Identification of unusual butanetriol dialkyl glycerol tetraether and pentanetriol dialkyl glycerol tetraether lipids in marine sediments.

RATIONALE: Glycerol serves as the principal backbone moiety bound to various acyl/alkyl chains for membrane lipids of Eukarya, Bacteria, and Archaea. In this study, we report a suite of unusual tetraether lipids in which one of the two conventional glycerol backbones is substituted by butanetriol or pentanetriol.
METHODS: Identification of these lipids was achieved via diagnostic fragments and their expected acetylation products using liquid chromatography/mass spectrometry (LC/MS), and their diagnostic ether cleavage products using gas chromatography/mass spectrometry (GC/MS).
RESULTS: We observed structural variations in the polyol backbones and alkyl chains and term these core lipid derivatives: isoprenoidal butanetriol dialkyl glycerol tetraethers (iso-BDGTs), isoprenoidal pentanetriol dialkyl glycerol tetraethers (iso-PDGTs), and hybrid isoprenoidal/branched BDGTs and PDGTs (ib-BDGTs, ib-PDGTs). Of these, iso-BDGTs were the most abundant with a methylation at either the sn-1 or sn-3 position of glycerol and were also found as part of intact polar lipids, adjoined to mono- or diglycosidic headgroups. Iso-BDGTs and iso-PDGTs are likely produced by Archaea, as indicated by the presence of the characteristic biphytanyl moieties.
CONCLUSIONS: Butanetriol- and pentanetriol-based tetraether lipids occur in modern estuarine and deeply buried subseafloor sediments, suggesting the presence of alternative backbones in archaeal lipids.