bioaccumulation

Mercury in the Arctic: An Invisible Threat to Polar Bears

Mercury is the famous metal that is liquid at room temperature that was commonly found in old fashioned thermometers. While it is cool to look at in chemistry class, mercury is very toxic to humans as well as animals causing damage to your brain and nervous system. Mercury pollution in the environment is mostly as a result of industrial activity such as burning coal, releasing into the air, and then depositing in aquatic ecosystems in the form of methylmercury (Dietz et al. 2013). Methylmercury eventually finds itself in fatty tissues of aquatic species and builds up as it is consumed over its lifetime, which is described as bioaccumulation (Branco et al. 2021). This is why you often hear warnings from doctors to watch your intake of ocean fish in order to avoid excessive mercury intake.

While the melting of ice caps due to global warming is the primary concern for the conservation of polar bears (Ursus maritimus), accumulation of contaminants in its body are a close second. Polar bears are a top predator in the Artic region of the planet, their diets mostly consisting of fatty seals and fish. This diet rich in fat causes them to be exposed to extremely high concentrations of contaminants of chemicals such as mercury as it travels up the food chain (Routti et al. 2013). Studies suggest that this accumulation of contaminants will cause polar bears to have weakened immune systems, making them vulnerable to disease (Routtie et al. 2013).

Mother and cubs on a small ice floe, taken near Svalbard. Image courtesy of Russel Millner and National Geographic Your Shot (2021).

High levels of mercury, in particular, can have many negative effects on polar bears. The primary danger of mercury is the risk of neurotoxicity, which is when the nervous system is damaged by toxins in the brain and brain stem (Branco et al. 2021). This can result in an impairment of many brain functions, which can impact the success of hunting. Prolonged high levels of mercury toxicity can impair mobility and the senses and can even result in the death of the polar bear (Branco et al. 2021). Studies show that developing mammals’ growth is also impaired when exposed to high levels of mercury (Branco et al. 2021).

Data shows that mercury concentrations in Arctic animals have increased over the last 150 years, showing that human activity is likely to blame (Dietz et al. 2013). This shows how our carbon emissions are continually discovered to have additional negative effects on our planet. Polar bears are amazing animals and as this blog shows, our damage to the environment is hurting and will continue to hurt them if we do not take action.

References:

Branco, V., Aschner, M., & Carvalho, C. (2021). Neurotoxicity of Mercury: An old issue with contemporary significance. Neurotoxicity of Metals: Old Issues and New Developments, 239–262. https://doi.org/10.1016/bs.ant.2021.01.001

Dietz, R., Sonne, C., Basu, N., Braune, B., O’Hara, T., Letcher, R. J., Scheuhammer, T., Andersen, M., Andreasen, C., Andriashek, D., Asmund, G., Aubail, A., Baagøe, H., Born, E. W., Chan, H. M., Derocher, A. E., Grandjean, P., Knott, K., Kirkegaard, M., … Aars, J. (2013). What are the toxicological effects of mercury in Arctic biota? Science of The Total Environment, 443, 775–790. https://doi.org/10.1016/j.scitotenv.2012.11.046

Millner, R. (2021). Will There Be Ice When We Grow Up? National Geographic Your Shot. National Geographic. Retrieved March 7, 2022, from http://yourshot.nationalgeographic.com/photos/2348856/.

Routti, H., Atwood, T. C., Bechshoft, T., Boltunov, A., Ciesielski, T. M., Desforges, J.-P., Dietz, R., Gabrielsen, G. W., Jenssen, B. M., Letcher, R. J., McKinney, M. A., Morris, A. D., Rigét, F. F., Sonne, C., Styrishave, B., & Tartu, S. (2019). State of knowledge on current exposure, fate and potential health effects of contaminants in polar bears from the Circumpolar Arctic. Science of The Total Environment, 664, 1063–1083. https://doi.org/10.1016/j.scitotenv.2019.02.030

Microplastics in Fish: Not a Microproblem

Plastics are everywhere. We humans use them in everything – from toys to food packaging to medical supplies and beyond. As a manmade material, plastics do not readily break down in the environment once we are finished using them. To make matters worse, there is an abundance of teeny, tiny pieces of plastics in our world. These tiny bits can occur as a byproduct of the production of plastic goods, or they may be a result of litter in the environment breaking down into smaller and smaller pieces (Barboza et al., 2020). Because they are so small, they spread easily. Today, little pieces of plastic are found everywhere: in our soil, on our beaches, in rivers and oceans, and even scarier… inside the bodies of animals.

