Five types of molecular species of C40 isoprenoid chains, having different numbers of cyclopentane rings, were detected in the ether core lipid of Thermoplasma acidophilum. The average cyclization number of the hydrocarbon chains in the lipids increased with increasing growth temperatures.

Archaea have unique membrane lipids typified by ether linkages of the glycerol-to-isoprenoid chains with sn-2,3 stereochemistry that runs against the naturally occurring sn-1,2 stereochemistry of the glycerophospholipids of Bacteria and Eukarya. Membrane lipids were extracted and analyzed from the hyperthermophilic archaeon, Thermococcus kodakaraensis, cultivated at various temperatures. At all growth temperatures examined, both the diphytanylglycerol diether (archaeol, C(20)) and diphytanyldiglycerol tetraether (caldarchaeol, C(40)) were identified as saturated forms, and no other lipids could be identified. The ratio of caldarchaeol to archaeol increased with increasing growth temperature, particularly at 93 degrees C. A larger amount of archaeol was detected from cells in the logarithmic phase than from those in the stationary phase at all temperatures examined. These results indicate that T. kodakaraensis modulated the membrane lipid composition depending on both the growth phase and the growth temperature, and suggest that the membrane fluidity to environmental change was maintained by altering the length of the hydrocarbon chains, and not by side-chain saturation such as double-bond hydrogenation nor by such a modification as cyclopentane ring formation.

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The order Thermoplasmales includes the wall-less strains in the Archaea. The genus Thermoplasma (two species), the genus Picrophilus (two species), and the genus Ferroplasma (one species) are present. The strains of Thermoplasma include the thermophilic acidophile (T. acidophilum grows between 45 and 62 , optimum 59 , pH 1~4, optimum 1~2 (1); T. volucanium does between 33 and 67 , optimum 60 , pH 1~4, optimum 2 (2)) and the cells are surrounded by a single triple-layer membrane, 5~10 nm thick. The family Picrophilaceae is the thermophilic hyperacidophile growing between 47 and 65 and between pH 0 and 3.5. This family is separated from the family Thermoplasmaceae based on their base sequence of the 16S rRNA gene, DNA-dependent RNA polymerase, and the presence of an S-layer (3). In contrast to these families, the family Ferroplasmaceae, also a wall-less Eur-yarchaeon, grows in the quite different temperature range between 15 and 45 , optimum 35 , pH 1.3~2.2, optimum 1.7 (4). Among these wall-less representatives, only the structure of the membrane lipids of T. acidophilum has been well investigated. The structure of the main membrane lipids of T. acidophilum is caldarchaeol (2, 2 , 3, 3-tetra-O-di(biphytanyl)-sn-diglycerol) (5, 6), one of the ether core lipids of the archaeal strains. Five types of molecular species of the C 40 isoprenoid chain, classified by the number of cyclopentane rings, have been detected in the caldarchaeol of the extreme thermophilic Crenarchaeon such as Sulfolobus solfataricus. These are acyclic-, monocyclic-, bicyclic-, tricyclic-, and tetracyclic-C 40 isoprenoid chains (7). It has been observed that a decrease in the number of double bond in the membrane depends on lowering the environmental temperature for controlling the membrane fluidity in Eucarya and Bacteria. In S. solfataricus, it has been

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The major ether-type lipid structures of Sulfolobus acidocaldarius (ATCC33909) were composed of caldarchaeol and calditoglycerocaldarchaeol. However, the characterization by nuclear magnetic resonance spectroscopy and mass spectrometry showed that the structure of calditol in calditoglycerocaldarchaeol is not nonitol, 2-(1',2',3'-trihydroxypropyl)1,2,3,4,5,6-hexahydroxyhexane, but 2-hydroxymethyl-1-(2,3-dihydroxypropoxy)2,3,4,5-cyclopentanetet raol with an ether linkage in the molecule. Such an intermolecular ether linkage was resistant to BCl3 treatment, but nonresistant to 57% HI degradation treatment conducted at 100 degrees C for 60 h, producing 2-hydroxymethyl-1,2,3,4,5-cyclopentanepentaol from calditol as reaction product. Further, it was confirmed that the structure of calditol is essentially a derivative of glycerol, and hydrocarbon chains were conjugated to the glycerol-like site in the structure. The calditol with an ether linkage in the molecule suggested an important role regarding the properties of heat-resistance and acid-resistance observed in Sulfolobales.

Although the phylogenetic taxonomy of the microorganisms is performed by two types of methods, the gene dependent and those of the phenotype dependent, the relation between a phylogenetic classification and membrane lipids is particularly important in Archaea. The specific membrane lipid of Archaea is capable of clearly distinguishing that of bacteria and eukaryota. That is, the membrane lipid of Archaea has a peculiar core lipid in which the isoprenoid chains are conjugated with ether bonds at the sn-2 and sn-3 sites of glycerol (1,2). In recent years, the isolation and identification of many hyperthermophilic Archaea obtained from the sea around the world have been carried out, and are being watched with keen interest. In hyperthermophlic Archaea, Thermococcales consist of three genera, the Pyrococcus, Thermococcus, and Palaeococcus (3). Most of these strains could propagate at 90 or more, and all of them were anaerobic microorganisms with the optimum pH around neutral. De Rosa et al. reported that the main core lipid of Thermococcus celer is the diether type archaeol (4). Furthermore, it was reported by Lanzotti et al. that the main core lipid of Pyrococcus wosei is archaeol (5). However, when we analyzed the lipid of Pyrococcus horikoshii, we detected caldarchaeol as the main core lipid and archaeol as the minor component at the cultivation temperature of 82-98 (6). Although the lipid