sand, or gravel to end up with about 1/50th of an ounce of gold. And digging or dredging out the ore is only the beginning.

As well as being one of earth's scarcest metals, gold is also one of its least reactive. One of the few chemicals it readily interacts with is cyanide, which can be a major pollutant in the air, water, and soil around gold mines. Difficult to dispose of cheaply, waste solutions containing cyanide are often kept in open ponds until the chemical is broken down by ultraviolet light.

In the traditional method of processing gold, cyanide is added to crushed ore and the dissolved gold mixture is passed through activated charcoal. Carbon in the charcoal attracts the gold compound from the solution, and the concentrated gold is later washed from the charcoal for final processing.

A difficulty crops up if the ore itself contains naturally occurring carbon—which it does in about 40 percent of the gold mines in the United States. In this case, the dissolved gold compound will bind onto the carbon in the ore and won't subsequently be attracted out of the mixture by charcoal. To prevent this from happening, carbon-bearing ores can be pretreated in a number of ways. Most commonly, the ore is finely ground and roasted at very high temperatures to burn off the carbon. Alternatively, chlorine gas is bubbled through a slurry of ground ore and water to oxidize the carbon. Both processes are costly and polluting, and can make mining operations uneconomical.

A way around the problem is offered by species of bacteria, fungi, and algae that produce and absorb cyanide ions. Adding these microbes to the crushed ore means that further pretreatment is unnecessary, and