MINING HASN'T CHANGED MUCH SINCE THE Bronze Age: to extract valuable metal from an ore, apply heat and a chemical agent such as charcoal. But this technique requires a lot of energy, which means that it is too expensive for ores with lower metal concentrations.

Miners are increasingly turning to bacteria that can extract metals from such low-grade ores, cheaply and at ambient temperatures. Using the bacteria, a mining firm can extract up to 85 percent of a metal from ores with a metal concentration of less than 1 percent by simply seeding a waste heap with microbes and irrigating it with diluted acid. Inside the heap Acidithiobacillus or Leptospirillum bacteria oxidize iron and sulfur for energy. As they eat, they generate reactive ferric iron and sulfuric acid, which degrade rocky materials and free the valued metal.

Biological techniques are also being used to clean up acidic runoff from old mines, extracting a few last precious bits of metal in the process. Bacteria such as Desulfovibrio and Desulfotomaculum neutralize acids and create sulfides that bond to copper, nickel and other metals, pulling them out of solution.

Biomining has seen unprecedented growth in recent years as a result of the increasing scarcity of high-grade ores. Nearly 20 percent of the world's copper comes from biomining, and production has doubled since the mid-1990s, estimates mining consultant Corale Brierley. “What mining companies used to throw away is what we call ore today,” Brierley says.

The next step is unleashing bacterial janitors on mine waste. David Barrie Johnson, who researches biological solutions to acid mine drainage at Bangor University in Wales, estimates that it will take 20 years before bacterial mine cleanup will pay for itself. “As the world moves on to a less carbon-dependent society, we have to look for ways of doing things that are less energy-demanding and more natural,” Johnson says. “That's the long-term objective, and things are starting to move nicely in that direction.”