Climate change may boost toxic mercury levels in sea life

The muddying of coastal waters by climate change could drastically increase levels of neurotoxic mercury in sea life, contaminating food supplies.

Shifting rainfall patterns may send 10 to 40 percent more water filled with dissolved bits of organic debris into many coastal areas by 2100. The material can cloud the water, disrupting marine ecosystems by shifting the balance of microbes at the base of the food web, new laboratory experiments suggest. That disruption can at least double methylmercury concentrations in microscopic grazers called zooplankton, researchers report January 27 in Science Advances.

The extra mercury could reverberate up the food web to fish that humans eat, warns study coauthor Erik Björn, a biogeochemist at Umeå University in Sweden. Even small amounts of methylmercury, a form of the metal easily absorbed by humans and other animals, can cause birth defects and kidney damage, he notes.

Pollution from human activities such as fossil fuel burning has already tripled the amount of mercury that has settled in the surface ocean since the start of the Industrial Revolution (SN: 9/20/14, p. 17). Climate changes spurred by those same activities are washing more dark organic matter into the oceans by, for instance, boosting wintertime rainfall in some regions.

Björn and colleagues replicated this increased runoff using 5-meter-tall vats filled with marine microbes and dashes of methylmercury. Vats darkened by extra organic matter showed an ecosystem shift from light-loving phytoplankton to dark-dwelling bacteria that eat the extra material, the researchers found.

Zooplankton nosh on phytoplankton, but they don’t directly eat the bacteria. Instead the bacteria are consumed by protozoa, which zooplankton then hunt. Methylmercury accumulates with each step up the food web. So the addition of the protozoa middle step, the researchers report, resulted in zooplankton methylmercury levels two to seven times higher than in vats without the extra organic matter. Methylmercury levels will continue to increase up the food web to fish and the humans who eat them, the researchers warn.

The results suggest that curbing mercury contamination is more complicated than simply controlling emissions, says Alexandre Poulain, an environmental microbiologist at the University of Ottawa. “First we need to control emissions, but we also need to account for climate change.”