractionation and deepen our understanding of Hg isotope fractionation in the biogeochemical cycling of MeHg.

Role of the rhizosphere of a flooding-tolerant herb in promoting mercury methylation in water-level fluctuation zones

The water-level fluctuation zone (WLFZ) has been considered as a hotspot for mercury (Hg) methylation. Flooding-tolerant herbs are gradually acclimated to this water-land ecotone, tending to form substantial root systems for improving erosion resistance. Accompanying rhizosphere microzone plays crucial but unclear roles in methylmercury (MeHg) formation in the WLFZ. Thus, we conducted this study in the WLFZ of the Three Gorges Reservoir, to explore effects of the rhizosphere of a dominant flooding-tolerant herb (bermudagrass) on MeHg production. The elevated Hg and MeHg in rhizosphere soils suggest that the rhizosphere environment provides favorable conditions for Hg accumulation and methylation. The increased bioavailable Hg and microbial activity in the rhizosphere probably serve as important factors driving MeHg formation in the presence of bermudagrass. Simultaneously, the rhizosphere environments changed the richness, diversity, and distribution of hgcA-containing microorganisms. Here, a typical iron-reducing bacterium (Geobacteraceae) has been screened, however, the majority of hgcA genes detected in rhizosphere, near-, and non-rhizosphere soils of the WLFZ were unclassified. Collectively, these results provide new insights into the elevated MeHg production as related to microbial processes in the rhizosphere of perennial herbs in the WLFZ, with general implications for Hg cycling in other ecosystems with water-level fluctuations.

The potential of mercury methylation and demethylation by 15 species of marine microalgae

Mercury (Hg) and its compounds are a kind of worldwide concerned persistent toxic pollutants. As the major primary producer in the ocean, microalgae are expected to play an important role in the cycling and accumulation of Hg in marine ecosystems by either uptake Hg species from seawater or involving in the transformations of Hg species. However, there is still lack of clear knowledge on whether microalgae can induce the methylation and demethylation of Hg in aquatic environments. In this study, Hg isotope dilution and isotope addition techniques were utilized to determine the methylation and demethylation potential of Hg at concentrations comparable to that in natural environments by 15 common marine microalgae (8 species of Diatoms, 4 species of Dinoflagellates, 2 species of Chlorophyta and 1 species of Chrysophyte). Methylation of inorganic Hg was found to be negligible in the culture of all tested marine microalgae, while 6 species could significantly induce the demethylation of methylmercury (MeHg). The rates of microalgae mediated MeHg demethylation were at the same order of magnitude as that of photodemethylation, indicating that marine microalgae may play an important role in the degradation of MeHg in marine environments. Further studies suggest that the demethylation of MeHg by the microalgae may be mainly caused by their extracellular secretions (via photo-induce demethylation) and associated bacteria, rather than the direct demethylation of MeHg by microalgae cells. In addition, it was found that thiol groups may be the major component in microalgal extracellular secretions that lead to the photo-demethylation of MeHg.

Cellular and genetic mechanism of bacterial mercury resistance and their role in biogeochemistry and bioremediation

Mercury (Hg) is a highly toxic element that occurs at low concentrations in nature. However, various anthropogenic and natural sources contribute around 5000 to 8000 metric tons of Hg per year, rapidly deteriorating the environmental conditions. Mercury-resistant bacteria that possess the mer operon system have the potential for Hg bioremediation through volatilization from the contaminated milieus. Thus, bacterial mer operon plays a crucial role in Hg biogeochemistry and bioremediation by converting both reactive inorganic and organic forms of Hg to relatively inert, volatile, and monoatomic forms. Both the broad-spectrum and narrow-spectrum bacteria harbor many genes of mer operon with their unique definitive functions. The presence of mer genes or proteins can regulate the fate of Hg in the biogeochemical cycle in the environment. The efficiency of Hg transformation depends upon the nature and diversity of mer genes present in mercury-resistant bacteria. Additionally, the bacterial cellular mechanism of Hg resistance involves reduced Hg uptake, extracellular sequestration, and bioaccumulation. The presence of unique physiological properties in a specific group of mercury-resistant bacteria enhances their bioremediation capabilities. Many advanced biotechnological tools also can improve the bioremediation efficiency of mercury-resistant bacteria to achieve Hg bioremediation.

