Isotope Fractionation from In Vivo Methylmercury Detoxification in Waterbirds

Abstract

The robust application of stable mercury (Hg) isotopes for mercury source apportionment and risk assessment necessitates the understanding of mass-dependent fractionation (MDF) due to internal transformations within organisms. Here, we used high energy-resolution XANES spectroscopy and isotope ratios of total mercury (δ 202 THg) and methylmercury (δ 202 MeHg) to elucidate the chemical speciation of Hg and the resultant MDF due to internal MeHg demethylation in waterbirds. In three waterbirds (Clark's grebe, Forster's tern, south polar skua), between 17-86% of the 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 δ 202 THg and δ 202 MeHg 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‰. δ 202 MeHg 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 2 reaction in birds, provide new insights on the internal transformations and cycling of MeHg and Hg(Sec) 4 , and allow for mathematical correction of δ 202 THg values due to the MeHg  Hg(Sec) 4 reaction.

Chemical Forms of Mercury in Pilot Whales Determined from Species-Averaged Mercury Isotope Signatures

“…One liver tissue of an adult pilot whale from the Li et al data is an outlier with δ 202 Hg = 0.33‰ for f (MeHg) = 0.08 (black arrow in Figure a). Importantly, the hockey stick shape of the δ 202 Hg t versus f (MeHg) plots (Figure ) suggests the occurrence of an intermediate species in the MeHg → HgSe reaction, which was previously documented to be Hg­(Sec) 4 . − The analysis here supports that Hg­(Sec) 4 is highly enriched in lighter Hg isotopes compared to MeHg, consistent with the observed MDF of mercury due to the demethylation of MeHg to Hg­(Sec) 4 , and to a lesser extent HgSe.…”

“…The isotopic data can be explained by the coexistence of MeHg with two inorganic species, Hg­(Sec) 4 and HgSe, with uniform isotopic signatures: δ 202 MeHg = 1.21 ± 0.04‰, δ 202 Hg­(Sec) 4 = −1.37 ± 0.09‰, and δ 202 HgSe = 0.00 ± 0.00‰. The δ 202 Hg­(Sec) 4 – δ 202 MeHg difference of 2.58 ± 0.13‰ is similar to that reported recently for birds (−2.2 ± 0.1‰) …”

“…The δ 202 Hg(Sec) 4 − δ 202 MeHg difference of 2.58 ± 0.13‰ is similar to that reported recently for birds (−2.2 ± 0.1‰). 37 A highlight of this study is the invariance of the three δ 202 Sp i values between the tissues of a same individual, across individuals of the same pod, and between pods sampled at different times in different geographic locations. A similar observation was made previously for the uniformity of δ 202 Hg in the blood of seven juvenile and seven adult whales (δ 202 Hg blood = 1.06 ± 0.05‰).…”

“…The first attempt to combine HR-XANES spectroscopy and 202 Hg fractionation was performed recently by Poulin et al 37 on piscivorous birds from lacustrine, estuarine, and marine environments. The demethylation of MeHg to Hg(Sec) 4 led to a δ 202 Hg(Sec) 4 − δ 202 MeHg = −2.2 ± 0.1‰ depletion of the 202 Hg isotope.…”

Response to Comment on “Mercury Isotope Fractionation by Internal Demethylation and Biomineralization Reactions in Seabirds: Implications for Environmental Mercury Science”: Principles and Limitations of Source Tracing and Process Tracing with Stable Isotope Signatures

“…2). The observed linearity in the isotope system in the petrels is consistent with observed two-component isotope systems that documented MeHg demethylation to inorganic Hg in waterbirds 5 and the brain of whales. 6 Further, similar isotopic results were obtained in a companion article on long-finned pilot whales.…”