Showing papers by "University of Bremen published in 2002"

Bayesian measures of model complexity and fit

Abstract: Summary. We consider the problem of comparing complex hierarchical models in which the number of parameters is not clearly defined. Using an information theoretic argument we derive a measure pD for the effective number of parameters in a model as the difference between the posterior mean of the deviance and the deviance at the posterior means of the parameters of interest. In general pD approximately corresponds to the trace of the product of Fisher's information and the posterior covariance, which in normal models is the trace of the ‘hat’ matrix projecting observations onto fitted values. Its properties in exponential families are explored. The posterior mean deviance is suggested as a Bayesian measure of fit or adequacy, and the contributions of individual observations to the fit and complexity can give rise to a diagnostic plot of deviance residuals against leverages. Adding pD to the posterior mean deviance gives a deviance information criterion for comparing models, which is related to other information criteria and has an approximate decision theoretic justification. The procedure is illustrated in some examples, and comparisons are drawn with alternative Bayesian and classical proposals. Throughout it is emphasized that the quantities required are trivial to compute in a Markov chain Monte Carlo analysis.

Amino acids from ultraviolet irradiation of interstellar ice analogues.

Abstract: Amino acids are the essential molecular components of living organisms on Earth, but the proposed mechanisms for their spontaneous generation have been unable to account for their presence in Earth's early history1. The delivery of extraterrestrial organic compounds has been proposed as an alternative to generation on Earth2,3,4,5, and some amino acids have been found in several meteorites6,7,8,9. Here we report the detection of amino acids in the room-temperature residue of an interstellar ice analogue that was ultraviolet-irradiated in a high vacuum at 12 K. We identified 16 amino acids; the chiral ones showed enantiomeric separation. Some of the identified amino acids are also found in meteorites. Our results demonstrate that the spontaneous generation of amino acids in the interstellar medium is possible, supporting the suggestion that prebiotic molecules could have been delivered to the early Earth by cometary dust, meteorites or interplanetary dust particles.

Dynamic oxidation of gaseous mercury in the Arctic troposphere at polar sunrise.

Abstract: Gaseous elemental mercury (Hg0) is a globally distributed air toxin with a long atmospheric residence time. Any process that reduces its atmospheric lifetime increases its potential accumulation in the biosphere. Our data from Barrow, AK, at 71 degrees N show that rapid, photochemically driven oxidation of boundary-layer Hg0 after polar sunrise, probably by reactive halogens, creates a rapidly depositing species of oxidized gaseous mercury in the remote Arctic troposphere at concentrations in excess of 900 pg m(-3). This mercury accumulates in the snowpack during polar spring at an accelerated rate in a form that is bioavailable to bacteria and is released with snowmelt during the summer emergence of the Arctic ecosystem. Evidence suggests that this is a recent phenomenon that may be occurring throughout the earth's polar regions.

Analysis of vasculature for liver surgical planning

Abstract: For liver surgical planning, the structure and morphology of the hepatic vessels and their relationship to tumors are of major interest. To achieve a fast and robust assistance with optimal quantitative and visual information, we present methods for a geometrical and structural analysis of vessel systems. Starting from the raw image data a sequence of image processing steps has to be carried out until a three-dimensional representation of the relevant anatomic and pathologic structures is generated. Based on computed tomography (CT) scans, the following steps are performed. 1) The volume data is preprocessed and the vessels are segmented. 2) The skeleton of the vessels is determined and transformed into a graph enabling a geometrical and structural shape analysis. Using this information the different intrahepatic vessel systems are identified automatically. 3) Based on the structural analysis of the branches of the portal vein, their vascular territories are approximated with different methods. These methods are compared and validated anatomically by means of corrosion casts of human livers. 4) Vessels are visualized with graphics primitives fitted to the skeleton to provide smooth visualizations without aliasing artifacts. The image analysis techniques have been evaluated in the clinical environment and have been used in more than 170 cases so far to plan interventions and transplantations.

