Microorganisms growing in highly acidic environments (pH values below three) 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 bacteria and archaea 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 bacteria and archaea, 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 generally caused by the activities of acidophilic microorganisms that oxidise 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 archaea 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.

Keywords: Acidithiobacillus; acid mine drainage; Acidophiles; metal leaching; Sulfolobus; thermoacidophiles

Figure 1. The red waters (pH∼2) of the Rio Tinto, Spain, coloured red by jarosite [HFe3(SO4)2(OH)6] formed by chemolithotrophic iron‐ and sulfur‐oxidising archaea. Courtesy of Extremophiles Lab, CAB, Madrid.
Figure 2. A simplified scheme showing interactions of the pH gradient, substrate oxidation and ATP synthesis in iron‐oxidising Acidithiobacillus ferrooxidans, with electrons from ferrous iron involved in proton removal in the cytoplasm after transport via cytochromes (cyt) and rusticyanin (ru).


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Further Reading

Angelov A and Liebl W (2007) Genomics of acidophiles. In: Gerdes C and Glansdorff N (eds) Physiology and Biochemistry of Extremophiles, pp. 279–292. Washington, DC: ASM Press.

Franke S and Rensing C (2007) Acidophiles: mechanisms to tolerate metal and acid toxicity. In: Gerdes C and Glansdorff N (eds) Physiology and Biochemistry of Extremophiles, pp. 271–278. Washington, DC: ASM Press.

Goebel BM, Norris PR and Burton NP (2000) Acidophiles in biomining. In: Priest FG and Goodfellow M (eds) Applied Microbial Systematics, pp. 293–314. Dordrecht: Kluwer Academic Publishers.

González‐Toril E, Llobet‐Brossa E, Casamayor EO, Amann R and Amils R (2003) Microbial ecology of an extreme acidic environment, the Tinto River. Applied and Environmental Microbiology 69: 4855–4865.

Hallberg KB and Johnson DB (2001) Biodiversity of acidophilic prokaryotes. Advances in Applied Microbiology 49: 37–84.

Johnson DB (1998) Biodiversity and ecology of acidophilic microorganisms. FEMS Microbiology Ecology 27: 307–317.

Johnson DB (2007) Physiology and ecology of acidophilic microorganisms. In: Gerdes C and Glansdorff N (eds) Physiology and Biochemistry of Extremophiles, pp. 257–270. Washington, DC: ASM Press.

Johnson DB and Schippers A (2017) Recent Advances in Acidophile Microbiology: Fundamentals and Applications. Frontiers Research Topics (e‐book). Frontiers in Microbiology. DOI: 10.3389/978-2-88945-163-0.

Norris PR and Johnson DB (1998) Acidophilic microorganisms. In: Horikoshi K and Grant WD (eds) Extremophiles: Microbial Life in Extreme Environments, pp. 133–153. New York: Wiley‐Liss.

Quatrini R and Johnson DB (eds) (2016) Acidophiles: Life in Extremely Acidic Environments. Poole: Caister Academic Press.

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Oren, Aharon(Sep 2018) Acidophiles. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000336.pub3]