MERCURIUS project : Mercury biogeochemistry

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**Summary

Mercury is a persistent and biomagnified neurotoxic pollutant along food webs, mainly due to its affinity for thiolate groups (-RSH) that are abundant in many proteins. According to a recent study carried out by one of the partners, the Hg(II) ion forms HgxSy clusters with cystic groups of dissolved organic matter (MOD) detrital (humic substances) of the type described in metalloproteins (metallothioneins, MT) for the detoxification of cadmium, copper and zinc by living organisms. The question then arises as to whether these abiotic organic forms are also bioavailable and cause the same genetic and cytological responses as free species in solution (such as CH3Hg+ and CH3HgCl), are degraded during biomagnification in the accumulation organs of animals, and are similar to Hg detoxification forms. These issues will be addressed through measurements on samples taken from the natural environment (fish and human hair) and laboratory experiments on zebrafish (Danio rerio), in order to test the following hypotheses :

a) Mercury associated with MOD modulates the transfer of methylmercury and divalent mercury into the water column and ultimately the overall bioavailability of this element.

b) The structure and quantity of HgxSy clusters in MOD depends on the quantity of cysteine type groups.

c) The structure and quantity of HgxSy clusters influence the bioaccumulation and toxicity of mercury.

d) The expression of the mt1 and mt2 genes encoding the MT-I and MT-II isoforms of metallothioneins in zebrafish is influenced by the speciation of mercury in MOD.

e) Mercury-sulfur clusters of detoxification metalloproteins (metallothioneins) produced by living organisms (fish and humans) are similar to the abiotic HgxSy clusters in MOD. There would then be a limited number of stable HgxSy forms in nature.

f) The nature of HgxSy clusters in MOD influences the ecotoxicity of mercury and modulates the adaptive genetic response, inhibition of mitochondrial function, and swimming behaviour of zebrafish.

g) Detoxification forms are not necessarily identical in the brains, livers and muscles of fish, and hair ; some accumulation organs could precipitate Hg as mercury selenide (HgSe) as in mammals (in addition to or in place of TMs).