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Persistent Pollutants

Polar bear poop provides clues to how pollutants build up in the body

A noninvasive method provides hints at how predators hoard contaminants from their food

by Carolyn Wilke, special to C&EN
June 7, 2022

Polar bear lying on its side on a rock
Credit: Toronto Zoo
Scientists collected samples of polar bear feces and food from the Toronto Zoo to measure how the animals absorbed polychlorinated biphenyl contaminants.

Predators’ bodies stash potentially harmful contaminants passed up the food chain from their prey. But studying the accumulation of these chemicals usually requires taking a chunk of tissue from an animal, which can’t be done for people or many endangered species. Now scientists have come up with a noninvasive alternative: By testing polar bears’ food and feces, they’ve gained insight on how these animals accumulate contaminants (Environ. Sci. Technol. 2022, DOI: 10.1021/acs.est.2c00310).

Biologists have a qualitative understanding of bioaccumulation in top predators such as bald eagles, killer whales, and polar bears but not a good explanation of how it happens, says Melissa A. McKinney, an ecotoxicologist at McGill University who was not involved in the study. Scientists have proposed several mechanisms by which chemicals may be absorbed from animals’ guts, such as diffusion from the intestine and micelle-mediated transport. With this work, she says, “we have a much better handle on the mechanism.”

University of Toronto Scarborough environmental chemists Frank Wania and Yuhao Chen and colleagues developed a sensing technique to study how the chemical environment of the gut may play a role in bioaccumulation. The researchers analyzed feces from polar bears at the Toronto Zoo, as well as samples of their food, and needs a fair amount of sample to work. Luckily, “polar bears eat a lot and poop a lot every day,” Chen says.

Chen and his colleagues placed samples of food or feces in jars lined with a layer of silicone and slowly rotated the jars with a roller mixer for up to 2 weeks. The contaminants absorbed into the silicone, reaching chemical equilibrium between the lining and the sample. Then the team measured the amount of polychlorinated biphenyl (PCB) pollutants in the silicone using gas chromatography-mass spectrometry. This provided a way to measure the fugacity, or potential for a given contaminant to move from one environment to another—for instance, from the food and into the gut. They also measured the lipid content of the food and feces samples to get a better sense of what drives contaminants to move.

The team determined that polar bears extract a whopping 99% of the lipids from their food. Some of what polar bears eat, such as seals or seal oil for these zoo bears, is contaminated with PCBs. PCBs like to hang around fats, so the depletion of lipids from the bears’ foods might provide the driving force for PCBs to diffuse from the bears’ guts and into other parts of their bodies.

This paper is the first to directly measure how differences in fugacity may drive diffusion inside a mammal during digestion, says Michael S. McLachlan, an environmental chemist at Stockholm University. But he notes that the chemical environment of the feces samples might differ from when they are in the polar bear’s gut.

“The bear is like a trap for these lipid-associated chemicals,” says Wania, who led the study. That combination of a diet high in lipids and a lipid-hoarding body makes polar bears particularly vulnerable to building up pollutants. This method could be applied to study bioaccumulating contaminants that are less understood, such as per- and polyfluoroalkyl substances (PFAS), McKinney adds.

Wania’s team is hoping to do similar bioaccumulation measurements with people. “Humans are very often at the end of the food chain, so it’s important for us,” Wania says. He expects that people may vary more in their efficiency at taking up lipids than polar bears, so it may be a challenge to understand what drives bioaccumulation in individuals.


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