Bioaccumulation of mercury from the bottom to the top of the food chain is a worldwide concern for ecosystem health and human food supply. Associations of Hg with natural organic matter (NOM), especially with reduced sulfur, to a great extent determine the bioavailability and toxicity of this hazardous element in the biosphere. There is, however, a huge unmet need for improved understanding of (1) how Hg is bound at the molecular-scale in NOM, (2) how its bonding environment influences its solubility, hence its mobility and subsequent uptake into the food chain, and (3) how it is bioaccumulated and detoxified in biological tissues such as fish (e.g., brain, liver, kidney, muscles) and hairs of human beings (medulla, cortex and cuticule). These fundamental questions are crucial to mitigate the impact of Hg on the biosphere. In this proposal, experiments relevant to natural processes and direct measurements on Hg-affected animals and humans are designed to examine (1) relationships between the speciation of Hg in dissolved organic matter (DOM) and its bioaccumulation and detoxification in the tissues of fish, taking Danio rerio (zebrafish) as the model animal in the laboratory, (2) the compartmentation of Hg among tissues down to cell organelles in D. rerio, in insectivorous fishes and fish eaters from French Guiana, and among the three major identifiable tissular regions of the hairs of contaminated Wayana and Wayampis Amerindians, and (3) the sequestration forms of Hg in all these tissues. The research is driven by six hypotheses, which will be tested by an interdisciplinary and international team of researchers : a) Hg is dominantly speciated as HgxSy clusters in DOM at concentrations relevant to realistic environmental processes, and their amounts depend on the concentration of cysteine-like thiolated sulphur groups. b) DOM-associated Hg modulates mercury and methylmercury transfer in the water column and in fine the overall bioavailability of this element. c) The detoxification form(s) of Hg differ, to an extent to be determined, (1) among organisms and tissues, in particular between the brain, liver, and muscles of fish; selenium may precipitate mercury in some tissues, as observed in mammals, and (2) through the food web from vegetarian/insectivorous to predatory fishes to humans. d) Hg-thiolate clusters in metalloproteins produced by cells to detoxify Hg are analogous to the HgxSy clusters in DOM. e) The expression of the mt1 and mt2 genes, which encode the two metallothionein isoforms MT-I and MT-II in D. rerio, is influenced by the speciation of Hg. f) The nature of the HgxSy clusters in DOM influences mercury ecotoxicity and modulates such outcomes as adaptive genetic response, mitochondrial impairment, swimming behaviour, and genotoxicity in zebrafish, and genotoxicity in S. subspicatus. Beyond the discovery of fundamental processes that control the bioavailability and toxicity of this global environmental contaminant and the mere production of high-quality scientific articles of wide interest to a large community of scientists and to media, the broader societal benefit of this proposal is to dissociate, for the first time, speciation and concentration in the evaluation of the bioaccumulation and toxicity of Hg. This knowledge is essential to improve the reliability and significance of assays for assessing mercury toxicity by taking into account the real form of Hg in the environment. So far, acute toxicity tests are performed with free ionic or small molecular Hg species, which are not relevant to natural conditions.