The relative importance of mercury methylation and demethylation in rice paddy soil varies depending on the presence of rice plants

Neurotoxic methylmercury (MeHg) accumulates in rice grain from paddy soil, where its concentration is controlled by microbial mercury methylation and demethylation. Both up- and down-regulation of methylation is known to occur in the presence of rice plants in comparison to non-vegetated paddy soils; the influence of rice plant presence/absence on demethylation is unknown. To assess the concurrent influence of rice plant presence/absence on methylation and demethylation, and to determine which process was more dominant in controlling soil MeHg concentrations, we maintained six rhizoboxes of paddy soil with and without rice plants. At the peak of plant growth, we simultaneously measured ambient MeHg, ambient inorganic mercury (IHg), and potential rate constants of methylation and demethylation (Kmeth and Kdemeth) in soil using stable isotope tracers and ID-GC-ICPMS. We also measured organic matter content, elemental S, and water-extractable sulfate. We found MeHg concentrations were differentially controlled by MeHg production and degradation processes, depending on whether plants were present. In non-vegetated boxes, MeHg concentration was controlled by Kmeth, as evidenced by a strong and positive correlation, while Kdemeth had no relation to MeHg concentration. These results indicate methylation was the dominant driver of MeHg concentration in non-vegetated soil. In vegetated boxes, Kdemeth strongly and negatively predicted MeHg concentration, indicating that demethylation was the dominant control in soil with plants. MeHg concentration, Kmeth, and % MeHg all had significantly less variance in vegetated than in non-vegetated soils due to a consistent elimination of greater values. This pattern suggests that reduced MeHg production capacity was a secondary control on MeHg concentrations in vegetated soils. We observed no difference in the magnitude or variance of Kdemeth between treatments, suggesting that demethylation was robust to soil chemical conditions influenced by the plant, perhaps because of a wider taxonomic diversity of demethylators. Our results suggest that methylation and demethylation processes could both be leveraged to alter MeHg concentrations in rice paddy soil.

High variability of mercury content in the hair of Russia Northwest population: the role of the environment and social factors

PURPOSE

The purpose of this work is to study mercury levels in the hair of different social and demographic groups of the population of the Vologda region in Northwest Russia. This region is selected due to a heterogeneous distribution of rivers and lakes-a resource base for fishing.

METHODS

The mercury content was determined in the hair from the root with a length of about 2 cm. The concentration of total mercury in human hair was determined by the atomic absorption method without preliminary sample preparation using an RA-915M mercury analyzer and a PYRO-915 + pyrolysis unit.

RESULTS

The average level of mercury in the human hair was 0.445 μg/g (median 0.220 μg/g). The concentration of mercury in the hair of people older than 44 years (0.875 μg/g) was three times higher than in the hair of children under 18 years of age (0.270 μg/g). People who eat fish less than once per month had a hair mercury concentration of 0.172 μg/g, for 1-2 times a month 0.409 μg/g, once a week 0.555 μg/g, and several times a week 0.995 μg/g. The concentration of mercury in the hair of smokers (0.514 μg/g) was higher than in the hair of non-smokers (0.426 μg/g).

CONCLUSION

Significantly higher concentrations of mercury were observed in the hair of participants from the western part of the region, where reservoirs are the main commercial sources of fish products. The data showed that the main source of people's mercury intake was fish.

Methylmercury Production and Degradation under Light and Dark Conditions in the Water Column of the Hells Canyon Reservoirs, USA

Methylmercury (MeHg) is a highly toxic form of mercury that can bioaccumulate in fish tissue. Methylmercury is produced by anaerobic bacteria, many of which are also capable of MeHg degradation. In addition, demethylation in surface waters can occur via abiotic sunlight-mediated processes. The goal of the present study was to understand the relative importance of microbial Hg methylation/demethylation and abiotic photodemethylation that govern the mass of MeHg within an aquatic system. The study location was the Hells Canyon complex of 3 reservoirs on the Idaho-Oregon border, USA, that has fish consumption advisories as a result of elevated MeHg concentrations. Our study utilized stable isotope addition experiments to trace MeHg formation and degradation within the water column of the reservoirs to understand the relative importance of these processes on the mass of MeHg using the Water Quality Analysis Simulation Program. The results showed that rates of MeHg production and degradation within the water column were relatively low (<0.07 d^-1 ) but sufficient to account for most of the MeHg observed with the system. Most MeHg production within the water column appeared to occur in the spring when much of the water column was in the processes of becoming anoxic. In the surface waters, rates of photodemethylation were relatively large (up to -0.25 d^-1 ) but quickly decreased at depths >0.5 m below the surface. These results can be used to identify the relative importance of MeHg processes that can help guide reservoir management decisions. Environ Toxicol Chem 2021;40:1829-1839. © 2021 SETAC. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Migration characteristics and potential determinants of mercury in long-term decomposing litterfall of two subtropical forests

