Higher order taxa:
Archaea; Euryarchaeota; Thermococci; Thermococcales; Thermococcaceae
Pyrococcus horikoshii, P. abyssi, P. furiosus, P. endeavori
Description and Significance
Pyrococcus has similar characteristics of other archaea such as Archaeoglobus, Thermoautotrophican, and Methanococcus in the respect that they are all thermophilic and anaerobic. Pyrococcus differs, however, because it's optimal growth temperature is nearly 100 oC and dwells at a greater sea depth than the other archaeons. Studying Pyrococcus helps give insight to possible mechanisms used to endure extreme environmental conditions like high temperatures and high pressure.
Three of the Pyrococcus species have been sequenced. P. furiosus is the largest containing 1908256 bp followed by P. abyssi with 1765118 bp and P. horikoshii with 1738505 bp. The genomes encode for many different metabolic enzymes which gives themselves a wider spectrum of living conditions because they can transport and metabolize a wide range of organic substances. Variation was detected between species as well.
Cell Structure and Metabolism
The cells of Pyrococcus are about 0.8- 2um and are slightly irregular cocci in shape. They show a polar grouping of flagella and are enveloped by an S-layer enclosing a periplasmic space around the cytoplasmic membrane. Pyrococcus species are anaerobic but vary slightly concerning their metabolism. Peptide fermentation is the principle metabolic pathway however, growth has been observed for P. furiosus and P. abyssi on starch, maltose, and pyruvate but not for P. horikoshii. While the presence of elemental sulfur is not needed for growth, growth is enhanced with the addition of So.
Pyrococcus species inhabit environments with extremely high temperatures such as undersea hot vents. Optimal growth conditions include a pH level of about 7, a salt concentration around 2.5%, and a temperature around 98oC. Growing in temperatures this high, it is easy to see why they are anaerobic since at these boiling temperatures hardly any oxygen will be available. In the example of undersea hot vents, where P. abyssi has been found, there is no sunlight and the pressure is around 200 atm in addition to the extremely high temperature.
Kawarabayasi et al. 1998. Complete Sequence and Gene Organization of the Genome of a Hyper-thermophilic Archaebacterium, Pyrococcus horikoshii OT3. DNA Research 5: 55-76
Cohen et al. 2003. An integrated analysis of the genome of the hyperthermophilic archaeon Pyrococcus abyssi. Molecular Microbiology 47: 1495-1512
Gonzalez et al. 1998. Pyrococcus horikoshii sp. nov., a hyperthermophilic archaeon isolated from a hydrothermal vent at the Okinawa Trough. Extremophiles 2: 123-130.