11.   Compounds of metal ions such as gadolinium can increase the brightness of the magnetic resonance imaging signal produced by neighboring water molecules.

12.   For example, a metal-on-metal prostheses may shed metal ions into the bloodstream and could increase risk of leukemia.

13.   The bacteria produce hydrogen sulfide, which combines with the metal ions to make them easier to precipitate.

14.   This makes the metal ions heavier, which means the most valuable metals, copper and zinc, precipitate out and could be sold.