Iso- and Anteiso-Branched Glycerol Diethers of the Thermophilic Anaerobe Thermodesulfotobacterium commune

Thermodesulfotobacterium commune is an extremely thermophilic, anaerobic, sulfate-reducing bacterium that grows optimally at 70 °C and neutral pH. Total lipids comprise about 6.7% of the cell dry weight and consist of 17.1% neutral lipids. 12.6% glycolipids and 70.3% phospholipids. Hydrophobic residues of the glycolipids and phospholipids were released by acid methanolysis and analysed by thin-layer and gas chromatography, infrared and mass spectrometry and by chemical analyses. The hydrophobic residues consisted of sn-l,2-dialkylglycerol diethers (82%), sn-1-alkylglycerol monothers (11%) and fatty acids (7%). Five principal diether species were present containing C(16): C(16), mixed C(16): C(17), C(17): C(17), mixed C(17): C(18), and C(18): C(18) alkyl side chains. The alkyl chains of the diethers and monoethers were comprised of primarily anteiso-C(17) and lesser amounts of iso-C(16), n-C(16), iso-C(17), iso-C(18), n-C(18), anteiso-C(18), iso-C(19) and n-C(l9) hydrocarbons. Among the neutral lipids were identified free fatty acids and a series of C(16) to C(35) hydrocarbons. In contrast to the isopranoid-branched sn-2,3-diphytanylglycerol diethers and dibiphytanyldiglycerol tetraethers that characterize the lipids of thermoacidophilic, halophilic and methanogenic archaebacteria, the diethers of Thermodesulfotobacterium are levorotary (M(D) -16.7°) indicating the sn-1,2-glycerol stereoconfiguration, and they contain O-alkyl chains of variable carbon number (C(16) to C(19)). The presence of glycerol diethers suggests that this organism may have had an evolutionary episode similar to archaebacteria, but the apparent stereochemistry and absence of isopranoid-branched side chains suggests that this organism is perhaps more likely a eubacterium capable of ether lipid biosynthesis. This new class of diethers indicates that a thorough chemical analysis is required when using ether lipids as a chemical marker for the identification of archaebacteria.