Acidophiles

Abstract Microorganisms growing in highly acidic environments (pH values below 3) are found in all three domains of life: Archaea, Bacteria and Eucarya. Such organisms thrive in acidic sulfur springs and in association with mining activities where microbial oxidation of pyrite and other reduced sulfur compounds lead to the formation of sulfuric acid. Acidophilic prokaryotes are also involved in the industrial leaching of copper and other metals from ores. Some acidophiles grow at high temperatures. Physiologically, the acidophiles are very diverse: there are aerobic and facultative anaerobic chemolithotrophs and different types of heterotrophic prokaryotes, photoautotrophic eukaryotes, predatory protozoa and others. Acidophilic microorganisms maintain their intracellular pH close to neutrality and their cytoplasmic membrane may support proton gradients up to five orders of magnitude; their membrane potential is often reversed in comparison with neutrophiles and alkaliphiles, with an intracellular positive charge. Key concepts: Microbial life is possible in strongly acidic environments and even at pH values below zero. The low pH of highly acidic environments in volcanic areas and environments associated with metal ore mining is caused by the activities of acidophilic microorganisms that oxidize reduced sulfur compounds to sulfuric acid. Acidophilic microorganisms are found in all three domains of life: Archaea, Bacteria and Eucarya. Unicellular eukaryotic green algae such as Dunaliella acidophila and Chlamydomonas acidophila perform photosynthesis at pH values down to 0–1 and the red alga Cyanidium caldarium grows as a photoautotroph in acidic hot springs up to 57°C and at pH<2–4. Some thermoacidophilic Archaea thrive at low pH up to very high temperatures. Sulfolobus and Acidanus spp. grow up to 96°C at pH 1–5; Picrophilus oshimae tolerates pH<0 and grows up to 65°C. Acidophilic microorganisms keep their intracellular pH at values close to neutrality and maintain a proton gradient over their cytoplasmic membranes of up to five orders of magnitude. In contrast to neutrophilic and alkaliphilic microorganisms that maintain an outside‐positive membrane potential, the membrane potential over the cytoplasmic membrane can be reversed, positive‐inside, in acidophilic microorganisms. Extracellular enzymes of acidophilic microorganisms are optimally active at low pH. Acid mine drainage is caused by the chemolithotrophic oxidation of pyrite and other reduced sulfur compounds in the ores by acidophilic prokaryotes such as Acidithiobacillus , Leptospirillum and Ferroplasma . The activities of the same types of organisms that cause acid mine drainage are exploited industrially for the bioleaching of copper and other metals from ores.