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 H
2/S
0 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.