Due to the very long half-life of Se-79 and its presence in used nuclear fuel, Se is an element of interest in the safety assessment for a deep geological repository (DGR) for used nuclear fuel. Sorption of radionuclides onto the surrounding host rocks and engineered barrier materials is a potential retardation mechanism for radionuclide transport. This thesis investigates the sorption of Se(-II) onto illite, bentonite (engineered barrier material), and shale under high ionic strength conditions relevant to a Canadian DGR. The ionic strength and pH dependence of Se(-II) sorption onto illite, bentonite, and shale were studied. A non-electrostatic surface complexation model was also developed for Se(-II) sorption onto illite and montmorillonite to investigate the sorption mechanisms by which Se(-II) sorbs onto these solids. / Thesis / Master of Applied Science (MASc)

No description available.

With World Health Organization estimation of population increase to over 9 billion by 2050, malnutrition and food insecurity are predicated to be two of the main challenges facing the world in the future. Fish is a source of protein that can have a significant contribution for solving the global food crisis. Fish for human consumption is provided by both the aquaculture industry and the capture fisheries, however, the flat-lined production of seafood from capture fisheries means aquaculture production should double in number to provide enough protein by 2050. Culturing fish at higher densities is being used as a way to increase the global aquaculture production. Modern aquaculture intensified practices expose the fish to various stressors that can negatively impact their welfare. Good nutrition can improve the health status of aquacultured fish while accumulation of specific nutrient can add value to the fillet. In line with that, the studies described in this dissertation aimed to examine the vital role of fish in human health and evaluate strategies to improve the sustainable aquaculture production of fish using novel aquafeeds. This was completed by conducting a survey of the essential minerals and heavy metals of commercially available catfish, salmon, and tilapia fillets in the United States marketplace and three fish nutritional trials to evaluate the impacts of novel aquafeeds on production and health of fish and the food quality of their respective fillets. Selenium with the established health benefit for human and fish was the trace mineral of interest in this dissertation. This element is the main component of a group of selenium containing proteins called selenoproteins that are antioxidant and maintain oxidative homeostasis of fish and human. Additionally, selenium is an antagonist for many heavy metals such as mercury. The market study revealed that catfish, salmon and tilapia samples available in United States marketplace are good source of selenium, zinc and phosphorous. The concentration of heavy metals in all the samples were lower than safety standards. Nile tilapia (Oreochromis niloticus) and rainbow trout (Oncorhynchus mykiss), which are two economically important aquacultured fish, were used as animal models in the fish nutritional trials. Different forms of selenium supplemented to Nile tilapia diet. The form of Dietary Se form did not have any significant (p>0.05) effect on growth, and plasma and hepatic GPX activity of Nile tilapia. However, organic selenium supplement showed significantly (p<0.05) higher bioavailability and accumulated in the fillets at the significantly higher concentration. In the second fish trial selenoneine, a novel selenium containing compound, was supplemented at various concentrations to rainbow trout diet. Other than growth parameters multiple other health biomarkers (lysozyme, glutathione peroxidase, superoxide dismutase activities) were examined to determine whether selenoneine supplementation had a positive impact on fish health. Selenoneine enriched diet significantly(p<0.05) increased the weight gain, lysozyme and oxidative enzymes activities of rainbow trout. Lastly rainbow trout feed was supplemented with commercially available probiotic (Actisaf®), prebiotic (Safmmann®), and their mixture (synbiotic). These dietary supplements did not have any significant effect on fish production but led to a significant increase in activity of oxidative enzymes in plasma when compared to control diet. These results suggest that feed supplements used in this dissertation can be used to promote the fish health raised in recirculating aquaculture systems and support an economically and environmentally sustainable aquaculture. / PHD / Fish contributes significantly to human’s diet in terms of high quality, easily digestible animal protein that helps fight malnutrition and food insecurity. More than half of the fish for human consumption is produced by the aquaculture industry and the growth of the industry is very critical for meeting the future needs for protein. Other than production of high-quality protein the aquaculture industry has made a great contribution to people’s livelihood, poverty alleviation, income generation, employment and, trade globally. To continue contributing effectively to global food security aquaculture industry should overcome various obstacles such as disease and sustainable production of high-quality feed. In this dissertation, important roles of fish in human diet was examined by measuring the mineral composition of commercially available catfish, salmon and tilapia in the United States market place. Also diets of Nile tilapia and rainbow trout were supplemented with various forms of selenium, prebiotics, and/or probiotics which have positive effect on health of both human and fish. These supplementations led to production of valued-added fish fillets enriched in selenium and improved the fish growth and health.

