Humans are affecting environmental change on local and global scales. This degree of change is troubling, as it involves material that is harmful and toxic to ecological systems as well as human health and well-being. Mercury is one such substance that is a particular concerning, because of its global circulation. Released to the environment through mining, fossil fuel combustion, and various other industrial activities, mercury travels through a global transport pathway and it doesn’t degrade in the environment. In fact, microorganisms in the environment transform mercury into an even more toxic form, methylmercury, a potent neurotoxin.
Methylmercury moves up the food web—or biomagnifies—from the microorganisms that are responsible for methylating inorganic mercury in aquatic systems up through fish as well as into terrestrial animals, including humans. This pattern is primarily due to dietary exposure or trophic transfer. Because it is physiologically difficult to eliminate mercury once it has been assimilated into tissue, methylmercury bioaccumulates, and when an animal is eaten by another, its burden of mercury is passed onto the consumer.
Due to the widespread deposition of mercury across landscapes, our government has issued fish consumption advisories for a staggering 42% of national lake acreage area and 36% of national rivers miles (EPA data from 2011). Consumption advisory guidelines are heightened for pregnant or nursing women and children because developing nervous systems are particularly susceptible to toxic injury. It is important to note, however, that fish can be a very important part of a balanced diet, providing a source of protein, essential nutrients, and omega-3 fatty acids, and should not necessarily be excluded outright. Additional information for those interested can be found at the EPA’s website.
There are many angles to explore scientifically in regards to methylmercury contamination and exposure and, fortunately, there is a growing wealth of knowledge on the subject. Although a significant effort is dedicated to human routes of exposure, little attention is given to the animals, other than fish, that are responsible for the movement of mercury through ecological systems. The health status of these other animals may have important implications for the transfer of mercury from one animal to another. My research, as part of Dr. Kathryn Cottingham’s Lab in the EEES graduate program, is focused on how environmental neurotoxins affect animal behavior. The main prediction behind my research is that behavioral impairments may modify ecological relationships. Specifically, if an animal’s ability to detect and respond to predation threats in their environment has been impaired by toxic stress (in this case, mercury), they may be at higher susceptibility of being consumed—thus, passing their contaminant burden to the consumer.
The Alumni Research Award supported the first chapter of my PhD dissertation research: the longitudinal assessment, or monitoring change through time, of crayfish behaviors that relate to defense and anti-predation after continuous exposure to sublethal dietary methylmercury exposure. Crayfish are an ideal system to look at the interface of neurobiology, behavior, and toxicology, because they have been a focal organism for foundational neurobiological studies. In fact, the neurological circuit controlling their characteristic tail flip swimming behavior has been described in detail. Shellfish consumers know, the tail is the tastiest part of the crayfish! Therefore, this tissue would contribute substantially to the dietary transfer of matter, energy, and mercury! The funding allowed me to send crayfish tail muscle tissue samples to Dr. Brian Jackson’s Trace Elements Lab at Dartmouth. Dr. Jackson’s facility received pre-dissected and prepared tail muscles for mercury analysis. This analysis included samples from experimental animals but also animals collected in the wild to determine background mercury contamination in crayfish from the White River watershed in Vermont. These analyses are ongoing—keep an eye out for a publication soon!
An indirect benefit of receiving funding from the Graduate School was, not only the ability to pay for samples, but also being able to outsource them to another internal lab. This freed up time that would have otherwise been dedicated to very solitary (but essential!) lab work. Instead, I was able to be in our lab more often working directly alongside a team of undergraduates who worked on this project. I was very fortunate to have a team of dedicated undergraduate researchers and volunteers; without their support this project would not have been possible! Because of their participation in my project, I would like to offer a very special thanks to Francine Mejia ’18, Rebecca Flowers ’19, Phoebe Cunningham ’20 and, Alexandra Urquiza ’20, the Office of Undergraduate Advising and Research (UGAR), and the Dartmouth Women in Science Program (WISP), in addition to the Graduate School and the Alumni Research Award fund.