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In dieser Arbeit wurden die Möglichkeiten und Grenzen für Zirkulardichroismus-Messungen mit Synchrotronstrahlung untersucht. Dazu wurde ein Messaufbau für Zirkulardichroismus-Messungen an zwei Strahlrohren am Berliner Elektronenspeicherring für Synchrotronstrahlung eingesetzt, die für Messungen im Bereich des ultravioletten Lichts geeignet sind. Eigenschaften der Strahlrohre und des Messaufbau wurden in einigen wichtigen Punkten mit kommerziellen Zirkulardichroismus-Spektrometern verglichen. Der Schwerpunkt lag auf der Ausdehnung des zugänglichen Wellenlängenbereichs unterhalb von 180 nm zur Untersuchung des Zirkulardichroismus von Proteinen in diesem Bereich. In diesem Bereich ist es nicht nur die Lichtquelle sondern vor allem die Absorption des Lichts durch Wasser, die den Messbereich bei der Messung biologischer Proben in wässriger Lösung einschränkt. Es wurden Bedingungen gefunden, unter denen der Messbereich auf etwa 160 nm, in einigen Fällen bis auf 130 nm ausgedehnt werden konnte. Dazu musste die Pfadlänge deutlich reduziert werden und verschieden Probenküvetten wurden getestet. Der Einfluss der dabei auftretenden Spannungsdoppelbrechung in den Probenküvetten auf das Messsignal konnte mit einem alternativen Messaufbau deutlich reduziert werden. Systematische Fehler im Messsignal und auftretende Strahlenschäden begrenzen jedoch die Zuverlässigkeit der gemessenen Spektren. Bei Proteinfilmen schränkt die Absorption von Wasser den Messbereich kaum ein. Es wurden jedoch meist deutliche Unterschiede zwischen den Spektren von Proteinfilmen und den Spektren von Proteinen in wässriger Lösung festgestellt. Solange diese Unterschiede nicht minimiert werden können, stellen Proteinfilme keine praktikable Alternative zu Messungen in wässriger Lösung dar.
Precipitation and land use in terms of livestock grazing have been identified as two of the most important drivers structuring the vegetation composition of semi-arid and arid savannas. Savanna research on the impact of these drivers has widely applied the so-called plant functional type (PFT) approach, grouping the vegetation into two or three broad types (here called meta-PFTs): woody plants and grasses, which are sometimes divided into perennial and annual grasses. However, little is known about the response of functional traits within these coarse types towards water availability or livestock grazing. In this study, we extended an existing eco-hydrological savanna vegetation model to capture trait diversity within the three broad meta-PFTs to assess the effects of both grazing and mean annual precipitation (MAP) on trait composition along a gradient of both drivers. Our results show a complex pattern of trait responses to grazing and aridity. The response differs for the three meta-PFTs. From our findings, we derive that trait responses to grazing and aridity for perennial grasses are similar, as suggested by the convergence model for grazing and aridity. However, we also see that this only holds for simulations below a MAP of 500 mm. This combined with the finding that trait response differs between the three meta-PFTs leads to the conclusion that there is no single, universal trait or set of traits determining the response to grazing and aridity. We finally discuss how simulation models including trait variability within meta-PFTs are necessary to understand ecosystem responses to environmental drivers, both locally and globally and how this perspective will help to extend conceptual frameworks of other ecosystems to savanna research.
