Diatoms are one of the major groups of algae which originated relatively recently and evolved in the Late Jurassic/Cretaceous. This thesis presents the results of a comprehensive study of diatom lipids in cultures and in the environment and their applications in the age determination of petroleum and in palaeoenvironmental studies. Diatom DNA sequences were analyzed in order to relate the phylogenetic positions of diatoms to the lipid chemotaxonomy. Forty four different sterols were identified in diatoms, with 24-methylcholesta-5,24(28)-dien-3-beta-ol being most common. 24-Methylcholesta-5,22E-dien-3-beta-ol, previously described as a diatom biomarker, was only the fifth most common sterol and absent in some major diatom groups. All identified sterols have been reported in other algae, but within the diatoms, some sterols and sterol compositions seem to be specific for specific phylogenetic clusters. Sterol compositions confirmed the separate phylogenetic position of the genus Attheya, as also indicated by molecular phylogeny and microscopy. 23-Methyl and 23,24-dimethyl sterols, often associated with dinoflagellate algae, were present in a substantial number of diatoms, suggesting that diatoms may also be a major source. Their phylogenetic position suggests that these diatoms originated from a single common ancestor which evolved in the late Jurassic. In addition to 23-methyl and 23,24-dimethyl sterols, the unusual sterol gorgosterol was found in two diatom cultures of the genus Delphineis. 24-Norsterols were found in the diatom species Thalassiosira aff. antarctica and in dinoflagellate cultures. The evolutionary history of dinoflagellates and diatoms explains the stepwise increases of 24-norsterane concentrations, diagenetic products of 24-norsterols, in petroleum. Long-chain 1,14-diols and 12-hydroxy methyl alkanoates were detected in Proboscia diatoms and may be used as indicators for high-nutrient conditions and upwelling. Their distributions varied between different Proboscia species and culture experiments showed increasing chain-lengths and a decreasing degree of unsaturation for 1,14-diols with increasing growth temperature. Lipid analyses from surface sediments from the eastern South Atlantic suggested a significant relationship between long-chain 1,14-diol chain-length and sea surface temperature, but the degree of unsaturation for 1,14-diols seems also determined by other factors. Sediment trap data from the Arabian Sea confirmed that Proboscia lipids can be used as proxies for upwelling conditions and also showed that long-chain 1,15-diols are not limited to upwelling conditions. A sediment core taken from the Somali continental slope showed strong fluctuations of long-chain 1,14- and 1,15-diols with time; 1,14-diols were relatively high during the Holocene when upwelling occurred and much lower during the Late Glacial Maximum and the last Glacial when upwelling was suppressed. Elevated 1,14-diol concentrations during the first half of Marine Isotope Stage 3 and at the end of Marine Isotope Stage 5.1 suggest intensified glacial upwelling during those periods. Analyses of long-chain diols in a sediment core from the North Western Antarctic Peninsula suggest that Proboscia diatom productivity in this area is associated with upwelling of Upper Circumpolar Deep Water at the shelf break. Comparison of the diol record with melt events in Siple Dome ice core indicates that this upwelling is driven by the same climatic processes that are responsible for changes in regional climate
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