The results presented in this thesis are parts of the multidisciplinary project
“Aerobic and anaerobic methane consumption in the central Baltic Sea water column” with a focus on sources, transport, distribution, and conservation of biomarkers. The aim of this thesis was to apply water column biomarker data in the identification of microbial communities and microorganisms involved in methanotrophy, and their occurrences and distribution in the water column.
Sediment samples were analyzed to assess the source and fate of biomarkers, particularly BHPs, in the sediment.
Initially, three common methods for the extraction of BHPs were tested. This study showed that the two phase solvent Bligh & Dyer extraction enabled a higher recovery of BHPs with an amino group, such as aminotriol, aminotetrol, aminopentol, or BHT cyclitol ether. Besides, the LC-MS separation and the signal of BHT II were improved because of less non-LC amenable material in the extract. The single phase solvent ultrasound and microwave extraction gave very similar results for individual concentrations. All three methods had an almost equal recovery of total BHPs supporting their general applicability for BHP extraction.
In the second study, water column samples were taken from the Landsort Deep, the deepest part of the Baltic Sea. Samples collected in summer 2011 covered the whole water column and were analyzed for biomarkers representing different members of the microbial community. An oxic zone community of cyanobacteria, algae, dinoflagellates and ciliates was identified but was restricted to the surface layer. In contrast, the deeper but still oxic cold winter water layer showed only low abundances of biomarkers. The suboxic zone biomarkers reflected abundant and diverse prokaryotic and eukaryotic microorganisms and the related biogeochemical processes such as aerobic methanotrophy, heterotrophy, sulfate reduction and the oxidation of H2S. The anoxic zone was dominated by sulfate reducing bacteria that were assumed to be the in situ source of BHPs in this water depth and, most likely, methanogenic archaea.
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The third study described the first results of the “Aerobic and anaerobic methane consumption in the central Baltic Sea water column” project. A multidisciplinary methods approach in the Gotland Deep in summer 2008 combined biomarker analysis with gas chemistry and molecular biology. 13C methane values substantiated the microbial origin of the methanein the anoxic water column.
Enriched 13C methanevalues and a strong decrease in methane concentrations indicated microbial consumption in the suboxic zone and first methane consumption rates could be calculated. DGGE analysis and the presence of indicative biomarkers with depleted δ13C values identified type I methanotrophic bacteria to be responsible for the effective aerobic oxidation of methane.
The following study also used the Gotland deep dataset collected in summer 2008.
The analysis of Biomarkers was extended to cover the oxic and suboxic water column and surface sediments as well. Type I Methanotrophic bacteria were found to be concentrated in the central suboxic zone but biomarker concentrations suggested a relatively low total contribution to the microbial community. No evidence for type II methanotrophs was found. Depleted δ13C values of relevant lipids additionally proved the process of aerobic methanotrophy. A BHT isomer that was suggested to be a marker for oxic-anoxic interfaces in other stratified systems was identified in the Gotland Deep water column. Its occurrence exclusively in suboxic to anoxic samples supported its relation to stratified systems. The Gotland Deep surface sediments reflected the BHP composition of the suboxic zone, including the BHT isomer. Therefore, BHPs were assumed to be a possible tool for the reconstruction of past stratified environments.
Since the BHP signal of the surface sediments mirrored the modern stratified water column, research was expanded to include a Gotland Deep sediment core comprising all stages of the Holocene Baltic Sea development. The analysis of sedimentary Corg
revealed low concentrations during the Baltic Ice Lake and Ancylus Lake stages, but a strong increase with the onset of the brackish-marine Littorina phase. The Medieval Climate Anomaly (MCA) and the Modern Warm Phase (MoWP) were marked by peaks in Corg concentrations. BHP concentrations reflected the Corg trend.
During Baltic Ice Lake and Ancylus Lake stages, BHP concentrations were low, but increased with the Littorina Transgression and during MCA and MoWP.
Furthermore, the BHT isomer indicative for water column stratification was absent
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in the early Baltic Sea stages and only occurred with onset of stratification. BHPs, thus, were successfully used to reconstruct the onset of the water column stratification in the Baltic Sea.
The studies presented in this thesis show that biomarkers are an important tool to gain information on the geobiology of the stratified water column of the central Baltic Sea. It was demonstrated that stratified water columns comprise various ecological niches for microorganisms that were able to adapt to these special conditions. The analysis of biomarkers in the Baltic Sea helped to identify different microbial communities, the related biogeochemical processes, and the vertical restriction of these ecological niches in the present and also the past environments.
However, the situation of the central Baltic Sea with lateral intrusions and larger inflows potentially oxidizing the whole water column has been highly dynamic since the onset of the stratification. Microorganisms were forced to not only adapt to the stable stratification but also to its disturbances. Consequently, future geobiological studies of these continuously changing interactions of geosphere and biosphere will be of great interest.
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