Européenne, (Anguilla anguilla) a conduit à une diminution du nombre d’anguille jaune dans les bassins versants. En 2007, le règlement (CE) instaure que 60% des civelles pêchées (<12 cm de long) doivent être dédiés à des opérations de repeuplement. Cependant, pour migrer et coloniser un nouvel environnement, les civelles doivent avoir une bonne condition énergétique. Or, les civelles ne s’alimentent pas ou peu durant leur migration estuarienne et la vitesse d’utilisation de leurs réserves dépend de divers facteurs (température, contaminants etc.). Dans l’estuaire de l’Adour, différents travaux ont mis en évidence de fortes concentrations en methylmercure (MeHg) dans les sédiments et une bioaccumulation de ce composé chez l’anguille jaune. L’exposition au MeHg entraîne la mise en place de processus de détoxication qui pourraient être couteux en énergie ce qui, chez des organismes jeûnant comme les civelles, conduirait à une perte de poids pouvant affecter leurs capacités de migration. Pour étudier l’impact du méthylmercure sur le potentiel de migration des civelles, nous avons tout d’abord (1) effectuer un état des lieux des concentrations en MeHg des civelles de l’estuaire de l’Adour, (2) évaluer l’effet du MeHg sur deux synchroniseurs de la migration estuarienne: le signal crépusculaire et l’alternance des marées chez les civelles grâce à l’utilisation de traceurs isotopiques et (3) étudier dans le milieu naturel (Adour) le comportement de migration, les taux de contamination au mercure et les mécanismes de détoxication des civelles. Nos résultats montrent des fluctuations temporelles et saisonnières de la concentration en MeHg des civelles prélevées en entrée et en milieu de l’estuaire. Par ailleurs, l’exposition au MeHg, en milieu contrôlé, a augmenté l'activité des civelles mais pas leur comportement de migration. Cependant, suite à la contamination, la structure mitochondriale et le métabolisme ont été modifiés suggérant un plus fort stress oxydant et une activation des défenses antioxydantes notamment des civelles ayant un faible potentiel de migration (non migrantes).En complément, l’étude en milieu naturel a mis en évidence une plus forte demande énergétique pour les civelles non migrantes entrainant une augmentation du fonctionnement des mitochondries et pouvant conduire à un plus fort stress oxydant. / Over the last three decades, the arrival of Anguilla anguilla glass eels has markedly declined which has lead to a decrease in the number of yellow eels in the watersheds. In 2007, the Regulation (EC) indicated that 60% of glass eels (<12cm long) caught by fisheries should be used for restocking. To migrate and colonize a new environment, glass eels must have a good energetic condition. However, during their estuarine migration, glass eels either feed very little or more likely not at all. The speed at which they expend their energy stores depends on various factors (temperature, contaminants etc.). In the Adour estuary, many studies have reported relatively high methylmercury (MeHg) concentrations in the sediments, and bioaccumulation in yellow eels. Exposure to MeHg requires energy for detoxification which, in starving organisms such as glass eels, might lead to weight loss, decreasing their ability to migrate up the estuary. To study the impact of MeHg on migratory behaviour in glass eels, we first conducted an inventory of MeHg concentrations of glass eels in the estuary of the Adour, (2) evaluate the effect of MeHg on two synchronizers of estuarine migration: dusk and tide on glass eels through the use of isotopic tracers and (3) to study the natural environment (Adour) migratory behavior, mercury contamination levels and detoxification mechanisms of glass eels. Our results show temporal and seasonal fluctuations in the concentration of MeHg glass eels collected input and middle of the estuary. Moreover, MeHg exposure, in a controlled environment, increased the activity of glass eels but not in response to migratory behavior. However, due to contamination, mitochondrial structure and metabolism have been modified suggesting a stronger oxidative stress and activation of antioxidant defences of glass eels, particularly those with low migration potential (non-migrant). In addition, the wild study showed a higher energy demand for non-migrating glass eels causing an increase in mitochondrial function which may lead to higher oxidative stress.

