93 search hits
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Einfluss von Wetterfaktoren und sozialer Umwelt auf den Endoparasitenbefall juveniler Wildkaninchen (Oryctolagus cuniculus L.)
(2009)
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Anett Starkloff
- In der vorliegenden Studie sollte untersucht werden, inwieweit abiotische und soziale Faktoren in der frühen Jungtierentwicklung den Endoparasitenbefall juveniler Wildkaninchen (Oryctolagus cuniculus) im Herbst beeinflussen. Dabei wurden im ersten Teil der Arbeit abiotische Parameter, wie der prozentuale Anteil an Regentagen in den ersten Wochen, nachdem die Kaninchen das Nest verlassen haben, auf den Befall mit verschiedenen Nematoden des Magen-Darm-Trakts sowie auf die Anzahl an Oozysten im Kot untersucht. Im zweiten Teil der Arbeit wurde weiterhin der Einfluss von sozialen Interaktionen mit Artgenossen auf den Endoparasitenbefall getestet. Die Studie wurde 2006 bis 2008 an einer unter semi-natürlichen Bedingungen lebenden Population von Europäischen Wildkaninchen durchgeführt. Die Entwicklung der Tiere wurden durch individuelle Markierung von Geburt an erfasst und mittels Verhaltensbeobachtungen die sozialen Interaktionen mit Artgenossen aufgenommen. Zum Ende der Vegetationsperiode im Herbst wurden die Jungtiere getötet und anschließend der Befall mit Endoparasiten im Magen-Darm-Trakt sowie im Kot quantifiziert. Drei verschiedene Nematodenarten konnten im Magen-Darm-Trakt nachgewiesen werden: der Rote Magenwurm Graphidium strigosum, der Haarstrongylid Trichostrongylus retortaeformis und der Pfriemenschwanz Passalurus ambiguus. Weiterhin konnten im Kot Oozysten von Kokzidien der Gattung Eimeria nachgewiesen werden, welche noch näher spezifiziert wurden in Oozysten von in der Leber vorkommenden Eimeria stiedai und Oozysten von im Darm lebenden Eimeria-Arten. Bei allen Endoparasiten ließ sich eine Altersabhängigkeit nachweisen, wobei bei G. strigosum und P. ambiguus die älteren Tiere einen höheren Befall aufwiesen im Gegensatz zu T. retortaeformis und den Oozysten im Kot, mit denen jüngere Tiere stärker belastet waren. Es zeigten sich deutliche Effekte der abiotischen Umwelt auf den Endoparasitenbefall der Tiere. Dabei war der prozentuale Anteil an Regentagen in den ersten acht Wochen nach Nestverlassen positiv korreliert mit dem Befall an Endoparasiten, wobei bei den Oozysten im Kot nur der Regen bei Temperaturen unter 10°C einen Einfluss hatte. Eine negative Korrelation konnte zwischen den soziopositven Interaktionen mit Artgenossen und sowohl dem Gesamtnematodenbefall als auch dem Befall mit P. ambiguus nachgewiesen werden: Tiere mit vielen soziopositiven Interaktionen mit Artgenossen in der frühen Jungtierentwicklung hatten eine geringere Wurmbelastung im Herbst. Hervorgerufen wurde dieser Effekt vor allem durch Interaktionen mit der Mutter und Wurfgeschwistern. Dieses Phänomen lässt den Schluss zu, dass durch social support mit Bindungspartnern die Endoparasitenbelastung der Jungtiere reduziert wird. Dafür spricht auch, dass viele soziopositive Interaktionen eine geringere Stressbelastung für die Tiere bedeuten. Zusammenfassend lässt sich sagen: Sowohl abiotische als auch soziale Umweltfaktoren bereits in der frühen Jungtierphase beeinflussen längerfristig die Parasitenbelastung der Tiere, was entscheidend für deren Überlebenswahrscheinlichkeit und Reproduktionserfolg sein kann.
