METHYLATION AND DEMETHYLATION ACTIVITY IN THE MERCURY CYCLING IN COASTAL WATERS (GULF OF TRIESTE, NORTHERN ADRIATIC)

4siThe Gulf of Trieste and adjacent Grado and Marano Lagoon (northern Adriatic Sea) are considered as an important “natural bioreactor” that can increase sedimentary methylmercury (MeHg) production by mobilizing the extensive mercury (Hg) pool as a result of historical (500 years) Hg mining and smelting activities in Idrija (NW Slovenia), the world’s second largest Hg mine. Even though the mine was definitely closed in 1995 the Soča/Isonzo River system still delivers about 1.5 t of Hg to the gulf annually and the MeHg is elevated in this marine environment. Research conducted by late M.E. Hines showed that Hg is readily methylated and demethylated in marine, lagoonary and freshwater sediments but the relative activities varied greatly with locality. Methylation activity increased from freshwater to the marine sites, conversely the highest demethylation was found in estuarine and lagoon sites. The methylation/demethylation ratios were consequently low in coastal sites but increased ib the gulf. Subsequent investigations in the southern, less contaminated (“pristine”) sediments of the gulf revealed that MeHg is also continuously produced especially in autumn in parallel with higher bottom water temperature and lower oxygen content. Sulphate reduction was found to control both processes. Hg methylation in sediments of coastal lagoon was controlled by rapid demethylation and the Hg bioavailability was affected by Hg adsorption and precipitation, In offshore marine sites, the sulphide produced by sulphate reduction lowers Hg methylation. Methylation of dissolved Hg, nearly 100% colloidally bonded, in the seawater column of the gulf was not detected (showing a pronounced Hg reduction potential) suggesting that sediments are the principal methylation site and the source of MeHg to the water column. It is likely that demethylation and reduction prevent the excessive accumulation of MeHg in the food webs of the Gulf of Trieste.noneopenJadran FAGANELI, Ingrid FALNOGA , Stefano COVELLI, Milena HORVATFaganeli, Jadran; Falnoga, Ingrid; Covelli, Stefano; Horvat, Milen