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	The Making of a Potent Toxic by Mika Pringle Tolson
				The release of toxic substances is a major source of concern, but the more important question is what happens to them in the environment, their environmental fate. When inorganic mercury is converted to an organic form, methylmercury, it becomes a serious threat to wildlife and humans as well. Understanding the mechanism of this conversion is a critical part of understanding the environmental mercury problem. Neal Buhler, a Microbiology doctoral student at UC Davis and an Ecotoxicology Lead Campus Program trainee, is looking at the microbial methylation of mercury - the biotic transformation of inorganic mercury to toxic methylmercury - at Clear Lake. Buhler explains, "Once mercury is methylated at the sediment interface, it moves into the lake's food chain bioaccumulating in the plankton, the fish which feed on the plankton, and birds which in turn feed on the fish." Clear Lake is a recreational area used by many people for a variety of different sports. "However, a potential problem of methylmercury exposure to humans exists where people who are fishing in the lake are also consuming the fish," says Buhler.
	There are other forms of organic mercury in the environment, but they are relatively insignificant in the system. As Buhler puts its, "The real culprit is methylmercury because it has been shown to be toxic in animals and humans." Inorganic mercury is found naturally occurring in geological strata, and mercury deposits can be associated with sulfur deposits in the mineral form known as cinnabar. When these deposits are mined, mercury becomes exposed to the environment, and if a group of bacteria known as "sulfate-reducing bacteria" happen to be in that same environment, methylation of the mercury occurs. "It's not a detoxification mechanism that sulfate-reducing bacteria use to remove inorganic mercury from their own environment, but it seems to be a kind of inadvertent side reaction of other biochemical pathways used by these bacteria," explains Buhler. Problems occur when sources of mercury rise to unnatural levels because of exposed mine tailings or runoff from mine sites. At Clear Lake, near the Sulphur Bank Mercury Mine, the inorganic mercury moves from the mine into the lake. When the highly acidic runoff water from the mine contacts the lake water, it forms a flocculent material made up of an aluminum silicate. This "floc" also contains inorganic mercury and appears to act as a matrix upon which the sulfate-reducing bacteria grow and methylate the inorganic mercury. The result is high levels of methylmercury production in the lake.
	The sulfate-reducing bacteria are thought to be largely responsible for mercury methylation. However, Buhler and colleagues have discovered that they may only contribute about 30 percent of the total methylmercury produced in Clear Lake. They are working on "analyzing the full microbial community found in the sediments of the most contaminated sites at Clear Lake to see which groups, aside from the sulfate-reducing bacteria, are present and responsible for methylation." Methanogens and acetogens are two other groups of bacteria that may also contribute to mercury methylation, but no one knows for sure.					"Floc", a precipitate of aluminum silicate, forms when acid mine drainage mixes with lake water in Clear Lake. Photo courtesy of William Shipp, TSR&TP fellowship recipient.
	It is estimated that in just one gram of soil, there are over 10,000 different species of bacteria. It is believed that less than 10% of the species have been characterized because at the present time there is no way to culture many of them in the lab. According to Buhler, "Microbiology is experiencing a renaissance right now because of new molecular techniques that are being used to identify unculturable bacteria." With DNA extraction and identification techniques, Buhler and colleagues are working to determine which species are present and which are most active and therefore most responsible for mercury methylation. Says Buhler, "Inhibiting the sulfate-reducing bacteria is one strategy to stop methylation of mercury from occurring," and limiting the production of methylmercury is one of the most important aspects of cleaning up sites such as Clear Lake. Determining the environmental fate of mercury lends a perfect opportunity to blend different disciplines such as toxicology, ecology, and microbiology, and to put new techniques to work in a practical application. Neal Buhler can be reached via email at ndbuhler@ucdavis.edu
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