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Iron oxidation in (Mg,Fe)0: Calibration of the Flank method on synthetic samples and applications on natural inclusions from lower mantle diamonds
(2009)
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Micaela Longo
- (Mg,Fe)O ferropericlase is the most common mineral found in diamonds originating in the lower mantle (more than 50% of occurrences). It is well known that the Fe3+ concentration in (Mg,Fe)O is sensitive to oxygen fugacity, even at high pressures. Therefore, the determination of Fe3+/Fetot in such inclusions provides a direct method for investigating lower mantle redox conditions during diamond formation. The goal of the present research is to calibrate the “flank method” by electron microprobe using synthetic (Mg,Fe)O, and then apply the method to determine in situ Fe3+/Fetot in ferropericlase inclusions from lower mantle diamonds. Up to now a calibration of the flank method is available only for garnets. Initially, the flank method was calibrated for garnets to test the reproducibility of the method on the Jeol XA-8200 electron microprobe in use at Bayerisches Geoinstitut. Results showed that for garnets a new calibration curve needs to be established at each working session. Then the flank method was calibrated for the Jeol XA-8200 electron microprobe in use at Bayerisches Geoinstitut for a homogeneous set of (Mg,Fe)O ferropericlase crystals over a wide range of composition (xFe = 2 to 60 at.%) and Fe3+/Fetot (1 to 15%). Samples were obtained by performing high pressure high temperature experiments in a multi anvil apparatus. In order to avoid compositional effects on flank method measurements, the high sample homogeneity was essential. Moreover, the determination of the Fe3+/Fetot ratio needed to be extremely accurate. For this purpose, a more accurate procedure for fitting the Mössbauer spectra of the final set of synthetic (Mg,Fe)O was adopted. The calibration curve determined is Fe2+ = 46.238 + 8.161 * ln (Fetot) - 137.01 * (Lbeta/Lalpha) + 85.57 * (Lbeta/Lalpha)2, for a Fe compositional range between 3 and 47 wt. %. A comparison of Fe3+/Fetot determined by flank method and values determined earlier by Mössbauer spectroscopy shows that results are generally consistent between the two different methods within the experimental errors. In contrast with garnet, the calibration curve established for ferropericlase does not need to be recalibrated at each microprobe session. Therefore, the calibration curve can be considered universal for the electron microprobe in use if the spectrometer adjustments remain identical with time. To explore applications of the flank method, a set of (Mg,Fe)O samples from diffusion studies was also investigated. Three (Mg,Fe)O crystals were measured by electron microprobe in order to test the sensitivity and accuracy of the flank method for small variations of bulk (Fetot)(wt%) as well as to measure Fe3+/Fetot along diffusion profiles. In the present work it is demonstrated how the flank method can be a powerful tool to measure small variations in Fe3+ content, with a spatial resolution of only few microns (2-3 µm) and a lower detection limit of Fetot of 3 wt%. Moreover, the measurement of Fe3+ content on the micron scale enables the study of the variation of oxygen fugacity conditions along diffusion gradients. A set of (Mg,Fe)O ferropericlase inclusions from ultra deep diamonds selected worldwide were analyzed by the flank method. The data set consists of eighteen (Mg,Fe)O ferropericlase samples from Juina, Brazil, Machado River, Brazil, and Ororoo, Australia. Inclusions are between 10 and 50 µm in size, therefore they are suitable to perform flank method measurements to determine Fe3+/Fetot. For the first time Fe3+/Fetot ratios were measured directly at the electron microprobe on inclusions of less than 50 µm in size. Results for the (Mg,Fe)O inclusions show good agreement with the theoretical trend described by the synthetic samples, which confirms high phase homogeneity for most of the samples. Flank method measurements show a large range of Fe3+/Fetot values for (Mg,Fe)O inclusions, which implies a large range of oxygen fugacities based on charge balance calculations. This large range of oxygen fugacities is similar to results for a suite of much larger inclusions from Kankan, Guinea, and São Luiz, Brazil, that were studied using Mössbauer spectroscopy. The variation of oxygen fugacity seems to be correlated to the geographical distribution of the inclusions studied, showing a redox gradient with more reducing conditions at Kankan, Guinea, and São Luiz, Brazil, and more oxidized in the case of Juina and Machado River, Brazil, and Eurelia, Australia. Such a correlation may be linked to the proto-pacific subduction mechanism, and the different ages combined with the geographic variation may indicate a difference in depth correlating with the large redox variation. Inclusions recovered from the same host diamond from Eurelia shows a strong redox gradient, which suggests a drastic change in the oxygen fugacity conditions during diamond growth. In order to provide information on the mechanisms able to control the redox conditions at lower mantle depths, a multi disciplinary study is suggested for further work.
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Surface Deformations of Magnetic Continua in Homogeneous Fields
(2009)
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Christian Gollwitzer
- In this thesis, experiments with magnetic liquids and gels are presented. Ferrofluids are synthetically created suspensions of magnetic nanoparticles in a carrier liquid. By adding a gelator, such a ferrofluid can be turned into a ferrogel. The magnetic properties of these substances are similar to a usual paramagnet with the important difference, that the susceptibility of the former is higher by a factor of 10^3 to 10^6. By the application of a homogeneous field, a transformation of the shape of a magnetic sample can be induced. In this thesis, four experiments on the surface deformation in homogeneous magnetic fields are presented. Two geometric configurations are considered: a horizontally extended flat layer with a free surface as well as a spherical sample. In both cases, the application of a homogeneous magnetic field leads to changes of the shape of the free boundary. In the case of the spherical geometry, the sample is deformed into a prolate ellipsoid under the action of the field, the so called magnetodeformational effect. In case of the extended flat layer, an abrupt shape transition into a patterned state takes place, the normal field or Rosensweig instability. In contrast to the smooth deformation of the sphere, this is an instability, which