“Microplastic” by Oregon State University is marked with CC BY-SA 2.0. To view the terms, visit https://creativecommons.org/licenses/by-sa/2.0/?ref=openverse

These miniscule bits of plastic measuring less than 5 mm are also known as microplastics, and they truly are ubiquitous in today’s environment. (So much so that they are now considered a contaminant of concern on a global scale (Barboza et al., 2020).) Ingestion of microplastics has been documented in over 700 marine animal species, including sea turtles, whales, dolphins, and fish (Wootton, 2021). This either happens when animals mistake microplastics as food and ingest them by accident, or they ingest another smaller prey organism that also has microplastics inside of its body (Wootton, 2021). Fish may also take in microplastics passively as they filter contaminated water through their gills. As a result, microplastics have been found in the digestive tracts, muscle tissues and even the gills of fish (Barboza et al., 2020).

So why should we care if fish are accumulating small particles of plastic in their bodies? Recent studies have demonstrated that there are multiple toxic impacts of microplastic ingestion in fish. These impacts include impaired development, decreased feeding and body mass (Naidoo and Glassom, 2019), damage to cells, changes in behavior, impaired reproductive capacity, and even death (Barboza et al., 2020). There are documented instances of neurotoxicity (or damage to the nervous system) as a result of microplastic ingestion in fish. Oxidative stress, or an imbalance of antioxidants and free radicals in the body, has also been found to result from accumulation of plastics in the body, and may lead to cell and tissue damage in fish (Barboza et al., 2020). All of these impacts have the potential to harm the overall population of a particular species of fish, and ultimately alter food webs.

Conceptual model illustrating capture, retention and internalization of microplastics by fish species (Barboza et al., 2020).

If none of that grabbed your attention, perhaps this will: since microplastics have been found in the edible muscle tissues of fish, humans are also at risk of accumulating small bits of plastic in their bodies after eating a fish meal. Further studies into human risk assessment of microplastic ingestion are warranted, and perhaps microplastic daily intake limits may be in our future once the research is more solid (Barboza et al., 2020). Also, if food webs are altered enough by reduced populations of fish impacted by microplastics, maybe your favorite type of fish will be a lot harder to come by in the grocery store years down the line. For now, take note next time you are on a walk around your neighborhood and see the tiny pieces of a broken up plastic bottle cap – think about the impacts to the fish in a nearby waterway once a heavy rain washes those microplastics downstream. If nothing else, perhaps this thought will motivate each of us to choose to use less plastic in some capacity in our daily lives.

 

References:

Barboza LG, Lopes C, Oliveira P, Bessa F, Otero V, Henriques B, Raimundo J, Caetano M, Vale C and Guilhermino L. (2020) Microplastics in wild fish from North East Atlantic Ocean and its potential for causing neurotoxic effects, lipid oxidative damage, and human health risks associated with ingestion exposure. Science of the Total Environment 717:1-14.

Naidoo T and Glassom D. (2019) Decreased growth and survival in small juvenile fish, after chronic exposure to environmentally relevant concentrations of microplastic. Marine Pollution Bulletin 145:254-259.

Wootton N, Reis-Santos P and Gillanders BM. (2021) Microplastic in fish – A global synthesis. Rev Fish Biol Fisheries 31:753-771.

Microplastics Impact on Northern Fulmar Birds

I have attached screenshots of my Twitter thread looking at how Northern Fulmar birds are negatively impacted by microplastics in the ocean. This thread discusses how these birds commonly ingest these small plastics, and how it impacts the birds physiologically, such as endocrine disruption. There are so many ways to reduce our use of single-use plastic, and I’m hoping that this thread will bring about more awareness to this issue!