Acute toxicity tests with selenium were conducted with three freshwater species. All data are expressed as selenite-selenium. Daphnia pulex had a 48 hr LC50 of 3.87 mg/l selenium. The 96 hr LC50 and EC50O values for Gambusia affinis and Physa sp. respectively, were 12.56 and 27.08 mg/l selenium. The sublethal effects of 0.2, 0.4, 0.6 and 0.8 mg/1 selenium on survival, growth and reproduction of Daphnia pulex were monitored for twenty-eight days. These results were analyzed statistically by brood. Appreciable mortality only occurred at 0.8 mg/l selenium. Growth, as measured by body length, was depressed at the highest concentration during the early instars and was slightly stimulated during the later instars. Number of live young per brood was depressed at 0.4, 0.6 and 0.8 mg/l selenium during the early broods and may have been stimulated in later broods. Reproductive dysfunction (i.e., dead young, deteriorated eggs, and abortions) only was significant at the higher concentrations in the early broods. It appeared that the Daphnia were acclimating to the selenium stress. Based on these studies, the MATC for selenite-selenium was 280 ug/l. The effects of selenium on oxygen consumption and filtering rate during 24 hr exposure were also tested at the above concentrations. There were no significant effects of selenium on oxygen consumption. Selenium slightly stimulated filtering rate at 0.2 mg/l and depressed it at the higher concentrations. An evaluation of the water quality criteria for selenium in fresh water indicates that all of the methods for deriving these proposed criteria are inadequate. Based on my evaluation a dual criterion seems appropriate: a strict criterion for selenate-selenium and lenient criterion for selenite- and biselenite-selenium. / Master of Science

As aquaculture continues to contribute high quality protein to a greater proportion of the worlds growing population, fish producers have been pressured to increase overall production. However, associated with elevated production is greater stress due to crowding, reduced water quality, and other factors. These stressors impact the health and welfare of the farmed animal which has become of increasing concern to a more environmentally aware and health conscious consumer. New strategies must therefore be developed and adopted by the aquaculture industry to counteract negative consumer perceptions of industrial fish production while also stabilizing the industry. Better nutrition may enhance disease resistance of farmed fish, while fillet accumulations of specific health-related nutrients may simultaneously add value to the final product. This thesis summarizes research undertaken in an effort to enhance the nutritional value of fish by increasing fillet levels of selenium (Se). In addition, various biomarkers of fish health (lysozyme, ceruloplasmin and glutathione peroxidase (GSH-Px) activities), were examined to determine whether dietary Se supplementation had a positive impact upon fish immunocompetence. Moreover, the effect of vaccination was also examined using lysozyme and growth as indicators of fish performance. Hybrid striped bass (HSB), the fourth most valuable farmed fish and fifth in tonnage produced in the United States, were employed as a model animal. Se, an essential component of the antioxidant enzyme, glutathione peroxidase with many established health benefits was supplemented to HSB diets at various concentrations but was found to be without effect upon serum immune proteins or GSH-Px activity. This finding likely reflected the use of fishmeal within the dietary formulation, which possessed relatively high Se levels, together with sufficient storage of tissue Se within the experimental animals. Nevertheless, these studies determined that organic sources of Se were more efficiently accumulated in HSB muscle than traditional inorganic sources. A linear response occurred up to the highest dose used (3.2 mg kg⁻¹) over a 6 week study. Fillet Se accumulation (r²=0.95) proved to be a better indicator than the liver (r²=0.87).Se enhanced fish therefor appear to offer a route of entry for fish producers into the lucrative designer food market - especially since many hundreds of millions of people worldwide are believed to be Se-deficient. Studies undertaken with Se-deficient HSB confirmed findings from the aforementioned research and also indicate that Se-enhanced fillets might be produced using a finishing feed containing 1.5 mg Se kg⁻¹ 6-8 weeks prior to harvest. Accumulation of Se using this strategy resulted in a 100g portion of HSB fillets containing between 33-109 µg Se, amounting to a dietary intake of between 25-80 µg Se; a level that would satisfy present daily intake recommendations. Vaccination of HSB with a Streptococcus iniae oil-in-water vaccine was examined for its potential negative impacts upon HSB production performance. Vaccinated fish did not exhibit any significant reductions in growth but microarray studies revealed that together with many hundreds of genes, four immune-related genes were impacted by this procedure. This thesis discusses the results obtained with regard to their practical implications to the industry and welfare of cultured fish. / Master of Science

