Microorganisms increase the solubility of metals by decreasing the pH of their milieu.
Metabolic activity of heterotrophic microorganisms produces protons and CO2, (production of carbonic acid), which help metal solubilization.
Acidification is also the result of the activity of chemolithotrophs such as the iron- and sulfur-oxidizing bacteria (e.g., Thiobacillusferrooxidaits, Thiobacillusthiooxidans; see Chapter 3).
Chemolithotrophs are used in the bioleaching of metals from contaminated sediments and biosolids (Crowley and Dungan, 2002).