Methylmercury found in seafood
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05/06/09 Seán Kinane
WMNF Drive-Time News Wednesday | Listen to this entire show:

Ocean fish, like tuna, and other seafood are a major contributor to exposure of the toxin methylmercury in humans. A study published last week in the journal Global Biogeochemical Cycles demonstrates how methylmercury originates in the ocean.

This afternoon, WMNF spoke with David Krabbenhoft, a research scientist with the U.S. Geological Survey (USGS) and a co-author of the research paper.

“Methylmercury is a specific form of mercury that exists in the environment largely through a process called methylation. And the reason scientists and the public are most concerned about that part of the total amount of mercury in the environment, is that’s the form of mercury that accumulates in food webs and thus presents the greatest threat in terms of toxicity and exposure.”

At mid-ocean depths, when bacteria decompose algae in the presence of mercury, methylmercury is produced. It can cause human health problems, including cognitive effects in children. Krabbenhoft says there “is still an area of active research to more fully understand” its effects.

“There are neurological concerns for methylmercury exposure. … Cardiovascular disease. … There’s been wildlife concerns for population effects due to diminished reproductive capacities. There are concerns, as you had mentioned, for unborn children because there appears to be a magnification effect across the uterus. Whereas maternal blood may have a concentration of X, the developing baby’s blood could have a concentration of 3X.”

The amount of methylmercury in seafood depends on the type of fish and where it was caught, Krabbenhoft says, because essentially all of them have some.

“Because mercury is truly a global contaminant and methylmercury formation is happening at some level just about in any environment where a fish would exist. Certainly there is variability and the amount of mercury in fish is going to vary mostly depending on their diet. So fish that eat other fish generally have more methylmercury in their tissues. So the top predator fish of the oceans generally have a relatively high methylmercury content as opposed to fish that maybe are plant eaters.”

Krabbenhoft and his team measured mercury levels at intermediate depths, from about 100 to 1000 meters, in the Pacific all the way from Hawaii to Alaska. They found a consistent increase in a short timeframe.

“We observed mercury levels in the Pacific Ocean that were about 30% higher than the last set of measurements that are published in the literature which was back in the 1990s.”

The model that Krabbenhoft and the other researchers created to help analyze their data predicts that those mercury levels will continue to go up unless something is done to reduce mercury emissions.

“The 30 percent increase isn’t the endpoint. It’s still on a trajectory upward in response to the increasing global emission pattern of mercury that’s currently estimated for the world. And if the current emission rates are sustained, our model estimates another 50 percent increase by the year 2050.”

The mercury that Krabbenhoft and his team found in the Pacific comes from local sources in Asia as well as global mercury emissions.

Krabbenhoft says he was caught completely off guard to learn from his team’s computer simulation model of large ocean circulation how quickly mercury is transported from the coast of Asia to the eastern Pacific.

Atmospheric mercury comes from a variety of sources in different places around the world, Krabbenhoft says, but most comes from human activities. This is especially true for Asia, where most of the contributing mercury is suspected to come from.

U.S. Secretary of the Interior Ken Salazar said in response to this research, “now we need to reduce the mercury emissions.” Krabbenhoft says that can be accomplished by reducing the amount of fossil fuels that are burned, especially coal.

Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models

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