Crenarchaeotes are phylogenetically distinct from Euryarchaeotes and contain representatives that live at both ends of natures's temperature extremes: boiling water and freezing water. Most cultured crenarchaeotes are hyperthermophiles (growth temperature optima above 80°C) and some actually have optima above the boiling point of water.
Habitats of Crenarchaeotes
Habitats of Crenarchaeota include very hot and very cold environments. Most hyperthermophilic Archaea have been isolated from geothermally heated soils or waters containing elemental sulfur and sufides and most species metabolize sulfur in one way or another. In terrestrial environments, sulfur-rich springs, boiling mud, and soils may have temperatures up to 100°C and are mildly to extremely acidic owing to production of sulfuric acid (H2SO4) from the biological oxidation of H2S and S0. Such hot, sulfur-rich environments are called solfataras, and are found throughout the world; extensive solfataras are present in Italy, Iceland, New Zealand, and Yellowstone National Park. Depending on the surrounding geology, solfataras can be slightly alkaline to mildly acidic, pH 5-8, or extremely acidic, with pH values below 1 not uncommon. Hyperthermophiles have been obtained from both types of environments, but the majority of these organisms inhabit neutral or mildly acidic habitats. In addition to these natural habitats, hyperthermophilic Archaea also thrive within artificial thermal habitats, in particular the boiling outflows of geothermal power plants.
Hyperthermophilic Crenarchaeota also thrive in undersea hot springs called hydorthermal vents. Submarine waters can be and often are much hotter than surface waters because the water is under pressure. Indeed, all hyperthermophiles with growth temperature optima above 100°C have come from submarine sources. The latter include both shallow (2-10 meters) vents such as those off the coast of Vulcano, Italy, to deep (2500-4000 meters) vents near ocean spreading centers. The latter are the hottest habitats so far known to yield living organisms.
Energy Metabolism
With a few exceptions, hyperthermophilic Crenarchaeota are obligate anaerobes. Their energy-yielding metabolism can be either chemoorganotrophic or chemolithotrophic (or both, for example Sulfolobus) and involves a wide diversity of different electron donors and acceptors. Fermentation is rare and most bioenergetic strategies are either aerobic or anaerobic respirations. Energy conservation during these respiratory processes occurs by the same general mechanism widespread in Bacteria: electron transfer within the cytoplasmic membrane leading to the formation of a proton motive force from which ATP is made by way of protontranslocating ATPases.
Many hyperthermophilic crenarchaeotes can grow chemolithotrophically under anoxic conditions with H2 as electron donor and S0 or NO3- as electron acceptor; a few can also oxidize H2 aerobically. H2 respiration with ferric iron (Fe3+) as electron acceptor also occurs in several hyperthermophiles. Other chemolithotrophic lifestyles include the oxidation of S0 and Fe2+ aerobically or Fe2+ anaerobically with NO3- as acceeptor. Only one sulfate-reducing hyperthermophile is known (the euryarchaeote Archaeoglobus). The only bioenergetic option apparently ruled out is photosynthesis. The most thermophilic phototrophic organism known can grow up to about 70°C - too cold for most hyperthermophiles!
Taken from the text Brock Biology of Microorganisms (10th ed.). Madigan, M.T., Martinko, J.M., and Parker, J. 2003. Prentice Hall. 461-463p.