Identification of homoglycerol- and dihomoglycerol-containing isoprenoid tetraether lipid cores in aquatic sediments and a soil

Lipid extracts from several aquatic sediments and a compost-fertilised soil contained higher homologues of widely reported archaeal diglycerol tetraether cores. Liquid chromatography–tandem mass spectrometry indicated that the structures are based on polyols not reported in archaeal membrane lipids, homoglycerol (GH; C4H8O3) or dihomoglycerol (GDH; C5H10O3) groups, which replace one of the terminal glycerol (C3H6O3) moieties in the diglycerol lipids. The homologues include monoalkyl, dialkyl and trialkyl tetraether cores, some of which were inferred to contain cyclopentyl rings. Distributional differences between diglycerol tetraethers and associated homologues in all the samples indicate a biogenic route and not a diagenetic route to the latter. The homologues are prominent components of tetraether distributions in some samples (up to ca. 22% of isoprenoid tetraether lipid cores), are preserved in ancient sediments (e.g. Jurassic shales, 160 Ma) and occur in disparate terrestrial and oceanic settings. Hence, their presence in other sedimentary archives can be expected. The components clearly encode different information from that encoded in the diglycerol tetraethers and may allow refinement of interpretations from environmental ether lipid distributions.