•Homologues of isoprenoid GDGT lipid cores found in sediments and soil.•Each contains a homoglycerol or dihomoglycerol group plus glycerol.•Cyclopentyl ring-containing structures apparent, as are monoalkyl and trialkyl structures.•Lipids are most likely biosynthetic products of archaea.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.

A glycerol dialkanol triol, similar in structure to glycerol dibiphytanyl glycerol tetraethers but devoid of the carbon atoms of one of the two glycerol termini, has been identified in Messinian sediments (∼6 Ma) and characterised using quadrupole ion trap and Fourier transform-ion cyclotron resonance liquid chromatography-tandem mass spectrometry.

Liquid chromatography–tandem mass spectrometry of membrane lipid cores from Sulfolobus species reveals isomeric forms of ring-containing isoprenoid glycerol dialkyl glycerol tetraether components not previously recognised via the use of NMR and liquid chromatography–mass spectrometry techniques. Equivalent isomerism was confirmed for the components in other hyperthermophilic genera and in sediments which contain the lipids of mesophilic archaea. The recognition of the isomeric structures in distinct archaeal clades suggests that profiles of tetraether lipids reported previously may have oversimplified the true lipid complexity in archaeal cultures and natural environments. Accordingly, the extent of variation in tetraether structures revealed by the work should direct more informative interpretations of lipid profiles in the future. Moreover, the results emphasise that tandem mass spectrometry provides a unique capability for assigning the structures of intact tetraether lipid cores for co-eluting species during chromatographic separation.Graphical abstractHighlights► Isomerism of GDGT lipids with respect to the distribution of cyclopentyl rings between isoprenoid chains in Sulfolobus. ► Isobaric GDGT structures with differing polarities also identified. ► Putative stereoisomers of isoprenoid chains containing two and three cyclopentyl rings released by ether-cleavage of tetraether lipids. ► Isomerism in lipids extends to other hyperthermophilic archaea and to mesophilic archaea living in moderate temperature environments.

The lipid cores from Ignisphaera aggregans, a hyperthermophilic Crenarchaeon recently isolated from New Zealand hot springs, have been profiled by liquid chromatography–tandem mass spectrometry. The distribution revealed includes relatively high proportions of monoalkyl (also known as H-shaped) tetraether cores which have previously been implicated as kingdom-specific biomarkers for the Euryarchaeota. Such high expression of monoalkyl tetraether lipids is unusual in the archaeal domain and may indicate that formation of these components is an adaptive mechanism that allows I. aggregans to regulate membrane behaviour at high temperatures. The observed dialkyl tetraether and monoalkyl tetraether lipid distributions are similar but not fully concordant, showing differences in the average number of incorporated rings. The similarity supports a biosynthetic route to the ring-containing dialkyl and monoalkyl tetraether lipids via a dialkyl tetraether core containing zero rings, or a closely related structural relative, as an intermediate. Currently, however, the precise nature of the biosynthetic route to these lipids cannot be deduced.

Reduction of the C-3 vinyl group to an ethyl substituent is a key, yet poorly understood, event in the diagenetic pathway linking chlorophyll a to sedimentary alkyl porphyrins. Laboratory simulation experiments employing conditions that mimic those of natural anoxic environments (aqueous media, low temperature, presence of H2S and limitation of oxygen) lead to partial conversion of the chlorophyll a derivative methyl pyrophaeophorbide a to mesopyrophaeophorbide a via reduction of the C-3 vinyl substituent mediated by H2S. The mild conditions employed, and the apparent susceptibility of methyl pyrophaeophorbide a to reductive transformation, implies that reduction mediated by H2S could be a widespread process occurring in anoxic depositional environments. Our findings have implications for the geological preservation of chlorins and other biological markers and provide clear evidence that reactions involving H2S are important in the geochemical reduction of sedimenting organic matter and can occur at temperatures within the range for natural environments.

A glycerol dialkanol triol, similar in structure to glycerol dibiphytanyl glycerol tetraethers but devoid of the carbon atoms of one of the two glycerol termini, has been identified in Messinian sediments (∼6 Ma) and characterised using quadrupole ion trap and Fourier transform-ion cyclotron resonance liquid chromatography-tandem mass spectrometry.