It is of great importance to elucidate the mechanism of mercury (Hg) migration in the forest litterfall so as to clearly understand global Hg deposition. However, it is still unclear for the migration and transformation of Hg in different forest litters during long-term decomposition. Therefore, the dynamics of total Hg (THg), methylmercury (MeHg), carbon, nitrogen, microbial biomass carbon and nitrogen in the litterfall of the evergreen broadleaf (EB) and mixed broadleaf-conifer (MBC) forests, southwest China were investigated, aiming to understand the migration characteristics of Hg in the two-year decomposing litterfall. Results showed that carbon decreased, while nitrogen accumulated slightly in the process of litterfall decomposition. THg levels in the second year of the EB and MBC forests decreased by 16.9% and 11.3%, while MeHg levels reduced by 141.4% and 210.7% respectively comparing with those in the first year. The total percentage of hydrochloric acid-soluble mercury (Hg-h) and water-soluble mercury (Hg-w) had a significant impact on the migration of THg and MeHg in the two forest stands. The C/N ratio in the EB forest bore a positive correlation with THg and MeHg levels, whereas that in the MBC forest was adverse. Besides, microbial biomass C and N were positively related with THg and MeHg levels in both the EB and MBC forests. It is proposed that THg and MeHg accumulation in the second year drastically decreased probably due to finite nutritional conditions, which implies that Hg accumulation risks alleviate with degradation time.

Toxic and essential trace element concentrations in fish species in the Lower Amazon, Brazil

The Lower Amazon region (Western Pará, northern Brazil) is greatly affected by mining exploitations (particularly artisanal gold mines) and other industrial and intensive agricultural activities with potentially strong impacts on aquatic ecosystems. Although such impacts include contamination with various toxic elements, to date only the effects of Hg have been considered. In this study, toxic and trace element concentrations were determined in the flesh of 351 fish specimens, including detritivores (Acarí, Pterygoplichthys pardalis), omnivores (Piranha, Pygocentrus nattereri; Pirarucu, Arapaima sp.) and carnivores (Caparari, Pseudoplatystoma fasciatum; Tucunaré, Cichla ocellaris), during the dry and wet seasons in 2015 and 2016. The range of concentrations of toxic element residues were 2-238 μg/kg fresh weight for As, 1-77 μg/kg for Cd, 4-1922 μg/kg for Hg and 1-30 μg/kg for Pb. Only the maximum concentrations of Hg established in the Brazilian legislation for fish destined for human consumption (0.5 mg/kg) were exceeded (in 16% of carnivorous species). The large between-species and seasonal differences observed for all these toxic elements are probably related to the seasonal behaviour and dietary habits of the different fish species. By contrast, essential trace element concentrations were low and not related to seasonal or dietary factors, and the observed differences may be at least partly related to the metabolism of each species. The associations between Hg and the essential trace elements Se, Fe, Co and Mn deserve special attention, as these trace elements may play a role in Hg cycling and methylation and merit further evaluation with the aim of reducing Hg toxicity in aquatic environments.

Photochemical behaviors of mercury (Hg) species in aquatic systems: A systematic review on reaction process, mechanism, and influencing factor

The fate and transport of Hg species in natural aquatic environment are strongly affected by photochemical transformation of Hg⁰, Hg²⁺, and MeHg. Migration of Hg is determined by its complexation with organic and inorganic ligands that are widely present in the water. The presence of dissolved organic matter (DOM) is closely related to photochemical reactions of Hg. DOM can strongly bind to mercury (e.g., Hg²⁺ and MeHg), thus affecting its speciation, mobility and toxicity, eventually dominating its bioavailability. This review summarizes extensive studies on photochemical behaviors of Hg including: (1) photo-oxidation; (2) photo-reduction; (3) photochemical methylation; and (4) MeHg photo-degradation. Photo-oxidation of Hg⁰ is mostly caused by oxidative free radicals (e.g., •OH, CO₃^•-, O₃, and ¹O₂), while photo-reduction of Hg²⁺ is more complicated and it involves two pathways: (1) primary processes (direct photolysis of Hg²⁺ or ligand-metal charge transfer of Hg²⁺-DOM complex); and (2) secondary processes (reduction of Hg²⁺-DOM complex induced by free radicals derived from DOM photolysis). Photochemical methylation of inorganic Hg occurs as follows: (1) Hg²⁺ complexes with methyl donors (e.g., acetic acid, tert-butyl, alcohols, etc.) to form intermediates, followed by (2) an intramolecular methyl transfer. MeHg photo-degradation is the leading pathway for MeHg demethylation and it primarily proceeds via four different pathways. The information on DOM was also mentioned, but DOM is not the only factor that affects the photochemical behaviors of Hg. Other influencing factors such as: (1) pH value; (2) dissolved oxygen; (3) cations (Fe³⁺, K⁺) and anions (NO₃^-, HCO₃^-, CO₃^2-, Cl^-); and (4) suspended substance cannot be ignored.

