Life in Extreme Environments-Some Like it Hot

We have found that the ability to reduce iron is a highly conserved characteristic of hyperthermophilic Archaea and Bacteria, suggesting that the last common ancestor(s) of extant life was probably an iron-reducing microorganism. Providing iron as an electron acceptor has proven to be a useful strategy for recovering hyperthermophiles from the environments that have previously resisted cultivation. The metabolic capabilities of hyperthermophiles are often increased when iron is provided as an electron acceptor; permitting respiratory growth in organisms previously thought to only have a ferementative metabolism in some organisms and increasing the range of electron donors utilized in other organisms. Studies with iron as the electron acceptor have demonstrated for the first time that hyperthermophiles can anaerobically oxidize acetate, aromatic compounds, and long-chain fatty acids. This significantly changes concepts about organic matter metabolism in hot microbial ecosystems. Hyperthermophilic Archaea can reduce a wide variety of metals other than iron, such as uranium, technetium, and even gold. Reduction of these metals by hyperthermophiles provides a likely explanation for a number of geologically and environmentally significant ore deposits.

One of the more interesting hyperthermophilic iron-reducing microorganisms is strain 121, Popular Science’s “Extremophile of the Year”. Strain 121 grows at temperatures higher than previously reported for any organism. Strain 121 grows at 121 °C (250 ° Fahrenheit), the temperature used in sterilization systems, such as autoclaves, to prepare microbe-free equipment and media. This temperature was previously thought to kill all living organisms. The finding that microorganisms have the ability to grow at these high temperature has implications for delimiting when and where life might have evolved on a hot, early Earth; the depth to which life exists in the Earth's subsurface; and the potential for life in hot, extraterrestrial environments. Furthermore, enzymes capable of functioning at high temperatures have a number of industrial applications.


Kashefi K, Lovley DR. Extending the upper temperature limit for life, 2003, Science 301(5635):93

Lovley DR. Dissimilatory Metal Reduction: from Early Life to Bioremediation, 2002, ASM News. 68(5):231-7.

Kashefi K, Holmes DE, Reysenbach AL, Lovley DR. Use of Fe(III) as an electron acceptor to recover previously uncultured hyperthermophiles: isolation and characterization of Geothermobacterium ferrireducens gen. nov., sp. nov., 2002, Appl Environ Microbiol. 68: 1735-42.

Vargas M, Kashefi K, Blunt-Harris EL, Lovley DR. Microbiological evidence for Fe(III) reduction on early Earth, 1998, Nature 39:65-7.

Press coverage

19 August: Microbe raises heat limits for life (National Public Radio)

15 August: Newly found microbe can take a lot of heat (Boston Globe)

15 August: Heat-loving microbe expands limits of life (Atlanta Journal-Constitution)

14 August: Microbe from Depths Takes Life to Hottest Known Limit (NSF Press Release)

14 August: Microbe Survives At Temperatures Above Sterilization Standard (UMass News Office)



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Kazem Kashefi pulling a live culture out of an autoclave previously considered to kill all living organisms with heat at 121 °C (250 °F). The magnet is used to test the iron to determine if strain 121 is converting non-magnetic iron oxide (rust) to the magnetic mineral magnetite. Photograph by Jessica Soules.
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Electron micrograph of heat-loving microorganism, strain 121, which grows and survives at temperatures higher than previously recorded for any form of life. The round organism has tail-like flagella used in propulsion. The black material is the iron on which strain 121 grows. Bar length, 1 µm. Photograph by Kazem Kashefi.
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Kazem Kashefi uses a magnet to show the production of magnetite as a result of the reduction of poorly crystalline Fe(III) oxide by strain 121. Strain 121 was able to reduce Fe(III) oxide after incubation at 121 °C, producing ultra-fine grained magnetic minerals called magnetite (right). Poorly crystalline Fe(III) oxide at the bottom of the uninoculated tube (left) was not attracted to the magnet. Photograph by Jessica Soules.
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Color images of strain 121.


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