Possible magnetic field variability during the 6.7 GHz methanol maser flares of G09.62+0.20

(Abridged) Recently, the magnetic field induced Zeeman splitting was measured for the strongest known 6.7 GHz methanol maser, which arises in the massive star forming region G09.62+0.20. This maser is one of a handful of periodically flaring methanol masers. The 100-m Effelsberg telescope was used to monitor the 6.7 GHz methanol masers of G09.62+0.20. With the exception of a two week period during the peak of the maser flare, we measure a constant magnetic field of B_||~11+-2 mG in the two strongest maser components of G09.62+0.20 that are separated by over 200 AU. In the two week period that coincides exactly with the peak of the maser flare of the strongest maser feature, we measure a sharp decrease and possible reversal of the Zeeman splitting. The exact cause of both maser and polarization variability is still unclear, but it could be related to either background amplification of polarized emission or the presence of a massive protostar with a close-by companion. Alternatively, the polarization variability could be caused by non-Zeeman effects related to the radiative transfer of polarized maser emission.Comment: 4 pages, 3 figures, accepted for publication Astronomy and Astrophysic

Accommodating the difference in students’ prior knowledge of cell growth kinetics

This paper describes the development and benefits of an adaptive digital module on cell growth to tackle the problem of educating a heterogeneous group of students at the beginning of an undergraduate course on process engineering. Aim of the digital module is to provide students with the minimal level of knowledge on cell growth kinetics they need to comprehend the content knowledge of the subsequent lectures and pass the exam. The module was organised to offer the subject matter in a differentiated manner, so that students could follow different learning paths. Two student groups were investigated, one consisting of students who had received their prior education abroad and one of students that had not. Exam scores, questionnaires, and logged user data of the two student groups were analysed to discover whether the digital module had the intended effect. The results indicate that students did indeed follow different learning paths. Also, the differences in exam scores between the two student groups that was present before the introduction of the digital module was found to have decreased afterwards. In general, students appreciated the use of the material regardless of their prior education. We therefore conclude that the use of adaptive digital learning material is a possible way to solve the problem of differences in prior education of students entering a course

