A remarkable paradox: Sulfurised freshwater algal (Botryococcus braunii) lipids in an ancient hypersaline euxinic ecosystem

Two relatively immature hypersaline sediments of Miocene/Pliocene age from the Sdom Formation, Dead Sea, Israel were studied using both GC–MS and irm–GCMS analyses. A novel series of extractable organosulfur compounds (OSC) derived from functionalised lipids of freshwater Botryococcus braunii algae races B and L were tentatively identified based on their mass spectra and Raney nickel desulfurisation products. Desulfurisation of the polar fractions released high amounts of apolar components, attributed to the major part of the macromolecular matrix being comprised of multiple sulfur-linked biomarkers derived from a limited number of highly functionalised lipids of Botryococcus. Most of the components appear to be early thermal released products and are not directly formed from sulfur incorporation into functionalised precursor lipids. One sample is mainly comprised of lipids derived from race B and possibly A and L, and the other sample is comprised of lipids derived from B. braunii races B and L. Carbon isotopic signatures of the individual biomarkers derived from the three races are widely dispersed. For example, δ values of components derived from the L race are ca. −21‰ and those from the B race are significantly enriched in 13C having values in the −10 to −13‰ range. Overall, B and L race derived components are 13–20‰ and 5–7‰, respectively, more enriched in 13C than phytoplanktonic biomarkers of marine origin. Stable carbon isotopic data of the B. braunii components point to differing bloom periods for the three races. The specific structures of the biomarkers (especially OSC) provide compelling evidence for the existence of freshwater algae in ancient hypersaline environments in a stratified water body.