Mercury isotopes of key tissues document mercury metabolic processes in seabirds

Abstract

Seabirds accumulate significant amounts of mercury (Hg) due to their long-life span together with their medium to high trophic position in marine food webs. Hg speciation and Hg isotopic analyses of total Hg in different tissues (pectoral muscles, liver, brain, kidneys, blood and feathers) were assessed to investigate their detoxification mechanisms. Three species with contrasted ecological characteristics were studied: the Antarctic prion (zooplankton feeder), the white-chinned petrel (pelagic generalist consumer) and the southern giant petrel (scavenger on seabirds and marine mammals). The difference of mass-dependent fractionation (MDF, δ 202 Hg) values between livers and muscles (up to 0.94 ‰) in all three seabirds strongly suggests hepatic methylmercury (MeHg) demethylation and redistribution of the isotopically heavier fraction of MeHg towards the muscles. Similarly, higher δ 202 Hg values in feathers (up to 1.88 ‰) relative to muscles and higher proportion of MeHg in feathers (94-97%) than muscles (30-70%) likely indicate potential MeHg demethylation in muscle and preferential excretion of MeHg (isotopically heavier) in the growing feathers during moult. The extents of these key detoxification processes were strongly dependent on the species-specific detoxification strategies and levels of dietary MeHg exposure. We also found higher mass-independent fractionation (MIF, Δ 199 Hg) values in feathers relative to internal tissues, possibly due to different integration times of Hg exposure between permanently active organs and inert tissues as feathers. Hg isotope variations reported in this study show evidence of detoxification processes in seabirds and propose a powerful approach for deep investigation of the Hg metabolic processes in seabirds.

The highest mercury concentrations ever reported in a South American bird, the Striated Caracara (Phalcoboenus australis)

“…As feather moult is the primary mechanism through which birds can excrete mercury (e.g. Renedo et al 2021), recently moulted feathers can provide insights into mercury exposure in the previous year. Annual moult occurs during the spring and summer (September to March) in this population (UB, NAL and ARR unpublished).…”

Spatiotemporal Characterization of Mercury Isotope Baselines and Anthropogenic Influences in Lake Sediment Cores

“…The tissues of biota including fish and birds have shown measurable MDF in the bioaccumulated mercury sources caused by metabolic processes (Kwon et al, 2013). The mercury isotope ratios of biota tissues also reflect a mixture of mercury sources, which may have been integrated over time and space (i.e., migratory sites) (Renedo et al, 2018(Renedo et al, , 2021. In fact, deciphering between policy signal and environmental noise has been one of the greatest challenges of evaluating the effectiveness of the MC.…”

Chemical Forms of Mercury in Pilot Whales Determined from Species-Averaged Mercury Isotope Signatures

“…Stable isotope ratios of Hg offer a powerful alternative to spectroscopic and microscopic approaches. 27 Chemical reactions induce a fractionation of isotopes between reactants and products, [28][29][30][31][32][33][34] and therefore Hg metabolic pathways can be investigated if species-specific stable isotope signatures are known. 35,36 The isotopic fractionation of 202 Hg relative to 198 Hg (denoted as δ 202 Hg) is well suited for this purpose because δ 202 Hg can vary by more than 3.5‰ in biological tissues.…”