Mercury levels in Pacific fish will likely continue rising for decades, a new study warns, based on a groundbreaking discovery of how the toxin infiltrates open-ocean ecosystems.

Researchers from Michigan and Hawaii have spent years investigating how a toxic compound called methylmercury winds up in moonfish, mahi mahi and other North Pacific species. Their findings, published this week in the journal Nature Geoscience, point to a bizarre collaboration between deep-sea bacteria and distant power plants, hinting at potential solutions but also suggesting the problem will only worsen in the near future.

"This study reinforces the links between mercury emitted from Asian countries and the fish that we catch off Hawaii and consume in this country," says lead author Joel Blum, an environmental scientist at the University of Michigan, in a statement about the research.

Up to 80 percent of all methylmercury in deep-feeding Pacific fish was produced well below the surface, the study found, most likely by bacteria clinging to organic debris as it sinks. But the problem begins much farther away: Using the chemical signature of mercury in fish tissue samples, Blum and his colleagues confirmed the toxin traveled thousands of miles by air before raining into the ocean, pointing to origins in Asia. The North Pacific fisheries are downwind from China and India, two fast-growing nations that increasingly burn coal — a major source of mercury emissions — to generate electricity.

Scientists have long known this mercury tends to fall and wash into water bodies, where certain microbes convert it to methylmercury. Since methylmercury lingers in body tissue, it then "biomagnifies" at higher and higher levels as it moves up the food chain. That's why big, predatory fish tend to be more mercury-laden than smaller fish, and why they're the main source of human exposure to methylmercury. The toxin can trigger debilitating neurological effects in humans, especially in fetuses, infants and young children.

But in 2009, University of Hawaii researchers made a confusing discovery: Deep-feeding pelagic fish contain more mercury than surface feeders, and feeding depth is nearly as relevant as food-chain rank to a species' contamination level. "We knew this was true," says University of Hawaii researcher Brian Popp, a co-author of the 2009 and 2013 studies, "but we didn't know why." Since biological activity is typically highest in the surface layer, scientists thought any open-ocean methylmercury production would happen there.

The new research, however, shows that up to 80 percent of North Pacific methylmercury is produced below the "surface mixed layer," which extends about 165 feet deep. Microbes are churning out the toxin as deep as 2,000 feet, suggesting it's the handiwork of anaerobic bacteria that cling to sinking bits of dead plant and animal matter. Thanks to faraway coal-fired power plants, that organic debris may also contain inorganic mercury, which is later converted to methylmercury in the feeding territory of deep-diving fish.

Not only does that help explain how mercury is getting into North Pacific wildlife, but it also hints at more contamination down the road. That's because Pacific mercury levels are already forecast to rise at intermediate depths in coming decades, possibly doubling by 2050. On top of that, the low-oxygen zones where anaerobic bacteria thrive are expanding globally, a process that's expected to worsen due to manmade climate change.

The immediate outlook may be discouraging, but Blum says the study could help ongoing efforts to curb global mercury emissions. The U.S. unveiled tougher mercury rules in 2011 after decades of debate, and a newly negotiated U.N. treaty aims to rein in future emissions from a variety of countries. As with greenhouse gases, rapidly industrializing economies like China and India are now some of the planet's top mercury emitters.

"The implications are that if we're going to effectively reduce the mercury concentrations in open-ocean fish, we're going to have to reduce global emissions of mercury, including emissions from places like China and India," Blum says. "Cleaning up our own shorelines is not going to be enough. This is a global atmospheric problem."

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