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

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.