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

Abstract

Marine mammals detoxify organic methylmercury (MeHg) as inorganic mercury selenide (HgSe), yet the nature of the reaction intermediate species and the tissue-specific redistribution of Hg species in the body are unknown. We report that the identity and proportion of the dominant Hg species in long-finned pilot whale (Globicephala melas) tissues can be obtained from the bulk variation of isotopic values of δ 202 Hg against the extent of demethylation (percentage of total Hg as MeHg, %MeHg) using an alternating regularized inversion method. Our analysis of isotope data from two previous studies supports that MeHg is demethylated as a tetraselenolate species (Hg(Sec)4), which further transforms into HgSe. Hg(Sec)4 occurs in the liver, kidneys, muscle, heart, and brain, whereas HgSe biomineralization occurs only in the liver and kidneys. This study provides a mathematical approach that facilitates probing the molecular-level chemistry of mercury in biological tissues using bulk isotopic data.

Comment on “New insights into the biomineralization of mercury selenide nanoparticles through stable isotope analysis in giant petrel tissues”

“…This interpretation conflicts with two recent Hg isotope studies performed on 22 petrel tissues from 8 individuals and 89 whale tissues from 28 individuals [4,15]. The δ 202 Hg value of a tissue (δ 202 Hgt) is the weighted sum of the MDF of each Hg species (Spi) expressed as follows:…”

“…They concluded that the δ 202 Hg values of the HgSe nanoparticles from each tissue of individual petrels are specific to the HgSe species alone and that the Hg(Sec)4 → HgSe reaction occurs without fractionation of the 202 Hg isotope. We show (1) that the HgSe nanoparticles are mixtures of MeHgCys, Hg(Sec)4, and HgSe, and therefore that the δ 202 Hg values are not species-specific, and (2) that the 202 Hg isotope is actually fractionated during the Hg(Sec)4 → HgSe reaction, and therefore that this isotope can be used to trace the Hg metabolic pathways between tissues in a single individual and in different animals.It was shown recently that the Clark's grebe, giant petrels, long-finned pilot whales, and blue marlins detoxify the organic methylmercury cysteinate (MeHgCys) complex by the stepwise MeHgCys → Hg(Sec)4 → Hgx(Se,Sec)y → HgSe demethylation reaction [2][3][4][5]. Hg(Sec)4 is a fourcoordinate selenocysteine complex and HgSe is tiemannite.…”

“…It was shown recently that the Clark's grebe, giant petrels, long-finned pilot whales, and blue marlins detoxify the organic methylmercury cysteinate (MeHgCys) complex by the stepwise MeHgCys → Hg(Sec)4 → Hgx(Se,Sec)y → HgSe demethylation reaction [2][3][4][5]. Hg(Sec)4 is a fourcoordinate selenocysteine complex and HgSe is tiemannite.…”

“…in which f(Spi)t is the fractional amount of species i in tissue t, and δ 202 Spi is the species-specific MDF. Through a mathematical inversion analysis of the petrel and whale isotopic data, Manceau et al [4,15] showed that the 22 δ 202 Hgt petrel values and the 89 δ 202 Hgt whale values, could be calculated with just three δ 202 Spi values, each associated with the MeHgCys, Hg(Sec)4, and HgSe species. Therefore, Equation 1 was reduced to We show below that the Queipo-Abad et al [1] data can be reinterpreted in light of the findings of the recent petrel and whale studies [4,15], which concluded that the three Hg species have a specific isotopic composition which is homogenized throughout the body.…”

Internal Dynamics and Metabolism of Mercury in Biota: A Review of Insights from Mercury Stable Isotopes

“… 36 , 70 − 73 , 83 The MeHg endmember has relatively uniform δ 202 Hg values across tissues, likely explained by the efficient exchange of MeHg between blood and tissues throughout the body. 70 , 71 , 73 …”