Magnetoviscous effects in ferrofluids

Abstract: The appearance of field and shear dependent changes of viscosity in ferrofluids opens possibilities for future applications e.g. in damping technologies. To enhance the effects, it is necessary to understand the observed magnitudes of magnetoviscosity in commercial ferrofluids from a microscopic point of view. Starting from experimental results, it is described how the magnetoviscous effects can be explained by chain formation of a small fraction of large particles in the fluid. With a dedicated experiment ferrofluids are separated into fractions with high and low amount of such particles. The rheological characterization of the fractions prove the aforementioned model. Furthermore it leads to additional information concerning viscoelasticity of the suspensions in a magnetic field.

Temperature effect on entrainment, phase shifting, and amplitude of circadian clocks and its molecular bases.

Abstract: Effects of temperature and temperature changes on circadian clocks in cyanobacteria, unicellular algae, and plants, as well as fungi, arthropods, and vertebrates are reviewed. Periodic temperature with periods around 24 h even in the low range of 1-2 degrees C (strong Zeitgeber effect) can entrain all ectothermic (poikilothermic) organisms. This is also reflected by the phase shifts-recorded by phase response curves (PRCs)-that are elicited by step- or pulsewise changes in the temperature. The amount of phase shift (weak or strong type of PRC) depends on the amplitude of the temperature change and on its duration when applied as a pulse. Form and position of the PRC to temperature pulses are similar to those of the PRC to light pulses. A combined high/low temperature and light/dark cycle leads to a stabile phase and maximal amplitude of the circadian rhythm-when applied in phase (i.e., warm/light and cold/dark). When the two Zeitgeber cycles are phase-shifted against each other the phase of the circadian rhythm is determined by either Zeitgeber or by both, depending on the relative strength (amplitude) of both Zeitgeber signals and the sensitivity of the species/individual toward them. A phase jump of the circadian rhythm has been observed in several organisms at a certain phase relationship of the two Zeitgeber cycles. Ectothermic organisms show inter- and intraspecies plus seasonal variations in the temperature limits for the expression of the clock, either of the basic molecular mechanism, and/or the dependent variables. A step-down from higher temperatures or a step-up from lower temperatures to moderate temperatures often results in initiation of oscillations from phase positions that are about 180 degrees different. This may be explained by holding the clock at different phase positions (maximum or minimum of a clock component) or by significantly different levels of clock components at the higher or lower temperatures. Different permissive temperatures result in different circadian amplitudes, that usually show a species-specific optimum. In endothermic (homeothermic) organisms periodic temperature changes of about 24 h often cause entrainment, although with considerable individual differences, only if they are of rather high amplitudes (weak Zeitgeber effects). The same applies to the phase-shifting effects of temperature pulses. Isolated bird pineals and rat suprachiasmatic nuclei tissues on the other hand, respond to medium high temperature pulses and reveal PRCs similar to that of light signals. Therefore, one may speculate that the self-selected circadian rhythm of body temperature in reptiles or the endogenously controlled body temperature in homeotherms (some of which show temperature differences of more than 2 degrees C) may, in itself, serve as an internal entraining system. The so-called heterothermic mammals (undergoing low body temperature states in a daily or seasonal pattern) may be more sensitive to temperature changes. Effects of temperature elevation on the molecular clock mechanisms have been shown in Neurospora (induction of the frequency (FRQ) protein) and in Drosophila (degradation of the period (PER) and timeless (TIM) protein) and can explain observed phase shifts of rhythms in conidiation and locomotor activity, respectively. Temperature changes probably act directly on all processes of the clock mechanism some being more sensitive than the others. Temperature changes affect membrane properties, ion homeostasis, calcium influx, and other signal cascades (cAMP, cGMP, and the protein kinases A and C) (indirect effects) and may thus influence, in particular, protein phosphorylation processes of the clock mechanism. The temperature effects resemble to some degree those induced by light or by light-transducing neurons and their transmitters. In ectothermic vertebrates temperature changes significantly affect the melatonin rhythm, which in turn exerts entraining (phase shifting) functions.

