No description available.

Mercury occurs naturally in the environment and is released by human activities. Mercury exists in gaseous, liquid, and solid phases, and all phases are of importance when fate s effects of mercury in terrestrial, fresh and marine water, and atmospheric environments. Mercury can be transformed to a highly toxic form of methylmercury. Humans are exposed to the toxicity of methylmercury by eating fish. Methylmercury is bioaccumulated up the food chain by transfer of residues of methylmercury in smaller organisms that are food for larger organisms in the chain. This sequence of process results in higher concentrations in organisms at higher levels in the food chain with human at the top of the food chain. This study is an evaluation of chemical speciation and biological processes that govern mercury distribution and transformation among three environmental media: atmosphere, water, and sediments. Understanding speciation of mercury and biological processes of methylmercury transformation plays an important part in toxicity and exposure of mercury to living organisms. Speciation also influences transport of mercury within and between environmental media while biological processes of methylmercury transformation influence methylmercury production and its transport to the biological communities. Study also covers the demethylation process that can convert methylmercury to inorganic mercury species. Demethylation and methylation processes therefore may occur in parallel further complicating the assessment of mercury fate in the environment. The study will provide integrated fundamental pathways of mercury species transformation through chemical and biological pathways and will contribute to an understanding of fate and transport of mercury species in environmental media. It will also provide a foundation for a state- and region-wide examination of mercury monitoring and control strategies.

The production of methyl mercury, an acute neurotoxin which readily accumulates in the tissue of organisms, is a biologically mediated process facilitated by sulfate reducing bacteria in aquatic sediments. In-situ capping is a frequently considered risk management strategy for contaminated sediments. Since placement of an in-situ cap will induce anaerobic conditions that are known to be favorable for the growth of sulfate reducing bacteria, there is justifiable concern that capping could increase mercury methylation in underlying sediments. This research builds an understanding of the effects of in-situ capping on underlying biogeochemical processes and elucidates their importance in controlling methyl mercury production. Laboratory experiments and mathematical models were implemented to simulate mercury methylation in redox conditions likely to be induced by capping using sediment from different environments. Mathematical descriptions of processes known to be involved in methylation were incorporated into the model to quantify the effects of these processes. Observations in both well-mixed slurry conditions and intact sediment columns showed that methyl mercury concentrations are strongly dependent upon biogeochemical conditions. Results from experiments with sediment spanning a range of redox conditions and organic contents suggested that sulfate reduction rates, aqueous speciation, and solid phase partitioning are involved in limiting methylation depending on bulk geochemical characteristics. A model with a mechanistic basis that incorporates the effects of these processes provides a useful means of qualitatively and quantitatively considering their cumulative impact in limiting methyl mercury production. High methyl mercury concentrations observed in some lab experiments suggest that there is reason to be concerned about anoxic conditions induced by capping; however, not all anoxic conditions led to equivalent increases in methyl mercury. Experimental and modeling results suggest that in a high organic environment, in-situ capping may produce conditions which accelerate methylation in (formerly) surficial sediment while in a low organic environment, with an overall lower potential for methylation, capping can be expected to have a less dramatic effect. Over time, two processes will temper capinduced increases in methyl mercury. Increases will only last until sulfide builds up to inhibitory levels in underlying sediment or until organic carbon is depleted and overall bacterial activity slows. By providing a more fundamental understanding of the effects of capping on mercury methylation, the results of this research will aid in identifying situations and conditions in which cap-induced increases in methyl mercury have the potential to limit the effectiveness of the management strategy. / text

The interactions of sulfate-reducing bacteria and Spartina alterniflora marsh grass have been established using a simulated salt marsh system and these interactions have been quantified using geochemical and molecular tools. Plant activities have a direct influence on mercury methylators and therefore control mercury transformation in the environment. Biogeochemical data show that sulfate and sulfide profiles change seasonally due to plant growth and senescence. Spartina alterniflora impact the two drivers for sulfate and sulfide transformation. The community of sulfate-reducing bacteria serve as the anaerobic driver and transform sulfate to sulfide (sulfate reduction). Sulfate-reducing bacteria have been identified as the principal methylators of mercury (Andersson, et al., 1990; Compeau and Bartha, 1985; Compeau and Bartha, 1984; Blum and Bartha, 1980; Gilmour and Capone). The aerobic driver is dissolved oxygen present in both porewater and plant root exudates, which transform sulfide back to sulfate (sulfide oxidation). Sulfate is not limiting in the vegetated sediment, even at the lower depths. Therefore, although sulfate reduction rates were high when plant activity was high, oxidative processes were also significant in the upper 4-cm of the sediment. In addition, demethylation of methylmercury to ionic Hg(II) in the porewater can occur through oxidative processes (Oremland et al., 1991). Therefore, the significance of sulfide oxidation may have strong implications for methylmercury demethylation in our marsh system.

