Isotopic and chemical characteristics of mercury in organs and tissues of fish in a mercury-polluted lake: Evidence for fractionation of mercury isotopes by physiological processes.

Organs and tissues of whitefish and trout from mercury (Hg)-polluted Lake Ontario were analyzed for Hg isotopes, methylmercury (CH3 Hg+ ), and inorganic Hg to investigate possible mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes by physiological processes of the fish. Isotope signatures of different body parts were defined by δ-values of 198 Hg/202 Hg, 199 Hg/202 Hg, 200 Hg/202 Hg, and 201 Hg/202 Hg ratios and by Δ-values representing effects of MIF on 199 Hg/202 Hg and 201 Hg/202 Hg ratios. The research yielded the following evidence for MDF and MIF, including MIF of isotopes with even as well as odd mass numbers, by metabolic activities: 1) anomalously low δ-values for whitefish kidneys but not for trout kidneys; 2) widely varying differences between the δ-values of different body parts of whitefish but practically uniform differences for those of trout; 3) different relationships between Δ199 Hg and Δ201 Hg for whitefish than for trout; 4) nonlinear correlation between δ198 Hg and δ200 Hg for whitefish but linear correlation for trout; 5) an inverse correlation between the δ199 Hg values and CH3 Hg+ concentrations of whitefish and trout; 6) an inverse correlation between the δ201 Hg/δ199 Hg and CH3 Hg+ /inorganic Hg ratios of trout kidneys and gills (and lipids of trout near the sources of pollution) but a positive correlation for muscle, liver, and gut; and 7) inverse correlations between Δ199 Hg and the CH3 Hg+ /inorganic Hg ratio for trout liver, kidneys, and gut. Environ Toxicol Chem 2018;37:515-529. © 2017 SETAC.