Novel Propyl and Isopropyl Alkenones in Saline Lakes and Their Potential Paleoclimate Significance

Polyunsaturated long chain methyl and ethyl alkenones with a C37-C40 linear acyl group, produced by certain haptophyte algae in ocean and lakes, are powerful biomarkers for paleotemperature reconstructions. For over thirty years, UK37 and UK37' indices, which are based on the degree of unsaturation in C37 methyl ketones linearly correlating with water temperature, have been successfully used for sea surface temperature reconstruction over time scales of millions of years. However, studies in the past few years demonstrate, molecular structures of long chain alkenones are more diverse than previously believed, especially for alkenones in lacustrine and estuarine environments. For example, alkenones with characteristic double bond positional isomers have been found in freshwater lakes that correspond exclusively to Group I Isochrysidales defined by 18S DNA sequences. In this study, we report the discovery of novel C38 and C39 propyl and isopropyl alkenones with up to 5 double bonds in two saline lakes: Lake Van (Turkey) and Lake Dagze (Tibetan Plateau). Mass spectral fragmentation patterns of some of these new alkenones are similar to alkenones previously reported in Lake Fryxell (Antarctica) (Jaraula et al., 2010, Org. Geochem. 41, 386-197). In comparison with typical methyl alkenones, the ratio of M-43 over M ions (M is molecular ion) is much higher (up to 10 times) in these propyl and isopropyl alkenones. Hydrogenation of the novel ketones yields distinct new peaks in subsequent GC-MS analyses that differ from hydrogenated methyl and ethyl ketones, indicating that these ketones are not methyl or ethyl ketones. Instead, the same strong M-43 ion peak is observed in the mass spectra, which is consistent with propyl and isopropyl alkenones. Efforts are underway to determine the exact double bond positions of these new alkenones. Since these compounds have now been found in three saline lakes, it is possible that they are produced by a common Group II Isochrysidales species. The specific environmental significance of these species is currently unknown. However, if only one single Group II Isochrysidales species produces these propyl and isopropyl ketones, the unsaturation ratio of these ketones may be used for paleotemperature reconstructions when UK37 and UK37' indices are compromised by mixed production of C37 methyl ketones from multiple Isochrysidales species in saline lakes.