Isotopic study of inorganic and methylmercury accumulation and distribution in Paralichthys olivaceus

Abstract

Studies of mercury bioaccumulation in fish continue to be an important scientific discipline for understanding the bioavailability and fate of mercury and for monitoring environmental levels of mercury in natural aquatic ecosystems. Many recent studies have utilized mercury stable isotope ratios in fish to monitor mercury sources and biogeochemical processes in diverse aquatic ecosystems. More experimental studies are however needed to precisely quantify the degree and mechanism(s) governing internal isotope fractionation of two mercury species, methylmercury (MMHg) and inorganic mercury (IHg), in fish. In order to better trace environmental sources of mercury using fish, we performed controlled experiments by raising Paralichthys olivaceus in captivity for 3 months and feeding them food pellets spiked with either MMHg or IHg (0.4 and 1.6 ug/g). Total mercury (THg) and MMHg concentrations and mercury isotope ratios (δ202Hg, Δ199Hg) were measured in the muscle, liver, kidney, and intestine tissues of fish. Fish fed with food pellets spiked with varying concentrations of MMHg equilibrated to the isotopic composition of the food regardless of the tissue type and MMHg concentration. In contrast, fish fed with food pellets spiked with varying concentrations of IHg showed a tissue specific variation. Kidney and muscle tissues with relatively large proportions of MMHg from THg (>60%) exhibited small isotopic shifts toward the IHg food pellets after 3 months. The mercury isotope ratios of intestine comprised mostly of IHg (>90%) equilibrated to the isotopic composition of the food. Interestingly, liver tissues comprised mostly of MMHg (~100%) showed δ202Hg value that is ~0.7 permil higher than the IHg food pellet, suggesting the presence of significant mass-dependent fractionation (MDF) in the fish liver. Our study demonstrates how the isotope ratios of different mercury species behave in the internal system of fish and that the mechanism responsible for MDF needs to be elucidated particularly for using fish tissues to monitor sites contaminated with IHg.