No description available.

Inorganic mercury (Hg) is converted to potential neurotoxic methylmercury (MeHg) by a natural process called Hg methylation. MeHg can be biomagnified in the food chain, thus the consumption of Hg-contaminated fish contributes to harmful human health issues. Selenium (Se) inhibits the Hg bioavailability to the methylating bacteria by forming mercuric selenide (HgSe). Pseudodesulfovibrio mercurii, a type of sulfate-reducing bacteria (SRB) cultures were grown in anaerobic environments using an estuarine sulfate lactate growth medium to evaluate the effects of Hg concentrations, bacterial growth phase, and sodium selenate in the Hg methylation process. Bacterial cultures contained two types of mercuric (II) nitrate, dissolved and nanoparticles with 1 nM, 2 nM, and 3 nM concentrations in anoxic conditions. In a different experimental batch, various concentrations of sodium selenate (VI) were added to the Hg-contained medium to evaluate the effect of Se in the Hg methylation process. Dissolved Hg produced higher net MeHg than nanoparticulate Hg throughout the incubation period in the culture medium. Bacterial culture medium stressed with high-level Hg concentrations showed increased MeHg production (pM) but decreased Hg methylation rate (%) for the dissolved Hg. During the methylation process in the presence of Se, net MeHg production was reduced significantly compared to the culture medium solely exposed to Hg. The significant reduction of MeHg generation suggests an interference in the Hg methylation process due to the presence of 50-, 75-, and 100-fold higher Se than Hg. This study reassures the antagonistic effect between Hg and Se at the molecular level. Moreover, this study represents a novel approach when the antagonistic effect of nanoparticulate Hg and selenate is observed at the bacterial level. These interactions between Hg and Se are crucial for a better understanding of the Hg methylation process. This research will help to provide a solid foundation for a better understanding of MeHg generation in anaerobic aquatic conditions.

<p>Methylmercury is a bioaccumulative neurotoxin that severely endangers human health. Humans are exposed to methylmercury through consumption of contaminated aquatic fish. To date, effective strategies for preventing and remediating methylmercury contamination have remained elusive, mainly due to the lack of knowledge in regard to how methylmercury is generated and degraded in the aquatic environment. The goal of this dissertation was to study the mechanisms of two transformation processes that govern the fate of methylmercury in natural settings: microbial mercury methylation and methylmercury photodegradation. The role of mercury speciation (influenced by environmental conditions) in determining the reactivity of mercury in these biological and photochemical reactions was the focus of this research.</p><p>Methylmercury production in the aquatic environment is primarily mediated by anaerobic bacteria in surface sediments, particularly sulfate reducing bacteria (SRB). The efficiency of this process is dependent on the activity of the methylating bacteria and the availability of inorganic divalent mercury (Hg(II)). In sediment pore waters, Hg(II) associates with sulfides and dissolved organic matter (DOM) to form a continuum of chemical species that include dissolved molecules, polynuclear clusters, amorphous nanoparticles and after long term aging, bulk-scale crystalline particles. The methylation potential of these mercury species were examined using both pure cultures of SRB and sediment slurry microcosms. The results of these experiments indicated that the activity of SRB was largely determined by the supply of sulfate and labile carbon, which significantly influenced the net methylmercury production in sediment slurries. The availability of mercury for methylation decreased during aging. Dissolved Hg-sulfide (added as Hg(NO3)2 and Na2S) resulted in the highest methylmercury production. Although the methylation potential of humic-coated HgS nanoparticles decreased with an increase in the age of nanoparticle stock solutions, nano-HgS was substantially more available for microbial methylation relative to microparticulate HgS, possibly due to the smaller size, larger specific surface area and more disordered structure of the nanoparticles. Moreover, the methylation of mercury derived from nanoparticles cannot be explained by equilibrium speciation of mercury in the aqueous phase (<0.2 <em>f</em>Ým, the currently-accepted approach for assessing mercury bioavailability for methylation). Instead, the methylation potential of mercury sulfides appeared to correlate with the extent of dissolution and their reactivity in thiol ligand exchange. Additionally, partitioning of mercury to a diverse group of bulk-scale mineral particles and colloids (especially FeS) may be an important process controlling the mercury speciation and subsequent methylmercury production in natural sediments.</p><p>In surface waters, sunlight degradation is believed to be the predominant pathway for the decomposition of methylmercury. The mechanism of this process was investigated in a series of photodegradation experiments under natural sunlight and UV-A radiation, and in the presence of DOM and selective quenchers for photo-generated reactive intermediates. The results suggested that singlet oxygen generated from photosensitization of DOM drove the photodecomposition of methylmercury. The rate of methylmercury degradation depended on the type of methylmercury (CH3Hg+) binding ligand present in the water. CH3Hg -thiol (e.g., glutathione, mercaptoacetate, DOM) complexes were significantly more reactive in photodegradation compared to other methylmercury complexes (CH3HgCl or CH3HgOH), which may be because thiol-binding can effectively decrease the activation energy and thus enhance the reactivity of methylmercury molecules toward the Hg-C bond breaking process. These findings challenge the long-accepted view that water chemistry characteristics do not affect the kinetics of methylmercury sunlight degradation, and help explain recent field observation that methylmercury photodegradation occurred rapidly in freshwater lakes (where CH3Hg-DOM dominate methylmercury speciation) but relatively slowly in sea water (where CH3Hg-Cl control methylmercury speciation).</p><p>Overall, this dissertation has demonstrated that chemical speciation of inorganic mercury and methylmercury determines their availability for microbial methylation and sunlight degradation, respectively. The abundance of these available mercury species is influenced by a variety of environmental parameters (e.g., DOM). This dissertation work contributes mechanistic knowledge toward understanding the occurrence of methylmercury in the aquatic environment. This information will ultimately help construct quantitative models for accurately predicting and assessing the risks of mercury contamination.</p> / Dissertation