Os materiais de referência certificados (MRCs) desempenham importante papel na obtenção de resultados de medições rastreáveis ao Sistema Internacional de Unidades (SI), por meio de uma cadeia contínua de comparações. Dessa forma, a demanda por novos materiais de referência certificados (MRC) aumenta a cada dia em todas as áreas do conhecimento. A disponibilidade de materiais de referência, principalmente no Brasil, ainda é incipiente, face a demanda que excede em muito a variedade disponível desses materiais. Entre as diversas áreas carentes de materiais de referência, destaca-se a área de alimentos pela importância desses produtos na balança comercial do país. A certificação de produtos alimentícios, destinados tanto para exportação como para o consumo interno, requer metodologias de análise que ofereçam resultados precisos e exatos a fim de assegurar a qualidade do produto. Este trabalho descreve a produção e certificação de um material de referência na matriz peixe em mercúrio e metilmercúrio. O estudo reúne desde a etapa de seleção do material, preparação, desenvolvimento dos estudos de homogeneidade e estabilidade e a caracterização. A certificação foi realizada por meio de medições utilizando duas técnicas analíticas, a Espectrofotometria de Absorção Atômica (AAS) com sistema de injeção em fluxo e geração de vapor frio (FIA-CV-AAS) e a Diluição Isotópica aplicada a Espectrometria de Massas (IDMS), sendo este um método primário. Neste trabalho as normas da série ISO 30 (ABNT 3034) e ISO Guide 35 foram utilizadas como base para a preparação e caracterização do material. Para o cálculo de incertezas foi utilizado o GUM e o guia Eurachem. Como resultado, foi produzido e certificado um lote do material em relação ao teor de mercúrio (Hg = 0,271±0,057 &mu;g g-1) e metilmercúrio (MeHg = 0,245±0,038 &mu;g g-1), além de valores informativos de chumbo e arsênio. / The certified reference materials (CRMs) play an important role in obtaining measurement results traceable to the International System of Units, through an unbroken chain of comparisons. Thus, the demand for new certified reference materials (CRMs) increases every day in all areas of knowledge. The availability of reference materials, mainly in Brazil is still incipient, given that the demand far exceeds the available variety of these materials. The amount of certified reference materials available in the country is insufficient to meet the need of the scientific community and demands for development of new methodologies. Among the many areas in need of reference materials, we highlight the importance for the food trade balance for these products within the country. The certification of food products, intended both for export and for domestic consumption, requires analysis methods that provide precise and accurate results to ensure product quality. This paper describes the preparation and certification of a reference material in the fish matrix in mercury and methylmercury. The study brings together since the stage of material selection, preparation, development of homogeneity and stability studies and characterization. The certification was performed by means of measurements using two analytical techniques, flow injection analysis - cold vapor atomic absorption spectrometry (FIA-CV-AAS) and isotope dilution applied to mass spectrometry (IDMS), which is a primary method. In this work the standards of the ISO 30 (ABNT 30-34) and ISO Guide 35 was used as the basis for the preparation and characterization of the material. For the calculation of uncertainties was used the GUM and Eurachem guide. As a result, was produced and certified a lot of material in relation to the concentration of mercury (Hg = 0.271 ± 0.057 mg g-1) and methylmercury (MeHg = 0.245 ± 0.038 mg g-1), and informational values of lead and arsenic .

