Tracking the neurotoxin methylmercury in marine environments
Creators: name: He, Xiaoshuai
Publisher: name: No Publisher Supplied
Year: 2022

description: Monomethylmercury (MeHg) is a highly bioaccumulated neurotoxin that poses threats to ecosystem health and seafood consumers. However, it is challenging to track the sources of MeHg in marine food chains, which contributes to difficulties in assessing ecosystem risk and human Hg exposure. In addition, aqueous photodemethylation, an important sink of MeHg in water column, remains a knowledge gap. To better understand the sources of MeHg to marine food web, this dissertation addresses three different aspects centered around this central goal.The first objective of this work was to evaluate the role of grass shrimp (Palaemonetes pugio) as potential link in the transfer of methylmercury (MeHg) from salt marsh sediments to transient young-of-the-year (YOY) fish in urban salt marshes. Using fish bioenergetic models, grass shrimp alone account for 12%-90% of MeHg observed in YOY striped bass and 6%-22% of MeHg in YOY summer flounder. Direct accumulation of MeHg from grass shrimp to YOY fish increased with MeHg levels in grass shrimp and sediment. However, in the most contaminated salt marshes with the highest levels of MeHg in grass shrimp and sediment, indirect accumulation of MeHg from grass shrimp by YOY summer flounder, whose diet is dominated by benthic forage fish (mummichog), is predicted to plateau because higher concentrations of MeHg in grass shrimp are offset by a lower proportion of grass shrimp in the mummichog diet. Our results demonstrate that grass shrimp are an important trophic link in the bioaccumulation of MeHg in salt marsh food webs, and that MeHg accumulation in YOY fish varies with both the concentration of MeHg in salt marsh sediments and benthic food web structure.The second objective of this work was to understand the sources of mercury (Hg) in adult Pacific bluefin tuna (Thunnus orientalis, PBT) in the Western Pacific Ocean. The primary sources of mercury to bluefin tuna are largely unknown due to its broad migration and feeding ranges. Here we used Hg stable isotopes in the muscle tissue of 91 adult PBT (ranging in age from 5 to more than 27 years) from the western North Pacific Ocean to examine the sources and transformation pathways of Hg before it enters the pelagic food web. Mercury concentrations (~100% MeHg), as well as δ²⁰²Hg and Δ¹⁹⁹Hg values in PBT increased significantly with PBT size and age. The observed increases in δ²⁰²Hg and Δ¹⁹⁹Hg with PBT age class indicate 1) a major shift in the dominant source of Hg between juvenile PBT from the eastern Pacific Ocean compared with adult PBT from the west, and 2) a gradually increasing reliance on epipelagic prey from the Kuroshio extension among adult PBT size classes as they continue to grow. Δ¹⁹⁹Hg/Δ²⁰¹Hg ratios of adult PBT (1.09±0.01) and lower trophic level fish from the Kuroshio Current east of Taiwan (1.13±0.04) and its extension east of Japan (1.12±0.04), which indicate the dominance of photoreduction of inorganic Hg(II) prior to Hg's entry into the food web, are significantly lower than those of oceanic fish from the central and eastern North Pacific (1.2-1.3), which indicate the dominance of photodegradation of MeHg . Overall, these results demonstrate that Hg in PBT from the western North Pacific is likely derived from different sources that undergo different photochemical reaction pathways than elsewhere in the North Pacific, which may explain the exceptionally high concentrations of Hg in PBT and other top predators from this region.The last objective of this dissertation was to investigate the effects of two environmentally important regions of ultraviolet radiation (UVA vs. UVB) on Hg isotope fractionation. Experiments were conducted using two different types of filtered seawater (saltmarsh and coastal ocean), as well as an isolated dissolved organic matter standard (Suwanee River Fulvic Acid). Experimental results show that for all experiments, UVB resulted in both higher energy normalized photo-decomposition rates as well as higher mass-independent enrichment of Δ¹⁹⁹Hg than UVA. However, no significant effect of wavelength on enrichment of δ²⁰²Hg was observed. Δ¹⁹⁹Hg/Δ²⁰¹Hg ratios (~1.36) were consistent across all experiments and were similar to values from previous studies of MeHg photodegradation in the presence of terrestrial dissolved organic matter, as well as MeHg in coastal food webs. Preliminary calculation using Δ¹⁹⁹Hg enrichment factors derived from this study suggests that UVA alone could account for 49 – 95% of the MIF signals remaining in the water after MeHg photodegradation. Overall, our results highlight the wavelength effects on Hg isotope fractionation during MeHg photodegradation in different water types and will help refine the use of Hg isotope ratios as a tracer of Hg biogeochemical cycles in various aquatic ecosystems.