The Crenarchaeota are a smaller group than the Euryarchaeota which contains the majority of the known Archaea.
Crenarchaeota are primarily found in extreme environments, either hot ones or cold ones.
Little is known about the cold-adapted species, except that they live in considerable numbers – 104 per ml. They are found in the Antarctic and probably the Arctic as well.
The high-temperature species occur in solfatoras and hydrothermal vents, naturally as well as in man-made habitats such as the effluent from Geothermal power stations.
There are 21 genera in the Crenarchaeota, most of which are monotypic. Even the most populous genus has only 6 species in it.
The following brief synopsis is an introduction to what little is known of the more studied genera.
Two genera found in volcanoes where high temperatures can easily occur are Sulfabolus and Acidianus. Both of these can live aerobically (in the presence of oxygen), but Acidianus can also live anaerobically (without oxygen).
Both are chemolithotrophs, with the potential to be chemoorganotrophs if the need arises. Sulfabolus can oxidise Fe2+ to Fe3+ as well as using sulphur compounds or organic molecules.
Sulfabolus lives at around 75(C and at a pH of 1 to 5. Acidianus will live at temperatures of 65-95(C but has an optimum of 88 (C. Both organisms are roughly spherical.
Two genera isolated from hot springs are Thermoproteus and Thermofilum, both of which are rod-shaped. Both are sulphur-reducing organisms and are strictly anaerobic. Both have optimum temperatures of 88(C and prefer a pH value of 5-6.
Several genera of hyperthermophiles are found only in non-marine environments, both shallow water thermal springs and deep sea hydrothermal vents.
Among these are some organisms whose optimum growth temperatures are 100°C or above.
The most spectacular organism in this group is Pyrolobus fumerii, which is the current (2000 AD) world record holder of high-temperature living – with an optimum growth temperature of 106°C.
P. fumerii dies at temperatures as low as 90°C. It can, however, survive autoclaving at 121°C for an hour (a process normally used to sterilize equipment and which is normally fatal even to highly resistant bacterial endospores).
Pyrolobus cells are an irregular coccoid shape and it is an obligate chemolithotroph.
In the deep sea hydrothermal vents where it lives, no sunlight ever penetrates and Pyrolobus and similar organisms are the primary producers – performing the same ecological role as green plants in most other habitats.
Another impressive species that doesn’t quite meet the record is Pyrodictium, an irregular disc-shaped organism with an optimum growth temperature of 105°C.
Pyrobaculum is an interesting genus because some of the species in it are capable of both aerobic and anaerobic respiration. Pyrobaculum sp has an optimum growth temperature of 100°C.
Other important genera of Crenarchaeota are Desulfurococcus, with an optimum temperature of 85°C, and Staphylothermus with an optimum temperature of 92°C. Staphylothermus is a widespread species with an irregular shape.
It takes its name from its habit of aggregating in groups of up to 100 cells, like the more well-known bacterium, Staphylococcus.