Vários estudos mostram que algumas regiões da Amazônia sofrem impacto por mercúrio (Hg), decorrente de processos naturais e antrópicos. O Parque Nacional do Jaú - PNJ, é o único Parque do Brasil que protege toda a bacia de um rio de água escura (Rio Jaú), terras inundáveis e reservas tropicais, condições que favorecem a metilação do Hg na biota aquática, portanto, expondo a população ribeirinha à contaminação e tornando a região passível de impactação natural por Hg. Estudos preliminares de dietas de préescolares de comunidades do PNJ, têm demonstrado concentrações preocupantes de Hg. O presente estudo avaliou as concentrações de Hg total, de micronutrientes (Ca, Fe, K, Na, Se e Zn) e de macronutrientes (proteínas, lipídeos, cinzas, energia e carbohidratos) em dietas de pré-escolares da região do PNJ e outras comunidades próximas. Avaliou-se também os níveis de Hg total e MeHg em amostras de cabelos dessas crianças e também, cabelos de crianças residentes em vários bairros da cidade de Manaus e pescados mais consumidos por essa população. A partir desses resultados, foi possível fazer uma avaliação nutricional das dietas e à exposição ao Hg e MeHg de crianças do PNJ e comunidades próximas, e da cidade de Manaus, AM. A quantificação de Hg total e MeHg foi feita por espectrometria de absorção atômica com geração de vapor frio (CV AAS). A determinação dos micronutrientes foi feita pela técnica de ativação neutrônica (AAN) e os macronutrientes, de acordo com as metodologias preconizadas pela AOAC (USA). Todos os métodos foram desenvolvidos e validados, quanto à precisão e exatidão, por meio da análise de materiais de referência com valores certificados para os elementos determinados. Além disso, foi realizada a avaliação das fontes de incerteza para a determinação de Hg e MeHg sendo calculada a incerteza padrão expandida. Os teores obtidos para Hg total nas amostras de dietas e Hg total e MeHg em cabelos do PNJ, estiveram bem acima dos valores encontrados em diferentes localidades da região Amazônica, bem como de localidades próximas ao PNJ. Para muitas crianças os valores de ingestão para Hg, ultrapassou-se o valor de 5 &#956;g de Hg/kg de peso corpóreo/semana (PTWI). As crianças da cidade de Manaus apresentaram teores muito menores de Hg nas amostras de cabelo, ao contrário do PNJ, mesmo sendo ecossistemas interligados. Concluise, portanto, que para a população que possui maior capacidade de escolha de produtos alimentícios fornecedores de proteínas, a ingestão de peixe é menor e conseqüentemente a ingestão de Hg também. Os resultados obtidos pela técnica de AAN forneceram valores de concentração confiáveis para elementos nutricionalmente importantes como Ca, Fe, K, Na, Se e Zn, os quais foram comparados aos valores de DRIs (Dietary Reference Intakes). De uma maneira geral, verificou-se prevalência de inadequação com relação aos micronutrientes e déficit protéico-calórico, para as dietas analisadas no presente estudo. Isso vem confirmar o alto nível de risco e a vulnerabilidade dessa população às deficiências nutricionais e à contaminação por Hg. Vários fatores externos podem influenciar a vulnerabilidade da população aos efeitos tóxicos do MeHg, tais como: idade, sexo, estado nutricional e de saúde, ingestão e interação dietética. Objetivando avaliar também a influência do processo de cocção na perda de nutrientes e de Hg total, em uma tentativa de contribuir para futuros estudos nutricionais, avaliou-se o teor de Hg e dos micronutrientes Ca, Fe, K, Na, Se e Zn em espécies de pescados mais consumidos pela população da cidade de Manaus e comunidades próximas, preparadas sob diferentes formas de cocção (in natura, cozido, frito e assado). Verificou-se que a variabilidade na perda dos elementos minerais e do Hg, para cada processo de cocção, parece estar mais em função da espécie do que a forma de preparo. Verificou-se também, que as espécies predadoras apresentaram os maiores teores de Hg e Se, conforme era esperado. Discute-se a correlação entre Hg e Se em peixes. Conclui-se que, embora as pesquisas atuais concordem que possa existir um BG (nível básico) de Hg na Amazônia, a exposição contínua ao Hg pela via alimentar, mesmo considerando os baixos teores do metal encontrados em algumas espécies de peixes, permitem supor que no decorrer do tempo esta situação possa