Study of microbes may hone predictions of mining impact

By tracing the abundance and distribution of bacteria in an abandoned California mine, scientists may have found a better way to predict the potential environmental consequences of mining metal ores.

Writing in the March 6 issue of the journal Science, a team of UW–Madison scientists presents the first in situ, molecular-level ecological study of the naturally occurring microbes that mediate some of the most severe pollution events associated with sulfide mining. The findings could provide the mining industry and others with a new predictive technology, one capable of estimating acid mine drainage from a given site.

Acid mine drainage is the flow of sulfuric acid into ground and surface water from metallic-ore mines. In addition to contributing sulfuric acid to nearby water supplies, the sulfuric acid itself facilitates the release and suspension of heavy metals into water, one of the most vexing consequences of sulfide mining.

In nature, minerals exposed to oxygen and water form sulfuric acid. Around mines, an abundance of minerals is exposed to the surface in tailings and the exposed surfaces of ore bodies, and they oxidize naturally. But contributing to the process are naturally-occurring bacteria which, like tiny factories, greatly accelerate the rate of oxidation. The bacteria are widely considered to be the microorganisms that control the production rate of acid mine drainage.

Knowing precisely where and under what conditions the microbes thrive in nature can be a powerful new tool to predict the effects of sulfide mining at a given site, said Katrina J. Edwards, a UW–Madison graduate student and a co-author of the study.

Prior to the new study, conducted at Iron Mountain, Calif., an abandoned and heavily polluted iron mine, two species of bacteria, Thiobacillus ferrooxidans and Leptospirillum ferrooxidans, were believed to be the primary microbial culprits involved in accelerating acid mine drainage.

But results of the study, the first to employ the techniques of modern molecular biology to assess the population dynamics of the two microbes in the wild, suggest that one of the microorganisms, Leptospirillum ferrooxidans, is a far more important contributor to mine pollution.

That finding, said Robert J. Hamers, a UW–Madison professor of chemistry and a co-author of the Science study, was a surprise.

“Thiobacillus is not the controlling or predominant player” it was presumed to be, said Hamers. “Inside the mine, where most acid drainage is found, it is essentially undetectable.”

On the other hand, Leptospirillum is a far more active player inside the mine, making up as much as 50 percent of all microbe species found in a vast network of underground tunnels.

The Wisconsin team explored two important environments at Iron Mountain where mining occurred both in above ground pits and in miles of tunnels below ground.

“There are two different types of environment, and geochemical conditions are different in both places,” said Hamers. “This gave us an opportunity to study all the different conditions of acid mine drainage.”

Results showed that Thiobacillus prefers moderate temperatures and lower levels of acidity. Leptospirillum survives at significantly higher concentrations of acidity and higher temperatures.

The fact that Leptospirillum thrives in such conditions, suggests its role in accelerating acid mine drainage is more significant because its chances of being in contact with the ore body are greater, said Hamers.

The study strongly suggests that “we can develop better models than the ones we currently have,” said Edwards. “We can not only identify how many are there, but we can show where they are” in nature.

Funded by the National Science Foundation, the study was conducted by an interdisciplinary team involving chemists, geologists and biologists. Other co-authors of the study included Matthew Schrenk, Robert Goodman and Jillian Banfield, all of UW–Madison.