Clostridioides difficile is a bacterial pathogen that causes pseudomembranous colitis and the majority of antibiotic-associated diarrheal cases. Broad-spectrum antibiotic usage disrupts the normal gut microbiota and thereby compromises colonization resistance, the main defense against C. difficile infection. Treatment options are limited to vancomycin, fidaxomicin, and the fecal microbiota transplant. Addressing the scarcity of these therapeutics, we documented two explorations in C. difficile drug discovery: (i) evaluation of antibacterial and toxin-suppressing activity of (+)-puupehenone and similar derivatives, and (ii) clarification of a discrepancy in the hypothesized mechanism of auranofin against C. difficile. A better understanding of how C. difficile colonizes and thrives in the gut can greatly benefit therapeutic development. Interestingly, C. difficile can scavenge nutrients such as amino acids and possibly even purines during infection. Amino acids including proline and glycine act as substrates for Stickland metabolism, a bioenergetics scheme that partially relies on enzymes containing selenium in the form of selenocysteine (e.g., D-proline reductase and glycine reductase). Purines such as xanthine and uric acid can be degraded by bacterial molybdenum hydroxylases harboring an uncharacterized form of selenium, though the role of these enzymes in C. difficile physiology is poorly understood. Selenium likely plays a key role in the scavenging of these nutrients during C. difficile infection. Our investigation of these selenium-dependent enzymes revealed two new findings in C. difficile biology: (i) a link between proline-dependent growth and D-proline reductase, characterized as an energy "addiction," and (ii) a previously uncharacterized selenium-dependent pathway involved in the catabolism of xanthine and uric acid. Overall, these physiological analyses of C. difficile provide promising candidates for therapeutics and key information regarding the organism's nutrient preferences.

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.

Vidros comerciais de base silicato do tipo sodo-cálcico, apresentam coloração pela presença de óxidos de metais de transição, tais como ferro e cobalto, ou pela presença de elementos não-metálicos, tais como o selênio. Geralmente, a presença de selênio metálico (Se°) num vidro silicato confere-lhe a cor rosa (depende de seu estado de oxidação). Desse modo, industrialmente, adiciona-se selênio à composição de vidros quando se deseja produzir vidros róseos ou, mais freqüentemente, quando se deseja mascarar a cor esverdeada conferida pela presença de ferro, o principal contaminante das matérias-primas naturais. Entretanto, o selênio é encontrado em pequenas quantidades na natureza, daí seu alto custo. Além disso, da quantidade inicial de selênio colocada na composição de vidros industriais sodo-cálcicos, quase 80% são vaporizados durante a etapa de fusão do vidro. Este trabalho é dedicado ao estudo do efeito da adição de diferentes compostos portadores de selênio como matéria-prima em substituição ao selênio metálico, visando a redução da volatilização de selênio durante a fusão. Além do selênio metálico, foram utilizados selenitos de sódio, de bário, de zinco e de ferro. Os vidros foram obtidos por fusões em escala de laboratório, em cadinhos de alta alumina, a 1500°C, em forno elétrico. A seguir determinou-se a composição química de cada vidro obtido, por fluorescência de raios X e sua cor, através da determinação de suas coordenadas cromáticas no sistema CIEL*a*b*. A análise dos resultados teve como principal conclusão que a concentração final de selênio no vidro é função apenas de sua quantidade no banho, independentemente do composto químico que lhe forneceu. / Commercial silicate glasses of the soda-lime system attain color by the presence of transition metal oxides, such as iron and cobalt, or by the presence of non-metallic elements, like selenium. Usually, the presence of metallic selenium (Se°) in a silicate glass results in a light pink coloration (depends on its oxidation state). Therefore, industrially, selenium is added to a glass composition when the objective is to obtain a pink color glass, or, more often, when the objective is to mask the greenish coloration resultant from the presence of iron, the most usual contaminant in the natural raw-materials. However, selenium occurs in the nature only in small quantities. Besides, from the initial selenium added to industrial soda-lime batch compositions, almost 80% volatilize during melting. This work is dedicated to the study of the effect of adding selenium from different raw-materials, substituting for the metallic selenium, with the principal objective of reducing the loss of this element during glass melting. Along with metallic selenium, sodium, barium, zinc, and iron selenites were employed. The glasses were obtained in laboratory scale meltings in alumina crucibles, at 1500°C, in an electrical furnace. After that, the chemical composition of each glass was obtained by X-ray fluorescence, and its color was measured according to the chromatic coordinates in the CIEL*a*b* method. The main conclusion that can be drawn from the analysis of the results is that the selenium concentration in the final glass is a function only of the quantity of selenium present in the melt, independently of the chemical compound which supplied such selenium.

