Predatory fish that feed deeper in the Pacific Ocean, including swordfish, accumulate higher levels of dangerous mercury than piscine predators at the surface, such as yellowfin tuna, but biologists weren’t sure why. Environmental scientists recently uncovered the cause and warned that the threat may increase in the future.

Oceanic bacteria transform the element mercury into toxic methylmercury. At the surface, sunlight destroys up to 80 percent of the methylmercury produced by these bacteria, according to a recent study published in Nature Geoscience. However, in murky waters between 165 to 2,000 feet (50-610 meters) below the ocean’s surface, methylmercury remains in the food web as bacteria devour mercury-contaminated detritus sinking from the surface.

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Those bacteria become food for other creatures and mercury thereby spreads through the Pacific’s food web. Methylmercury accumulates higher in the food web because prey pass on mercury contamination to predators, which lack a biological means to dump the poison.

Humans sit at the top of the oceans’ food web and accumulate methylmercury after making lunch of oceanic predators, such as mahi-mahi fish. Mercury itself harms humans, but people face the greatest threat from methylmercury consumption because our bodies lack sufficient defenses against the toxin, according to a U.S. Geological Survey fact sheet. Severe exposure can kill a person, while lower-level doses cause neurological problems, including reduction in motor skills and sensation.

People likely produce the mercury that eventually contaminates seafood.

The mercury entering the Pacific ecosystem bore the chemical signature of pollution from coal plants and other human-related sources. This suggests the mercury menace may increase in the future. Pollution in the winds blowing from Asia onto the Pacific will continue to carry ever-increasing amounts of mercury as the regions’ industrial production and demand for energy increases.

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“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,” lead author Joel Blum of the University of Michigan said in a press release. “Cleaning up our own shorelines is not going to be enough. This is a global atmospheric problem.”

The rising mercury in thermometers may cause a rise in mercury levels in seafood. Marine biologists have observed an expansion of oxygen-deprived waters below 1,300 feet. Mercury munching bacteria thrive in these oxygen-poor regions. Climate change may be accelerating the expansion of these regions.

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“In the next few decades there will be changes in mercury concentrations in the Pacific Ocean, and those changes are likely to be different for surface waters than for deep waters,” co-author Brian Popp of the University of Hawaii said in a press release. “Understanding the competing processes that produce and destroy monomethlylmercury at different depths in the ocean is critical to tracing its bioaccumulation in fishes and the potential consequences for human food supply.”

IMAGE: A swordfish skeleton appears on display at the National Museum of Natural History in Washington. (Postdlf from w, Wikimedia Commons)