Abstract
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 of the genera Nodularia and Aphanizomenon. A multi-core and a gravity core from the Gotland Basin were analyzed to determine the abundance and distribution of a suite of selected HGs at a 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, including those isolated from Baltic Sea waters, revealing high similarity. However, the abundance of HGs dropped substantially with depth, and this may be caused by either a decrease in the occurrence of the cyanobacterial blooms or diagenesis, resulting in partial destruction of the HGs. The record also shows that the HG distribution has remained stable since the Baltic turned into a brackish semi-enclosed basin ∼ 7200 cal. yr 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 and Yoldia Sea phases), 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 a specific biomarker of heterocystous cyanobacteria in paleo-environmental studies.
Highlights
Cyanobacteria are a broad and diverse group of photoautotrophic bacteria; they are found in many terrestrial and aquatic environments (Whitton and Potts, 2012)
Bauersachs et al (2017) have recently analyzed the Heterocyst glycolipids (HGs) of eight representative heterocystous cyanobacterial strains isolated from the Baltic Sea, and the six C6 HGs targeted in our study www.biogeosciences.net/14/5789/2017/
The record shows that the HG distribution has remained stable since the Baltic turned into a brackish semi-enclosed basin ∼ 7200 cal. yr BP
Summary
Cyanobacteria are a broad and diverse group of photoautotrophic bacteria; they are found in many terrestrial and aquatic environments (Whitton and Potts, 2012). They can exist as benthos or plankton and be unicellular or filamentous with or without branches, free-living or endosymbionts (Rippka et al, 1979) and are of biogeochemical significance due to their role in the cycling of carbon and nitrogen through photosynthesis and the fixation of N2. Some N2fixing cyanobacteria can negatively impact aquatic ecosystems due to their role in harmful algal blooms (HABs): exceptional events of phytoplankton growth causing anomalous feedbacks on food webs, alterations in the geochemical features of the water column (e.g., anoxia) and sometimes the release of harmful toxins into the environment. Sollai et al.: The Holocene sedimentary record of cyanobacterial glycolipids in the Baltic Sea for exacerbating the phenomenon (Paerl, 1988; Paerl et al, 2011; Paerl and Huisman, 2009)
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