Biology
In recent years, lakes have faced rising pressure from anthropogenic activities and climate warming, and the aquatic communities of some lake ecosystems are reshaping in ways that can form harmful algal blooms. It is crucial to understand how lake phytoplankton communities respond to environmental stressors under varying environmental conditions. The cell size of phytoplankton has multiple important implications for the dynamics, diversity, and productivity of a phytoplankton community. Empirical investigations in lakes showed that the size composition of phytoplankton communities differs with inorganic nutrient conditions, grazing pressure (usually quantified by zooplankton abundance), and water temperature. However, it is not clear how these three factors interact to shape the size composition of lake phytoplankton. In this thesis, I use size-based plankton modelling to elucidate how a trade-off mechanism, dependent on inorganic nutrient availabilities and zooplankton size-specific grazing strategies, shapes the dynamics, the size composition, and the exclusion pattern of phytoplankton in a generic temperate lake. Lastly, I recast the model to a specific Swiss lake, Greifensee, by using high-frequency data comprising phytoplankton cell size (biovolume) and plankton abundances. In summary, this thesis investigates the interactive effects of inorganic nutrient regimes and zooplankton grazing strategies on the community dynamics and compositions of lake phytoplankton and offers a glimpse into the future size compositions of phytoplankton and nutrient and plankton dynamics of Greifensee. The results not only advance our understanding of plankton communities in temperate lakes, but they also identify hypotheses related to zooplankton grazing strategies that can be further tested experimentally. The data-driven modelling approach presented here can contribute to strategic conservation and management plans for mitigating the effects of ongoing environmental change.
The Arctic is experiencing warming to a much higher degree compared to other regions on Earth. The annual mean surface temperature between 1971 and 2019 was three times higher than the global average (AMAP 2021). While previous warm episodes like the Early Arctic warming occurring from the 1920s to 1960s are known to be driven by natural processes like changes in the North Atlantic Oscillation (NAO), recent studies provide evidence that the ongoing Arctic warming process differs from the earlier phases and that it is caused by anthropogenically induced large-scale global warming. Consequences for the local marine ecosystem can be, among other things, a shift in species abundance and distribution from a polar towards a more boreal community. In this context, fish species like the Atlantic cod (Gadus morhua) are reported to have risen in abundance in the Arctic region over the last decades.
This dissertation focuses on the effects of climate change and climate variability on the population structure and spatial distribution of Atlantic cod in the fjords of Svalbard. A combination of different methods was used and included fishing campaigns in several locations on the coast of Svalbard, genetic studies on the ecotype composition of the catches, otolith studies on subpopulation structure, and year-round in situ camera observations on the occurrence of juvenile cod in the shallow waters of the Kongsfjorden ecosystem.
Based on the combined use of observation, otolith shape, and genetic tools, a local ecotype, the “Svalbard coastal cod (SCC)” could be identified as a potential permanent resident in Svalbard fjords. Eggs or larvae of Atlantic cod could not be identified in the catches, nevertheless, eggs of long rough dab have been found during the ichthyoplanktonic surveys indicating spawning in the area. Both, Atlantic cod, and the long rough dab share a similar lifestyle and demonstrate the recent invasion of boreal species into the Arctic marine ecosystem.
Microplastics (MP) have received increased scientific, political and societal attention due to their environmental omnipresence. This thesis aims to provide comprehensive data on aquatic MP pollution through the application of state-of-the-art analytical methods, and compares data outputs from two data pipelines.
Within the River Weser–North Sea transitional system, small MP (<500 µm) predominated, with a notable abundance of suspected paint particles. Large MP (>500 µm) exhibited low abundances, mainly composed of the common plastic polymers polyethylene and polypropylene. The estuary’s turbidity maximum zone showed the highest MP concentrations, then declining towards the North Sea, possibly influenced by increased vertical and horizontal export or dilution in the larger marine water body. Additionally, this thesis evaluated two wastewater treatment plants as potential riverine MP point sources. Interference by post-processing residual material required an adaptation of the FTIR reference database by the inclusion of new reference material. Results showed that polyolefins were prevalent in the effluent, and that observed temporal patterns in MP concentrations could be partially explained by technical and environmental parameters. Input of MP into the River Weser via effluent is likely, necessitating more research to understand the full dynamics of MP pollution within this river system. Furthermore, the MP analysis pipeline comparison study showed discrepancies for certain polymer types, possibly due to different polymer grouping methods, or overestimation effects. By excluding these polymer types, both datasets generally were in accordance, suggesting a harmonization of both pipelines should be undertaken to improve comparability of MP data.
In summary, this thesis provides a detailed foundation for understanding MP dynamics in the River Weser–North Sea system and highlights methodological challenges inherent in the field of MP pollution research.