Epitaxial strain and superconductivity in La2-xSrxCuO4 thin films.

Abstract: The report that ${T}_{c}$ was doubled in underdoped ${\mathrm{L}\mathrm{a}}_{2\ensuremath{-}x}{\mathrm{S}\mathrm{r}}_{x}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4}$ films under compressive epitaxial strain has stirred great interest. We show that such films are extremely sensitive to oxygen intake, even at very low temperature, with startling consequences including colossal lattice expansion and a crossover from semiconductor to metallic behavior. We can bring ${T}_{c}$ up to 40 K in ${\mathrm{L}\mathrm{a}}_{2}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4}$ films on ${\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}$ substrates---without any Sr doping and under tensile strain. On $\mathrm{L}\mathrm{a}\mathrm{S}\mathrm{r}{\mathrm{A}\mathrm{l}\mathrm{O}}_{4}$ substrates, we reached ${T}_{c}=51.5\text{ }\text{ }\mathrm{K}$, the highest so far in ${\mathrm{L}\mathrm{a}}_{2\ensuremath{-}x}{\mathrm{S}\mathrm{r}}_{x}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4}$.

Lipid biomarkers indicate different ecological niches and trophic relationships of the Arctic hyperiid amphipods Themisto abyssorum and T. libellula

Abstract: The hyperiid amphipods Themisto libellula and T. abyssorum are important components of Arctic pelagic ecosystems. Both species are carnivorous and prey on mesozooplankton. They represent a substantial food source for marine vertebrates and are a key link between zooplankton secondary production and higher trophic levels. We present data on the total lipid content, lipid class and fatty acid composition of T. libellula and T. abyssorum from northern Fram Strait and the central Arctic Ocean. Both species had moderate to high lipid contents of 14–42% of body dry mass. In T. abyssorum, total lipid content was correlated to body mass, while T. libellula showed sex-related differences in lipid content. Despite their smaller body size, females of T. libellula had higher lipid contents than males. Wax esters represented the major lipid class in both species with 41–43% of total lipid, while triacylglycerols contributed 23–32%. The fatty acid composition was dominated by the long-chain polyunsaturated moieties 20:5(n-3) and 22:6(n-3), short-chain saturated compounds (16:0 and 14:0) and monounsaturated fatty acids of varying length, i.e. 16:1(n-7), 20:1(n-9), 18:1(n-9) and 22:1(n-11). Species-specific and geographic variations in the fatty acid and alcohol patterns were apparently linked to differences in diet and life-cycle. High amounts of the fatty acids and alcohols 20:1(n-9) and 22:1(n-11) in T. libellula indicate predation on herbivorous Calanus copepodids. In addition, elevated levels of 20:5(n-3) in T. libellula indicate a close connection with ice-algal production and the importance of cryo-pelagic coupling processes (i.e. exchange processes between the sea ice and the pelagic communities) for the nutrition of this high-Arctic epipelagic species. In contrast, T. abyssorum is characterised by lower amounts of 20:5(n-3) and its biomarker ratios indicate a higher trophic level. This observation is consistent with the subarctic-boreal origin of T. abyssorum and its occurrence in deeper layers of the Arctic Ocean, where it may feed on omnivorous and/or carnivorous prey.

DNA‐Directed Assembly of Bienzymic Complexes from In Vivo Biotinylated NAD(P)H:FMN Oxidoreductase and Luciferase

