During the last 27 years, super-heat loving Archaea (and a few Bacteria) have been isolated which grow optimally at temperatures between 80C and 106C, where mesophiles and usual thermophiles are quickly killed (e.g. in the Pasteurization process). I named these organisms hyperthermophiles, which represent life at the upper temperature border. Hyperthermophiles have been discovered in water-containing terrestrial and submarine environments of active volcanism and in geothermally heated subterranean rocks. Members of the (non-spore-forming) genera Pyrodictium and Pyrolobus survive one hour autoclaving at 121C. Hyperthermophiles are adapted to their extreme environments by their physiological demands. A great deal depends only on inorganic nutrients. The energy-yielding reactions represent anaerobic and aerobic types of respiration. Within the small subunit r-RNA phylogenetic tree, hyperthermophiles occupy all the short deep branches closest to the root.
In line with their great phylogenetic diversity, hyperthermophiles display a variety of different unusual cell morphologies like disks, lobes, networks, "golf clubs", antler-like branched rods, "needles", and tiny cocci. Cells of the Nanoarchaeota consist of minicocci, only 0.4 um in diameter. Cultivation of Nanoarchaeum equitans requires the presence of a crenarchaeal host. Its small subunit r-RNA gene sequence remained undetectable in ecological studies based on the commonly used polymerase chain reaction. The N. equitans genome is among the smallest known to date (490,885 bp). Based on environmental ss r-RNA sequences (Barns and Pace 1994), the Korarchaeota had been originally suggested to have an ancestral relationship to all known Archaea although this assessment had been disputed. A complete genome sequence obtained from enrichment cultures revealed an unprecedented combination of genes thought to be characteristic of either the Crenarchaeota, Euryarchaeota, or Eukarya (Elkins et al., submitted).
