Priya M. Ganguli, Ph.D. Candidate for Earth and Planetary Sciences and UCSC graduate handles multiple samples of Mercury before attaching them to the automatic sampler Mercury detector. Photo by Chelsea McKeown.

The fog that hydrates coastal Californian vegetation during the hot summer months is bringing in more than just moisture. UC Santa Cruz researchers confirm that the fog is a carrier of toxic methylmercury.

A team within the university’s department of Microbiology and Environmental Toxicology headed the research. The study that outlines the discovery, Total and Monomethyl Mercury in fog water from the Central California Coast, was published in the Geophysical Research Letters in February of this year.

“Up until now, [mercury] was only measured in rain,” said Peter Weiss-Penzias, the environmental toxicologist who led the research. “We didn’t even know there was anything in the fog.”

According to the study Weiss-Penzias and his team authored, eight fog water samples were gathered during the summer months of 2011 from four different locations in the Monterey Bay area. In these samples, methylmercury concentrations ranged from about 1.5 parts per trillion to 10 parts per trillion, averaging at 3.4 parts per trillion — five times higher than concentrations formerly observed in rainwater.

Weiss-Penzias said that although these very low levels are not hazardous to breathe, the toxin is problematic for the ecosystem as a whole.

Michael Loik, a UCSC environmental studies professor who specializes in plant physiology and ecosystem responses to climate change, said that fog precipitation plays a vital role in ecosystem functionality.

“The fog lands on the needles [of redwood trees] and it drips off and lands on the soil,” Loik said. “That water keeps the soil moist and then the roots of the redwoods take it up.”

Loik said that if the methylmercury concentration levels in the fog water increase, the toxin could be harmful to the plants’ physiological processes and negatively influence photosynthesis. He said mercury places an additional burden on plants and animals that can hinder the ability of ecosystems to sustain themselves.

Priya Ganguli, a UCSC Ph.D. candidate in the earth and planetary sciences department who works in Weiss-Penzias’ lab, said that as methylmercury ends up in the soil, lakes and ponds, it is introduced into the aquatic food web and its concentration can magnify to levels unsafe for organisms and human consumption in a process known as bioaccumulation. In this process, a small organism might ingest the low level of methylmercury in the fog, but when a larger organism eats the smaller one, it also takes up the toxin. Moving up the food chain, the concentration increases.

Ganguli added that this form of mercury elicits concern because it is especially toxic to the food web.

“One of the key issues of methylmercury is it passes the blood-brain barrier, so it’s a very potent neurotoxin,” Ganguli said. “Additionally, it binds to protein so it takes months for an organism to expel it.”

Prior to this study, the source of methylmercury in aquatic organisms like fish had been considerably debated.

“I think that this could explain the elevated levels [of mercury] that we’re seeing in the food web,” Weiss-Penzias said. “Up until this point, it’s been a mystery as to why there is so much mercury in fish, for example.”

The next step in understanding the impact of methylmercury is identifying how it gets into the fog. The study suggests that mercury is perhaps carried to the surface of the ocean through upwelling, a process by which wind drives denser and cooler nutrient-rich water upwards. From there, the volatile toxin can diffuse into the fog.

To further investigate the mechanism of mercury assimilation into fog, Weiss-Penzias and his team are examining spatial and temporal distributions of mercury in ocean sediments, insects and rainwater. By identifying the source of the mercury and how it is distributed into the fog, Weiss-Penzias said he hopes that ways to prevent the increase of mercury in fog will become more apparent.

“Mercury is a problem,” Weiss-Penzias said, “and if we can understand where it is and where it goes in some of these mechanisms, it will allow us to better understand why our environment is polluted.”