Abstract: The DNA-directed assembly of proteins offers a promising route to the generation of spatially ordered multienzyme complexes (MECs), which are not accessible by conventional chemical crosslinking or genetic engineering. MECs with several catalytic centers arranged in a spatially defined way are abundant in nature. Mechanistic advantages of MECs are revealed during the multistep catalytic transformation of a substrate since reactions limited by the rate of diffusional transport are accelerated by the immediate proximity of the catalytic centers. Furthermore, the TMsubstrate-channeling∫ of intermediate products avoids side reactions. Artifical multienzymes would allow the development of novel catalytic systems for enzyme process technology that are capable of regenerating cofactors, as well as multistep chemical transformations; they are also useful for exploration of proximity effects in biochemical pathways. Herein we report the initial steps towards the development of artificial multienzyme complexes through the DNA-directed assembly of two enzymes, NAD(P)H:FMN Oxidoreductase (NFOR) and Luciferase (Luc), which catalyze two consecutive reaction steps (Figure 1). NFOR reduces flavin mononucleotide Figure 1. Schematic representation of the bienzymic reaction cascade catalyzed by NAD(P)H:FMN Oxidoreductase (NFOR) and Luciferase (Luc). R CH3(CH2)10 .

Labrador Sea Water: Pathways, CFC Inventory, and Formation Rates

Abstract: In 1997, a unique hydrographic and chlorofluorocarbon (CFC: component CFC-11) dataset was obtained in the subpolar North Atlantic. To estimate the synopticity of the 1997 data, the recent temporal evolution of the CFC and Labrador Sea Water (LSW) thickness fields are examined. In the western Atlantic north of 50°N, the LSW thickness decreased considerably from 1994–97, while the mean CFC concentrations did not change much. South of 50°N and in the eastern Atlantic, the CFC concentration increased with little or no change in the LSW thickness. On shorter timescales, local anomalies due to the presence of eddies are observed, but for space scales larger than the eddies the dataset can be treated as being synoptic over the 1997 observation period. The spreading of LSW in the subpolar North Atlantic is described in detail using gridded CFC and LSW thickness fields combined with Profiling Autonomous Lagrangian Circulation Explorer (PALACE) float trajectories. The gridded fields are also used to calculate the CFC-11 inventory in the LSW from 40° to 65°N, and from 10° to 60°W. In total, 2300 ± 250 tons of CFC-11 (equivalent to 16.6 million moles) were brought into the LSW by deep convection. In 1997, 28% of the inventory was still found in the Labrador Sea west of 45°W and 31% of the inventory was located in the eastern Atlantic. The CFC inventory in the LSW was used to estimate the lower limits of LSW formation rates. At a constant formation rate, a value of 4.4–5.6 Sv (Sv ≡ 106 m3 s−1) is obtained. If the denser modes of LSW are ventilated only in periods with intense convection, the minimum formation rate of LSW in 1988–94 is 8.1–10.8 Sv, and 1.8–2.4 Sv in 1995–97

The temperature and pressure dependence of the absorption cross-sections of NO2 in the 250–800 nm region measured by Fourier-transform spectroscopy

Abstract: The absorption cross-sections of NO2 at atmospheric temperatures (223–293 K) and pressures (100 and 1000 mbar) were measured in the 250–800 nm (12500–40000 cm −1 ) region using Fourier-transform spectroscopy, at spectral resolutions of 0.5 cm −1 above 435 nm and 1.0 cm −1 below 435 nm (corresponding to about 8 and 16 pm at this wavelength). The wavenumber accuracy of the new cross-sections is better than 0.1 cm −1 (about 0.5 pm at 250 nm and about 6.4 pm at 800 nm), validated by recording of I2 absorption spectra in the visible using the same experimental set-up (light source, beam splitter, interferometer optics). The NO 2 absorption spectra were recorded at five different sample temperatures between 223 and 293 K, and at each temperature at two total pressures (100 and 1000 mbar) using pure N2 as buffer gas. Despite the weakness of this effect compared to the density of the NO2 absorption structures, pressure-broadening was clearly observed at all temperatures. The pressure-broadening was partially modeled using a convolution of the low-pressure NO2 absorption spectra with a Lorentzian lineshape. The pressure-broadening coefficient increases significantly with decreasing temperature, as already observed in the mid- and near-infrared vibration–rotation spectra of NO2. This effect is of importance for high-resolution spectroscopy of the earth’s atmosphere in the UV–visible region. © 2002 Elsevier Science B.V. All rights reserved.