se agravar, principalmente em comunidades cuja dieta não é diversificada apresenta deficiências nutricionais sérias e se constitua principalmente de pescado. O presente estudo pretendeu também, além da avaliação do estado nutricional e da exposição ao Hg de crianças do Parque Nacional do Jaú e da cidade de Manaus, AM, contribuir de forma a subsidiar futuras ações de políticas públicas. / Studies show that some regions of the Amazon region suffer mercury (Hg) impacts as a direct result of both natural and anthropogenic processes. Jaú National Park (PNJ) is the only National Park in Brazil that protects an entire black water basin (Jaú River), flood land and tropical reserve. These conditions favor Hg methylation in the aquatic biota. This in turn, exposes riverine populations to Hg contamination as well as the adjacent regions. Preliminary studies of pre-school diets from PNJ communities have shown that these diets have a worrisomely high Hg content. The present study assessed total Hg content, micronutrients (Ca, Fe, K, Na, Se and Zn) and macronutrients (proteins, lipids, ash, energy, carbohydrate) in pre-school diets in the PNJ and surrounding communities. Furthermore, total and MeHg levels were also determined in hair samples of these children as well as those living in several neighborhoods of the city of Manaus. Included in this determination were the fish most consumed by these populations. From these results it was possible to evaluate the nutritional content of the diets and the exposure of the children to Hg and MeHg. Cold vapor atomic absorption spectrometry was used to quantify total and MeHg. Micronutrient determination was performed using neutron activation analysis technique (NAA) and Macronutrient through AOAC methodologies (USA). All analytical methods were developed and validated for precision and accuracy by means of reference materials analyses with certified values for the elements determined. Furthermore, the uncertainty sources for Hg and MeHg determination were assessed and the expanded uncertainties were calculated. Total Hg levels in diets and total and MeHg levels for hair samples, were well above those values found in different localities of the Amazon region. This also holds true for those surrounding areas of the JNP. For many children Hg intake values passed the 5 &#956;g Hg/body weigh/week (PTWI). Children of the city of Manaus presented much lower Hg levels in hair samples in contrast to those of the PNJ even though both groups shared the same ecosystem. It can thus be concluded that populations that have a wider choice of food products for protein, have a lesser intake of fish and as a consequence less Hg intake. Obtained NAA results furnished reliable concentration values for important nutritional elements, mainly Ca, Fe, K, Na, Se and Zn. From these values the daily intake was calculated and compared to the DRIs (Dietary reference intakes). In general, the diets analysed in this study presented inadequate and deficient protein-caloric levels, confirming these populations´ high risk and vulnerability to nutritional deficiencies and to Hg exposure. Several external factors can obviously influence the vulnerability of these populations to the toxic effects to MeHg such as: age, sex, health and nutritional status, intake and dietary interaction. In order to evaluate the influence of the cooking processes (in natura, cooked, fried, baked) regarding micronutrient and total Hg losses, in order to contribute to future nutritional studies, Hg levels and micronutrients in fish species most consumed by Manaus residents and surrounding communities were assessed. Micronutrients and Hg loss variability through the different cooking processes seem to be more related to fish species rather than cooking process. Furthermore, predatory species presented higher Hg and Se levels as was expected. The Hg and Se correlation in fish is also discussed. Although research shows a high Hg background in the Amazon the continuous exposure through food can in time worsen especially in communities that do not have diet diversification, have serious nutritional deficiencies and whose diets are mainly fish. It is hoped that this study can also contribute to future public political actions and policies in terms of reducing dietary hazards due to Hg and MeHg exposure.