breaks the translational symmetry, and the transition occurs at a certain threshold value of the magnetic induction. Each of the four experiments in this thesis is briefly summarized in the following paragraphs. Part I of the thesis considers ferrofluids. In chapter 2, the ideal geometry of an infinitely extended flat layer is intentionally reduced to a cylinder such that only a single spike in the centre exists, and the solution space becomes rotationally symmetric. This makes the problem very feasible for experimental methods and numerical simulations. Two measurement techniques are applied and compared to each other, namely an X-ray technique, where the surface deformation is extracted from radioscopic images, and a laser technique, which focuses a laser spot onto the surface. The experiments and the simulations, the latter performed in close cooperation with a group in Athens, show a convincing agreement within a few percent. It remains an open question, whether the result can be deduced in analytic form, however. In chapter 3, a highly viscous ferrofluid is utilized to study the nonlinear dynamics of the normal field instability at very low Reynolds numbers. The linear growth rate for the growth and decay of the pattern at small amplitudes is extracted from the measurements and compared with an existing theoretical model. In addition, the measurement technique provides the reconstruction of a fully nonlinear amplitude equation, which is qualitatively compared to model equations. These nonlinear amplitude equations can only describe the dynamics of the growth in the immediate vicinity of the critical point so far. For a quantitative comparison, there is a need for a model with an extended range of validity. Additionally, localized patterns are observed which arise spontaneously in the neighbourhood of the unstable solution branch, which have previously been observed with the help of an external disturbance Part II of the thesis deals with thermoreversible ferrogels. Chapter 4 studies the magnetodeformational effect. A ferrogel sphere is exposed to homogeneous magnetic field. When the field is applied suddenly, the sphere not only elongates in the direction of the field, but also vibrates about the new equilibrium. On a longer time scale, the deformation continuously increases due to the viscoelastic properties of the gel. Both phenomena can well be described by a harmonic oscillator model, where the spring constant changes with time. From the deformation parallel and perpendicular to the applied field, Poisson´s ratio can be calculated, which turns out to be close to the limit of incompressibility. The absolute values of the deformation are compared to recent theoretical models. The resulting deviation of about 10% is attributed to the viscoelastic properties of the ferrogel, which are not taken into account in the static models. In chapter 5, the normal field instability is realized for the first time with a ferrogel. A flat layer of a thermoreversible ferrogel is exposed to a homogeneous magnetic field at different temperatures, where the gel is viscoelastic. This is a consequence of the need for a very soft material, such that the growth of the pattern is not completely suppressed by the elastic forces. The magnetic field is periodically modulated in time, and the amplitude of the instability is measured, which is modulated with the same frequency. The comparison with rheological measurements reveals a scaling of the modulated amplitude with the complex viscosity of the ferrogel. A comparison with the theoretical model for a ferrogel is difficult due to the viscoelasticity of the gel.
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Auswirkungen der RNA-Interferenz auf allatoregulierende Neuropeptide und Lokalisation der B-Typ Allatostatine bei Spodoptera frugiperda
(2009)
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Manuela Griebler
- Zusammenfassung Bei Insekten regulieren Peptidhormone (Neuropeptide) viele physiologische und entwicklungsrelevante Prozesse, wie Wachstum, Häutung, Metamorphose, Reproduktion, Diapause, Fressverhalten und den Metabolismus. Allatoregulierende Neuropeptide hemmen (Allatostatine, AS) oder fördern (Allatotropine, AT) die Juvenilhormonbiosynthese in den Corpora allata, können aber auch andere Funktionen aufweisen, wie z.B. eine myoregulatorische Aktivität. In dieser Arbeit wurde zunächst die Lokalisation der B-Typ Allatostatine [W(X)6W-amide] aus Spodoptera frugiperda immunologisch untersucht. In Gehirnen adulter, 2 Tage alter Weibchen trat eine deutliche Immunfärbung in medianen neurosekretorischen Zellen des Protocerebrums, im oberen Bereich des Zentralkörpers und in den Antennalloben (Glomeruli) auf. In den Thorakal- und Abdominalganglien von Larven des letzten Larvenstadiums wurde eine intensive Färbung im Bereich des Neuropils und eine schwächere Immunreaktion im Bereich der lateral gelegenen neurosekretorischen Zellen beobachtet. Starke Färbung war auch in den Malpighischen Gefäßen sichtbar, die wahrscheinlich der apikalen Membran zwischen den Hauptzellen zuzuordnen ist, sowie im Mitteldarm und in larvalen Hoden. Anschließend wurden die physiologischen Funktionen der bereits charakterisierten allatoregulierenden Neuropeptide AT 2 und AS-C-Typ aus S. frugiperda nach einer Hemmung der Genexpression mittels der RNA-Interferenz Methode untersucht. Es wurden Expressionsstudien durchgeführt, um die Wirksamkeit und die Spezifität der RNA-Interferenz in verschiedenen Geweben zu überprüfen. Zudem wurden die Auswirkungen der RNA-Interferenz auf verschiedene physiologische Parameter untersucht. Dabei lag der Schwerpunkt der Experimente auf der Regulierung der Juvenilhormonbiosynthese, der Reproduktion, der Verpaarung und der Metamorphose. Nach Injektion und auch nach Fütterung von AS-C- bzw. AT 2-doppelsträngiger RNA (dsRNA) war die Expression der mRNA im Gehirn und Darm von Larven reduziert. Zudem führte die Injektion von AS-C- bzw. AT 2-dsRNA auch bei adulten Weibchen und Männchen unterschiedlicher Altersstufen zu einer Reduktion der Expression im Gehirn. Die Reduktion der Transkription bei Larven ging mit einem erhöhten Juvenilhormon(JH)-Titer in der Hämolymphe der Tiere einher. Dies lässt eine allatostate Aktivität der beiden Peptide zu diesem Zeitpunkt vermuten. Die Folge des erhöhten JH-Titers war eine Verlängerung des letzten Larvenstadiums. Bei adulten Weibchen war der Effekt der Injektion von AS-C- bzw. AT 2-dsRNA auf den JH-Titer vom Alter der Tiere abhängig und variierte innerhalb der JH-Homologen (JH I bis JH III). Diese Ergebnisse lassen vermuten, dass beide Peptide bei adulten Weibchen unterschiedliche Wirkungen auf unterschiedliche JH-Homologe in spezifischen Stadien der Entwicklung