The phylum Euryarchaeota

This phylum also includes extremophiles but also numbers mesophiles including some of the methanogens between its members. The methanogens include like genera as Methanobacterium, Methanococcus and Methanosarcina. The phylum is physiologically diverse with representatives in the extreme thermophiles example for Picrophilus and the cell wall frees Thermoplasma, the halophiles Halococcus, Natronococcus and several oligotrophic marine Archaea. The phylum may also include the organism with the smallest known Archaeal genome, Nanoarchaeum equitans. This intracellular parasite of Ignicoccus an extreme thermophile isolated from an undersea hot water system off the coast of Iceland has been the subject of some debate. Some taxonomists maintain which the organism should be classified in its own phylum Nanoarchaeota.

Pyrococcus furiosus is a very thermophilic euryarchaeote and was the source of the Pfu polymerase used as an instead to Taq polymerase in the PCR reaction. Although Taq, of bacterial origin, and Pfu, of Archaeal origin, can be substituted for one another in the in vitro technique of PCR, the two proteins illustrate a difference among the two kingdoms. Pfu is the main replicative DNA polymerase of Pyrococcus  furiosus, but  has little  in general with  the  eubacterial equivalent, DNA polymerase. Taq is a DNA polymerase I-type protein, with a function in DNA repair but it is not since known whether Pfu or other proteins perform the equivalent role in P. furiosus. P. furiosus is attractive for PCR as it has the quickest doubling time of the Archaea, rep- licating its genome in only 37 minutes. To do this it must be growing at among 70 and 103?C and studies on this organism reveal which even at its maximum growth temperature the DNA within the cell remains double-stranded without breaks, something which cannot be achieved if pure DNA is boiled in water. It is also almost as resistant to radiation as Deinococcus.

The methanogens are a diverse group of the Euryarchaeota including acidophiles, methanogenic thermophiles, halophiles and mesophiles. They are linked through the ability to produce methane from C1 compounds or acetate and an obligately anaerobic way of life. The model organism for the genus is Methanocaldococcus jannaschii one of various methanogens to have had their genomes sequenced. Comparative genomics reveals that M. jannaschii has same enzymes to the Bacteria in its central metabolic pathways but a greater similarity to the Eukaryotes in DNA transcription, replication and translation. Not with- standing 50percent of the genes of M. jannaschii have absolutely no counterpart in any organism so far sequenced. The organism was named after Holger Jannasch, a pioneer in the field of deep sea microbiology. The organism was isolated from a white smoker in the East Pacific and consequently the growth requirements of M. jannaschii are strict:  carbon dioxide, no oxygen, hydrogen a few mineral salts, temperature of around 85?C and a significant amount of pressure. This barophile can withstand up to 200 atmospheres of pressure.