Mass spectrometric studies of ether lipids in Archaea and sediments

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been implemented as a means to separate and detect tetraether lipid cores derived from the complex lipids of Archaea. Distinct dissociation pathways during tandem mass spectrometry were noted for the lipid cores, providing information regarding their structure on-line. Analysis of cellular material from species of Methanothermobacter and Sulfolobus revealed tetraether lipid cores which contain up to four cyclopentyl rings per etherified alkyl chain, including structures identified previously in each genera. Identical structures were similarly identified in novel isolates from New Zealand hot springs. Product ions in the MS/MS spectra of the lipid cores include those formed from individual losses of both ring-containing C40 alkyl chains, allowing the reported structures to be verified with respect to the distribution of the rings within the two chains. A number of additional, hitherto unreported isomers and higher homologues of the ring-containing structures were resolved, both chromatographically and/or by characteristic product ions in MS/MS. Structures in which the two chains appear to be conjoined by a covalent link were also identified in Ignisphaera aggregans, the first such identifications in a Euryarchaeote. The array of structures revealed highlights both the complexity of the archaeal lipidome, which is more extensive than has been attributed previously, and the potential of LC-MS/MS as a powerful tool for probing tetraether lipid core structure. Ether lipid cores extracted from ancient aquatic sediments and contemporary soil were used to investigate the scope of LC-MS/MS for profiling of extremely complex distributions sourced from ecological communities as opposed to single organisms. Over 100 ether lipid components in total were identified during the studies, the vast majority of which represent novel structures. These include isoprenoid lipid cores of known archaeal origin and structures which may represent their transformation products; triolic structures in which one of the two capping glycerol moieties has been lost and chain or glycerol methylated higher homologues. A wealth of non-isoprenoid lipid cores were similarly identified, with inferred structures suggestive of a eubacterial or mixed eubacterial/archaeal origin. The components, once constrained to more specific origins, may be of chemotaxonomic value for use in modern environmental profiling or in palaeoecological reconstructions made using fossilised lipid cores.