haben. Adulte Männchen hatten nach einer Injektion von AS-C- bzw. AT 2-dsRNA an bestimmten Tagen einen erhöhten JH-Titer. Bei der Verpaarung werden mit der Spermatophore verschiedene Substanzen, darunter auch JH, vom Männchen auf das Weibchen übertragen. Bei mit Ringer injizierten Männchen konnte vor der Verpaarung JH in den akzessorischen Drüsen (AD) detektiert werden. Nach der Verpaarung nahm der JH-Titer in den männlichen AD stark ab. Bei unverpaarten Weibchen war kein JH in der Bursa copulatrix nachweisbar. Wurden die Weibchen jedoch verpaart, konnten große Mengen JH in der Bursa copulatrix gefunden werden. Mit AS-C-dsRNA injizierte Männchen zeigten vor der Verpaarung einen erhöhten JH-Titer in ihren AD, der nach der Verpaarung wieder niedriger war. Die mit diesen Männchen verpaarten Weibchen hatten einen erhöhten JH-Titer in der Bursa copulatrix im Vergleich zu mit Ringer injizierten verpaarten Weibchen. Die Ergebnisse lassen auf eine allatostate Wirkung der AS-C-Peptide auf den JH-Titer in den AD schließen. Bei mit AT 2-dsRNA injizierten Männchen war der JH-Titer in den AD, im Vergleich zu den Kontrollen, sowohl vor als auch nach der Verpaarung erniedrigt, ebenso wie in der Bursa copulatrix der mit diesen Männchen verpaarten Weibchen. Weibchen, die mit AS-C- bzw. AT 2-dsRNA injiziert worden waren, zeigten eine reduzierte Eiablage (Oviposition), ebenso wie Ringer injizierte Weibchen, die mit AS-C- bzw. AT 2-dsRNA injizierten Männchen verpaart worden waren. Diese Beobachtungen deuten darauf hin, dass die Fertilität der Männchen und Weibchen durch Neuropeptide unterschiedlich beeinflusst wird, und damit auch die Eiproduktion und Eiablage der Weibchen. Insektenneuropeptide eignen sich als (Bio)Insektizide, da sie die Physiologie und das Verhalten der Insekten während der Entwicklung und Reproduktion beeinflussen bzw. regulieren. Eine Expression von spezifischen dsRNAs in transgenen Pflanzen, gerichtet gegen allatoregulierende Neuropeptidgene, könnte die gezielte Bekämpfung von Schadinsekten, wie z. B. des Agrarschädlings S. frugiperda, ermöglichen.
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Crystalline Morphologies of Poly(butadiene)-b-Poly(ethylene oxide) Block Copolymers in n-Heptane
(2009)
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Adriana Mirela Mihut
- This thesis reports the development of micellar crystalline morphologies in a selective solvent. The phase diagram of solution morphologies as a function of the molecular composition of the semicrystalline poly(butadiene)-b-poly(ethylene oxide)(PB-b-PEO) block copolymers was investigated. The crystalline morphologies discussed here have been generated from selective solvent condition (70°C in n-heptane) via two thermal pathways: (A) by direct immersion into liquid nitrogen, where n-heptane becomes a poor solvent for both blocks at very low temperatures, and (B): by quenching to the crystallization temperature of the PEO, i.e., determined by the length of PEO block. In pathway B, n-heptane is a poor solvent only for the PEO block. At 70°C, the block copolymers self-assembled into micellar structures consisting of a PEO molten core and a soluble PB corona. As crystallization takes place in the PEO core, a fast quenching into liquid nitrogen results in the formation of crystalline micelles retaining the shape present in the molten state at 70°C (pathway A). In the case of pathway B, the competition between the PEO core crystallization and the self-assembly of the micellar units, is the driving force that dictates the morphological development, therefore crystallization breaks out the melt morphology. These studies, demonstrated that the PB-b-PEO block copolymers are a promising system models for developing a general route towards tunable crystalline morphologies. In a symmetric PB-b-PEO block copolymer, crystalline morphologies like spheres and meanders formed upon quenching into liquid nitrogen and at 30°C, respectively. The meander morphology consisting of branched lamellae with a crystalline PEO ribbon-like core and ellipsoidal endings was observed for the first time in solution. Investigations of the crystal development revealed that this structure formed via crystallization-induced aggregation of spherical micelles upon cooling. A systematic study of the effect of crystallization kinetics on the formed morphology upon crystallization-induced aggregation of spherical micelles of a symmetric PB-b-PEO block copolymer was discussed. We demonstrated that the resulting morphology is controlled by two competitive effects, namely, by the nucleation and growth of the PEO micellar core: at lower crystallization temperatures (Tc ≤ 30°C), a high nucleation rate leads to a meander-like morphology formation, whereas at higher crystallization temperatures (Tc > 30°C), a low nucleation rate favors the formation of twisted lamellae. For a highly asymmetric PB-b-PEO block copolymer, crystallization at -30°C induced the formation of crystalline micelles retaining the spherical shape present in the molten state at 70°C. However, a quenching into liquid nitrogen facilitated a transition to rod-like micelles caused by changes of solvent quality for the PB coronar chains. This triggers the onset of an interfacial instability, therefore the spherical micelles preferred to reorganize into a morphology with a smaller interfacial curvature. The low crystallinity of the PEO core imposed a stronger tendency of the rods to aggregate and to thicken into more stable morphologies as needle-like structures, with a preferred growth direction along the long axis. Finally, the micellar morphology diagram of the PB-b-PEO block copolymers has been studied as a function of the crystallization temperature and molecular composition of the blocks via two thermal pathways. Pathway A allowed morphological transitions from spheres to rods, worms or twisted cylinders with the increase of the crystalline content of the PEO core. In Pathway B, the sequence of spheres, cylinders, lamellae, platelets and dendrites structures is observed with the increases of the PEO block length. The aggregation number of the spherical micelles is affected by the weight fraction and crystallinity of the PEO block. Moreover, an increased chain folding was observed at a high PEO composition which reduced the lamellar thickness of the crystals. The competition between the PEO core crystallization and the aggregation of the micellar units leads to coexistence regions of lamellae with platelets and cylinders with platelets. The novelty of this thesis relies on the development of novel crystalline morphologies in a selective solvent, as well as, in the detailed analysis of the major parameters that govern morphological formation in a controlled manner.
