A strange whistling hum fills the air in Bourne. It’s a tell-tale sign that hundreds of periodical cicadas are all around, in the trees and crawling up bushes, hoping to attract a mate after 17 years underground.
The bugs are part of Brood XIV, group of periodical cicadas that emerged recently on the Cape and across a large swath of the eastern U.S.
It’s more than their reappearance after nearly two decades that makes the insects special: These cicadas may hold clues to unravel big mysteries about mercury.
Making sense of the toxin
There is a lot scientists don’t understand about mercury — namely, how it becomes its most toxic form and why our bodies don’t get rid of it.
All forms of mercury are toxic, and humans and animals have evolved to flush toxins out of their systems, including most forms of mercury.
But bodies can’t seem to expel the methylmercury, which is the element’s most toxic form. Because of this, the compound builds up in the food web, leading to high mercury levels in fish.
At high concentrations, methylmercury can cause neurological damage in humans. Parents can pass mercury to their babies during pregnancy, leading to developmental delays and growth deficits.
“ Despite being the oldest element ever studied in history, we don’t really understand how it became this toxic compound. The chemistry of it doesn’t make sense,” said Laura Motta, an assistant scientist at the Woods Hole Oceanographic Institution specializing in marine chemistry. “So it showcases how little we truly understand our environment and how much is left to be known of it.”
Mercury exists naturally in the environment. Volcanoes emit mercury into the air. But mercury is also released into the environment from human sources, like coal-fired power plants, waste burning, mining and industrial production. A 2023 Harvard study found that emissions from humans have increased the presence of mercury in the atmosphere sevenfold from its natural state.
These mercury emissions can travel the globe and get absorbed into the ocean. But that form of mercury is not methylmercury, most toxic form. And scientists don’t understand what makes the mercury transition to its highly toxic form.
Motta is hoping cicadas can help piece that together.
How cicadas fit in
Motta doesn’t generally study bugs. Her work focuses primarily on mercury in plankton and the ocean. But she says cicadas are an analogue to zooplankton, tiny aquatic animals like krill, in the food web.
She began collecting cicadas as a postdoctoral researcher in Maryland in 2021. She had read earlier research that indicated periodical cicadas could have high mercury levels and decided to collect a sample of the insects to study when she had her own lab.
Now with a lab at WHOI, Motta has collected cicadas from different broods in Maryland, Illinois, Michigan and Alabama. The Cape Cod collection is the last sample before Motta and her team begin analyzing mercury levels.
The cicadas aren’t hard for the scientists to find. Their inch-long black bodies and bright red eyes are visible on almost any tree in this woodsy area near a housing development.
Research assistant Corinne Richard holds a live adult cicada next to her ear and gives it a shake. She listens, waiting to hear if it makes a hum, signifying that it’s a male. When the bug stays silent, she drops it into a plastic bag labeled “female.”
Nearby, Motta plucks papery cicada exoskeletons off a branch and places them in a bag.
“By using the cicadas, we’re trying to figure out how much mercury they gained while they were underground feeding from the tree? What happens after they molt? Where does the mercury go?” said Motta. “If you think of a food web, how much would a bird that would eat them get?”
In a few weeks, the researchers will be back in the field. They’re aiming to collect newly hatched cicada nymphs as they make their way down to the ground for their 17-year stay. Motta is hoping that they can figure out how much toxic mercury a female cicada passes to its offspring, which can help understand how humans pass mercury to babies during pregnancy.
Once back in the lab, researchers will wash the insects and chill them in a freezer kept at -30 Celcius (about -22 Fahrenheit). Then, the bugs will be ground up into samples, separated by sex. The fine-ground cicadas are then tested for mercury levels.
They will also freeze, grind and test cicada exoskeletons. When cicadas emerge from underground, they shed an exoskeleton and transition into adulthood. By analyzing the shells left behind, Motta is hoping to see how much mercury cicadas expel in a molt, and whether they rid any of the most toxic form.
Key to the research are the isotopes of mercury, which help scientists track the history of elements.
“It’s like fingerprints that are stuck, frozen in time, in a geological example, that could tell us a story of the chemistry that took place when that element got in there,” said Motta.
In looking at the mercury isotopes, Motta and her team hope to piece together where the element came from and when it changed from one form of mercury to another.
This study is a part of a broader effort to tamp down on toxic mercury pollution. The United States is among the 152 nations signed on to the United Nations’ Minamata Convention, which works to reduce mercury pollution and understand where it it goes.
And the work comes at a key moment: Mercury build up in fish is only predicted to increase as climate change advances.
Melting arctic ice is likely to release more long-stored mercury into the ocean. And study from 2019 showed that as water temperatures warm, fish are likely to eat more, creating higher toxic mercury concentrations in their bodies.
But Motta says without understanding the chemistry, scientists won’t be able to accurately predict these increases.
“ Unless we understand,” she said, “we won’t be able to know how to prepare for this.”
This article was originally published on WBUR.org.
Copyright 2025 WBUR