Vários estudos mostram que algumas regiões da Amazônia sofrem impacto por mercúrio (Hg), decorrente de processos naturais e antrópicos. O Parque Nacional do Jaú - PNJ, é o único Parque do Brasil que protege toda a bacia de um rio de água escura (Rio Jaú), terras inundáveis e reservas tropicais, condições que favorecem a metilação do Hg na biota aquática, portanto, expondo a população ribeirinha à contaminação e tornando a região passível de impactação natural por Hg. Estudos preliminares de dietas de préescolares de comunidades do PNJ, têm demonstrado concentrações preocupantes de Hg. O presente estudo avaliou as concentrações de Hg total, de micronutrientes (Ca, Fe, K, Na, Se e Zn) e de macronutrientes (proteínas, lipídeos, cinzas, energia e carbohidratos) em dietas de pré-escolares da região do PNJ e outras comunidades próximas. Avaliou-se também os níveis de Hg total e MeHg em amostras de cabelos dessas crianças e também, cabelos de crianças residentes em vários bairros da cidade de Manaus e pescados mais consumidos por essa população. A partir desses resultados, foi possível fazer uma avaliação nutricional das dietas e à exposição ao Hg e MeHg de crianças do PNJ e comunidades próximas, e da cidade de Manaus, AM. A quantificação de Hg total e MeHg foi feita por espectrometria de absorção atômica com geração de vapor frio (CV AAS). A determinação dos micronutrientes foi feita pela técnica de ativação neutrônica (AAN) e os macronutrientes, de acordo com as metodologias preconizadas pela AOAC (USA). Todos os métodos foram desenvolvidos e validados, quanto à precisão e exatidão, por meio da análise de materiais de referência com valores certificados para os elementos determinados. Além disso, foi realizada a avaliação das fontes de incerteza para a determinação de Hg e MeHg sendo calculada a incerteza padrão expandida. Os teores obtidos para Hg total nas amostras de dietas e Hg total e MeHg em cabelos do PNJ, estiveram bem acima dos valores encontrados em diferentes localidades da região Amazônica, bem como de localidades próximas ao PNJ. Para muitas crianças os valores de ingestão para Hg, ultrapassou-se o valor de 5 &#956;g de Hg/kg de peso corpóreo/semana (PTWI). As crianças da cidade de Manaus apresentaram teores muito menores de Hg nas amostras de cabelo, ao contrário do PNJ, mesmo sendo ecossistemas interligados. Concluise, portanto, que para a população que possui maior capacidade de escolha de produtos alimentícios fornecedores de proteínas, a ingestão de peixe é menor e conseqüentemente a ingestão de Hg também. Os resultados obtidos pela técnica de AAN forneceram valores de concentração confiáveis para elementos nutricionalmente importantes como Ca, Fe, K, Na, Se e Zn, os quais foram comparados aos valores de DRIs (Dietary Reference Intakes). De uma maneira geral, verificou-se prevalência de inadequação com relação aos micronutrientes e déficit protéico-calórico, para as dietas analisadas no presente estudo. Isso vem confirmar o alto nível de risco e a vulnerabilidade dessa população às deficiências nutricionais e à contaminação por Hg. Vários fatores externos podem influenciar a vulnerabilidade da população aos efeitos tóxicos do MeHg, tais como: idade, sexo, estado nutricional e de saúde, ingestão e interação dietética. Objetivando avaliar também a influência do processo de cocção na perda de nutrientes e de Hg total, em uma tentativa de contribuir para futuros estudos nutricionais, avaliou-se o teor de Hg e dos micronutrientes Ca, Fe, K, Na, Se e Zn em espécies de pescados mais consumidos pela população da cidade de Manaus e comunidades próximas, preparadas sob diferentes formas de cocção (in natura, cozido, frito e assado). Verificou-se que a variabilidade na perda dos elementos minerais e do Hg, para cada processo de cocção, parece estar mais em função da espécie do que a forma de preparo. Verificou-se também, que as espécies predadoras apresentaram os maiores teores de Hg e Se, conforme era esperado. Discute-se a correlação entre Hg e Se em peixes. Conclui-se que, embora as pesquisas atuais concordem que possa existir um BG (nível básico) de Hg na Amazônia, a exposição contínua ao Hg pela via alimentar, mesmo considerando os baixos teores do metal encontrados em algumas espécies de peixes, permitem supor que no decorrer do tempo esta situação possa