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Direct Amino Acid Uptake by Plants related to Grassland Diversity - methodological and ecological Investigations
(2009)
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Leopold Sauheitl
- Uptake of intact amino acids by plants has been identified as an alternative nitrogen (N) source for plants in a number of ecosystems and soil types. Up to now it is assumed that this uptake strategy is of particular relevance in ecosystems with low mineral N (Nmin) contents due to insignificant microbial activity or due to poorly developed soils. However, it has also been discussed that amino acid uptake might enable plants to lower intracspecific competition for mineral N and shortcut the microbial mineralization of organic N in systems were competition is exceptionally high. The positive effect of plant diversity on plant productivity is known to induce conditions of intense N competition and thus amino acid uptake might explain how plant communities enable higher productive with increasing diversity. However, the ecological importance of organic N uptake has also been questioned due to the high competitive power of microbes in soils of the temperate zone and due to a number of flaws in the commonly used method to proof and quantify direct amino acid uptake. In this, dual labelled (13C and 15N) amino acids are injected into the soil and direct tracer uptake is quantified via bulk isotope measurement of 13C and 15N enrichment in plant tissues, which recently has been challenged to exclusively reflect direct amino acid uptake. The first objective therefore was to identify and reduce methodological influences on the direct amino acid uptake by plants. Thus the effect of changed amino acid concentrations on amino acid uptake was investigated by application of different tracer amounts. Next, the accuracy and precision of commonly used bulk isotope measurements were compared to compound specific measurements with respect to the determination of direct amino acid uptake. It was shown that the use of high tracer amounts accompanied by high Nmin release reduces direct amino acid uptake via plant internal down regulation of amino acid transporters. This corroborates the importance of minimizing tracer amounts and suggests that plants can actively increase amino acid uptake when N availability in soil is low. Bulk measurements turned out to overestimate direct amino acid uptake by a factor of up to six, as they were not able to separate uptake of intact tracer molecules from uptake of tracer fragments or inorganic carbon. At the same time compound specific isotope measurements proofed to be an accurate and precise tool to demonstrate and quantify uptake of intact amino acids. Using these optimized methods, the importance of amino acid uptake for the N-nutrition of plants with respect to changing plant diversity was investigated. The uptake of amino acids and mineral N by plants as well as the competition between plants and microbes for amino acid N was investigated in grassland communities with 1 to 16 grassland species. Microbes were superior competitors for amino acid derived nitrogen, irrespective of plant diversity and took up 54% of the applied amino acid N in average within 24 h. In contrast, plants only incorporated 2.7% of the applied N and were thus less effective by a factor of 20 in short term N acquisition than microbes. In addition, plant mineral N uptake decreased with increasing plant diversity while uptake of intact amino acids increased. Thus the contribution of amino acid uptake to the overall plant N nutrition increased from 1.5 to 7.0% in which amino acid uptake was mainly controlled by plant N concentration shoot biomass and rooting density while mineral N uptake was controlled by microbial competition. In detail amino acid uptake increased with decreasing plant N concentration while mineral N uptake decreased with increasing microbial abundance and microbial N uptake. Thus, the boosted importance of amino acid uptake for plant N nutrition has to be seen as a reaction on increased N competition with increasing plant diversity. Additionally, plant diversity stimulated microbial diversity which was most likely due to the bottom up effect of increased root exudation and litter input caused by increasing N competition and plant productivity, respectively. While the microbial community was dominated by bacteria (54.7%) the abundance of litter and soil organic matter decomposing gram positive bacteria and fungi as well as protozoan abundance increased with increasing plant diversity. Protozoa are known to stimulate turnover of bacteria which was indicated by higher tracer incorporation of this microbial group and an overall increase of deaminase activity with increasing plant diversity. As higher microbial turnover is associated with increased loss of microbial N to plants, we have to expect higher N availability for plants in the long term. The positive feedback of a plant-induced higher microbial turnover rate on N availability in soil together with an increased amino acids uptake might therefore be an important model to explain the positive effect of plant diversity on plant productivity.
