REPORT / INTERNATIONAL NEWS

Fog blankets the coast of central California each summer, hydrating the region’s majestic redwood trees and chilling beachgoers. New research out of the University of California, Santa Cruz shows that the moist air also carries methylmercury, an especially toxic form of the heavy metal mercury.

“Is it dangerous to breathe the fog? Of course not — we’re talking very low levels,� said Peter Weiss-Penzias, a chemist and the lead author of the study, published in the latest issue of the journal Geophysical Research Letters.

Yet over time, the researchers’ analysis suggests, significant amounts of methylmercury could be deposited along the coastline, nearly all of it from fog during the rainless summer months. “This toxic form of mercury is basically raining down in a place where redwood forests, for example, are collecting a lot of fog precipitation,� Dr. Weiss-Penzias said in a telephone interview.
Mercury, a neurotoxin, occurs naturally in air, water and soil, and coal releases it when it is burned. Bacteria in soil and sediments convert mercury to methylmercury, which is both organic and soluble in water — a potent combination, Dr. Weiss-Penzias said. “It can be in water, taken up by organisms, stored in fatty tissues and cross the blood-brain barrier,� which prevents most other toxins from entering the brain, he explained.

When animals higher up in the food chain eat smaller organisms, they take up methylmercury, too. Moving up the food chain, methylmercury concentrations increase, in a process known as bioaccumulation.

It is not known exactly how mercury from fog might work its way through a terrestrial ecosystem. Still, it is clear that mercury is bring mobilized into the food web, Dr. Weiss-Penzias said. Imagine trees collecting the fog water that is dripping down to the forest floor and putting it into the soil, where it can be taken up by organisms and plants, he said. “As you go up the food chain it can become concentrated,� he said.

Dr. Weiss-Penzias’s team measured methylmercury and total mercury levels in eight fog water samples collected last summer around Monterey Bay, Calif., and in five samples of rainwater. (The researchers discarded another 17 fog water samples deemed unusable due to low volumes.) In fog, methylmercury concentrations ranged from about 1.5 parts per trillion to 10 parts per trillion, with an average of 3.4 parts per trillion — about five times higher than the highest levels previously recorded in rainwater.

For comparison, the threshold level in fish considered safe for consumption is 0.3 parts per million. What the researchers found in fog is only a tiny fraction of that. Nonetheless, the discovery of the toxin in such high concentrations in fog, relative to rain, suggests that fog is an “important, and previously unrecognized source� of methylmercury for coastal ecosystems, the researchers wrote.
For Dr. Weiss-Penzias, “the big scientific question� is how the methylmercury gets into the fog. A preliminary analysis of the data suggests that the ocean is one source, with methylmercury from sediments brought to the surface by upwelling, which is driven by strong winds. When the winds relax and the air cools, it enhances the concentration of methylmercury into fog, he said. “We’d like to understand how, and how much, and when.�

At this point, his team is collecting insects to check their methylmercury levels. The scientists are also gathering rainwater from underneath trees and comparing it with rainwater collected in the open to see if there is an accumulation of mercury on foliage that may wash off.

Such investigations on the role of fog in coastal ecosystems could become more and more important as climate change unfolds.

“As the land grows hotter, it will increase or stimulate that sucking in of the fog from the cool ocean area,� Dr. Weiss-Penzias said. Combine that with less rainfall in California, and fog could make up a larger fraction of annual precipitation. “We don’t know these things for sure,� he said, “but it’s certainly something that we should be understanding.