to grow. The microbes oxidize the sulfide in the ore and convert the copper to a soluble form. The dissolved copper is washed out of the ore and collected. In this way, the Thiobacillus ferrooxidans improves the recovery rates, allowing copper to be extracted economically from low-grade ores and tailings—an increasingly important consideration as much of the high-grade ores have been mined out in the past.

Bioleaching is now routinely used in copper mines in the United States, Canada, Australia, Chile, and South Africa, producing about one-quarter of all copper worldwide. With a value of more than $1 billion annually, bioprocessed copper is one of the most important applications of biotechnology in the mid- 1990s.

The potential usefulness of oxidizing bacteria doesn't stop with copper extraction. In Japan, these bacteria are being used to remove iron from mine drainage waters and to treat toxic gases such as hydrogen sulfide. The bacteria oxidize the iron compounds into a form that is much cheaper to manage.

Wherever oxidizing bacteria are used to solve a problem, they also create a pollution problem—they make sulfuric acid. Calcium carbonate added to the acid mine water can neutralize the acid but it forms solid wastes, which must be removed. Acid drainage from abandoned mines is also a serious environmental problem. Here, again, biotechnology is lending a hand in the form of competing bacteria. Reducing bacteria, the counterpart of oxidizing bacteria, can be added to water near old mine sites to counteract the effects of the oxidizing bacteria (see Figure 5.3).

While the acid pollution problems caused by T. ferrooxidans are coming under control, biotechnologists