Ahyperthermophile is an organism that thrives in extremely hot environments—from 60 °C (140 °F) upwards. An optimal temperature for the existence of hyperthermophiles is often above 80 °C (176 °F).[1] Hyperthermophiles are often within the domain Archaea, although some bacteria are also able to tolerate extreme temperatures. Some of these bacteria are able to live at temperatures greater than 100 °C, deep in the ocean where high pressures increase the boiling point of water. Many hyperthermophiles are also able to withstand other environmental extremes, such as high acidity or high radiation levels. Hyperthermophiles are a subset of extremophiles. Their existence may support the possibility of extraterrestrial life, showing that life can thrive in environmental extremes.
Hyperthermophiles isolated from hot springs in Yellowstone National Park were first reported by Thomas D. Brock in 1965.[2][3] Since then, more than 70 species have been established.[4] The most extreme hyperthermophiles live on the superheated walls of deep-sea hydrothermal vents, requiring temperatures of at least 90 °C for survival. An extraordinary heat-tolerant hyperthermophile is Strain 121,[5] which has been able to double its population during 24 hours in an autoclave at 121 °C (hence its name). The current record growth temperature is 122 °C, for Methanopyrus kandleri.
Although no hyperthermophile has shown to thrive at temperatures >122 °C, their existence is possible. Strain 121 survives 130 °C for two hours, but was not able to reproduce until it had been transferred into a fresh growth medium, at a relatively cooler 103 °C.
Early research into hyperthermophiles speculated that their genome could be characterized by high guanine-cytosine content; however, recent studies show that "there is no obvious correlation between the GC content of the genome and the optimal environmental growth temperature of the organism."[6][7]
The protein molecules in the hyperthermophiles exhibit hyperthermostability—that is, they can maintain structural stability (and therefore function) at high temperatures. Such proteins are homologous to their functional analogs in organisms that thrive at lower temperatures but have evolved to exhibit optimal function at much greater temperatures. Most of the low-temperature homologs of the hyperthermostable proteins would be denatured above 60 °C. Such hyperthermostable proteins are often commercially important, as chemical reactions proceed faster at high temperatures.[8][9]
Due to their extreme environments, hyperthermophiles can be adapted to several variety of factors such as pH, redox potential, level of salinity, and temperature. They grow-similar to mesophiles-within a temperature range of about 25–30 °C between the minimal and maximal temperature. The fastest growth is obtained at their optimal growth temperature which may be up to 106 °C.[10] The main characteristics they present in their morphology are:
Hyperthermophiles have a great diversity in metabolism including chemolithoautotrophs and chemoorganoheterotrophs, while there are not phototrophic hyperthermophiles known. Sugar catabolism involves non-phosphorylated versions of the Entner-Doudoroff pathway some modified versions of the Embden-Meyerhof pathway, the canonical Embden-Meyerhof pathway is present only in hyperthermophilic Bacteria but not Archaea.[13]
Most of informations about sugar catabolism came from observation on Pyrococcus furiosus. It grows on many different sugars such as starch, maltose, and cellobiose, that once in the cell they are transformed in glucose, but they can use even others organic substrate as carbon and energy source. Some evidences showed that glucose is catabolysed by a modified Embden-Meyerhof pathway, that is the canonical version of well-known glycolysis, present in both eukaryotes and bacteria.[14]
Some differences discovered concerned the sugar kinase of starting reactions of this pathway: instead of conventional glucokinase and phosphofructokinase, two novel sugar kinase have been discovered. These enzymes are ADP-dependent glucokinase (ADP-GK) and ADP-dependent phosphofructokinase (ADP-PFK), they catalyse the same reactions but use ADP as phosphoryl donor, instead of ATP, producing AMP.[15]
As a rule, hyperthermophiles do not propagate at 50 °C or below, some not even below 80 or 90º.[16] Although unable to grow at ambient temperatures, they are able to survive there for many years. Based on their simple growth requirements, hyperthermophiles could grow on any hot water-containing site, even on other planets and moons like Mars and Europa. Thermophiles-hyperthermophiles employ different mechanisms to adapt their cells to heat, especially to the cell wall, plasma membrane and its biomolecules (DNA, proteins, etc.):[12]
The hyperthermophilic archaea appear to have special strategies for coping with DNA damage that distinguish these organisms from other organisms.[17] These strategies include an essential requirement for key proteins employed in homologous recombination (aDNA repair process), an apparent lack of the DNA repair process of nucleotide excision repair and a lack of the MutS/MutL homologs (DNA mismatch repair proteins).[17]
In June 1965, Thomas Brock, a microbiologist at Indiana University, discovered a new form of bacteria in the thermal vents of Yellowstone National Park. They can survive at near-boiling temperatures. At that time the upper temperature for life was thought to be 73 °C. He found that one particular spring, Octopus Spring, had large amounts of pink, filamentous bacteria at temperatures of 82–88 °C.