The temporal and spatial dynamics of the sublittoral fish community of Kongsfjorden, Spitsbergen
(2021)
The Arctic is one of the areas that is most affected by global climate change (IPCC 2014). As a result of anthropogenic Arctic warming, the Arctic fish community might change, and species from temperate areas are expected to invade (Cheung et al. 2009). In this context, it is critical that “Arctic marine fishes are indispensable to ecosystem structuring and functioning, but they are still beyond credible assessment due to lack of basic biological data“ (Christiansen et al. 2014). Especially in the shallow-water zones (3 - 12 m) of Arctic fjord systems only qualitative data on fish community composition are available. In other ecosystems, it was shown that the structured shallow-water zone has special ecological functions (Seitz 2014, Pondella et al. 2015). Therefore, the objective of this study is to increase our scientific knowledge on the fish assemblage of this special area by performing a quantitative first-time assessment of its species composition and abundance as well as the size and age structure of selected species.
As study site, Kongsfjorden (79°N, 12°E) at the west coast of the Svalbard archipelago was chosen. It is one of the best investigated fjords in the Arctic, and the local AWIPEV research base provides one of few sites where this project could be logistically supported. Despite the local infrastructure, access to the field was restricted due to the challenging climatic regime of Kongsfjorden. A thorough risk assessment resulted in the finding that no fishing from small boats can be performed safely during the polar night. Fish assessments were therefore conducted with two complementary methodologies. The first method was seasonal fyke net fishing in June/July and September of the years 2012 to 2014. The second method was a year-round assessment via a stereo-optic imaging system, which was connected to an underwater observatory.
In the course of evolution, the structure and topology of cells have changed greatly. From simple cells surrounded only by a single membrane to cells containing multiple membrane-enclosed organelles. The advantage of individual reaction compartments is offset by the fact that proteins required there are almost exclusively nuclear-coded. They have to be targeted to and imported into their compartment post-translationally. One of the protein import systems involved, which can even transport large fully folded proteins, is the twin-arginine translocation (TAT) system. It operates both in the thylakoidal membrane and the plasma membrane of a wide range of bacteria. The thesis focuses on Tha4, the potential channel forming subunit of the TAT system of Arabidopsis thaliana. To verify formation of homo-oligomeric membrane channels purified Tha4 protein was used. The oligomeric topology of fluorescence-labelled Tha4 was investigated using fluorescence fluctuation spectroscopy (FFS). The functional properties of the membrane reconstituted Tha4 was investigated using the electrophysiological bilayer technique both in the vertical setup and with the combined fluorescence-opto-electrical horizontal bilayer (HLB) technique. Electrophysiological measurements with artificial planar lipid bilayers revealed cation-selective ion-channels for reconstituted Tha4 with a pore width of 1 nm. Similar results were obtained for co-reconstituted Tha4-Hcf106 proteins in vertical planer bilayers. The investigation of the oligomeric state in solution and in artificial planar lipid bilayers using FFS at the single molecule level with fluorescence-labelled proteins revealed that addition of soluble, NBD-labelled Tha4 showed a strong accumulation in the membrane. Subsequent lifetime measurements disclosed that the NBD-labelled parts are located more close to the polar part of the membrane, in close vicinity of the membrane-water interface.
Microplastics (MP) in the marine environment are of global concern. The European Marine Strategy Framework Directive (MSFD), targeting marine pollution, provides a legal framework for MP research in the North Sea. Utilizing state-of-the-art methods, this thesis presents a comprehensive dataset of concentrations, polymer types, size distribution, weathering status, and spatial distribution of MP (11–5000 µm) in sediments and surface waters of the southern North Sea. In surface waters, low-density polymers like polyethylene (PE) and polypropylene (PP) dominated, with the highest MP concentration (245 particles m-3) detected close to the English Channel and the Rhine delta. In sediments, PP was omnipresent but less abundant, accompanied by varnishes and rubbers, and the highest concentration (1189 particles kg-1) found in the central southern North Sea. Weathering status, assessed through carbonyl indices for PE and PP, differed significantly for the compartments and was on average lower for large MP (500–5000 µm) than for small MP (10–500 µm), potentially indicating a weathering dependent fragmentation. Surface water samples were also analyzed for paraffin waxes of 500 to 5000 µm size, which were detected at 14 of 24 stations. Their synthetic origin was confirmed by spectroscopic complemented with thermoanalytical techniques. Sediments of an urban fjord in Norway were analyzed with harmonized methods. The fjord’s MP concentrations ranged from 12×103 to 205×103 particles kg-1, exceeding the highest concentration in the southern North Sea 10 to 100 times. This thesis confirmes the significant influence of regional input processes and highlights that harmonized methods providing comprehensive data are indispensable for inter-study comparisons. It emphasizes that future studies and monitoring efforts should analyze small MP, which showed much higher concentrations and more diverse polymer compositions than large MP, to allow for reliable environmental risk assessment data
Coral polyps of the order Scleractinia are tiny anemone-like invertebrates interconnected through a common gastrovascular system. Scleractinian corals accrete a carbonate exoskeleton and act as primary builders of limestone structures called coral reefs in the shallow, well-lit and nutrient-poor waters of the tropics. A myriad of other organisms benefits from reef structures, making coral reefs one of the most diverse and productive ecosystems on Earth. The most puzzling aspect about corals is that they thrive in nutrient-poor waters of the tropics. The reason is inherent to a symbiotic association that they form with unicellular photoautotrophs known as zooxanthellae, located in membrane-bound vacuoles, the symbiosomes, in the corals' endodermal cells. Corals host millions of zooxanthellae algae and benefit from the carbohydrates produced from algal photosynthesis. As a result, corals do not rely exclusively on external nutrient sources. Increasing sea surface temperature induces a breakdown of the coral-algae association, causing the whitening of the corals due to a loss of zooxanthellae cells or zooxanthellae pigments, a process called bleaching. Consequently, in a warming world, the future of corals and the rich ecosystem they contribute to create is a matter of great concern. The work presented in this thesis investigates the acclimation capacity of corals under global warming and proposes potential mechanisms for explaining symbiont shuffling. These new model theories can be tested with laboratory experiments thus contributing to the development of strategies for the preservation and restoration of coral reef ecosystems.