All photos are from the references listed below or CreativeCommons.

References

Costa M,  Ivar do Sul J (2013) The Present and Future of Microplastic Pollution in the Marine Environment. El Sevier, Environmental Pollution, 185: 352-364.

Galloway T, Lewis C (2016) Marine Microplastics Spell Big Problems for Future Generations. PNAS, 113 (9): 2331-2333

Herzke D et al. (2016) Negligible Impact of Ingested Microplastics on Tissue Concentrations of Persistent Organic Pollutants in Northern Fulmars off Coastal Norway. Environmental Science & Technology, 50 (4): 1924-1933.

Law KL, Thompson RC (2014) Microplastics in the Seas. Science, 345 (6193): 144-145.

Obbard R et al. (2014) Global Warming Releases Microplastic Legacy Frozen in Arctic Sea Ice. Earth’s Future, 2 (6): 315-320.

Bioaccumulation & Biomagnification: A Heavy (Metals) Topic

In 1962 Rachel Carson published what is now recognized as one of the most important books in conservation literature, Silent Spring. Based on original research conducted in response to reports linking an observed decline in bird populations with widespread use of DDT as a pesticide in the 1950’s, the book broadly asserted that liberal use of DDT was contributing to considerable detrimental impact on the environment through trophic interactions. Although the book was met with considerable opposition from the chemical industry and lobbyists at the time, the academic community, along with the general public, defended the work. The book also emboldened the environmentalist movement, and eventually resulted in a ban on DDT in 1972, following president Nixon’s creation of the Environmental Protection Agency (EPA).

Bioaccumulation is a phenomenon that occurs when chemicals or toxins build in the tissues of organisms over time. One of the key problems with DDT is that at low concentrations it appeared to be harmless to vertebrate organisms, while being lethal to common pest invertebrates such as mosquitoes and flies. Thus, the negative effects of DDT on larger organisms such as mammals and birds were not initially apparent. DDT is not easily broken down through the metabolic pathway, yet readily passes across the gastrointestinal barrier. Organisms that consume organic material containing DDT will therefore accumulate the chemical in their fat stores, until eventually concentrations become lethal. However, such compounds can also be “passed up” the food chain in a process known as biomagnification. Insects such as mosquitoes that die from DDT exposure may find their way into water, where they are consumed by fish. Those fish then sequester the chemical in their tissues, and may then be consumed by larger predators such as eagles, osprey and falcons (Carson, 1962).

Although DDT has been banned for use in the U.S. except for in emergency situations, there are a number of other chemicals and compounds that have been known to bioaccumulate and biomagnify. Heavy metals such as mercury (Hg) are highly toxic to most living organisms. In particular, mercury causes permanent damage to a class of molecules called thioredoxin reductases (Carvalho, et. al., 2008), enzymes that are essential for proper cell growth and in counteracting oxidative damage from metabolic activity (Linster & Van Schaftingen, 2007). In an ecosystem-level study in Connecticut, it was shown that increases in mercury concentrations of fish were correlated with body size and age. Additionally, predators occupying the top of the food chain accumulated mercury the fastest, regardless of species identity (Neumann & Ward, 1999). Similar patterns of bioaccumulation and biomagnification have been observed in sharks in marine ecosystems (Maz-Corrau, et al., 2012), as well as montane stream ecosystems (Chasar, et al., 2009).

However, bioaccumulation of pollutants in aquatic ecosystems may have potentially devastating consequences for non-aquatic organisms as well. In particular, fish-eating sea birds are very susceptible to poisoning from industrial pollutants, as some compounds such as polychlorinated biphenyls (PCBs) are remarkably persistent in tissues (Walker, 1990). In a study conducted  at Kesterson Reservoir in California, bioaccumulation of selenium (Se) was observed in several species of birds, and correlated with reductions in adult body weight, and embryonic mortality (Ohlendorf, et al., 1990).

