| Summary: | The majority of planktonic species, including those that are informative in the reconstructions of past marine environmental conditions, do not produce diagnostic features (e.g., cysts, spores, or lipid biomarkers) and would therefore escape identification from the fossil record using traditional paleoecological tools (microscopy or lipid biomarker geochemistry). However, several studies have recently demonstrated that fossil DNA of planktonic species can be preserved for thousands of years and can be used for species-specific characterization using molecular biological techniques. The objectives of this thesis were to investigate the potential fate of fossil DNA and to what extent it can be used as a qualitative and quantitative biomarker for paleoecological and paleoenvironmental reconstructions. For this goal, fossil DNA of several groups of planktonic protists was analyzed in various marine settings, and validated by comparing the results to microscopical identification and lipid biomarker analyses. A Holocene sediment core from Ellis Fjord, Antarctica, spanning 2700 years of deposition was used to study taxon-specific variation in the level of preservation of fossil DNA. The results showed that post-depositional fragmentation of DNA was highest for dinoflagellates, followed by diatoms and lowest for phototrophic green sulfur bacteria (GSB) stemming from the ancient sulfidic chemocline. Dinoflagellate cysts were rare in the sediment record and despite an exponential decline in fossil dinoflagellate DNA, paleogenetics was the only approach that revealed an important shift in dinoflagellate communities around 1850 years ago, indicative of colder climate and an increased ice cover. In similar aged anoxic sediments from the much deeper (2000 m) permanently stratified Black Sea, a significant fraction (~30%) of fossil DNA of the calcifying haptophyte Emiliania huxleyi, an important species involved in global oceanic C and N cycling and a source of alkenones as a proxy for past oceanic sea surface temperatures ...
|
|---|