Discovery may jump-start mine remediation efforts

Probing the microscopic life found in the submerged recesses of an abandoned Wisconsin lead and zinc mine, scientists have found compelling evidence that microorganisms play a key role in the formation of mineral deposits.

The finding not only sheds light on biology’s role in the formation of some metal ores, but could help jump-start new remediation efforts for contaminated mining sites.

Writing in the Friday, Dec. 1, edition of the journal Science, researchers led by Jillian Banfield, a professor of geology and geophysics, describe the discovery and characterization of natural biofilms that seem to concentrate zinc sulfide.

The biofilms, found deep in an abandoned mine, are heavily populated with bacteria, some of which help convert sulfate or sulfuric acid, a pervasive contaminant associated with the mining of metal ores, and zinc from ground water into zinc sulfide.

“These results show how microbes control metal concentrations in ground water and wetland-based remediation systems and suggest biological routes for formation of some low-temperature zinc sulfide deposits,” the researchers write in the latest Science, the nation’s leading scientific journal.

The biofilms were first discovered by recreational scuba divers exploring the dark, flooded tunnels of an old mine in southwestern Wisconsin, the historic lead mining region near the Mississippi River. Team members trained the divers in the use of sterile collection systems to retrieve samples of the biofilm for laboratory analysis.

In the paper, the Wisconsin team described a process by which tiny zinc sulfide crystals rapidly accumulate within natural biofilms populated by species of sulfate-reducing bacteria. The reduction of sulfate to sulfide, according to Matthias Labrenz, the lead author of the Science paper, is linked to the oxidation of organic matter: as the microbes metabolize organic material, they release sulfide ions into the solution around the biofilm. This leads to saturation in the neighborhood of the biofilm and, as a result, zinc sulfide rapidly accumulates as microscopic crystals in the biofilm.

That process, says Banfield, is “what has been envisaged for acid-mine drainage treatment using constructed wetlands which are rich in organic matter. Maybe our work can be used to refine development of these strategies.”

The worldwide problem of contamination around mining sites is significant. Acid mine drainage, for example, is a threat to surface and ground water near mines. It occurs when metal-sulfide ores are exposed to air and water and the sulfide is transformed to sulfuric acid. Moreover, metals such as zinc are toxic and can leach into ground water and contaminate wells and other drinking water supplies.

Working with Ken Kemner and colleagues at the Advanced Photon Source at Argonne National Laboratory, the Wisconsin team used the most finely focused high energy X-ray beam in the world to date to show that small but significant quantities of other toxic ions, arsenic and selenium, for example, as well as zinc are extracted from ground water and concentrated in the biofilms.

Another group, led by UW–Madison professor of physics Gelsomina De Stasio, used a novel X-ray microscope to perform a chemical analysis showing that the sulfur compounds found in the biofilms were indeed sulfides.

“The capability of the microscope enabled us to determine that there are a lot of sulfates in the sample, everywhere, whereas localized on the bacterium were sulfide deposits,” De Stasio says, “This leads us to think for the first time that these bacteria metabolize sulfates and precipitate sulfides as byproducts.”

In essence, then, what Banfield and her group have found is a natural process by which microbes – bacteria from a family known as Desulfobacteriaceae – produce sulfide that scavenges zinc and other toxic metals from the surrounding ground water.

In some cases, the newly discovered biofilms can bring zinc-rich solutions of ground water “down to values well below safe drinking water levels,” says Banfield. “Many bioremediation schemes reduce contaminants, but not to a low enough degree that the environment is safe. But our findings provide detailed insights into how metal removal occurs, thus they may be useful for improvement of (bioremediation) strategies currently under development.”

At a more fundamental level, the discovery made by Banfield’s group and colleagues broadens the scientific understanding of the role that microbes play in the formation of mineral deposits. “This is not just a skip forward,” says Argonne’s Kemner. “We’ve taken a very large step in understanding mineral microbe interactions. The interaction of bacteria with solids is ubiquitous and has implications not just in geochemistry but in other areas from astrobiology to environmental science.”

It has long been established, for example, that microbes form some iron sulfide deposits, but the role of bacteria and other microorganisms in the formation of ore minerals has been little studied, says Banfield.

“In the case of some of the biggest zinc sulfide deposits, the temperature of formation has been inferred to be too high (above 100 degrees Celsius) to warrant serious consideration of microbes,” says Banfield, noting that the biggest such deposits are found in sediments.

Prevailing theories suggest thermal processes, possibly linked to the decomposition of organic matter, are responsible, but they rule out the direct involvement of microbes.

Clouding the picture is the fact that there is some uncertainty about the temperatures needed to form ore deposits, or the range of temperatures that occur during the process. Moreover, in recent years scientists have discovered sulfide-reducing microbes that thrive at temperatures close to 100 degrees Celsius, suggesting that researchers cannot dismiss the possibility of microbes laying down mineral deposits.

The new work, says Banfield, helps fill in the picture: “We now know how it is possible to form pure zinc sulfide deposits (through the intervention of microbes), and to do this from dilute ground water solutions,” she says. “These biofilms appear to be a major sink for ground water zinc.”

Banfield’s group is now investigating the details of the zinc sulfide crystals: “We hope that some of the newly discovered microstructural characteristics can be detected in ancient zinc sulfide deposits. This will be important for making the case for the biogenicity of these ores.”

In addition to Banfield, Labrenz, De Stasio and Kemner, contributors to the paper in Science include Gregory Druschel, Susan Welch, Benjamin Gilbert, and Philip Bond, all of UW–Madison; Tamara Thomsen-Ebert, Diversions Scuba, Madison; Barry Lai, and Shelly Kelly of Argonne National Laboratory; and Graham Logan and Roger Summons of the Australian Geological Survey Organisation. The work was primarily funded through the U.S. Department of Energy’s Basic Energy Science Program.