Extremely low pH environments have global distribution, may be natural or caused by human activities, and are frequently associated with the oxidation of elemental sulfur (e.g. in volcanic areas) or sulfide minerals (e.g. in sites impacted by metal mining). Microorganisms that grow optimally in environments that have pH values at or below 3.0 are classed as extreme acidophiles. These are surprisingly diverse, and include many unrelated species of archaea, bacteria and eukaryotes, which interact with each other in various ways in low pH ecosystems. In addition to their tolerance of extreme acidity, some species can also thrive under other environmental extremes, such as extremes of temperature or high salinity, and grow in the presence or absence of oxygen. The mechanisms used by extreme acidophiles to grow at low pH and in the presence of elevated concentrations of transition metals and metalloids have been widely studied. As with other extremophiles, acidophiles are being increasingly exploited in biotechnologies, ranging from the use of bacterial and archaeal species in extracting and recovering metals from ores and wastes (biomining), bioremediation (eukaryotic and prokaryotic species) and using acidophilic algae as food-stocks and as sources of industrially-important organic compounds.