Deficiencies of selenium (Se) and iodine (I) are widespread in livestock all over Europe. They have an impact on the animals health. Since the clinical signs of the deficiencies are rarely pathognomic, auxiliary exams, based on blood and milk samples are needed for the confirmation of the diagnosis. To evaluate the Se and I status, the plasmatic Se content, the erythrocytic glutathione peroxydase activity (GPX), and the inorganic plasmatic I (IIP) content are measured routinely. Other analyses, like e.g. the dosage of tri-iodothyronine (T3) or thyrotropine (bTSH) can be used. Once the deficiency is diagnosed, it can be corrected by several methods. The first aim of the study was to evaluate the zinc, copper, Se, and I status of Wallonian dairy and beef herds and to correlate their trace element status to their health. The trace element status of the herds with pathologies was less good than that of healthy herds. Further, more herds with pathologies had deficiencies when compared to healthy ones. Dairy herds had a better trace element status than beef herds. Se and I deficiencies are among the most important ones and have the most severe sequels. Therefore, the subsequent parts of the study focussed on these two trace elements. The second aim was the establishment of a technique for the dosage of bTSH and of reference values in healthy cattle. Reference ranges for bTSH and for thyroxine (T4) have been determined in healthy adult cows and in healthy calves. Thereafter, the next aim was to compare the concentration of bTSH in newborn calves with goitre to those obtained in healthy calves, in order to validate a diagnostic test for this pathology. The bTSH allowed the discrimination of the two groups and to approve the diagnosis of hypothyroidism in some of the calves. The threshold value of bTSH for the diagnosis of hypothyroidism in the newborn calf has been fixed at 35 µU/ml. The fourth aim was to compare the I (IIP) and Se (plasmatic Se, GPX) status as well as the thyroid status (bTSH, T4, T3, rT3) in dried pregnant cows and their calves and in non-pregnant cows, that received normal diet and a diet enriched in Se and I. In those receiving a Se and I enriched diet, the T4 and the bTSH decreased while the IIP, the T3, and the GPX activity increased. In the group that received a diet with normal Se and I contents, only the GPX activity increased. At birth, calves from mothers receiving the Se and I enriched diet, had a higher IPP content and GPX activity, and a lower bTSH concentration than calves from the other group. The last aim was to compare the effects of two different forms of Se (sodium selenite versus seleno-methionine) and two different doses of Se (0.1 versus 0.5 ppm) on the health and the Se status of Se deficient Belgian Blue cows and their calves. The first two groups of cows received a ration with 0.1 and 0.5 ppm, respectively, of Se in the form of sodium selenite (Na-Se 0.1 and Na-Se 0.5), while the third group received 0.5 ppm of Se in the form of seleno-methionine (Y-Se 0.5). The Se content of plasma, colostrum, and milk was higher in the cows of group Y-Se 0.5 when compared to the two other groups. The Se content of the plasma was higher in calves from group Y-Se 0.5 when compared to the two other groups. The daily weight gain of the Y-Se 0.5 group was higher than those of the group Na-Se 0.1. The incidence of diarrhoea among calves in group Na-Se 0.1 was higher than in group Y-Se 0.5. In conclusion, trace elements deficiencies are common in Wallonia and often they are multiple. They play a major role in the aetiology of multifactorial diseases diagnosed in the cattle herds. Deficiencies in Se and in I are most commonly implicated in clinical problems. The diagnosis of these deficiencies is determined by blood analyses. Therefore, the tests need to be differentiated according to their capacity to test the nutritional or the thyroid status. A simultaneous supplementation with I and Se, as well