Speciation of mercury in dolphin liver: A two-stage mechanism for the demethylation accumulation process and role of selenium

Liver samples of Stenella coeruleoalba dolphins, beached along the Adriatic and Ionian coasts of Apulia (southern Italy), have been analysed for their mercury, methyl mercury and selenium contents. Clear experimental evidence is provided for the existence of a two-stage mechanism for the demethylation/accumulation process of mercury in dolphin liver, as well as a threshold Hg level (ca 100 μg Hg/g) above which co-accumulation of selenium occurs. Only above the threshold is a 1:1 molar ratio observed between selenium and mercury fraction not released by an acid hydrolysis step at room temperature. The existence of HCl soluble inorganic compounds such as mercury selenite or selenate could be excluded. Different binding sites can be hypothesized for mercury, based on the evidence that different amounts of the metal were recovered depending on the particular physico-chemical treatment of the liver sample. The demethylation/accumulation process of mercury probably involves the formation of HgSe and selenocompounds with Se containing residues of specific proteins.