SAN FRANCISCO—It is well known nowadays that people should be careful around broken thermometers and moderate their consumption of tuna to avoid contact with mercury, a highly potent neurotoxin.

Now, scientists have figured out one of the things that makes water-borne mercury even more toxic—dead stuff.

The decayed remains of plants and other organic materials may help convert mercury in waterways to forms that are highly toxic to humans, a new study shows.

Mercury is present throughout Earth's environment—it is found in small quantities in rocks and in watery environments, including lakes, wetlands and oceans. Pollution, especially from the burning of coal for electricity, adds to these levels in the environment.

Mercury also can accumulate in fish—and in larger quantities in big fish such as tuna—that live in contaminated waters, eventually posing a health risk to those who eat tainted fish, especially to children, pregnant women and women planning a pregnancy. Another source of mercury exposure is broken CFL bulbs.

Dissolved organic carbon (DOC) is also present in the environment and forms from dead and decaying plant and animal material that accumulates on the ground and in waterways. The new study confirms what scientists had suspected: certain materials, such as DOC, in the water with mercury affect how it accumulates in the food chain and how potent a toxin it becomes.

Certain forms of mercury, particularly one called methylmercury, are more of a threat because they are more easily taken up by living tissues.

Bacteria that naturally occurs in lakes and wetlands proliferate in the presence of their food—decaying plants and animals—and take up environmental mercury and turn it in to methylmercury. Study author John Moreau, a geomicrobiologist at the University of Wisconsin-Madison, calls the bacteria "little methylmercury factories."

"The bacteria take mercury from a form that is less toxic to humans and turn it into a form that is much more toxic," Moreau said. "[Methylation] increases mercury’s toxicity by essentially putting it on a fast train into your tissue—it increase its mobility."

Moreau and his colleagues studied how DOC influences how quickly these bacteria process the mercury. They presented their research last week at a meeting of the American Geophysical Union.

DOC can tint wetlands and streams shades of yellow and dark brown. Moreau’s teams looked at DOC-rich environments in the Suwannee River and Florida’s Everglades and found that DOCs make the bacteria churn out methylmercury faster—meaning that areas particularly rich in organic matter pose a higher mercury toxicity threat to those environments and to the humans around them.