as the form of the supplemented Se, may modify the interpretation of the nutritional and the thyroid status. Better reproduction performances and a better health have been observed in herds with a normal trace element status. Furthermore, the advantage of the supplementation with Se in the form of seleno-methionine has been demonstrated in comparison to sodium selenite in deficient Belgian Blue cattle. This study opened numerous perspectives. The measurement of bTSH should be implemented in laboratories in order to offer it as a routine analysis to the practitioning veterinarian, who could use this tool in the framework of many diseases other than goitre. From a fundamental point of view, the dosage of deiodinases would allow the understanding of the regulation and of the synthesis of the thyroid hormones in bovines, and identifying the role of Se and I in this process. Finally, following the discoveries concerning the seleno-methionine, the effect of organic forms of other trace elements in bovine supplementation should be investigated. / Les carences en sélénium (Se) et en iode (I) sont répandues en Europe chez le bétail et ont des répercussions sur leur santé. Les signes cliniques de carence sont rarement pathognomoniques, ce qui nécessite le recours à des examens de sang ou de lait afin de confirmer le diagnostic. Pour évaluer le statut en Se et en I des bovins, le dosage du Se plasmatique, de lactivité de la glutathion peroxydase érythrocytaire (GPX) et de lI inorganique plasmatique (IIP) est réalisé en routine. Lévaluation du statut thyroïdien se fait principalement via la détermination de la thyroxine (T4) dans le plasma. Dautres analyses sont utilisables à cette fin, telles que la tri-iodothyronine (T3) ou la thyrotropine (bTSH). Une fois le diagnostic de carence posé, la carence peut être corrigée de diverses façons. Le 1er objectif de ce travail a été dévaluer les statuts en zinc, cuivre, Se et I dans les exploitations bovines laitières et viandeuses en Wallonie et de mettre en corrélation ces statuts avec létat de santé des troupeaux étudiés. Le statut en oligo-éléments (O-E) dans les troupeaux avec pathologies était moins bon que celui des troupeaux sains. De même, davantage de troupeaux avec pathologies étaient carencés par rapport aux troupeaux sains. Les troupeaux laitiers bénéficiaient de meilleurs statuts par rapport aux troupeaux viandeux. Les carences en Se et en I sont parmi les plus importantes et les plus lourdes de conséquences. La suite du travail sest donc focalisé sur ces 2 oligo-éléments. Le 2ème objectif a consisté à mettre au point un dosage de la bTSH et à établir des valeurs de référence chez des bovins adultes en bonne santé. Un intervalle de référence pour la bTSH et la T4 a été établi pour des vaches adultes saines et des veaux nouveau-nés sains. En corollaire, lobjectif suivant a été de comparer la concentration en bTSH trouvée chez des veaux nouveau-nés atteints dun goitre avec celle obtenue chez des veaux nouveau-nés en bonne santé, afin de valider un test diagnostique pour cette pathologie. La bTSH a permis de discriminer ces 2 groupes de veaux et détablir le diagnostic dhypothyroïdie chez certains veaux goitreux. Une valeur seuil de bTSH pour poser le diagnostic dhypothyroïdie chez des veaux nouveau-nés a été établie à 35 µU/ml. Le 4ème objectif a été de comparer les statuts en I (IIP) et Se (Se plasmatique, GPX) mais également le statut thyroïdien (bTSH, T4, T3, rT3) de vaches taries gestantes ou non et, le cas échéant, de leur veau, qui ont reçu une ration normalement pourvue ou enrichie en I et Se. Chez les vaches recevant une ration enrichie en Se et I, la T4 et la bTSH ont diminué alors que lIIP, la T3 et lactivité de la GPX ont augmenté. Dans le groupe recevant une ration normalement pourvue en Se et I, seule lactivité de la GPX a augmenté. A la naissance, les veaux provenant des mères ayant reçu une ration enrichie en Se et I avaient une concentration en IIP et