No description available.

Forestry has been found to increase the accumulation of methyl mercury (MeHg), a highly neurotoxic compound, in forest soils. However, little is known about how forestry influences catchment processes that governs the mercury (Hg) methylation process. Logging residues are used in harvested catchments in stick roads to reduce soil disturbances caused by forestry machinery. Logging residues left on site after harvest have been suggested to act as a source of high-quality organic matter that stimulates the activity of the microorganisms that carry out the methylation of Hg. In addition, logging residues might influence the activity and abundance of methylating bacteria by reducing the temperature fluctuations in soils below residues and by increasing the soil moisture content. To evaluate the impact of logging residues on the accumulation of MeHg, an experimental field study was carried out in three sites, one in Uppland and two in Västerbotten. The concentration of MeHg was compared between soils covered with residues and soils without residues, and between the lower and upper parts residue piles. Logging residues were not found to influence the levels of MeHg in soils. However, an increased accumulation of MeHg was found in the lower part of residue piles. The accumulation of MeHg in the lower parts was accompanied by a reduced temperature amplitude and an increased water content compared to the upper part of the piles. The increased formation of MeHg might have been mediated by an increased water content in the lower part of the residue piles, possibly by increasing the abundance and activity of Hgmethylating microorganisms due to suboxic/anoxic conditions within biofilms around decomposing needles. The dissolved organic matter composition in soil water differed in soils below residues compared to without residues, but the organic matter signature in soil water under residues was not found to be compliant with an elevated mercury methylation rate. As MeHg accumulated in the lower part of residue piles could become mobilized and transferred to surface water, the suitability of using logging residues in stick roads depends on the location within the catchment. The removal of logging residues could prevent the potential mobilization of MeHg from residue piles. Though, as soil disturbances may cause an increased Hg methylation rate and mediate MeHg export to surface waters, other form of protection, e.g. logging mats, should be used if logging residues are not used to protect soils. / Mänsklig aktivitet har lett till förhöjda halter av kvicksilver (Hg) i atmosfären. Genom långväga transport och deposition har detta orsakat förhöjda halter i svensk natur. Den huvudsakliga exponeringsvägen av Hg för människan sker genom konsumtion av fisk. Halterna av Hg i svensk insjöfisk överstiger EU:s gränsvärden för god kemisk status samt Världshälsoorganisationens riktlinjer för konsumtion i majoriteten av svenska vatten. Ackumuleringen av Hg i biota sker främst i form av metylkvicksilver (MeHg) som är starkt neurotoxiskt. Avverkning av skog tros bidra till en ökad bildning av MeHg i skogsmark genom att skapa miljöer som gynnar etableringen och aktiviteten av de mikroorganismer som omvandlar icke-organiskt kvicksilver (Hg(II)) till organiskt kvicksilver (MeHg). Dessutom kan skogsbruk bidra till en ökad export av MeHg till följd av ändrade hydrologiska förhållanden samt markskador. En ökad bildning av MeHg är oönskad då mobilisering sedermera kan ske till vattendrag där MeHg kan ackumuleras i akvatisk biota. Dock är kunskapen om hur skogsbruk påverkar specifika processer som är av betydelse för metyleringen av Hg begränsad. Inom skogsbruk används avverkningsrester, bestående av till exempel grenar och toppar, i rishögar för att skydda marken mot körskador. Avverkningsrester som lämnas kvar på området efter avverkning tros bidra till en ökad metylering genom att utgöra en källa av högkvalitativt organiskt material vilket kan stimulera bakteriell aktivitet. Dessutom kan avverkningsrester bidra till en ökad metylering av Hg genom att minska temperaturfluktuationerna i mark täckt med ris samt öka markens vattenhalt, vilket kan bidra till en ökad etablering samt att stimulera aktiviteten av Hg-metylerande mikroorganismer. För att utvärdera effekten av avverkningsrester på ackumuleringen av MeHg genomfördes en experimentell fältstudie där koncentrationen av MeHg jämfördes i mark täckt med ris och mark utan ris. Dessutom undersöktes huruvida en ökad metylering kan ske i den nedre delen av riset, vilket skulle kunna möjliggöras av bakterier i biofilmer kring ris under nedbrytning. Koncentrationen MeHg var högre i den nedre delen av rishögarna jämfört med den övre delen. I den nedre delen av riset var temperaturfluktuationerna lägre och vattenhalten högre än i den övre delen av riset. Ackumuleringen av MeHg kan ha skett via biofilmer i de nedre delarna av riset, där tillgången till hög kvalitativt organiskt material samt mer syrefria förhållanden kan ha gynnat etableringen och aktiviteten av de bakterier som utför metyleringen av Hg. Däremot hade typen av marktäcke, dvs mark täckt med ris och mark utan ris, inte någon påverkan på metyleringen av Hg. Karaktären av organiskt material i markvatten skiljde sig mellan typ av marktäcke, men visar inte på att avverkningsrester utgör en källa av högkvalitativt organiskt material. Vattenhalten i mark under rishögar skiljde sig inte jämfört med mark utan ris, men temperaturen var lägre med mindre variation i mark under rishögar. Därmed visar resultatet på att avverkningsrester kan bidra till en ökad bildning av MeHg. En potentiell mobilisering av MeHg som bildats i rishögarna skulle därför kunna bidra till en ökad export av MeHg från avverkade områden. Användningen av avverkningsrester, för att skydda marken vid körning med skogsmaskiner, anses ändå vara fördelaktig för att minimera utläckage av MeHg. Körskador kan skapa stående vattensamlingar där MeHg kan bildas samt kompaktera mark och skapa snabba ytliga flödesvägar i området. Däremot tyder resultatet på att användningen bör ske med hänsyn till att förhindra eventuell transport av MeHg till vattendrag. Ifall avverkningsrester inte används för att motverka markskador bör ett substitut användas, exempelvis stock-mattor, då körskador kan bidra till en ökad metylering och mobilisering av MeHg. Dock saknas kunskap om eventuell mobilisering av MeHg från rishögar till vattendrag och detta bör utvärderas i framtida studier.

