Synthesis of Maternal Transfer of Mercury in Birds: Implications for Altered Toxicity Risk

Abstract

Maternal transfer is a predominant route of methylmercury (MeHg) exposure to offspring. We reviewed and synthesized published and unpublished data on maternal transfer of MeHg in birds. Using paired samples of females’ blood (n = 564) and their eggs (n = 1814) from 26 bird species in 6 taxonomic orders, we conducted a meta-analysis to evaluate whether maternal transfer of MeHg to eggs differed among species and caused differential toxicity risk to embryos. Total mercury (THg) concentrations in eggs increased with maternal blood THg concentrations; however, the proportion of THg transferred from females to their eggs differed among bird taxa and with maternal THg exposure. Specifically, a smaller proportion of maternal THg was transferred to eggs with increasing female THg concentrations. Additionally, the proportion of THg that was transferred to eggs at the same maternal blood THg concentration differed among taxonomic orders, with waterfowl (Anseriformes) transferring up to 382% more THg into their eggs than songbirds (Passeriformes). We provide equations to predict THg concentrations in eggs using female blood THg concentrations, and vice versa, which may help translate toxicity benchmarks across tissues and life stages. Our results indicate that toxicity risk of MeHg can vary among bird taxa due to differences in maternal transfer of MeHg to offspring.

Isotope Fractionation from In Vivo Methylmercury Detoxification in Waterbirds

“…2−4 The toxicological risks of MeHg to aquatic and terrestrial organisms are governed by in vivo transformations, intertissular exchanges, and depuration rates and pathways of MeHg and other biologically relevant forms of mercury. 1 In birds, MeHg can be demethylated in the liver, 5,6 depurated into feathers during molt or to offspring by maternal transfer, 7 and excreted. 1 Stable isotope ratios of mercury are a central tool for ecologic risk assessment and mercury source apportionment to organisms, 8−14 yet critical questions remain on the isotopic fractionation of mercury by in vivo transformations.…”

“…Higher trophic level organisms (e.g., birds, fish, and mammals) are exposed to methylmercury (MeHg) through dietary sources, which is assimilated in the digestive tract, circulated in the bloodstream, and retained in the protein of tissues as a MeHg–cysteine complex (MeHg–Cys). − The toxicological risks of MeHg to aquatic and terrestrial organisms are governed by in vivo transformations, intertissular exchanges, and depuration rates and pathways of MeHg and other biologically relevant forms of mercury . In birds, MeHg can be demethylated in the liver, , depurated into feathers during molt or to offspring by maternal transfer, and excreted . Stable isotope ratios of mercury are a central tool for ecologic risk assessment and mercury source apportionment to organisms, − yet critical questions remain on the isotopic fractionation of mercury by in vivo transformations.…”

Avian eggshell thickness in relation to egg morphometrics, embryonic development, and mercury contamination

“…Organochlorine compounds, such as DDT (dichlorodiphenyltrichloroethane), were widely used beginning in the 1940s and were found to decrease eggshell thickness and affect egg survival (Cooke, 1973; Hickey & Anderson, 1968). It is possible that Hg within the egg itself, which also indicates Hg within the female developing the egg (Ackerman et al., 2020), may also influence eggshell thickness, but studies are less conclusive for this contaminant. We found that eggshell thickness, measured either at the equator or the sharp pole, was not correlated with egg content THg concentrations.…”

Internal Dynamics and Metabolism of Mercury in Biota: A Review of Insights from Mercury Stable Isotopes

“… 58 − 61 The toxicological risk of MeHg to birds is ultimately governed by the intake of dietary MeHg relative to internal demethylation of MeHg and depuration of solely MeHg into feathers during molt 62 or to offspring by maternal transfer. 63 Environmental studies of birds using stable Hg isotopes provide critical toxicological information on the above-mentioned processes and highlight the need to consider internal transformations of Hg when Hg stable isotopes are used to source- or process-track Hg in the environment. As such, birds have been proposed to behave as open isotope systems with continuous inputs of MeHg (the “reactant”), several internal transformations that induce isotopic fractionation in products, and selective depuration of primarily MeHg.…”