Extreme Thermophiles


Extreme thermophiles are microorganisms adapted to temperatures normally found only in hot springs, hydrothermal vents and similar sites of geothermal activity. These microorganisms include diverse archaea and bacteria and represent a wide range of metabolic strategies. The geothermal environments populated by extreme thermophiles provide chemical resources for microbial metabolism. Various molecular features enable the cells of extreme thermophiles to function optimally at these temperatures, which kill other cells. These features include low‐molecular weight compounds that stabilise the conformations of proteins and nucleic acids, enzymes with intrinsically stable folded conformations and unusual lipids that form highly impermeable membranes. Metabolic activities and intrinsically stable enzymes of extreme thermophiles offer advantages for industrial and diagnostic processes ranging from ore processing to molecular genotyping.

Key Concepts

  • Bacteria and archaea consist of very simple (prokaryotic) cells but they vary greatly with respect to metabolic capabilities, physiological limits and other fundamental properties.
  • The term ‘extreme thermophile’ usually denotes a microorganism which requires the temperatures found in geothermal environments for its optimal growth.
  • Geothermal environments provide inorganic nutrients and sources of chemical energy that certain bacteria and archaea can utilise.
  • Extreme thermophiles use two general strategies to avoid thermal denaturation of their enzymes: extrinsic stabilisation, conferred by certain small molecules, and intrinsic stabilisation, conferred by the specific structure and conformation of the enzyme itself.
  • The intrinsically thermostable enzymes of extreme thermophiles are useful for biotechnology because they allow chemical reactions to be catalysed with high specificity at high temperatures or under other harsh conditions.

Keywords: optimal growth temperature (Topt); molecular phylogeny; microbial biogeography; enzyme denaturation; extrinsic stabilisation; intrinsic stabilisation; DNA damage; proton‐motive force (PMF); bioleaching; polymerase chain reaction (PCR)

Figure 1. The three cardinal temperatures of microbial growth.
Figure 2. A universal molecular phylogeny. The tree depicts the relatedness of small‐subunit ribosomal RNAs among all cellular organisms (Woese et al., ). The total branch length between two organisms (end points) reflects the number of differences between the two nucleotide sequences. The heavy lines mark the lineages of extreme thermophiles and hyperthermophiles. The irregular box (blue) indicates the molecular divergence of multicellular organisms (plants, animals and fungi).
Figure 3. The cellular structure of some extreme thermophiles: (a) Thermoanaerobacter ethanolicus. The cell is approximately 1 μm long, and layers corresponding to cytoplasmic membrane and cell wall are visible around the periphery (ultrathin section, electron micrograph). Wiegel and Ljungdahl . Reproduced with permission of Springer Nature. (b) Dictyoglomus thermophilium strain Rt8N2 (Patel et al., ; light microscopy, phase contrast). At this stage of growth, very long rod‐shaped cells are arranged around the periphery of a spherical membrane or envelope of approximately 20 μm in diameter. Courtesy of Morgan and Wiegel. (c) Thermotoga maritima (electron micrograph, ultrathin section). The cell body is approximately 1 μm long, densely stained and bounded by a cytoplasmic membrane and cell wall. The ‘toga’ is an outer membrane that surrounds the cell, leaving large spaces at the ends. Courtesy of Drs Reinhard Rachel and Karl O. Stetter. (d) Thermosipho species (electron micrograph, platinum shadowing). Several cells (marked by arrows) are enclosed in one long sheath, or toga, which becomes visible between the cells (intercellular regions of ‘empty’ toga marked by arrowheads). The total length of sheathed cells shown is approximately 20 μm. Courtesy of Drs Reinhard Rachel and Karl O. Stetter. (e) Sulfolobus acidocaldarius (electron micrograph, ultrathin section). Two irregular cells are shown in contact, each with an envelope that creates a characteristic ‘picket fence’ pattern in the cross‐section. This represents an S‐layer canopy (outer stained layer) held approximately 20 nm away from the cytoplasmic membrane by thin supports (visible in certain regions of the cell margin). The width of the panel corresponds to approximately 1 μm.


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Further Reading

Cavicchioli R (ed.) (2007) Archaea: Cellular and Molecular Biology. ASM Press: Washington, DC.

Robb FT, Antranikian G, Grogan D and Driessen A (eds) (2008) Thermophiles: Biology and Technology at High Temperatures. CRC Press, Taylor and Francis: Boca Raton, USA.

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Grogan, Dennis W(Mar 2020) Extreme Thermophiles. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000392.pub3]