Isotope Fractionation from In Vivo Methylmercury Detoxification in Waterbirds

The robust application of stable mercury (Hg) isotopes for mercury source apportionment and risk assessment necessitates the understanding of mass-dependent fractionation (MDF) as a result of internal transformations within organisms. Here, we used high energy-resolution X-ray absorption near edge structure spectroscopy and isotope ratios of total mercury (δ²⁰²THg) and methylmercury (δ²⁰²MeHg) to elucidate the chemical speciation of Hg and the resultant MDF as a result of internal MeHg demethylation in waterbirds. In three waterbirds (Clark’s grebe, Forster’s tern, and south polar skua), between 17 and 86% of MeHg was demethylated to inorganic mercury (iHg) species primarily in the liver and kidneys as Hg–tetraselenolate [Hg­(Sec)₄] and minor Hg–dithiolate [Hg­(SR)₂] complexes. Tissular differences between δ²⁰²THg and δ²⁰²MeHg correlated linearly with %iHg [Hg­(Sec)₄ + Hg­(SR)₂] and were interpreted to reflect a kinetic isotope effect during in vivo MeHg demethylation. The product–reactant isotopic enrichment factor (ε_p/r) for the demethylation of MeHg → Hg­(Sec)₄ was −2.2 ± 0.1‰. δ²⁰²MeHg values were unvarying within each bird, regardless of Hg­(Sec)₄ abundance, indicating fast internal cycling or replenishment of MeHg relative to demethylation. Our findings document a universal selenium-dependent demethylation reaction in birds, provide new insights on the internal transformations and cycling of MeHg and Hg­(Sec)₄, and allow for mathematical correction of δ²⁰²THg values as a result of the MeHg → Hg­(Sec)₄ reaction.