Estimation of Hg(II) in Soil Samples by Bioluminescent Bacterial Bioreporter E. coli ARL1, and the Effect of Humic Acids and Metal Ions on the Biosensor Performance

Mercury is a ubiquitous environmental pollutant of dominantly anthropogenic origin. A critical concern for human health is the introduction of mercury to the food chain; therefore, monitoring of mercury levels in agricultural soil is essential. Unfortunately, the total mercury content is not sufficiently informative as mercury can be present in different forms with variable bioavailability. Since 1990, the use of bioreporters has been investigated for assessment of the bioavailability of pollutants; however, real contaminated soils have rarely been used in these studies. In this work, a bioassay with whole-cell bacterial bioreporter Escherichia coli ARL1 was used for estimation of bioavailable concentration of mercury in 11 soil samples. The bioreporter emits bioluminescence in the presence of Hg(II). Four different pretreatments of soil samples prior to the bioassay were tested. Among them, laccase mediated extraction was found to be the most suitable over water extraction, alkaline extraction, and direct use of water-soil suspensions. Nevertheless, effect of the matrix on bioreporter signal was found to be severe and not possible to be completely eliminated by the method of standard addition. In order to elucidate the matrix role, influences of humic acid and selected metal ions present in soil on the bioreporter signal were tested separately in laboratory solutions. Humic acids were found to have a positive effect on the bioreporter growth, but a negative effect on the measured bioluminescence, likely due to shading and Hg binding resulting in decreased bioavailability. Each of the tested metal ions solutions affected the bioluminescence signal differently; cobalt (II) positively, iron (III) negatively, and the effects of iron (II) and nickel (II) were dependent on their concentrations. In conclusion, the information on bioavailable mercury estimated by bioreporter E. coli ARL1 is valuable, but the results must be interpreted with caution. The route to functional bioavailability bioassay remains long.

Mercury methylation-related microbes and genes in the sediments of the Pearl River Estuary and the South China Sea

Methylmercury (MeHg) is a toxicant that mainly originates from in situ microbial methylation of inorganic mercury (Hg) in the environment and poses a severe health risk to the public. However, the characteristics of the Hg-methylating microbial community and its relationship with MeHg production in various environments remain to be understood. In the present study, Hg-methylating microbial communities and genes (hgcAB cluster) in the sediments of the Pearl River (PR), Pearl River Estuary (PRE) and South China Sea (SCS) were investigated at a large spatial scale using high-throughput sequencing-based approaches. The results showed that sulfur-reducing bacteria (SRB) and iron-reducing bacteria (IRB) were consistently the dominant microbial strains responsible for the methylation of inorganic Hg in all three regions investigated. The abundance and diversity of Hg-methylating communities and genes were both found to be higher in the PR sediments compared to that in the PRE and SCS sediments, and in good agreement with the spatial distribution of MeHg. Furthermore, a significant correlation was observed between the MeHg concentration and the abundance of both hgcA and hgcB genes in the sediments of the PR, PRE and SCS regions. Overall, the present study suggested that there was the presence of a close link between MeHg and Hg-methylating communities or genes in the ambient aquatic environment, which could be used to reflect the potential of in situ MeHg production.

Biotic and Abiotic Degradation of Methylmercury in Aquatic Ecosystems: A Review

Mercury (Hg) methylation and demethylation is supposed to simultaneously exist in the environment and form a cycle, which determines the net production of methylmercury (MeHg). Exploring the mechanisms of MeHg formation and degradation, and its final fate in the natural environment is essential to understanding the biogeochemical cycle of Hg. However, MeHg demethylation has been less studied in the past years comparing with Hg methylation, particularly in anaerobic microorganisms whose demethylation role has been under-evaluated. This review described the current state of knowledge on biotic (microorganisms) and abiotic demethylation (photodegradation, chemical degradation) of MeHg. The decomposition of MeHg performed by microorganisms has been identified as two different pathways, reductive demethylation (RD) and oxidative demethylation (OD). Anaerobic and aerobic microorganisms involved in the process of RD and OD, influencing factors as well as research background and histories are systematically described in this review. It is predicted that the photodegradation mechanism, as well as anaerobic microorganisms involved in MeHg formation and degradation cycle will be the focus of future research.

A Review on the Distribution and Cycling of Mercury in the Pacific Ocean

With the rapid development of economy in surrounding land, the Pacific Ocean is facing a number of serious environmental challenges, including mercury (Hg) pollution. Over the past several decades, a number of studies have been conducted on investigating the cycling of Hg in this ecosystem. This review summarizes recent studies on the distribution of Hg species in the water, sediment, and biota and the important processes controlling Hg cycling in the Pacific Ocean. Although a lot of studies have been conducted in this system, more efforts should be made on Hg speciation and cycling in the Pacific Ocean, especially some areas that have rarely studied so far. There is a need to measure the rates of important biogeochemical processes in this ecosystem. Application of multiple methods expected to give a better estimation of the sources and sinks of Hg species in the Pacific Ocean in future studies.