se agravar, principalmente em comunidades cuja dieta não é diversificada apresenta deficiências nutricionais sérias e se constitua principalmente de pescado. O presente estudo pretendeu também, além da avaliação do estado nutricional e da exposição ao Hg de crianças do Parque Nacional do Jaú e da cidade de Manaus, AM, contribuir de forma a subsidiar futuras ações de políticas públicas. / Studies show that some regions of the Amazon region suffer mercury (Hg) impacts as a direct result of both natural and anthropogenic processes. Jaú National Park (PNJ) is the only National Park in Brazil that protects an entire black water basin (Jaú River), flood land and tropical reserve. These conditions favor Hg methylation in the aquatic biota. This in turn, exposes riverine populations to Hg contamination as well as the adjacent regions. Preliminary studies of pre-school diets from PNJ communities have shown that these diets have a worrisomely high Hg content. The present study assessed total Hg content, micronutrients (Ca, Fe, K, Na, Se and Zn) and macronutrients (proteins, lipids, ash, energy, carbohydrate) in pre-school diets in the PNJ and surrounding communities. Furthermore, total and MeHg levels were also determined in hair samples of these children as well as those living in several neighborhoods of the city of Manaus. Included in this determination were the fish most consumed by these populations. From these results it was possible to evaluate the nutritional content of the diets and the exposure of the children to Hg and MeHg. Cold vapor atomic absorption spectrometry was used to quantify total and MeHg. Micronutrient determination was performed using neutron activation analysis technique (NAA) and Macronutrient through AOAC methodologies (USA). All analytical methods were developed and validated for precision and accuracy by means of reference materials analyses with certified values for the elements determined. Furthermore, the uncertainty sources for Hg and MeHg determination were assessed and the expanded uncertainties were calculated. Total Hg levels in diets and total and MeHg levels for hair samples, were well above those values found in different localities of the Amazon region. This also holds true for those surrounding areas of the JNP. For many children Hg intake values passed the 5 &#956;g Hg/body weigh/week (PTWI). Children of the city of Manaus presented much lower Hg levels in hair samples in contrast to those of the PNJ even though both groups shared the same ecosystem. It can thus be concluded that populations that have a wider choice of food products for protein, have a lesser intake of fish and as a consequence less Hg intake. Obtained NAA results furnished reliable concentration values for important nutritional elements, mainly Ca, Fe, K, Na, Se and Zn. From these values the daily intake was calculated and compared to the DRIs (Dietary reference intakes). In general, the diets analysed in this study presented inadequate and deficient protein-caloric levels, confirming these populations´ high risk and vulnerability to nutritional deficiencies and to Hg exposure. Several external factors can obviously influence the vulnerability of these populations to the toxic effects to MeHg such as: age, sex, health and nutritional status, intake and dietary interaction. In order to evaluate the influence of the cooking processes (in natura, cooked, fried, baked) regarding micronutrient and total Hg losses, in order to contribute to future nutritional studies, Hg levels and micronutrients in fish species most consumed by Manaus residents and surrounding communities were assessed. Micronutrients and Hg loss variability through the different cooking processes seem to be more related to fish species rather than cooking process. Furthermore, predatory species presented higher Hg and Se levels as was expected. The Hg and Se correlation in fish is also discussed. Although research shows a high Hg background in the Amazon the continuous exposure through food can in time worsen especially in communities that do not have diet diversification, have serious nutritional deficiencies and whose diets are mainly fish. It is hoped that this study can also contribute to future public political actions and policies in terms of reducing dietary hazards due to Hg and MeHg exposure.

