Total Mercury, Total Selenium, and Monomethylmercury Relationships in Multiple Age Cohorts and Tissues of Steller Sea Lions (Eumetopias jubatus)

Abstract

Steller sea lion (Eumetopias jubatus) tissue mercury concentrations increasingly above thresholds of concern occur in regions of Alaska where lack of population recovery is noted. Selenium-monomethylmercury interactions may mitigate toxicosis but may also result in functional selenium deficiency, impacting essential selenium-dependent processes. Physiologically driven differences in tissue concentrations (organotropism) of total mercury ([THg]), total selenium ([TSe]), and monomethylmercury ([MeHg + ]) confound interpretation for various age cohorts. Archived tissues from Alaska Steller sea lions (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016) were used to compare [THg], [MeHg + ], and [TSe] across age cohorts and tissue types. Liver [THg] ranged from 0.05 to 63.7 µg/g. Fetal and pup livers had significantly lower [THg] and [TSe], higher percentage MeHg + , and greater range of molar TSe:THg than subadult and adult livers. Molar Se:MeHg + ratios, including Se in excess of nonmethylmercury, were dependent on [MeHg + ] in fetuses and pups. While [THg] varied significantly by muscle type (heart vs. skeletal) and anatomical location, concentrations were strongly correlated. Biomagnification and/or bioaccumulation of THg in liver of older animals confounded comparison with other tissues; however, in fetal and pup liver [THg] correlated with other tissues. In contrast, liver [MeHg + ] correlated with other tissues across all age classes. Fetal and pup tissues, which reflect in utero exposure and are subject to limited bioaccumulation, are ideal for assessing mercury exposure related to dam diet, including intertissue comparison, and represent key cohorts of concern. Evaluating [MeHg + ] and [TSe] in tissues from multiple age cohorts allows better intertissue comparison, providing insight into time courses, routes of exposure, and potential for adverse effects.

How small can you go? Using a direct mercury analyzer to measure mercury in vibrissae of Steller sea lions

“…Charapata et al (2023) have recently paired [THg] measured by LA‐ICP‐MS with stable isotope measurements along the vibrissae of leopard seals ( Hydrurga leptonyx ) as a tool to monitor the health of pinniped populations and ecosystem changes, including climate change. There has been a growing body of research on [THg] in North Pacific pinnipeds and their piscivorous prey measured using direct mercury analyzers (Castellini et al, 2022; Cyr, López, Wooller, et al, 2019; Rea et al, 2020). Thus, in this study we investigated the feasibility and reliability of measuring [THg] along the length of Steller sea lion (SSL; Eumetopias jubatus ) vibrissae using a direct mercury analyzer to provide directly comparable [THg] data in this longitudinally grown tissue.…”

“…For both animals, the first vibrissae were cut into approximately 2 mm section lengths and the second vibrissae were cut into approximately 1 mm section lengths. Most methods using a direct mercury analyzer for [THg] determination rely on sample masses of 10–40 mg (e.g., Castellini et al, 2022; Correa et al, 2014; van der Velden et al, 2013). We chose to examine two different section lengths to determine if the section length currently used for stable isotope analysis (1 mm) in this species provided sufficient mass for detection of [THg], or if a larger section length provided more reliable and repeatable data.…”

Traceable and continuous flow calibration method for gaseous elemental mercury at low ambient concentrations

“…Nguyen et al, 2019). Deposited GOM can be subsequently methylated to the most toxic Hg species, monomethyl mercury (Castellini et al, 2022). Assessment and monitoring of chemical transformations and physical processes require high…”