The Holocene sedimentary record of cyanobacterial glycolipids in the Baltic Sea: Evaluation of their application as tracers of past nitrogen fixation
Abstract. Heterocyst glycolipids (HGs) are lipids exclusively produced by heterocystous dinitrogen-fixing cyanobacteria. The Baltic Sea is an ideal environment to study the distribution of HGs and test their potential as biomarkers because of its recurring summer phytoplankton blooms, dominated by a few heterocystous cyanobacterial species. A multicore and a gravity core from the Gotland basin were analyzed to determine the abundance and distribution of HGs at high resolution to investigate the changes in past cyanobacterial communities during the Holocene. The HG distribution of the sediments deposited during the Modern Warm Period (MoWP) was compared with those of cultivated heterocystous cyanobacteria, revealing high similarity. However, the abundance of HGs dropped substantially with depth and this may be caused by either a decrease of the cyanobacterial blooms or diagenesis, resulting in partial destruction of the HGs. The record also shows that the HGs distribution has remained stable since the Baltic has turned into a brackish semi-enclosed basin ~ 7200 yrs BP. This suggests that the heterocystous cyanobacterial species composition remained relatively stable as well. During the earlier freshwater phase of the Baltic (i.e. the Ancylus Lake phase) the distribution of the HGs varied much more than in the subsequent brackish phase and the absolute abundance of HGs was much lower than during the brackish phase. This suggests that the cyanobacterial community adjusted to the different environmental conditions in the basin. Our results confirm the potential of HGs as specific biomarker of heterocystous cyanobacteria in paleo-environmental studies.