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April 28, 2025 This article has been reviewed according to Science X's editorial process and policies . Editors have highlightedthe following attributes while ensuring the content's credibility: fact-checked peer-reviewed publication trusted source proofread by Krisy Gashler, Cornell University What if counteracting mercury contamination in waterways could be as simple as adding selenium? That proposition has gained traction among some scientists and governments, but new research from Cornell and the U.S.
Geological Survey (USGS) suggests the idea is likely too good to be true. In the new study , published April 16 in Environmental Science & Technology , researchers exposed mayflies to varying levels of mercury and selenium to test how a species at the base of the food web would respond. Researchers found that at low levels of mercury, selenium additions did seem to help mayfly larva avoid accumulating mercury.
But at high mercury levels—the condition in which environmental remediation is most needed—selenium actually made mercury accumulation worse. "Mayflies are what's called the primary consumer—they're the first thing eating the aquatic plants contaminated with mercury and pulling that contamination into the rest of the food web," said Jacqueline Gerson, assistant professor of biological and environmental engineering in the College of Agriculture and Life Sciences and first author of the paper. "Understanding what's happening at the base of the food web has implications for all the organisms above it, including fish and, ultimately, the humans consuming the fish.
" Especially in fetuses and young children , exposure to mercury, a potent neurotoxin , can harm the nervous system , kidneys, lungs and other organs. Previous research on larger fish species that humans consume had found that the trace mineral selenium counteracts the effects of mercury, reducing its toxicity. That finding led to calls to explore the possibility of treating mercury-contaminated waterways with selenium.
But other research by the same authors also found that the more selenium mayflies were exposed to, the more flies died and the longer they took to emerge from their larval stage, among other negative impacts. Globally, the biggest sources of mercury contamination are the burning of fossil fuels and gold mining. For their study, researchers used mercury levels measured in nature.
The high levels mimicked concentrations found in ponds near gold mines in Peru, and the low levels replicated concentrations measured in bodies of water in the U.S. without any particular pollution source nearby.
The researchers also explored how mercury contamination changes over mayflies' life stages. Like mosquitoes and dragonflies, mayflies hatch from eggs laid in the water, go through an aquatic larval stage and emerge as flies. The research found that even though mayflies leave behind other contaminants in their larval stage , they retain mercury as flies.
"We've primarily been worried about mercury bioaccumulation in aquatic ecosystems , but what we found is that, no matter what, the flies always had higher mercury concentrations than the larva did," Gerson said. "That means terrestrial predators, like frogs, snakes and birds, are being exposed to more mercury than even the aquatic predators. So these emergent insects are a way to basically shunt the contaminant from an aquatic ecosystem into a nearby terrestrial ecosystem.
" Future research should seek to understand more about how these elements interact at the food web base and microbially, to determine how beneficial interactions could be encouraged and to prevent potentially harmful interventions in nature, Gerson said. "Based on the research we have right now, adding selenium to mercury-contaminated systems would be premature and could lead to really large negative impacts on the aquatic ecosystems," she said. "The consequences are so severe, I wouldn't see any circumstance where we could safely say we should be remediating in this way at the moment.
" More information: Jacqueline R Gerson et al, Selenium Differentially Influences Methylmercury Retention across Mayfly Life Stages, Environmental Science & Technology (2025). DOI: 10.1021/acs.
est.5c00338 Journal information: Environmental Science & Technology Provided by Cornell University.
