Xiao-Lei Liu

Identification and significance of unsaturated archaeal tetraether lipids in marine sediments

RATIONALE Studies of archaeal glycerol dibiphytanyl glycerol tetraethers (GDGTs) in the environment and cultures have exclusively focused on compounds with fully saturated alkyl moieties. Here we report a number of novel unsaturated GDGTs (uns-GDGTs) whose alkyl chains contain up to six double bonds and zero to two cyclopentyl moieties. METHODS The identification of these lipids was achieved via comparison of lipid distribution before and after hydrogenation, characteristic retention time patterns, and diagnostic ions using liquid chromatography/tandem mass spectrometry (LC/MS/MS), and ether cleavage products using gas chromatography/mass spectrometry (GC/MS). Isomerism resulting from different unsaturation patterns in the alkyl moieties was observed and specific positions of double bonds in the biphytene and biphytadiene moieties were tentatively assigned. RESULTS Uns-GDGTs were detected in sediment and microbial mat samples as both core lipids (CLs) and intact polar lipids (IPLs) associated with mono- or diglycosyl or phosphatidylglycerol headgroups. However, these lipids were overlooked in past investigations because conventional methods for archaeal lipid analysis are unsuitable for uns-GDGTs. Samples from distinct marine environments (Black Sea, Cariaco Basin, Discovery Basin, Eastern Mediterranean Sea, upwelling area off NW Africa, and seep sites off Crimea and Pakistan) were screened for uns-GDGTs using a new LC/MS protocol. The results show that uns-GDGTs contribute significantly to the archaeal lipid pool in anoxic methane-rich environments (Black Sea, Cariaco Basin, and both seep sites) but they were barely detected in the oxic or hypersaline settings. CONCLUSIONS The characteristic distribution of uns-GDGTs implies that they are attractive targets for future studies aiming at the chemotaxonomy of uncultivated archaea and regulation of uns-GDGT biosynthesis.

Xeropreservation of Functionalized Lipid Biomarkers in Hyperarid Soils in the Atacama Desert

Our understanding of long-term organic matter preservation comes mostly from studies in aquatic systems. In contrast, taphonomic processes in extremely dry environments are relatively understudied and are poorly understood. We investigated the accumulation and preservation of lipid biomarkers in hyperarid soils in the Yungay region of the Atacama Desert. Lipids from seven soil horizons in a 2.5 m vertical profile were extracted and analyzed using GC-MS and LC-MS. Diagnostic functionalized lipids and geolipids were detected and increased in abundance and diversity with depth. Deeper clay units contain fossil organic matter (radiocarbon dead) that has been protected from rainwater since the onset of hyperaridity. We show that these clay units contain lipids in an excellent state of structural preservation with functional groups and unsaturated bonds in carbon chains. This indicates that minimal degradation of lipids has occurred in these soils since the time of their deposition between >40,000 and 2 million years ago. The exceptional structural preservation of biomarkers is likely due to the long-term hyperaridity that has minimized microbial and enzymatic activity, a taphonomic process we term xeropreservation (i.e. preservation by drying). The degree of biomarker preservation allowed us to reconstruct major changes in ecology in the Yungay region that reflect a shift in hydrological regime from wet to dry since the early Quaternary. Our results suggest that hyperarid environments, which comprise 7.5% of the continental landmass, could represent a rich and relatively unexplored source of paleobiological information on Earth.