In times of ongoing biodiversity loss, understanding how communities are structured and what mechanisms and local adaptations underlie the patterns we observe in nature is crucial for predicting how future ecological and anthropogenic changes might affect local and regional biodiversity. Aquatic zooplankton are a group of primary consumers that represent a critical link in the food chain, providing nutrients for the entire food web. Thus, understanding the adaptability and structure of zooplankton communities is essential. In this work, the genetic basis for the different temperature adaptations of two seasonally shifted (i.e., temperature-dependent) occurring freshwater rotifers of a formerly cryptic species complex (Brachionus calyciflorus) was investigated to understand the overall genetic diversity and evolutionary scenario for putative adaptations to different temperature regimes. Furthermore, this work aimed to clarify to what extent the different temperature adaptations may represent a niche partitioning process thus enabling co-existence. The findings were then embedded in a metacommunity context to understand how zooplankton communities assemble in a kettle hole metacommunity located in the northeastern German "Uckermark" and which underlying processes contribute to the biodiversity patterns we observe. Using a combined approach of newly generated mitochondrial resources (genomes/cds) and the analysis of a candidate gene (Heat Shock Protein 40kDa) for temperature adaptation, I showed that the global representatives of B. calyciflorus s.s.. are genetically more similar than B. fernandoi (average pairwise nucleotide diversity: 0.079 intraspecific vs. 0.257 interspecific) indicating that both species carry different standing genetic variation. In addition to differential expression in the thermotolerant B. calyciflorus s.s. and thermosensitive B. fernandoi, the HSP 40kDa also showed structural variation with eleven fixed and six positively selected sites, some of which are located in functional areas of the protein. The estimated divergence time of ~ 25-29 Myr combined with the fixed sites and a prevalence of ancestral amino acids in B. calyciflorus s.s. indicate that B. calyciflorus s.s. remained in the ancestral niche, while B. fernandoi partitioned into a new niche. The comparison of mitochondrial and nuclear markers (HPS 40kDa, ITS1, COI) revealed a hybridisation event between the two species. However, as hybridisation between the two species is rare, it can be concluded that the temporally isolated niches (i.e., seasonal-shifted occurrence) they inhabit based on their different temperature preferences most likely represent a pre-zygotic isolation mechanism that allows sympatric occurrence while maintaining species boundaries. To determine the processes underlying zooplankton community assembly, a zooplankton metacommunity comprising 24 kettle holes was sampled over a two-year period. Active (i.e., water samples) and dormant communities (i.e., dormant eggs hatched from sediment) were identified using a two-fragment DNA metabarcoding approach (COI and 18S). Species richness and diversity as well as community composition were analysed considering spatial, temporal and environmental parameters. The analysis revealed that environmental filtering based on parameters such as pH, size and location of the habitat patch (i.e., kettle hole) and surrounding field crops largely determined zooplankton community composition (explained variance: Bray-Curtis dissimilarities: 10.5%; Jaccard dissimilarities: 12.9%), indicating that adaptation to a particular habitat is a key feature of zooplankton species in this system. While the spatial configuration of the kettle holes played a minor role (explained variance: Bray-Curtis dissimilarities: 2.8% and Jaccard dissimilarities: 5.5%), the individual kettle hole sites had a significant influence on the community composition. This suggests monopolisation/priority effects (i.e., dormant communities) of certain species in individual kettle holes. As environmental filtering is the dominating process structuring zooplankton communities, this system could be significantly influenced by future land-use change, pollution and climate change.
Zooplankton carcasses are ubiquitous in marine and freshwater systems, implicating the importance of non-predatory mortality, but both are often overlooked in ecological studies compared with predatory mortality. The development of several microscopic methods allows the distinction between live and dead zooplankton in field samples, and the reported percentages of dead zooplankton average 11.6 (minimum) to 59.8 (maximum) in marine environments, and 7.4 (minimum) to 47.6 (maximum) in fresh and inland waters. Common causes of non-predatory mortality among zooplankton include senescence, temperature change, physical and chemical stresses, parasitism and food-related factors. Carcasses resulting from non-predatory mortality may undergo decomposition leading to an increase in microbial production and a shift in microbial composition in the water column. Alternatively, sinking carcasses may contribute significantly to vertical carbon flux especially outside the phytoplankton growth seasons, and become a food source for the benthos. Global climate change is already altering freshwater ecosystems on multiple levels, and likely will have significant positive or negative effects on zooplankton non-predatory mortality. Better spatial and temporal studies of zooplankton carcasses and non-predatory mortality rates will improve our understanding of this important but under-appreciated topic.