The Wildlife Toxicology Laboratory at the University of Alaska, Fairbanks (UAF) considers it a great opportunity and privilege to work with the Sarasota Dolphin Research Program as we investigate mercury (Hg) in our ocean systems. Working with colleagues who are experts in marine mammals offers a window (sentinel species) for us, especially when high quality samples are obtained in a well studied population using exceptional handling techniques. This research into Hg in our oceans is similar to some of our efforts in Alaska.
We have addressed aspects of Hg in dolphins in the past related to their feeding ecology and changes in their outer skin (epidermis). Our investigations have turned to addressing Hg and selenium (Se, an essential element) concentrations in blood compartments. We have determined concentrations in serum, plasma, whole blood, and packed cells (e.g., “blood compartments”). We do this in part because Se has been hypothesized to be protective against the effects of Hg, but is likely dependent on species and tissue-type. The dogma considers an abundance of Se relative to Hg as potentially protective or ameliorating to the adverse effects to Hg. In this study we examine Hg and Se on a mass (weight) and molar (number of atoms, more of a count) concentration basis in various blood compartments of bottlenose dolphins to determine how these elements partition in blood and the potential for specific compartmental interactions. One can appreciate that the count of the Hg and Se atoms tells us more about how they interact as elements, especially when we determine the ratio of Se:Hg. However, when we discuss nutritional status and potential toxicity we use mass (ppm) as our measure and since Hg and Se have different atomic weights we need to consider both measures (mass and count).
The distribution within the blood compartments of Se and Hg can provide insight on the potential association of Hg with selenoproteins (Se binding sites, essential enzymes that require Se, etc.) in blood. For concentrations of both Hg and Se, serum=plasma < whole blood < packed cells (e.g., whole blood has lower concentrations than packed cells), however there was a higher proportion of Se in serum and plasma compared to Hg. The Se:Hg molar (count, not mass) ratio was greater than 1.0 in all compartments, with the highest ratios found in serum and plasma and the lowest in whole blood and packed cells. This indicates an abundance of Se relative to Hg in these compartments. As expected age of the dolphin was positively correlated to the concentration of Hg in all blood compartments and to Se concentration in serum, plasma and whole blood only. Age was negatively correlated to Se:Hg molar ratios in all blood compartments indicating a relative change where protective mechanisms may be decreased with aging animals (speculation as functional measures are required to better assess this). The feasibility of calculating packed cell concentrations of Hg and Se using hematocrit (routine measure of blood cellular components relative to whole blood volume) measurements in combination with whole blood and plasma concentrations was validated, making it possible to routinely assess compartmentalization of Hg and Se within erythrocytes (red blood cells) using common clinical measurements.
This research, combined with other animal models, is advancing our understanding of how a toxicant and nutrient interact. Concern is driven by observations in some marine systems that Hg concentrations may be increasing over time. One question is “Will Se and other protective systems continue to offset the potential toxicity of Hg in fish consumers?” Marine mammals may offer some insights into these mechanisms and potential effects, including how nutrients may protect from toxicity.
This article was published on page 6 in the January 2013 Nicks n Notches.