Microalgal cultures in photobioreactors are essential in biological life support systems for space flight. However, photobioreactor cultures are sensitive to environmental parameters outside of their tolerance range, and crew time for repair is limited. This work aimed to increase resilience and decrease vulnerability of photobioreactor cultures by exploring methods for restarting photobioreactor cultures after heat shock, with as little use of crew time as possible.
Using the microalga Chlamydomonas reinhardtii, two paths for restarting a photobioreactor culture were explored. In the first path we tested the heat resistance of C. reinhardtii biofilms, hypothesizing that microalgal biofilms would have a higher heat tolerance than planktonic cultures. That would make it possible to use a subset of microalgal culture, grown as a biofilm inside the planktonic photobioreactor culture, to restart the planktonic culture after a heat shock event, making it essentially self-restarting.
As biofilm substrate we used alumina (Al2O3). Several combinations of porosities and surface functionalization were tested for biofilm growth and potential harm to cells. Alumina with pore sizes around the size of a C. reinhardtii cell had the most cells attaching to them. To test heat tolerance of C. reinhardtii biofilm, and compare it to that of planktonic cultures, we designed a heat stress test setup. We could not, however, detect any difference in heat stress tolerance between C. reinhardtii biofilm and C. reinhardtii planktonic culture.
The second path explored the suitability of a silica sol-gel, manufactured with a novel method, for encapsulation of C. reinhardtii cells for medium term storage. The silica sol gel manufacturing method has been designed to be less stressful to biological cells and also consists of fewer steps than other similar methods described in literature. The method proved successful, the algal cells survived and stayed healthy for several weeks.
What explains phytoplankton dynamics? An analysis of the Helgoland Roads Time Series data sets
(2018)
Phytoplankton is a diverse group of organisms that account for almost 50% of the global primary production. This thesis aims to investigate the role of environmental variables in structuring phytoplankton communities at Helgoland Roads, North Sea. It contains a detailed description of long-term changes in nutrients, calculates the phytoplankton-carrying capacity of the German Bight, and explores the response of phytoplankton diversity to environmental changes. Moreover, the importance of different biotic and abiotic factors in explaining the variability of phytoplankton abundance is discussed here. This thesis, for the first time, estimates the phytoplankton K of the German Bight, showing a high degree of variability over time. It also establishes a causal relationship between ecosystem variability and biodiversity; it explains the coexistence of the phytoplankton species in a system limited by multiple resources. In addition, this thesis establishes a pattern of seasonal phytoplankton dynamics in relation to biotic and abiotic factors. Overall, the results of this thesis will expand our understanding of the effect of long-term environmental changes on the dynamics of the phytoplankton community in the North Sea.
Protists form a major part of the Arctic ecosystem and are of great importance for biodiversity and productivity of the Central Arctic Ocean (CAO). The sea ice is considered as one of the main factors that influence protist communities. Current changes in sea ice conditions are assumed to alter protist community biodiversity and composition in ice-influenced habitats with further implications for the overall productivity of the CAO and carbon sequestration to the deep sea. Protist taxa that are adapted to habitat-specific environmental conditions or available food sources might be affected most by the recent sea ice retreat. However, little is known about how drastic sea ice loss might affect biodiversity and composition of sea ice biota and phytoplankton. So far, in-depth knowledge about protist community structure and protist ecology in variable habitats of the CAO is still scarce and mainly based on the analysis of conventional approaches (e.g. light microscopy). Therefore, this thesis analyzed 18S amplicon sequence data in a total of 97 samples collected from the deep-chlorophyll maximum water depth, under-ice water, sea ice and melt pond water. Samples were collected in the CAO during August and September in 2011 and 2012 to investigate the environmental factors driving protist community structure and the potential consequences of sea ice retreat on protists in the CAO.
The following questions will be addressed:
1. What are the influences of different sequence processing procedures on the resulting diversity and composition of protists obtained from environmental samples?
2. How does sea ice retreat and sea ice origin impact protist communities in the sea ice and water column of the CAO?
3. How habitat-specific are Arctic protist communities and is there any protist exchange between sea ice-influenced habitats in the CAO?