Intestinal Methylation and Demethylation of Mercury

Mercury (Hg) is a global pollutant, which is linked with different diseases. The methylation of Hg and demethylation of methylmercury (MeHg) in the environment were extensively studied and summarized; however, the transformation of Hg in the intestine is less presented. In this review, the research progress and the perspectives on the intestinal transformation of Hg were discussed. Studies found that MeHg could be formed when exposed to inorganic Hg by the gut microbiota in aquatic organisms, terrestrial invertebrates, and mammals, etc. hgcAB genes could be used as indicators for predicting Hg methylation potential. In vitro studies using fecal specimen, intestinal contents, and the isolated intestinal microbes confirmed the intestinal demethylation of MeHg. The investigation on the effects of Hg exposure to the abundance and diversity of intestinal microbes and their metabolites could shed light on the mechanism of the toxicity of Hg, especially the neurotoxicity of MeHg, which deserves further study.

Microbial Diversity of Mer Operon Genes and Their Potential Rules in Mercury Bioremediation and Resistance

Background:

Mercury is a toxic metal that is present in small amounts in the environment, but its level is rising steadily, due to different human activities, such as industrialization. It can reach humans through the food chain, amalgam fillings, and other sources, causing different neurological disorders, memory loss, vision impairment, and may even lead to death; making its detoxification an urgent task.

Methods:

Various physical and chemical mercury remediation techniques are available, which generally aim at: (i) reducing its mobility or solubility; (ii) causing its vaporization or condensation; (iii) its separation from contaminated soils. Biological remediation techniques, commonly known as bioremediation, are also another possible alternative, which is considered as cheaper than the conventional means and can be accomplished using either (i) organisms harboring themeroperon genes (merB,merA,merR,merP,merT,merD,merF,merC,merE,merHandmerG), or (ii) plants expressing metal-binding proteins. Recently, differentmerdeterminants have been genetically engineered into several organisms, including bacteria and plants, to aid in detoxification of both ionic and organic forms of mercury.

Results:

Bacteria that are resistant to mercury compounds have at least a mercuric reductase enzyme (MerA) that reduces Hg+2to volatile Hg0, a membrane-bound protein (MerT) for Hg+2uptake and an additional enzyme, MerB, that degrades organomercurials by protonolysis. Presence of bothmerA andmerB genes confer broad-spectrum mercury resistance. However,merA alone confers narrow spectrum inorganic mercury resistance.

Conclusion:

To conclude, this review discusses the importance of mercury-resistance genes in mercury bioremediation. Functional analysis ofmeroperon genes and the recent advances in genetic engineering techniques could provide the most environmental friendly, safe, effective and fantastic solution to overcome mercuric toxicity.

Methylmercury production in soil in the water-level-fluctuating zone of the Three Gorges Reservoir, China: The key role of low-molecular-weight organic acids

As important parts of dissolved organic matter, low-molecular-weight organic acids (LMWOAs) typically play important roles in desorbing Hg(II) from the soil solid-phase, which may directly or indirectly impact methylmercury (MeHg) production. However, the mechanism of these processes remains unclear. To better understand the effects of LMWOAs on Hg methylation in the soil, a field study was conducted to investigate the distribution of LMWOAs and their relationship with soil MeHg in a seasonally inundated area in the Three Gorges Reservoir (TGR), China. Meanwhile, laboratory simulation experiments were performed to determine the potential mechanism of LMWOAs in Hg methylation. The field investigation detected considerable amounts of LMWOAs in soil, among which tartaric acid and oxalic acid were dominant components. Among which, tartaric acid and oxalic acid were dominant components. Also, a seasonally and spatially heterogeneous distribution of LMWOAs in soil was observed. Notably, a significant positive relationship was found between MeHg concentrations and LMWOA pools in soil (r = 0.969, p < .01), implying that LMWOAs could promote soil MeHg production. The simulation experiments confirmed that the MeHg levels in soil were largely elevated with the addition of LMWOAs, which occurred mainly in oxygen-deficient environment and was mediated by biotic factors. The soluble Hg-LMWOA complexes, which were formed by the enhanced desorption of Hg(II) from solid-phase, were mostly responsible for the elevated MeHg production in soil. Moreover, those LMWOAs with more carboxylic groups were believed to enhance the net production of MeHg. The generated MeHg in sediment could diffuse into the overlying water, which thus poses a potential threat to the aquatic food web. Therefore, the enhanced Hg methylation caused by LMWOAs should be given more attention, especially in a seasonally inundated ecosystem, where the MeHg exposure is usually related to fishery activities.

Calls to Florida Poison Control Centers about mercury: Trends over 2003-2013

OBJECTIVE

The aim of this analysis was to contrast trends in exposure-report calls and informational queries (a measure of public interest) about mercury to the Florida Poison Control Centers over 2003-2013.