Contamination of methylmercury (MeHg) in aquatic products has been a wide spread health concern. The objective of this study is to determine total mercury (THg) and MeHg concentrations in different species of aquatic products from major offshore farms in China, and to assess health impacts from consumption. Results showed that the concentrations of THg and MeHg ranged 5.6–328.4 ng/g (wet weight) and 4.3–303.6 ng/g (wet weight) in aquatic products, respectively, and were very variable among species and origin sources. Target hazard quotient (THQ) suggested that MeHg exposure via consumption posed high health risks to children aged 2–7 and higher income families. Residents above the age of 13 and with low income have relatively lower health risk of MeHg exposure. Health impacts on heart attacks and newborns’ IQ from MeHg exposure were evaluated using dose-response relationships. Results showed that mother’s consumption of aquatic products (at 6 ounce per day) may cause a loss of 0.38 IQ points for newborns. For non-pregnant, consumption of aquatic products may cause an increase rate of mortality and morbidity of heart attacks at 10.59 and 78.45 per 100,000 persons, respectively. The negative health impact of consuming seawater fish was higher than freshwater fish.

Methylmercury (MeHg) bioaccumulation in freshwater aquatic systems is impacted by anthropogenic stressors, including climate change and excess nutrients. The goal of this study was to determine how warmer water temperatures and excess nutrients would impact zooplankton communities and phytoplankton concentrations, and in turn increase or decrease MeHg concentrations in freshwater zooplankton. I used a 2x2 factorial design to determine if the interaction of temperature and nutrients would impact plankton metrics and zooplankton MeHg concentrations. Mesocosms were filled with Hg-contaminated water and plankton from Cottage Grove Reservoir, Oregon, a waterbody that has experienced decades of elevated MeHg concentrations and corresponding fish consumption advisories due to run-off from Black Butte Mine tailings, located within the watershed. Treatment combinations of warmer temperature (increased by 0.5°C) and nutrient addition (a single pulse of excess nitrogen and phosphorous), control, and a combination of temperature and nutrients were applied to mesocosms. While plankton did respond to treatments, zooplankton biomass and phytoplankton concentrations did not have significant relationships to MeHg concentrations. However, a significant interactive effect of nutrients and temperature was present: nutrients appeared to buffer against increased MeHg concentrations when temperature was elevated. The mechanisms for this interaction appear to be related to a shift to larger body size and an increase in abundance of Daphnia over copepods. Findings suggest that community composition and species-specific differences in both zooplankton and phytoplankton could play a role in MeHg transfer to higher trophic levels.

<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

The effects of different types and amounts of dissolved organic matter (DOM) on the isotope fractionation of Hg isotopes during aqueous photo-reduction of monomethylmercury (MMHg) were investigated to assess whether mass-independent fractionation (MIF) signatures can be used to track photo-degradation of MMHg in natural waters. From experiments conducted with different amounts of reduced organic sulfur (Sred-DOM), it appears that MIF during photo-reduction may be dependent on whether MMHg is dominantly bound to Sred-DOM. Similar fractionation factors were observed for experiments where Sred-DOM was in far excess of MMHg, while significantly lower fractionation factors were observed with lower Sred-DOM. We also characterized the signature of MIF (i.e. Δ199Hg/Δ201Hg) during MMHg photo-degradation to assess if it was similar in different matrices. The experimental Δ199Hg/Δ201Hg was very similar for different matrices. However, the experimental slope is slightly but statistically different than the slope observed in freshwater fish, which preserve MMHg in nature.

The effects of different types and amounts of dissolved organic matter (DOM) on the isotope fractionation of Hg isotopes during aqueous photo-reduction of monomethylmercury (MMHg) were investigated to assess whether mass-independent fractionation (MIF) signatures can be used to track photo-degradation of MMHg in natural waters. From experiments conducted with different amounts of reduced organic sulfur (Sred-DOM), it appears that MIF during photo-reduction may be dependent on whether MMHg is dominantly bound to Sred-DOM. Similar fractionation factors were observed for experiments where Sred-DOM was in far excess of MMHg, while significantly lower fractionation factors were observed with lower Sred-DOM. We also characterized the signature of MIF (i.e. Δ199Hg/Δ201Hg) during MMHg photo-degradation to assess if it was similar in different matrices. The experimental Δ199Hg/Δ201Hg was very similar for different matrices. However, the experimental slope is slightly but statistically different than the slope observed in freshwater fish, which preserve MMHg in nature.

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