Isotope Fractionation from <i>In Vivo</i> Methylmercury Detoxification in Waterbirds
Brett A. Poulin, Sarah E. Janssen, Tylor J. Rosera, David P. Krabbenhoft, Collin A. Eagles‐Smith, Joshua T. Ackerman, A. Robin Stewart, Eun Hee Kim, Zofia Baumann, Jeong‐Hoon Kim, Alain Manceau
Abstract
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 (εp/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.