MATERIALS AND METHODS

Poison-control specialists coded calls to Florida Poison Control Centers by substance of concern, caller demographics, and whether the call pertained to an exposure event or was an informational query. For the present study, call records regarding mercury were de-identified and provided along with daily total number of calls for statistical analysis. We fit Poisson models using generalized estimating equations to summarize changes across years in counts of daily calls to Florida Poison Control Centers, adjusting for month. In a second stage of analysis, we further adjusted for the total number of calls each day. We also conducted analyses stratified by age of the exposed.

RESULTS

There was an overall decrease over 2003-2013 in the number of total calls about mercury [Ratio per year: 0.89, 95% CI: (0.88, 0.90)], and calls about mercury exposure [Ratio per year: 0.84, 95% CI: (0.83, 0.85)], but the number of informational queries about mercury increased over this time [Ratio per year: 1.15 (95% CI: 1.12, 1.18)]. After adjusting for the number of calls of that type each day (e.g., call volume), the associations remained similar: a ratio of 0.88 (95% CI: 0.87, 0.89) per year for total calls, 0.85 (0.83, 0.86) for exposure-related calls, and 1.17 (1.14, 1.21) for informational queries.

CONCLUSION

Although, the number of exposure-related calls decreased, informational queries increased over 2003-2013. This might suggest an increased public interest in mercury health risks despite a decrease in reported exposures over this time period.

Survey of mercury in boreal chorus frog (Pseudacris maculata) and wood frog (Rana sylvatica) tadpoles from wetland ponds in the Prairie Pothole Region of Canada

Tadpoles are important prey items for many aquatic organisms and often represent the largest vertebrate biomass in many fishless wetland ecosystems. Neurotoxic mercury (Hg) can, at elevated levels, decrease growth, lower survival, and cause developmental instability in amphibians. We compared total Hg (THg) body burden and concentration in boreal chorus frog ( Pseudacris maculata) and wood frog ( Rana sylvatica) tadpoles. Overall, body burden and concentration were lower in boreal chorus frog tadpoles than wood frog tadpoles, as expected, because boreal chorus frog tadpoles consume at lower trophic levels. The variables species, stage, and mass explained 21% of total variation for body burden in our models but had negligible predictive ability for THg concentration. The vast majority of the remaining variation in both body burden and THg concentration was attributable to differences among ponds; tadpoles from ponds in three areas had considerably higher THg body burden and concentration. The pond-to-pond differences were not related to any water chemistry or physical parameter measured, and we assumed that differences in wetland geomorphology likely played an important role in determining Hg levels in tadpoles. This is the first report of Hg in frog tadpoles in the Prairie Pothole Region of North America.

Recent advances in the study of mercury methylation in aquatic systems

With increasing input of neurotoxic mercury to environments as a result of anthropogenic activity, it has become imperative to examine how mercury may enter biotic systems through its methylation to bioavailable forms in aquatic environments. Recent development of stable isotope-based methods in methylation studies has enabled a better understanding of the factors controlling methylation in aquatic systems. In addition, the identification and tracking of the hgcAB gene cluster, which is necessary for methylation, has broadened the range of known methylators and methylation-conducive environments. Study of abiotic factors in methylation with new molecular methods (the use of stable isotopes and genomic methods) has helped elucidate the confounding influences of many environmental factors, as these methods enable the examination of their direct effects instead of merely correlative observations. Such developments will be helpful in the finer characterization of mercury biogeochemical cycles, which will enable better predictions of the potential effects of climate change on mercury methylation in aquatic systems and, by extension, the threat this may pose to biota.

Role of Free Radicals/Reactive Oxygen Species in MeHg Photodegradation: Importance of Utilizing Appropriate Scavengers

A variety of free radicals (FR)/reactive oxygen species (ROS) have been proposed to dominate methylmercury (MeHg) photodegradation, primarily based on the results of FR/ROS scavenger addition experiments. However, in addition to eliminating FR/ROS, the added scavengers may also affect the experimental results by altering some water chemical properties, resulting in a misleading assessment of the importance of FR/ROS. In this study, 20 common FR/ROS scavengers were evaluated in terms of their influence on light absorbance, pH, MeHg analysis, MeHg-dissolved organic matter (DOM) complexation, and the scavenger-induced degradation of MeHg. Only nine scavengers were identified to be appropriate for investigating MeHg photodegradation. By utilizing these appropriate scavengers, direct photodegradation of MeHg-DOM complexes was found to be the major pathway of MeHg photodegradation in Laoshan Reservoir water and Stone Old Beach seawater. In contrast, MeHg photodegradation in Ink River water primarily occurs through both ·OH and ³DOM* mediated indirect pathways and direct photodegradation of MeHg-DOM complexes. The diverse pathways of MeHg photodegradation in the tested water may be due to differences in water chemical properties. A severe overestimation of the role of FR/ROS was observed when several improper but commonly used scavengers were adopted, highlighting the necessity of utilizing appropriate scavengers.

