L'uranium est un radioélément naturel, généralement retrouvé à l'état de traces, mais dont la concentration peut être significativement augmentée à proximité de certaines installations du cycle du combustible nucléaire ou de zones d'agriculture intensive. En raison de son association dans les minerais avec différents éléments traces métalliques tels que le cadmium, l'U est la plupart du temps présent en mélange avec d'autres contaminants dans l'environnement. L'étude de leurs effets combinés est indispensable afin de mieux appréhender le risque engendré par les contaminations métalliques multiples pour les écosystèmes. L'évaluation de la toxicité des mélanges et l'identification des interactions synergiques ou antagonistes sont généralement réalisées sur la base de modèles d'additivité de référence intégrés à des approches descriptives, purement statistiques et sans réelles considérations biologiques. Plus récemment, des modèles mécanistes ont été proposés en alternative afin de mieux rendre compte de la dynamique des processus biologiques et des mécanismes de toxicité des contaminants. Ceux-ci n'ont cependant été mis en pratique que dans un nombre limité de cas d'étude de la toxicité des mélanges. L'objectif de ce projet de thèse a été d'étudier les effets combinés chroniques de l'U et du Cd sur les traits de vie du nématode Caenorhabditis elegans, en utilisant en parallèle une approche descriptive (MixTox) et une approche mécaniste (DEBtox). Pour cela, une exposition des nématodes à différentes concentrations d'U et de Cd, seuls ou en mélange, a été réalisée durant onze jours. Une interaction antagoniste importante entre l'U et le Cd a été identifiée pour les critères d'effet de croissance et de reproduction, à partir des deux approches. L'étude du transfert de l'U et du Cd du milieu vers la nourriture ainsi que de leur bioaccumulation par C. elegans nous a permis de montrer que la coprésence d'U diminuait la fraction de Cd disponible pour l'exposition des nématodes. Afin de déceler la présence d'une éventuelle interaction au niveau des organismes, durant les phases toxicocinétique ou toxicodynamique, les données d'effet ont été réanalysées à partir des concentrations d'U et de Cd dans la nourriture, supposées plus proches des concentrations disponibles pour C. elegans. Des effets combinés globalement additifs, sans interaction notable, ont été mis en évidence pour l'U et le Cd même si des conclusions légèrement contrastées ont été obtenues à partir des approches descriptives et mécanistes. La présente étude permet d'illustrer la complexité de l'étude des effets des mélanges et de l'identification des interactions entre les substances. Malgré quelques difficultés d'application, l'approche mécaniste DEBtox semble particulièrement prometteuse pour décrire la toxicité de mélanges au cours du temps et tester d'éventuels mécanismes d'interaction. À terme, le perfectionnement des outils d'analyse des effets combinés des contaminants devrait permettre une meilleure prise en compte de la problématique des mélanges dans les démarches d'évaluation des risques. / Uranium is a natural radioactive trace element for which elevated concentrations can be found in the vicinity of some nuclear fuel cycle facilities or of intensive farming areas. Due its co-occurrence with different trace metals, such as cadmium, in geological ores, U is generally found associated with other contaminants in the environment. The study of their combined effects on ecosystems is of interest to better characterize such multi-metallic polluted sites. The mixture toxicity assessment and the identification of synergistic or antagonistic interaction are generally performed on the basis of additive reference models integrated to descriptive and purely statistical approaches with no real biological basis. Recently, mechanistic models were proposed to better account for the dynamics of biological and toxicological processes. However, such models have only been put into practice in a few number of mixture toxicity case studies. The aim of this PhD project was to assess the chronic U/Cd combined toxicity on the life history traits of the nematode Caenorhabditis elegans using both a descriptive (MixTox) and a mechanistic (DEBtox) approach. To do so, nematodes were exposed during eleven days to different U and Cd concentrations, alone or in mixture. A strong antagonistic interaction between U and Cd was identified for length increase and brood size endpoints on the basis of both approaches. From the study of the U and Cd media-to-food transfer and of the U and Cd bioaccumulation by C. elegans, we showed that the co-presence of U reduced the available Cd fraction for nematodes. To identify a possible interaction at organism level, occurring during the toxicokinetic or toxicodynamic steps, data were re-analyzed on the basis of U/Cd concentrations in food, assumed to be more closely related to available concentrations for C. elegans. Overall additive effects, without interaction, were identified between U and Cd, even if slightly contrasted conclusions were obtained on the basis of the descriptive and mechanistic approaches. The present study underlines the complexity of studying mixture toxicity and identifying chemical interactions. Despite some application problems, the mechanistic approach DEBtox is particularly promising to describe the toxicity of chemical mixtures over time and to test hypothetical interaction mechanisms. In the future, the improvement of tools to analyze the combined toxicity of contaminants would allow to better address the issue of mixtures in ecotoxicological risk assessment processes.

