The biogeochemical cycling of mercury
Presenta: Laura Fantozzi
CNR Istituto per lo Studio degli Ecosistemi
Mercury (Hg) is one of the most investigated natural elements and potential contaminants in the world and in the Mediterranean area in particular. It is considered a priority pollutant due to its unique properties and adverse effects on human health and environment, and is a major driving force for international action. Differently to other metals, the atmosphere is the central pathway of distribution for Hg. It evaporates easily and exists in the atmosphere almost in the gaseous phase (>95%). In this form, called gaseous elemental mercury (GEM), it is very stable, with a residence time in air of 6-24 months. As a result, Hg can travel long distances, and persists in environments where it circulates between air, water, sediments, soil, and living organisms. Its organic form, methylmercury (MeHg), readily bioaccumulates and biomagnifies, causing neurological, immunological, and cardiovascular problems at high levels. Human exposure to MeHg is mainly through the consumption of contaminated fish, and is a concern even in relatively pristine environments, as Hg can be readily transported through the atmosphere. It is of even greater concern in areas that have high background levels of Hg, as is the case in the Mediterranean area, due to its distinct climatic, meteorological, geomorphological, and anthropogenic framework. Atmospheric Hg enter marine and fresh waters by wet and dry deposition mostly as inorganic Hg (Hg(II)). Hg(II) can be reduced by biological and photochemical processes to volatile dissolved forms (90% elemental Hg), named Dissolved Gaseous Mercury (DGM), that are readily re-emitted. The removal of these volatile forms ultimately reduces the pool of Hg that could be converted to MeHg and bioaccumulated in marine and fresh water food webs. Water surfaces contribute more than 53% of global Hg emissions that comes from natural processes and sources. In particular, in the Mediterranean area the aquatic evasion represents the largest natural source of Hg into the atmosphere (60-77 t/year). The purpose of this seminar is to give an insight to the cycling of Hg in the aquatic environment and at the air-water interface. In particular, the formation and distribution of DGM, air-water Hg flux, and the contribution of the eukaryotic and prokaryotic microorganisms to the production of DGM in aquatic systems will be discussed.
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