The influence of sample drying and storage conditions on methylmercury determination in soils and sediments

The separate influences of drying and storage conditions on methylmercury (MeHg) concentrations in soil and sediment samples were investigated. Concentrations of MeHg and total Hg were determined in various soil and sediment samples that had been stored or dried under differing conditions. The influence of drying conditions (oven-drying (40 °C) versus freeze-drying) on MeHg concentrations in marine sediments, river sediments, soils, and paddy field soils was investigated (n = 43). The ratio of the MeHg concentration in oven-dried sub-samples divided by the concentration in freeze-dried sub-samples ranged from 0 to 336%. In order to confirm the production of MeHg during storage in some samples, Hg²⁺ was added at 15 mg kg^-1 to a paddy soil, and the sample was then stored at 30 °C. The concentrations of MeHg at 1-h, 1-day, 4-days and 7-days after Hg²⁺ spiking were 2.0 ± 0.1, 13.8 ± 1.0, 36.0 ± 5.0, and 24.9 ± 1.6 μg kg^-1 (n = 3), respectively. The concentration of MeHg at 4-days after Hg spiking and sterilizing (121 °C, 30 min) was 1.8 μg kg^-1, similar to the original value. These results indicate that bacterial Hg methylation and MeHg demethylation occurred within days in the soil. In addition, tests of the stability of MeHg in wet and dry samples during storage were also performed. Overall, our results indicate that the best way to preserve MeHg in soil and sediment samples is to freeze the samples immediately after collection, followed subsequently by freeze-drying, grinding, homogenization, and storage of the dry material in cool, dark conditions until analysis.

Occurrence and photodegradation of methylmercury in surface water of Wen-Rui-Tang River network, Wenzhou, China

The spatial distribution and seasonal variations of methylmercury (MeHg) in Wen-Rui-Tang (WRT) River network were investigated by monitoring the MeHg concentrations in surface water samples collected from 30 sites across the river network over four seasons. Detection frequencies and concentrations of MeHg were generally higher in January, indicating that low sunlight irradiation, wind speed, and temperature conditions might enhance the persistence of MeHg in surface water. The MeHg levels varied with sampling locations, with the highest concentrations being observed in the industrial area especially around wastewater outfall, revealing that the mercury contamination in WRT River mainly comes from the industrial wastewater. Photodegradation of MeHg in WRT River surface water and the effects of natural constituents such as fulvic acid (FA), ferric ions (Fe³⁺), nitrate (NO₃^-), and dissolved oxygen on the MeHg photodegradation in aqueous solutions were studied under the simulated sunlight. The experimental data indicated that the indirect photodecomposition of MeHg occurred in WRT River surface water. Photodegradation of MeHg in FA solution was initiated by triplet ³FA* or MeHg-FA* via electron transfer interaction under light irradiations. The Fe³⁺ and NO₃^- can absorb light energy to produce ·OH and enhance the photochemical degradation of MeHg. The MeHg photodecompositions in FA, nitrate, and Fe³⁺ solutions were markedly accelerated after removing the dissolved oxygen.

Methylmercury production and accumulation in urban stormwater ponds and habitat wetlands

Stormwater management ponds and created habitat wetlands effectively manage erosion, flooding, and pollutant loadings while providing biodiversity and aesthetic benefits, but these structures are also potential sources of methylmercury (MeHg), a bioaccumulative neurotoxin. While MeHg accumulation has been confirmed in habitat wetlands, the extent of MeHg production and accumulation in stormwater ponds is unknown. Additionally, the fine-scale spatial variation in MeHg in these wetlands has never been explored despite the possibility that cycles of wetting and drying, and the presence of aquatic plants may stimulate methylation at their margins. To address these knowledge gaps, we compared MeHg and inorganic mercury concentrations, the percent of total mercury present as MeHg (%MeHg), and potential mercury methylation rate constants (K_meth) in the sediments of terrestrial-aquatic transects through several stormwater and habitat wetlands. We present novel evidence confirming the in situ production of MeHg in both stormwater ponds and habitat wetlands, but observe no systematic differences across the terrestrial-aquatic gradient, suggesting that routine variations in water level do not alter MeHg production and accumulation. Stormwater ponds effectively trap mercury while converting relatively little to MeHg, as evidenced by lower MeHg concentrations, %-MeHg, and K_meth values than habitat wetlands, but often greater inorganic Hg concentrations. The relationship of aquatic vegetation to MeHg accumulation is weak and ambiguous, suggesting plants are not strong drivers of MeHg biogeochemistry in these systems. Although the MeHg hazard associated with individual artificial wetlands is low, they may be important sources of MeHg at the landscape level.

