Mercury Isotope Fractionation by Internal Demethylation and Biomineralization Reactions in Seabirds: Implications for Environmental Mercury Science

A prerequisite for environmental and toxicological applications of mercury (Hg) stable isotopes in wildlife and humans is quantifying the isotopic fractionation of biological reactions. Here, we measured stable Hg isotope values of relevant tissues of giant petrels (Macronectes spp.). Isotopic data were interpreted with published HR-XANES spectroscopic data that document a stepwise transformation of methylmercury (MeHg) to Hg-tetraselenolate (Hg(Sec)(4)) and mercury selenide (HgSe) (Sec = selenocysteine). By mathematical inversion of isotopic and spectroscopic data, identical delta Hg-202 values for MeHg (2.69 +/- 0.04 parts per thousand), Hg(Sec)(4) (-1.37 +/- 0.06 parts per thousand), and HgSe (0.18 +/- 0.02 parts per thousand) were determined in 23 tissues of eight birds from the Kerguelen Islands and Ade ' lie Land (Antarctica). Isotopic differences in delta Hg-202 between MeHg and Hg(Sec) 4 (-4.1 +/- 0.1 parts per thousand) reflect mass-dependent fractionation from a kinetic isotope effect due to the MeHg. Hg(Sec)(4) demethylation reaction. Surprisingly, Hg(Sec)(4) and HgSe differed isotopically in delta Hg-202 (+1.6 +/- 0.1 parts per thousand) and mass-independent anomalies (i.e., changes in Delta Hg-199 of <= 0.3 parts per thousand), consistent with equilibrium isotope effects of mass-dependent and nuclear volume fractionation from Hg(Sec)(4) -> HgSe biomineralization. The invariance of species-specific delta Hg-202 values across tissues and individual birds reflects the kinetic lability of Hg-ligand bonds and tissue-specific redistribution of MeHg and inorganic Hg, likely as Hg(Sec)(4). These observations provide fundamental information necessary to improve the interpretation of stable Hg isotope data and provoke a revisitation of processes governing isotopic fractionation in biota and toxicological risk assessment in wildlife.

Automated summary

The study measured stable Hg isotope values in relevant tissues of giant petrels, finding similar isotopic fractionation values for methylmercury, Hg-tetraselenolate and mercury selenide across different tissues. Differences in isotopic values between MeHg and Hg(Sec)(4) were attributed to mass-dependent fractionation from the demethylation reaction of Hg(Sec)(4). Additionally, differences in isotopic values and mass-independent anomalies between Hg(Sec)(4) and HgSe were observed, likely due to equilibrium isotope effects of biomineralization from Hg(Sec)(4) to HgSe.