une activité de la GPX supérieures et une concentration en bTSH inférieure par rapport aux veaux de lautre groupe. Le dernier objectif a été de comparer les effets sur la santé et le statut en Se de vaches BBB carencées et de leur veau de deux formes (sélénite de soude versus séléno-méthionine) et de deux doses différentes de Se (0,1 versus 0,5 ppm). Les deux premiers groupes de vaches ont reçu une ration avec respectivement 0,1 et 0,5 ppm de Se sous forme de sélénite de soude (Na-Se 0,1 et Na-Se 0,5), alors que le troisième groupe de vaches a reçu 0,5 ppm de Se sous forme de séléno-méthionine (Y-Se 0,5). Les concentrations en Se dans le plasma, le colostrum et le lait étaient plus élevées chez les vaches du groupe Y-Se 0,5 par rapport aux 2 autres groupes. La concentration en Se plasmatique était plus importante chez les veaux du groupe Y-Se 0,5 par rapport à celles des autres groupes. Le gain quotidien moyen des veaux du groupe Y-Se 0,5 était plus important par rapport à celui du groupe Na-Se 0,1. La prévalence de diarrhée des veaux du groupe Na-Se 0,1 était plus élevée par rapport à celle du groupe Y-Se 0,5. En conclusion, les carences en oligo-éléments sont fréquentes en Wallonie et souvent multiples. Elles interviennent de manière importante dans létiologie des troubles multifactoriels constatés dans les exploitations bovines. Les carences en Se et en I sont celles qui occasionnent le plus de répercussions cliniques. Le diagnostic de ces carences en particulier repose sur lutilisation de dosages sanguins. Il faut y distinguer ceux qui mesurent le statut nutritionnel en I et Se de ceux qui évaluent plutôt le statut thyroïdien. Une supplémentation simultanée en I et en Se peut modifier linterprétation des statuts nutritionnel et thyroïdien, de même que la forme sous laquelle le Se est apporté aux bovins. De meilleures performances zootechniques et une meilleure santé sont constatées dans les troupeaux supplémentés en O-E qui jouissent de statuts corrects. De plus, de ce point de vue, la supériorité de la supplémentation en Se sous forme de séléno-méthionine a été démontrée par rapport au sélénite de soude chez des bovins BBB carencés. De nombreuses perspectives se dégagent de ce travail. Le dosage de la bTSH est à implémenter dans des laboratoires en vue den faire une analyse de routine à disposition des vétérinaires praticiens qui pourraient ainsi utiliser cet outil dans le cadre de nombreuses autres pathologies que le goitre congénital. Dun point de vue plus fondamental, le dosage des désiodases permettrait daffiner la compréhension de la régulation de la synthèse des hormones thyroïdiennes chez le bovin, en précisant les rôles respectifs de lI et du Se à ce propos. Enfin, à linstar des découvertes concernant la séléno-méthionine, lintérêt annoncé des formes organiques des autres O-E chez les ruminants devrait être investigué plus avant.

Model organisms are often used in ecotoxicological studies and environmental risk assessments; however, species differences in responses to toxicants exist. A meta-analysis identified normal biomarker ranges for rainbow trout (RT) and brook trout (BT), and showed that RT had greater whole-body lipids and plasma T4 levels than BT. Exposure to selenium inhibited cortisol secretion of trout adrenocortical cells; however, RT were more sensitive than BT. To investigate species vulnerability at the individual level, RT and BT were stocked into reference and selenium-contaminated pit lakes. Fish accumulated more Se from selenium-contaminated than reference lakes, and selenium accumulation was similar between species. Chronic selenium exposure had a greater energetic cost for RT than BT, but this was mitigated by food availability. Chronic selenium exposure decreased plasma T3 and T4 levels, but did not alter other endocrine or oxidative stress biomarkers. This project highlights the need for both species- and site-specific risk assessments. / xiv, 171 leaves : ill., maps ; 29 cm