A literature review of metal mixture interaction analyses identified that there was not a consistent method to determine the impact of metal mixtures on an aquatic organism. The review also revealed that a majority of the research on mixtures made use of water concentrations only. Therefore research was conducted to determine the relationship between exposure, bioaccumulation and chronic effects of the four elements As, Co, Cr and Mn individually. Mechanistically based saturation models of bioaccumulation and toxicity were determined for the benthic invertebrate Hyalella azteca, from which lethal water concentrations and body concentrations were also determined. These models were then combined with those previously done for the metals Cd, Cu, Ni, Pb, Tl and Zn to model the impact of 10 metal mixtures on bioaccumulation in short term (1-week) exposures and on bioaccumulation and toxicity in chronic (4-week) exposures at “equi-toxic” concentrations. Interactions between the metals were identified in which; Cd, Co and Ni bioaccumulations were significantly inhibited, Tl and Zn bioaccumulations were marginally inhibited, there was no impact on Cr, Cu or Mn bioaccumulation, and both As and Pb bioaccumulation were enhanced by some mixtures of metals. It was determined that strict competitive inhibition may be a plausible mechanism of interaction affecting Co, Cd and Ni bioaccumulation but not for any of the other metals. However, it is possible that other interactions such as non-competitive or anti-competitive inhibition may have been responsible. A metal effects addition model (MEAM) was developed for Hyalella azteca based on both the bioaccumulation (body concentrations) to effects and the exposure (water concentration) to effects relationships developed from the single metal only studies The MEAM was used to predict the impact of metal mixture exposures on mortality. Toxicity was under-estimated when based on measured water or body concentrations, however, its best prediction was based on body concentrations. The MEAM, when based on measured body concentrations, takes bioavailability into account, which is important since the chemical characteristics of water can greatly alter the bioavailability and therefore toxicity of metals. The MEAM was compared to the traditional Concentration Addition Model (CAM), which calculates toxic units based on water concentrations and LC50s or body concentrations and LBC50s. The CAM overestimated toxicity, but had its best prediction when based on water concentrations. Over all, the best fit to observed mortality was the prediction by the MEAM, based on body concentrations. The measurement of bioaccumulated metals and the use of the MEAM could be important in field site assessments since it takes into account changes in bioavailability due to different site water chemistries whereas the traditional CAM based on water concentration does not.

A literature review of metal mixture interaction analyses identified that there was not a consistent method to determine the impact of metal mixtures on an aquatic organism. The review also revealed that a majority of the research on mixtures made use of water concentrations only. Therefore research was conducted to determine the relationship between exposure, bioaccumulation and chronic effects of the four elements As, Co, Cr and Mn individually. Mechanistically based saturation models of bioaccumulation and toxicity were determined for the benthic invertebrate Hyalella azteca, from which lethal water concentrations and body concentrations were also determined. These models were then combined with those previously done for the metals Cd, Cu, Ni, Pb, Tl and Zn to model the impact of 10 metal mixtures on bioaccumulation in short term (1-week) exposures and on bioaccumulation and toxicity in chronic (4-week) exposures at “equi-toxic” concentrations. Interactions between the metals were identified in which; Cd, Co and Ni bioaccumulations were significantly inhibited, Tl and Zn bioaccumulations were marginally inhibited, there was no impact on Cr, Cu or Mn bioaccumulation, and both As and Pb bioaccumulation were enhanced by some mixtures of metals. It was determined that strict competitive inhibition may be a plausible mechanism of interaction affecting Co, Cd and Ni bioaccumulation but not for any of the other metals. However, it is possible that other interactions such as non-competitive or anti-competitive inhibition may have been responsible. A metal effects addition model (MEAM) was developed for Hyalella azteca based on both the bioaccumulation (body concentrations) to effects and the exposure (water concentration) to effects relationships developed from the single metal only studies The MEAM was used to predict the impact of metal mixture exposures on mortality. Toxicity was under-estimated when based on measured water or body concentrations, however, its best prediction was based on body concentrations. The MEAM, when based on measured body concentrations, takes bioavailability into account, which is important since the chemical characteristics of water can greatly alter the bioavailability and therefore toxicity of metals. The MEAM was compared to the traditional Concentration Addition Model (CAM), which calculates toxic units based on water concentrations and LC50s or body concentrations and LBC50s. The CAM overestimated toxicity, but had its best prediction when based on water concentrations. Over all, the best fit to observed mortality was the prediction by the MEAM, based on body concentrations. The measurement of bioaccumulated metals and the use of the MEAM could be important in field site assessments since it takes into account changes in bioavailability due to different site water chemistries whereas the traditional CAM based on water concentration does not.