Mercury-methylating genes dsrB and hgcA in soils/sediments of the Three Gorges Reservoir

Previous research found that the water-level fluctuating zone (WLFZ) of the Three Gorges Reservoir (TGR) was an Hg-sensitive area. However, little research has been conducted on the distribution of Hg-methylating microorganisms in this area. The goal of this research was to provide an initial description of the distribution of the dsrB (for sulfate-reducing bacteria) and hgcA (one gene confirmed for Hg methylation) genes. Different types of soil were selected to analyze the abundance of the dsrB and hgcA in different periods, in inundated soil (SI, ≤155 m, which becomes sediment during the wet period, SS) and in non-inundated soil (≥175 m, SN) from Shibao, a typical WLFZ of the TGR. A significant positive correlation was observed between dsrB and hgcA abundance and MeHg concentrations, suggesting that microorganisms with these genes contribute to Hg methylation. Principal component analysis (PCA) indicated that dsrB diversity was highest in SI, followed by SS; SS had the highest diversity of hcgA. Six phylogenetic trees were constructed and showed that more strains were present in SI than in SS. HgcA sequences in SS were confined to three evolutionarily distant clades, δ-Proteobacteria, a methanogen group, and a Clostridia group, which was relatively rare among most clades.

Mercury and methylmercury distribution in the intertidal surface sediment of a heavily anthrophogenically impacted saltwater-mangrove-sediment interplay zone

Total mercury (THg) and methylmercury (MeHg) concentrations were determined from sediment samples collected from thirty sampling stations in Port Klang, Malaysia. Three stations had THg concentrations exceeding the threshold effect level of the Florida Department of Environmental Protection and the Canadian interim sediment quality guidelines. THg and MeHg concentrations were found to be concentrated in the Lumut Strait where inputs from the two most urbanized rivers in the state converged (i.e. Klang River and Langat River). This suggests that Hg in the study area likely originated from the catchments of these rivers. MeHg made up 0.06-94.96% of the sediment's THg. There is significant positive correlation (p < 0.01) between THg and MeHg concentrations. Significant positive correlation (p < 0.05) was also observed between fine sediment particles (i.e. clay and silt) with MeHg concentrations. Sediment particle size, however, was not found to have any influence on THg concentrations in the sediment in the study area.

One-step fabrication and characterization of a poly(vinyl alcohol)/silver hybrid nanofiber mat by electrospinning for multifunctional applications

Preparation and application of a poly(vinyl alcohol)/silver hybrid nanofiber mat by electrospinning.

Transportation and transformation of mercury in a calcine profile in the Wanshan Mercury Mine, SW China

Calcination of Hg ores has resulted in serious contamination of mercury (Hg) in the environment. To understand the mobilization of Hg in the calcine pile, the speciation of Hg in a profile of a large calcine pile in the Wanshan Mercury Mine, SW China was investigated using the X-ray absorption spectroscopy (XANES), to understand the mobilization of Hg in the calcine pile. Higher concentrations of Hg were observed at the 30-50 cm depth of the profile, corresponding to a cemented layer. This layer is observed in the entire pile, and was formed due to cementation of calcines. Hg species in calcines include cinnabar (α-HgS), metacinnabar (β-HgS), elemental Hg(0), and minor mercuric chloride (HgCl₂), but these Hg species show dramatic changes in the profile. Variations in Hg speciation suggest that extensive mobilization of Hg can occur during weathering processes. We show that the cemented layer can prevent the leaching of Hg and the emission of Hg(0) from the pile. High MeHg concentrations were found near the cemented layer, indicating Hg methylation occurs. This study provides important insights into the environmental risk of Hg in mining areas.

Major correlates of mercury in small fish and common loons (Gavia immer) across four large study areas in Canada

We investigated mercury (Hg) concentrations in small fish (mainly yellow perch, Perca flavescens; ∼60% of fish collected) and in blood of common loons (Gavia immer) that prey upon them during the breeding season on lakes in 4 large, widely separated study areas in Canada (>13 lakes per study area; total number of lakes = 93). Although surface sediments from lakes near a base metal smelter in Flin Flon, Manitoba had the highest Hg concentrations, perch and other small fish and blood of common loon chicks sampled from these same lakes had low Hg concentrations similar to those from uncontaminated reference lakes. Multiple regression modeling with AIC analysis indicated that lake pH was by far the most important single factor influencing perch Hg concentrations in lakes across the four study areas (R(2) = 0.29). The best model was a three-variable model (pH + alkalinity + sediment Se; Wi = 0.61, R(2) = 0.85). A single-variable model (fish Hg) best explained among-lake variability in loon chick blood Hg (Wi = 0.17; R(2) = 0.53). From a toxicological risk perspective, all lakes posing a potential Hg health risk for perch and possibly other small pelagic fish species (where mean fish muscle Hg concentrations exceeded 2.4 μg/g dry wt.), and for breeding common loons (where mean fish muscle Hg concentrations exceeded 0.8 μg/g dry wt., and loon chick blood Hg exceeded 1.4 μg/g dry wt.) had pH < 6.7 and were located in eastern Canada.