Environmental Origins of Methylmercury in Aquatic Biota and Humans

Abstract

Methylmercury (MeHg) is a neurotoxin found in fish and shellfish, that poses risks to human and ecological health. Exposure to MeHg adversely affects neurodevelopment of children and cardiovascular health in adults. Seafood consumption is the primary exposure route to MeHg in North America. An understanding of the link between environmental MeHg sources and human exposures is needed to determine the impacts of ongoing environmental change. However, few data exist for relating environmental exposures to human health outcomes. Imprecision in dietary recall data on fish consumption and variability in MeHg concentrations within and across seafood species consumed have made it challenging to accurately identify sources of human MeHg exposure. In addition, the diverse environmental sources of MeHg production in ecosystems make it more difficult to quantitatively attribute human exposures to specific environments where methylation is taking place. My doctoral dissertation uses naturally occurring mercury (Hg) stable isotopes to characterize sources of MeHg exposure in aquatic biota and human populations. The objectives of my work are to (1) explore the utility of Hg stable isotopes in human hair as a novel method for tracing sources of MeHg exposure to humans; (2) examine drivers of the internal body burden of MeHg in frequent seafood consumers; (3) refine understanding of environmental MeHg sources for estuarine fish. My first dissertation chapter characterizes the magnitude of mass-dependent fractionation between seafood and consumers and shows Hg stable isotopes in human hair is a promising tool for estimating different Hg exposure sources (e.g., coastal vs. oceanic fish). My second chapter uses dietary survey data and Hg isotopes in hair from high-frequency seafood consumers to show that differences in in vivo demethylation do not explain variability in biomarker concentrations. I infer that absorption efficiencies for MeHg in seafood are very low for some high-frequency fish consumers and hypothesize that this is caused by interactions with co-ingested foods. The last chapter investigates diverse Hg stable isotope signatures in benthic, riverine and pelagic estuarine fish and uses these signatures to better characterize the relative importance of different environmental MeHg sources.