Finding Gold in a Zinc Mine

Mining for zinc. A microscopic image of the biofilms shows zinc sulfide particles.

Deep in a flooded, abandoned mine shaft, scuba divers have discovered common bacteria doing something unheard of: extracting zinc from the surrounding water and storing it in large, remarkably pure zinc-sulfide films. The phenomenon, reported in the 1 December issue of Science , p. 1744 might help scientists devise better ways to clean up wastewater and may also solve the mystery of how some ancient ore deposits were formed.

The discovery comes courtesy of dive shop owner Tamara Thomsen-Ebert. After finding strange, gray, filmlike structures on the walls of an abandoned zinc and lead mine in Tennyson, Wisconsin, she showed pictures to a research team at the University of Wisconsin, Madison. Intrigued, the scientists asked her to get samples. "They looked really fascinating and turned out indeed to be," says geochemist Greg Druschel, a team member. Using electron microscopes and other tools, the team discovered that the samples contained bacteria and micrometer-sized particles of a zinc-sulfide mineral called sphalerite.

The bacteria turned out to belong to the family Desulfobacteriaceae, a group of sulfate-reducing organisms that are common in soil, ocean sediments, and other environments. Nobody had ever suspected that these bacteria could produce massive films of zinc-sulfide particles. In the process, they had reduced zinc concentrations to below drinking water levels.

"They're finding something unique," says George Luther, a marine chemist at the University of Delaware, Lewes. Studying the interaction between the mine water and the microbes may help scientists develop more effective microbial wastewater treatment systems, says team leader Jill Banfield. Under different conditions, the bacteria likely could purify water of pollutants such as lead, iron, or arsenic, Banfield adds.

The study may also challenge the notion that all ancient ore deposits were formed nonbiologically, through chemical reactions during extreme heat. At least in the case of zinc, bacteria seem to be able to produce unusually high concentrations of metal. Further studies should show whether microbial activity actually played a role in the formation of some zinc and other ore deposits, says Banfield.

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