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Self-interaction and charge transfer in organic semiconductors
(2009)
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Thomas Körzdörfer
- The fascinating properties of organic molecular semiconductors paved the way for a new class of electronic devices such as organic light-emitting diodes, transistors, or solar cells. Despite an inferior efficiency as compared to commonly used silicon-based technologies, organic semiconductors promise the advent of fully flexible devices for large-area displays and solar cells, printed transistors as low-cost radio frequency identification (RFID) tags, displays for electronic books, and disposable measuring instruments for medical diagnosis. Hence, the investigation of organic molecular semiconductors has emerged as a vibrant field of development both in industry and in academia, spanning a wide range of subjects from physics, chemistry, and materials science to engineering and technology. Theoretical physicists can contribute to this progress by developing methods that allow to determine the electronic properties of organic semiconductors from first principles and thus deepen our knowledge of the underlying electronic processes in organic electronic devices. The calculation of the electronic properties of molecular semiconductors issues a serious challenge to theoretical physicists and chemists. Typically, organic semiconductor molecules employ several hundreds of electrons. For systems of that size, approaches that work with model Hamiltonians are typically not accurate enough in predicting many important electronic properties. However, solving the many-particle Schrödinger-equation by employing highly accurate perturbation theory approaches is often numerically too expensive to be considered as a convenient alternative. Hence, density functional theory (DFT) naturally arises as the method of choice. However, although in theory DFT yields an exact formulation of quantum mechanics, the quality of the results obtained from DFT calculations in practice strongly depends on the used approximations to the so-called exchange-correlation functional. This work concentrates on the problem of self-interaction, which is one of the most serious problems of commonly used approximative density functionals. As a major result of this work, it is demonstrated that self-interaction plays a decisive role for the performance of different approximative functionals in predicting accurate electronic properties of organic molecular semiconductors. This is particularly true for the calculation of ionization potentials, photoelectron spectra, dissociation, and charge-transfer processes. In search for a solution to the self-interaction problem, a new concept for correcting commonly used density functionals for self-interaction is introduced and applied to a variety of systems, spanning small molecules, extended molecular chains, and organic molecular semiconductors. It is further shown that the performance of functionals that are not free from self-interaction can vary strongly for different systems and observables of interest, thus entailing the danger of misinterpretation of the results obtained from those functionals. The underlying reasons for the varying performance of commonly used density functionals are discussed thoroughly in this work. Finally, this thesis provides strategies that allow to analyze the reliability of commonly used approximations to the exchange-correlation functional for particular systems of interest. This cumulative dissertation is divided into three parts. Part I gives a short introduction into DFT and its time-dependent extension (TDDFT). Part II provides further insights into the self-interaction problem, presents a newly developed concept for the correction of self-interaction, gives an introduction into the publications, and discusses their basic results. Finally, the four publications on self-interaction and charge-transfer in extended molecular systems and organic molecular semiconductors are collected in Part III.
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Tailoring Fluorene-based Oligomers for micron and sub-micron sized Photopatterning
(2009)
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Esther Scheler
- This thesis describes the work on tailor-made synthesis, characterization and application of well-defined fluorene oligomers for photoptatterning. Two types of fluorene oligomers are presented: pure fluorene oligomers and fluorene cooligomers incorporating various comonomers for adjusting the conductive properties towards electron and hole conduction. Since possible applications for these materials feature organic light emitting diodes and organic field effect transistors we focused on the preparation of well-defined and defect free oligomers and the preservation of their electro-optical properties during photopatterning. Further on the requirements for material synthesis are easy procedures and large quantities. Therefore we developed an approach, which produces large quantities combined with the adjustment of the desired properties in one single step. The synthetic strategy throughout the thesis comprises the addition of an endcapping species in aryl-aryl polymerization reactions. The tailormade endcapper fulfils three tasks at once, the control of the molecular weight, the introduction of polymerizable acrylate moieties and the avoidance of undesired endgroups. As aryl-aryl coupling methods the nickel catalyzed Yamamoto and palladium catalyzed Suzuki condensations were applied. The completeness of the aryl-aryl coupling and the endcapping was proven with Maldi-ToF mass spectrometry. With this approach the properties of the oligomers can be easily adjusted in view of the optimization of their photopolymerization behaviour. The first oligomer series deals with the effect of the molecular weight on the properties and photopatterning behaviour of pure fluorene oligomers. The molecular weights were controlled by the amount of functionalized endcapper, which carried the polymerizable acrylate groups. As coupling method the Yamamoto coupling was applied. The molecular weights defined the temperature range of the nematic mesophases. An increase of the average chain length leads to higher transition temperatures (e.g. clearing temperatures) and to better film forming properties. The photopolymerization is usually performed in the nematic state to achieve a sufficient mobility of the acrylates. The irradiation conditions had major consequences