Evolution of the plankton paleome in the Black Sea
Thus far, sedimentary paleomes have only been studied for the presence of specific plankton species such as haptophytes using small clone libraries. In a first attempt to reconstruct complete past planktonic ecosystems we explored the use of high-throughput next generation sequencing (NGS) methods: Roche 454 pyrosequencing of sedimentary 18S rRNA genes revealed that 150 out of 2710 species showed a statistically significant response to four environmental stages in the Black Sea since the last 11,400 years. Diversity and relative abundance of species quickly changed at important late-Pleistocene and Holocene climate shifts. Vice versa, the plankton paleome enabled us to determine the exact timing of these climate shifts for this part of continental Europe: Freshwater taxa predominated during the dry Preboreal (11.4-9.0 ka BP), although a co-presence of previously unidentified marine taxa indicated that the Black Sea was to some extend influenced by the Marmara Sea since 9.6 ka cal BP, six centuries earlier as reported previously. Major changes in various coastal marine taxa occurred at the transition from the warm and most Holocene climatic optimum (HCO; 9.0-5.2 ka cal BP) and the dry Subboreal (5.2-2.5 ka cal BP) as well as with the onset of the wet and colder Subatlantic climate (last 2500 years)2. Changes in temperature and salinity associated with these environmental and ecological shifts were confirmed using the alkenone unsaturation index (SST) and D/H isotopic ratios (SSS) (Giosan et al., 2012). A final shift in diversity occurred since the last few centuries linked to a strong increase in anthropogenic perturbations (Coolen et al., in review at PNAS). Statistical analysis showed that 25% of the quantitative paleome data could be explained by the past environmental changes. In addition this study showed for the first time that relatively long DNA fragments (~560 bp) of past plankton species were preserved even in the oldest TOC-poor lacustrine sediments, which were deposited under fully oxygenated conditions.