Mercury (Hg) is a global contaminant of major concern to human and ecosystem health. Mercury has a tendency to be naturally converted into the organometallic toxin methylmercury (MeHg, CH3Hg) which is characterized by elevated neurotoxicity, persistence, and biomagnification properties in food webs. Marine fish consumption represents the most important exposure pathways for humans to neurotoxic MeHg. MeHg health and socioeconomic costs are estimated to billions € per year worldwide. Despite important progress in environmental Hg science, we still lack answers to some of the most fundamental questions on where, how and when MeHg is formed in the global ocean. Current Hg analytical tools offer only limited molecular resolution and throughput to capture, characterize, and quantify complex in situ biogeochemical processes operating at ultratrace levels. The aim of the MERTOX project is to tackle the paramount and yet unanswered question so as to what exactly controls marine MeHg formation, fate and accumulation in pelagic fish. Central to the proposal is the development and application of innovative organometallic stable isotopic tools. These new tools aim at fingerprinting metal/organic matter source - interaction mechanisms, exemplified here with MeHg toxin. We are convinced that the combined stable isotopic signatures of both the Carbon (C) (d13C) and Hg (d202?199Hg) atoms embedded in MeHg holds key information about its marine source origin and fate. Laboratory experiments to explore the fundamental mechanisms driving the organometallic isotopic variations of MeHg, in situ field observations to capture MeHg source and fate across organic and redox marine biogeochemical interfaces, and MeHg stable isotopic analyses of marine top predator tissue collections from the global ocean will be conducted. Particular attention will be paid to the yet unexplored isotopic carbon side of the MeHg cycle, an original and promising new approach to capture the origin of this toxin at the global ocean scale using marine d13C isoscapes of different marine organic carbon components.