on the preservation of the characteristic electro-optical properties of the fluorenes, the harsher the conditions the higher was the probability to destroy the chemical structure by photooxidation. Further on since each chain only carries two acrylate functionalities attached to the endcappers the total number of acrylates is different for high molecular weight and low molecular weight mixtures. The lowest molecular weight mixture contains the most acrylates and shows the lowest transition temperature, which leads to the best micron sized photopatterns. The second generation of pure fluorene oligomers demonstrate how different contents of polymerizable groups affect the photopolymerization behaviour. Here the molecular weights were kept constant around 5000 g/mol by equal amounts of endcapper and the acrylate groups were introduced by the fluorene monomers. The Yamamoto coupling was used and upon cooligomerization with a non-acrylate fluorene monomer the acrylate content was changed from 10% to 100%. The photopolymerization times strongly depend on the acrylate content, the 100% acrylate oligomer could be photopatterned in 30 seconds, whereas the 80% and 60% mixtures needed 2-5 minutes. In the best case crosslinking is 20 times faster than found for the preceding series, which ensures the preservation of the electro-optical properties. With the highest acrylate content a photocrosslinking even at room temperature became possible. The third oligomer series describes the incorporation of various comonomers such as TPD and bithiophene via Yamamoto reaction. Taking the knowledge of the two preceding generations into account we exploited the acrylate monomer from series two and introduced 30% comonomer. This ensured a sufficient content of acrylates for a fast photopatterning and enough comonomer for a shift of the electronic properties. We found that the electronic structure of the comonomer strongly affected the behaviour in the Yamamoto reaction. The HOMO and LUMO energy values were shifted towards hole or electron conduction. The photopatterning conditions were similar as found for the corresponding pure oligofluorenes with a 60% acrylate content. 2-5 minutes exposure produced highly emissive micro patterns. Thus a change of the electro-optical properties does not affect the photopolymerization behaviour and vice versa. Since we found differences in the incorporation of comonomers during Yamamoto coupling we applied the Suzuki coupling, which ensures an alternately linkage of monomers. Here protective groups had to be used since the Pd-catalyst does not tolerate acrylate functionalities. We found that the comonomers were incorporated quantitatively, but the major difficulty proved to be the polymeranalogous reactions following the polycondensation. The energy values were shifted towards electron and hole conduction and the photopolymerization behaviour was similar to the Yamamoto oligomers. An exposure time of 2-5 minutes produced patterns with a maximum resolution of 700 nm. To conclude the endcapping strategy combined with the Yamamoto coupling is a most effective tool for the adjustment of properties within one single step. The acrylate content as well as the molecular weight can be precisely tuned, which allows a good control of the photopatterning properties. In some cases e.g. with electron withdrawing comonomers the Suzuki cross coupling is the method of choice.
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Refugee woman and the experiences of local integration in Nairobi, Kenya
(2009)
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Rosemary Jaji
- It seems trendy for current studies to argue that the term refugee is no more than a policy category which does not reflect the circumstances of the people that it subsumes. Such studies further argue that the circumstances of refugees are not necessarily different from those of local populations. This study argues that theoretical positions emanating from such observations do not have a universal application as illustrated in Nairobi where the term refugee is not merely a policy category or legal label but also experiential. Understanding the concept refugee is very much an outcome of empirical enterprise which locates those who bear the refugee status in specific contexts. The study draws attention to cases of targeted rape, raids, exclusionary discourses epitomised by negative stereotyping and xenophobia as well as refoulement which are specifically aimed at refugees in Nairobi. The refugee status is intertwined with other variables such as refugees´ ethnic, national and religious identities in ways that restrict inclusion of refugees into the host country. As a gendered experience, exile impacts on intra-household dynamics and transforms gender roles and relations within refugee households in ways that are simultaneously enabling and challenging for refugee women. Although local women also experience Sexual and Gender-Based Violence (SGBV), for refugee women this intersects with the vulnerability that the refugee status entails resulting in refugee women being abused even by police officers and officials who are tasked to protect them thus leaving them with limited channels for recourse. SGBV thus becomes salient because of its targeted nature. Emphasised in the study is the fact that refugee women are heterogeneous such that it is more appropriate to refer to refugee women´s experiences rather than the refugee woman experience. Exile as occupation of marginal space is however not solely about constraints as it also creates opportunities and possibilities that may not have been available to the women prior to flight. Contradictory as it may seem, the refugee status is mediated by the same variables that lead to exclusion at a macro level in ways that facilitate inclusion at a micro, interpersonal level characterised by interaction between refugee women and locals as fellow congregants or as neighbours who share the same plight of poverty in Nairobi´s slums. This is coupled with refugee women´s agency by which they convert obstacles into resources and create space for themselves in a country which advocates encampment and expects refugees to reside in the designated areas. Through their own agency, refugee women are able to navigate structural barriers meant to deter integration in ways that demonstrate that the absence of an official integration policy does not necessarily deter integration; individual agency has a countervailing impact on measures instituted to deter integration.
