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

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 δ202Hg 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.