Recently the Minamata Convention on Mercury (2013) recognized mercury (Hg) as a chemical of global concern. Owing to its long-range atmospheric transport and persistence in the environment Hg can ultimately cause significant negative effects on human health. Although anthropogenic Hg emissions have been reduced by half in the last decades, ongoing contamination is still a worldwide problem and elevated Hg concentrations occur in many parts of the world. As a consequence, the European Commission recommended the implementation of a management strategy for mercury and the EPA (USA) considered mercury to be the third most dangerous pollutant. Notwithstanding almost 70 years of scientific research on Hg cycling, the understanding of the biogeochemical processes that govern the environmental fate of this element are far from being fully comprehended, particularly in complex ecosystems like wetlands. Whereas the behaviour of Hg has been extensively studied in freshwater systems, much less is known about the speciation and fate of mercury in salt-marshes. However, these ecosystems are of vital importance due to the high biological productivity, hydrological flux regulation, biogeochemical cycling of metals and nutrients and habitat for fish and wildlife. Moreover, these coastal areas are becoming more vulnerable according to several climate change scenarios that predict marsh degradation (particularly in temperate areas) due to sea level rise or long periods of droughts. This degradation may induce mercury (and other contaminants) remobilization with potentially severe impact on the surrounding aquatic systems and ecosystem services. Previous Hg studies in salt-marshes suggested high accumulation of Hg in plants and a substantial increase of monomethylmercury (MMHg, the most toxic Hg form) in the rhizosphere environment. More recently, it was shown that Hg0 can be emitted from plant leaves, indicating an efficient Hg translocation process inside the plants and resulting in a low accumulation of Hg in the plant aerial parts. Despite all these isolated studies, there are still many questions that need to be clarified. Major knowledge gaps relate to (i) the effect of plant activity on Hg methylation and MMHg demethylation rates, (ii) the Hg (and MMHg) toxic-kinetic and toxic-dynamic mechanisms inside the plant and (iii) the amount of Hg species that are remobilized during marsh degradation and their consequent impacts. This project PLANTA II aims to answer these major questions and will develop a model describing the impact of vulnerable marsh degradation on Hg fluxes and budgets. Therefore, the outcomes of this proposal will provide crucial scientific information not only to better understand the Hg cycle in wetlands but also to provide sound scientific evidence for decision makers and stakeholders.