La formation de méthylmercure, la forme la plus toxique du mercure, est due à l’activité bactérienne anaérobie. Afin de connaître la contribution des compartiments microbiens (biofilms, eaux brutes, sédiments) dans la méthylation du mercure, nous avons évalué les vitesses de méthylation d’échantillons de mares de région tempérée (Lorraine) et subarctique (Québec, Canada). Si les bactéries des biofilms ne semblent pas plus méthylantes que d’autres, le sédiment apparait comme le compartiment le plus méthylant en lien avec la concentration en nutriments ainsi qu’avec la température. Ainsi, les changements climatiques actuels, en augmentant la température de l’eau et en favorisant l’activité biologique, peuvent faire de ces mares des sites préférentiels de la méthylation du mercure en milieu subarctique. Enfin, l’activité des biofilms a mené à la formation de rouille verte, un minéral capable de réduire HgII en mercure élémentaire, concurrençant ainsi la méthylation bactérienne / Monomethylmercury formation, the neurotoxic form of mercury, is mainly linked to anaerobic microbial activity. In order to assess the relative contribution of several microbial compartments (biofilms, raw water and sediment) we evaluated methylation of samples from ponds of temperate area (Lorraine, France) and from subarctic ponds (Nunavik, Quebec). Biofilms were not found to specifically promote mercury methylation, whereas sediment emerges as the main compartment involved in mercury methylation. The formation of methylmercury is positively linked to the temperature and to nutrients. Thus, by increasing the open water period, the water temperature and of the microbial activity, current climate changes may turn these ponds in preferential location for mercury methylation in the subarctic ecosystem. Finally, the reactivity of green rust, a mineral which can be produced by bacterial activity of environmental biofilms, may compete with mercury methylation by reducing HgII into Hg0