Simple method for reduction of Autofluorescence in fluorescence microscopy

Abstract: Autofluorescence of aldehyde-fixed neural tissue often complicates the use of fluorescence microscopy. Background fluorescence can be notably reduced or eliminated by irradiation with light before treatment with fluorescence probes, resulting in a higher contrast without adversely affecting the staining probabilities.

The Ligand-binding Domain of CD22 Is Needed for Inhibition of the B Cell Receptor Signal, as Demonstrated by a Novel Human CD22-specific Inhibitor Compound

Abstract: CD22 is a B cell–specific transmembrane protein of the Siglec family. It binds specifically to α2,6-linked sialic acid (Sia) residues, which are also present on glycoproteins on the B cell surface. CD22 acts as a negative regulator in B cell receptor–mediated signaling by recruitment of Src homology 2 domain–containing tyrosine phosphatase (SHP)-1 to its intracellular tail. To analyze how ligand-binding of CD22 influences its intracellular signaling domain, we designed synthetic sialosides as inhibitors for the lectin domain of CD22. One of these compounds inhibited binding of human CD22-Fc to target cells over 200-fold better than Sia and was highly selective for human CD22. When Daudi cells or primary B cells were stimulated with anti-immunoglobulin (Ig)M in presence of this sialoside inhibitor, a higher Ca2+ response was observed, similar to CD22-deficient B cells. Accordingly, a lower tyrosine-phosphorylation of CD22 and SHP-1 recruitment was demonstrated in presence of the sialoside. Thus, by interfering with ligand binding of CD22 on the B cell surface, we have shown for the first time that the lectin domain of CD22 has a direct, positive influence on its intracellular inhibitory domain. Also, we have developed a novel low molecular weight compound which can enhance the response of human B cells.

Analysis for BrO in zenith‐sky spectra: An intercomparison exercise for analysis improvement

Abstract: [1] The analysis for BrO using the technique of differential optical absorption spectroscopy as applied to spectra of light scattered from the zenith sky has historically presented something of a challenge, leading to uncertainty about the accuracy of measurements. This has largely been due to the large sensitivity of the measurement to many analysis parameters and due to the small size of the absorption features being measured. BrO differential slant columns have been measured by six different groups taking part in an intercomparison exercise at Observatoire de Haute-Provence in France from 23 to 27 June 1996. The data are analyzed in a collaborative attempt to improve the overall analysis for BrO through investigation of a series of sources of errors in the instrumentation, calibration, input to the analysis, and the spectral analysis itself. The study included comprehensive sensitivity tests performed using both actual measurements and synthetic data. The latter proved invaluable for assessing several aspects of the spectral analysis without the limitations of spectral quality and instrument variability. The most significant sources of error are identified as the wavelength calibration of several of the absorption cross sections fitted and of the measured spectra themselves, the wavelength region of the fitting, the temperature dependence of the O3 absorption cross sections, failure to adequately account for the so-called I0 effect, inadequate offset correction, and inadequate measurement of the individual instrument slit functions. Recommendations for optimal analysis settings are presented, and comparing the results from the analysis of the campaign data shows BrO differential slant column observations from the various groups to be in agreement to within 4% on average between 87° and 90° solar zenith angle, with a scatter of 16%.

Love and Death in Germany: The Marital Biography and Its Effect on Mortality.