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Atmospheric Distribution and Seasonality of Airborne Polyfluorinated Compounds: Spatial and Temporal Concentration Variations from Ship-and Land-Based Measurements in Northern Germany, the Atlantic Ocean, and Polar Regions
(2009)
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Annekatrin Dreyer
- Per- und polyfluorierte Verbindungen (PFC) sind vielfältig verwendete chemische Substanzen, die mittlerweile weltweit und in allen Umweltmedien nachgewiesen werden konnten. Einige PFC, vor allem Perfluorcarboxylate (PFCA) und Perfluorsulfonate (PFSA), sind persistent, bioakkumulativ und toxisch. Der atmosphärische Transport und Abbau von neutralen volatilen PFCA und PFSA Vorläufersubstanzen könnte die Existenz der persistenten perfluorierten Säuren in quellenfernen Gebieten erklären. Obwohl erste Einzelmessungen von volatilen PFC in Luftproben einen atmosphärischen Transport bestätigten, fehlen bisher Studien, die die Verbreitung dieser Verbindungen umfassend darstellen. Aus diesem Grund wurden räumliche und zeitliche Variationen von PFC-Konzentrationen untersucht. Um PFC in Luftproben im Ultraspurenbereich bestimmen zu können, wurde die analytische Methode optimiert. Schließlich wurden PFC in der Luft auf Glasfaserfiltern (partikuläre Phase) und geeigneten Adsorbtionsmaterialien (Gasphase), hier Polyurethanschaum und XAD-2, angereichert. Neutrale volatile PFC wurden mit Aceton:Methyl-tert-butylether (1:1) extrahiert und mittels GC-MS detektiert. Perfluorierte Säuren wurden mit Methanol extrahiert und mit HPLC-MS/MS detektiert. Die optimierte Methode wurde angewendet, um Fluortelomeralkohole (FTOH), Fluortelomeracrylate (FTA), Perfluoralkylsulfonamide (FASA), and Perfluoralkyl-sulfonamidoethanole (FASE) in der Gas- und Partikelphase sowie PFCA und PFSA in der partikulären Phase zu bestimmen. Die Luftkonzentrationen von PFC wurden in Proben aus verschiedenen Probenahme-kampagnen der Jahre 2007 und 2008 bestimmt. Als Probenahmeplattformen dienten Dauermessstellen bei Hamburg (Deutschland) und verschiedene Forschungsschiffe, die in atlantischen und antarktischen Gewässern sowie der Nord- und Ostsee operierten. Die perfluorierten Säuren (PFCA, PFSA) wurden ausschließlich in geringen Konzentrationen in der partikulären Phase bestimmt. Die Gasphasenkonzentrationen ihrer neutralen volatilen Vorläufer (FTOH, FTA, FASA, FASE) bis zu zwei Größenordnungen höher und variierten zwischen ein und zwei Größenordnungen auf räumlicher und zeitlicher Ebene. PFC Summenkonzentrationen variierten zwischen 4.5 pg/m3 im antarktischen Ozean und 335 pg/m3 in Quellengebieten (marine Luft) und zwischen17 und 972 pg/m3 (permanente Stationen bei Hamburg). Mit durchschnittlich mehr als 80 % dominierten die FTOH das Substanzspektrum in der Gasphase. Perfluoroctylethanol (8:2 FTOH) war die Substanz, die in höchsten Konzentrationen vorgefunden wurde. Die Einzelstoffkonzentrationen in der partikulären Phase lagen meist unter 1 pg/m3. Während einer 14monatigen Probenahmekampagne an zwei Dauermessstationen bei Hamburg wurden die zeitlichen Variationen von PFC Konzentrationen untersucht. Die beobachteten starken Schwankungen über diesen Zeitraum verdeutlichten die limitierte Aussagekraft von Einzelmessungen. Der Konzentrationsverlauf war durch eine fluktuierende Grundlinie gekennzeichnet, die durch einzelne, unregelmäßig vorkommende Ereignisse stark erhöhter Konzentrationen unterbrochen wurde. Diese Ereignisse könnten durch lokale Quellen verursacht worden sein. Die Bereinigung der Zeitreihen von diesen Einzelereignissen führte zu weniger gestörten Konzentrationsverläufen, bei denen die PFC Konzentrationen im Sommer höher als die im Winter waren. Temperaturabhängige Emissionen von diffusen Quellen könnten für diesen Konzentrationsverlauf verantwortlich sein. Die Luftmassenherkunft hatte einen starken Einfluss auf die PFC Konzentrationen dieser Luftproben. Eine Trajektorienanalyse ließ auf westlich und südwestlich von Hamburg gelegene Quellenregionen mit hoher Bevölkerungs- und Industriedichte schließen. Trajektorien-, Korrelations- und Clusteranalysen von Luftproben aus der Deutschen Bucht bestätigten, dass Mittel- bis Langstreckentransport einen wichtigen Einfluss auf das detektierte PFC Niveau in Luftproben aus Norddeutschland und der Nordsee hat. PFC konnten in allen Luftproben von der Arktis bis zur Antarktis nachgewiesen werden, wobei ihre Konzentrationen von kontinentalen und küstennahen Regionen zur offenen See und von Mitteleuropa zu den Polen abnahmen. Die Einzelstoffkonzentrationen waren in der Nordhemisphäre höher als in der Südhemisphäre. Auf Grundlage der zeitlichen und räumlichen Konzentrationsvariationen konnten atmosphärische Verweilzeiten im Bereich von 20 bis 60 Tagen für die analysierten PFC berechnet werden. Die Ergebnisse dieser Arbeit liefern neue Erkenntnisse über atmosphärisch transportierte PFC. Sowohl die atmosphärischen Verweilzeiten als auch die festgestellte weltweite Verbreitung von PFC in der Luft bestätigen, dass diese Gruppe von Chemikalien von hauptsächlich nordhemisphärischen Quellenregionen über weite Strecken in entlegene Gebiete transportiert werden können.