But bioaccumulation doesn’t just stop at birds and fish. Contamination of fisheries with metabolically stable compounds and chemicals has been reported across the globe, from Romania (Bravo, et. al., 2010) to China (Feng, et al., 2007). Even in the United States there is mounting concern that deposition of atmospheric heavy metals from industrial manufacturing and coal-fired power plants can reach fisheries via hydrologic processes such as runoff, and eventually affect human health (Driscoll, et al., 2007). After all, humankind has in many ways found its way to the top of the global food chain. So while the effects of bioaccumulation may seem a distant or alien concern in our isolated human ecosystem, it may not be long before heavy metals make their way onto our dinner plate.

Sources:

Bravo, A. G., Loizeau, J. L., Bouchet, S., Richard, A., Rubin, J. F., Ungureanu, V. G., … & Dominik, J. (2010). Mercury human exposure through fish consumption in a reservoir contaminated by a chlor-alkali plant: Babeni reservoir (Romania). Environmental Science and Pollution Research, 17(8), 1422-1432.

Carson, R. (1962). Silent spring. Houghton Mifflin Harcourt.

Carvalho CM, Chew EH, Hashemy SI, Lu J, Holmgren A (2008). “Inhibition of the human thioredoxin system: A molecular mechanism of mercury toxicity.”. Journal of Biological Chemistry. 283 (18): 11913–11923.

Chasar, L. C., Scudder, B. C., Stewart, A. R., Bell, A. H., & Aiken, G. R. (2009). Mercury cycling in stream ecosystems. 3. Trophic dynamics and methylmercury bioaccumulation. Environmental science & technology, 43(8), 2733-2739.

Driscoll, C. T., Han, Y. J., Chen, C. Y., Evers, D. C., Lambert, K. F., Holsen, T. M., … & Munson, R. K. (2007). Mercury contamination in forest and freshwater ecosystems in the northeastern United States. BioScience, 57(1), 17-28.

Feng, X., Li, P., Qiu, G., Wang, S., Li, G., Shang, L., … & Fu, X. (2007). Human exposure to methylmercury through rice intake in mercury mining areas, guizhou province, china. Environmental science & technology, 42(1), 326-332.

Linster, C.L.; Van Schaftingen, E. (2007). “Vitamin C: Biosynthesis, recycling and degradation in mammals.”. FEBS Journal. 274 (1): 1–22.

Maz-Courrau, A., López-Vera, C., Galvan-Magaña, F., Escobar-Sánchez, O., Rosíles-Martínez, R., & Sanjuán-Muñoz, A. (2012). Bioaccumulation and biomagnification of total mercury in four exploited shark species in the Baja California Peninsula, Mexico. Bulletin of Environmental Contamination and Toxicology, 88(2), 129-134.

Neumann, R. M., & Ward, S. M. (1999). Bioaccumulation and biomagnification of mercury in two warmwater fish communities. Journal of Freshwater Ecology, 14(4), 487-497.

Ohlendorf, H. M., Hothem, R. L., Bunck, C. M., & Marois, K. C. (1990). Bioaccumulation of selenium in birds at Kesterson Reservoir, California. Archives of Environmental Contamination and Toxicology, 19(4), 495-507.

Walker, C. H. (1990). Persistent pollutants in fish-eating sea birds—bioaccumulation, metabolism and effects. Aquatic Toxicology, 17(4), 293-324.

 

Images (In Order of Appearance):

1st edition copy of Rachel  Carson’s Silent Spring. (https://thebreakthrough.org/archive/silent_spring_environmentalism_and_fear)

Illustration of the process of bioaccumulation and biomagnification. World Wildlife Fund. (http://sustainable-nano.com/2013/12/17/the-cautionary-tale-of-ddt-biomagnification-bioaccumulation-and-research-motivation/)

American Coot, one of the many species detrimentally impacted by bioaccumulation of selenium (https://www.allaboutbirds.org/guide/American_Coot/id)

Coal fired power plant. Shutterstock. (https://thinkprogress.org/7-out-of-10-americans-want-their-states-to-comply-with-the-epas-climate-plan-8ad8f9fe8e74)