Abstract: In this article, we analyze how the marital biography is affecting mortality in Germany today (N = 12,484). We find support for temporal selection into marriage for both genders, but the effect is stronger for men. In addition, protection through marriage results from long-term accumulation of survival advantages and from the attenuation of higher mortality risks that occur immediately after a transition into or out of a marriage. Moreover, women are more likely to keep survival advantages from previous marriages and to forget about survival disadvantages from divorces and widowhood. Key Words: event history analysis, life course, marital history, marriage, mortality, selectivity. In 1830, Benoisten de Chateauneuf observed a difference in life expectancy between married and unmarried people. Since then, numerous studies have confirmed this result within many countries (Cheung, 2000; Hemstrom, 1996; Hu & Goldman, 1990; Klein, 1993; Lillard & Waite, 1995). Researchers hold two distinct processes responsible for such disparities. The first process is selection into marriage. Various studies have shown that those who find a partner on the marriage market are healthier than those who remain unmarried (Goldman, 1993; Goldman, Korenman, & Weinstein, 1995). The second process refers to the protective mechanism of marriage: Being married provides emotional support, constrains risk-taking behavior, stimulates a healthy lifestyle, provides additional resources, buffers critical experiences, and partly replaces professional health care. Both explanations are highly plausible, but they do not capture an important element of today's family biographies. Men and women are much more likely today to move into and out of different unions over the life course than they were 50 years ago. Most people live without a partner for extensive periods of their adult lives (Settles, 1999). Marital timing patterns influence the wealth of individuals (Wilmoth & Koso, 2002), but what about their effect on health and mortality? Do married men and women preserve their survival advantage over singles, divorced persons, widowers, and widows over their entire life spans? Is a marriage at the age of 40 as protective as a marriage at the age of 20? Is being married for 20 years twice as protective as being married for 10 years? Does a marriage after a previous divorce prolong one's life expectancy by more time than a marriage after a previous widowhood? In this article, we examine the influence of marital changes on mortality throughout the life course. A growing body of literature suggests that early life events (e.g., Barker, 1998; Doblhammer & Vaupel, 2001; Elo & Preston, 1992) and midlife conditions (e.g., Hart, Smith, & Blane, 1998; Manton, Stallard, & Corder, 1997) are important determinants of mortality later in life. All of these studies focus on single life events, but they rarely analyze the consequences of these events. In addition, they often neglect marriage as a factor. In this article, we trace people over long periods of time and focus on key events that shape their marital biographies. Analyzing the chronology of these events should help us to better understand when and why married people have higher life expectancies than do divorced, widowed, or single people. We distinguish four patterns of causation: Timing: The effect of marital transitions may be different if they occur at certain critical junctures of the life course. Accumulation: Some marital states may have positive or negative effects on mortality that accumulate over time. Attenuation: Some marital states may have positive or negative effects on mortality that are lost over time. Sequencing: Some events may have a varying effect on mortality depending on the sequence in which they occur. In this article, we first map the research background of marital transitions and mortality differences. …

Feeding and energy budgets of Antarctic krill Euphausia superba at the onset of winter—II. Juveniles and adults

Abstract: The overwintering success of Euphausia superba is a key factor that dictates population size, but there is uncertainty over how they cope with the scarcity of pelagic food. Both nonfeeding strategies (reduced metabolism, lipid use, or shrinkage in size) and switching to other foods (carnivory, ice algae, or detritus) have been suggested. We examined these alternatives in the southwest Lazarev Sea in autumn (April 1999), when sea ice was forming and phytoplankton was at winter concentrations. Both juveniles and adults had a very high lipid content (36% and 44% of dry mass, respectively) of which .40% was phospholipid. However, their low O : N ratios suggested that these reserves were not being used. Results from gut contents analysis and large volume incubations agreed that juveniles fed mainly on phytoplankton and adults fed on small (,3 mm) copepods. This dietary difference was supported possibly by elevated concentrations of 20 : 1 and 22 : 1 fatty acids in the adults. The feeding methods also confirmed that feeding rates were low compared with those in summer. Even when acclimated to high food concentrations, clearance and ingestion rates were ,30% of summer rates. Respiration and ammonium excretion rates of freshly caught krill were 60%‐80% of those in summer and declined significantly during 18 d of starvation. These findings suggest both switch feeding and energy conservation strategies, with a trend of reduced and more carnivorous feeding with ontogeny. This points to a ‘‘compromise’’ strategy for postlarvae, but there are alternative explanations. First, the krill may have reduced their feeding in an autumn transition to a nonfeeding mode, and, second, some of the population may have maintained a high feeding effort whereas the remainder was not feeding.