Phytanyl-glycerol ethers and squalenes in the archaebacteriumMethanobacterium thermoautotrophicum

Summary

The lipids of a thermophilic chemolithotroph,Metbanobacterium thermoautotropbicum, have been analyzed by chromatographic techniques and identified by infrared spectrometry and combined gas chromatography-mass spectrometry. Of the total chloroform soluble lipids 79% and 21% are polar and non-polar lipids, respectively. The major components of the polar lipids are dialkyl ethers of glycerol or its derivatives. The nature of the glycerol ether alkyl groups was found to be that of the saturated tetraisoprenoid hydrocarbon phytane. The non-polar lipids of the chloroform soluble fraction consist principally of three series of C₂₀, C₂₅ and C₃₀ acyclic isoprenoid hydrocarbons, the major components being squalene and a continuous range of hydrosqualene derivatives, from dihydrosqualene up to and including decahydrosqualene. These data establish thatM. tbermoautotropbicum contains predominantly non-sapo-nifiable lipids as doHalobacterium, Halococcus, Sulfolobus andTbermoplasma. In particular, the composition of the chloroform soluble lipids ofM. tbermoautotropbicum is quite similar to that ofHalobacterium cutirubrum. The results strongly support the recent proposal, based on 16S rRNA sequence homologies, that the extreme halophiles and methanogens share a common ancestor. In addition, it is pointed out that the occurrence of phytane and related polyisoprenoid compounds in ancient sediments can no longer be considered unequivocally as indicative of past photosynthetic activity. Finally, speculations are made concerning the possible role of and evolutionary significance of the presence of squalene and hydrosqualenes in these organisms. To our knowledge this is the first report of squalene and hydrosqualenes in a strictly anaerobic microorganism.