Open AccessA case for in vivo mass‐independent fractionation of mercury isotopes in fish
Author(s) -
Das Reshmi,
Salters Vincent J. M.,
Odom A. Leroy
Publication year - 2009
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2009gc002617
Subject(s) - isotope , mercury (programming language) , geology , fractionation , fish <actinopterygii> , in vivo , mass independent fractionation , isotope fractionation , geochemistry , fishery , chemistry , biology , chromatography , nuclear physics , physics , microbiology and biotechnology , computer science , programming language
The recent discovery of mass‐independent fractionation of mercury isotopes allows new constraints to be placed on the mercury cycle. Here we report new Hg isotopic analyses of zooplankton and fish from different trophic levels of a freshwater lake (Lake Jackson, Florida) bearing systematic mass‐independent fractionation of mercury isotopes. Fish muscle tissues show a progressive enrichment in the odd‐mass mercury isotopes having odd atomic mass numbers (199 and 201) with increasing trophic level. Trophic level was determined based on nitrogen isotopic composition as well as fish stomach content. Zooplankton in the lake contain mercury with Δ 199 Hg and Δ 201 Hg values of +0.43 (±0.07)‰ and +0.44 (±0.07)‰, respectively. The Δ 199 Hg values increase by ∼1‰ from ∼+0.4‰ in zooplankton, juvenile bluegill, and several other small fishes to Δ 199 Hg = +1.36‰ for the Florida gar, which is the top predator fish in the lake. Previous observations of odd‐mass‐number isotope enrichment of mercury have been explained by photoreduction and demethylation of methyl mercury in the water column or as isotope effects related to microbial methylation. However, our data and the data of Jackson et al. (2008) are also consistent with in vivo production of mass‐independent fractionation. Considering the alternatives, mass‐independent fractionation by metabolic processes is the most straightforward explanation for the mercury isotope data. There are two known mechanisms for mass‐independent fractionation of mercury, i.e., the nuclear volume effect and the magnetic isotope effect. While the data are insufficient to serve as proof, the magnitude of the mass‐independent effect and the nearly equal enrichment of 199 Hg and 201 Hg seem most suggestive of a magnetic isotope effect.