Thermal Organism: uncultured Ignicoccus sp.

Ignicoccus is a novel hyperthermophile that may contain a true "outer membrane" similar to the outer membrane of Bacteria. The outer membrane of Ignicoccus is unusual, however, in that it is formed at some distance form the cytoplasm of the cell; this arrangement yields an unusually large periplasm. Indeed, the volume of the periplasm of Ignicoccus is some 2-3 times that of its cytoplasm, in contrast to that of gram-negative Bacteria, where periplasmic volume is about 25% that of the cytoplasm. The periplasm of Ignicoccus also contains membrane-bound vesicles that may funtion in exporting substances outside the cell. Ignicoccus grows optimally at 90°C, and its metabolism is H₂/S⁰ based, as is that of so many hyperthermophilic Archaea. The function of the unusual periplasm of Ignicoccus is unclear, and chemical studies of its outer membrane are needed to determine whetehr it contains a lipopolysacharide component similar to that of Bacteria. However, regardless of the outcome of such studies, Ignicoccus stands out among hyperthermophilic Archaea (and indeed, all known prokaryotes) as being structurally unique.

Taken from the text Brock Biology of Microorganisms (10th ed.). Madigan, M.T., Martinko, J.M., and Parker, J. 2003. Prentice Hall. 467-468p.