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Abstract
High dietary methylmercury (MeHg) exposures are often associated with increased accumulation of selenium (Se), particularly in tissues with rapid rates of selenocysteine synthesis. Conversely, increased dietary Se intakes result in increased accumulation of Hg, possibly because of mutual sequestration in insoluble HgSe complexes. In the current study, results from Hg X-ray absorption fine structure (XAFS) spectroscopy of lyophilized liver and pituitary tissues from beluga whales, coupled with instrumental neutron activation analysis determinations of their Hg and Se contents, show that Hg occurs as a mixture of HgS and HgSe. In the liver tissues studied, the proportion of Hg as HgSe varied from 38% to 77%, whereas it was higher in the pituitary tissues (85%–90%). Selenium XAFS spectra showed that Se as HgSe also varied from dominant to minor among Se forms in the same tissues. The distribution of Se between HgSe and a biological form was estimated from analysis of the Hg derivative XANES spectra and from the Se EXAFS spectra. These alternative estimates for the Se distribution were reasonably consistent. These findings confirm that dietary exposure of beluga whales to Se and MeHg results in the formation of HgSe, a potential bioindicator for non-toxic mercury, in their tissues.