Probing nod factor perception in legumes by fluorescence microspectroscopy

Plants of the family of legumes are capable of forming a symbiosis with Rhizobium bacteria. These Gram-negative bacteria invade the root system of a host legume and fix nitrogen in a specialized organ, the so-called root nodule. In exchange for sugars, the bacteria convert atmospheric nitrogen to ammonia which can be used by the plant. This remarkable alliance allows the plant to grow independently from nitrogen sources provided by the soil. Examples of leguminous plants are clover, pea, and soybean.The symbiosis is initiated by a molecular dialogue. The plant produces flavonoid compounds which are recognized by the bacterial NodD protein. The signaling pathway which is activated leads to the synthesis and secretion of lipo-chitooligosaccharides which are also called Nod factors. The production of Nod factors by the Rhizobium bacteria is an essential step for accomplishing symbiosis and also determines host specificity. The general structure of Nod factors comprises a chitin backbone of three to five b-1,4-linked N-acetylglucosamine units. A fatty acid of 16-20 carbon atoms is N-linked to the terminal non-reducing sugar residue. The exact molecular structure can comprise different acyl chains and a variety of decorations on the chitin backbone depending on the Rhizobium species.After successful recognition of the bacteria by the legume, a remarkable morphogenic process takes place, which is known as root hair curling. The root hair curls around the Rhizobium colony by which the bacteria are entrapped within the so-called shepherd's crook. Subsequently, the rhizobia enter the root hair through an infection thread, starting from the center of the curl. Via the infection thread several cell layers are crossed after which the bacteria are released in nodule primordium cells, where they differentiate into bacteroids that fix nitrogen.Nod factors in the absence of bacteria, either purified from Rhizobium cultures or chemically synthesized can elicit a wide variety of responses on a compatible legume host. When Nod factors are applied to roots, the earliest visible response takes place in root hairs. Root hairs are single tip-growing cells that develop from the epidermis of a root and grow perpendicular from the longitudinal axis of the root. Generally, root hairs that are terminating growth are susceptible to Nod factors and respond by swelling of the tip of the root hairs, followed by the re-initiation of tip growth in a random direction. This typical Nod factor response is referred to as root hair deformation and can be observed with a microscope 2-3 hours after addition of Nod factors.The perception of Nod factors by the plant, and the downstream signaling cascades that are activated are major research topics in the Rhizobium-legume interaction. The low concentration (down to 10-12 M) at which Nod factors can still induce root hair deformation and the dependence of the bioactivity on specific decorations of the Nod factor suggest that these molecules are perceived by receptors at the root hair. However, to date no such receptors are characterized. Moreover, it is far from clear where Nod factor recognition by root hairs takes place. Therefore an approach was taken in which fluorescent Nod factor derivatives are synthesized, allowing to probe the ligand binding sites on legume root hairs.The research described in this thesis focuses on the quantification, characterization and perception by legumes of Nod factors. In order to detect Nod factors at physiologically relevant concentrations sensitive techniques are required. A number of fluorescence spectroscopy and microscopy based techniques can be used to study fluorescent derivatives of signaling molecules. In chapter 1, the use of fluorescence microspectroscopic techniques available in the laboratory are discussed. Examples how these techniques can be used for the study of root hairs and other living cells are described.In chapter 2, two methods to quantify purified Nod factors are described. An enzymatic step which is crucial for the first method was analyzed in detail. The second method was optimized and validated using fluorescent and radiolabeled Nod factor derivatives. The chapter describes in detail how the two optimized methods can be used for quantifying Nod factors as well as potential pitfalls.In chapter 3, the spectral properties of three novel fluorescent Nod factor derivatives are described. It is checked whether these fluorescent Nod factors can still elicit root hair deformation on Vicia sativa roots. The properties of the amphiphilic signaling molecules were characterized in vitro in the absence and presence of micelles and model membrane systems using fluorescence spectroscopy. Time-correlated single photon counting fluorescence spectroscopy was used to measure rotational mobility of the fluorophore. These experiments are complemented with fluorescence correlation spectroscopy to examine diffusional mobility of the Nod factors. A lipid transfer assay was used to measure the rate of intermembrane transfer and intramembrane flip-flop of Nod factors.In chapter 4, a detailed study is reported describing the sites at which the fluorescent Nod factors accumulate. Fluorescence microscopy is used to examine the location of fluorescent Nod factors on root hairs during the initial perception and during root hair deformation. Subsequently, the diffusional mobility of the fluorescent Nod factors is measured in vivo using fluorescence correlation microscopy (FCM), allowing quantification of molecular mobility and concentration of fluorescent Nod factors in living root hairs at a molecular level. This study is continued in chapter 5 in which also novel sulfated fluorescent Nod factors are used and characterized, enabling a direct comparison between sulfated and non-sulfated Nod factors on a host and non-host legume. Also, the origin of the molecular mobility of the Nod factors is studied in more detail.In chapter 6 a novel approach towards manipulating phospholipid second messengers in single cells with spatiotemporal control is presented. The synthesis of a fluorescent and caged derivative, NPE-phosphatidic acid, which releases phosphatidic acid upon exposure to UV is described. The release of phosphatidic acid from the caged compound is studied in vitro and in vivo. The use of photoreleasable phosphatidic acid for studying phospholipid signaling in vivo is evaluated.Chapter 7 summarizes the conclusions that can be drawn from the results described in this thesis. The implications for Nod factor secretion by the bacterium and subsequent perception by legume root hairs are discussed. Based on the results presented in this thesis, it is tempting to speculate that spatial restriction of signaling molecules in plants is achieved by immobilization in the cell wall. Subseqent perception of Nod factor takes place either in the plasma membrane or within the cell wall as is illustrated by two proposed modes of perception. The results of this thesis are discussed with respect to these two models.</p

The effect of canopy position on growth and mortality in mixed sapling communities during self-thinning

This research investigates how species in the sapling phase differ in growth and survival depending on light availability (as estimated by canopy position) by means of tree-ring analysis and modelling mortality. We harvested 120 live and 158 dead saplings in self-thinning communities consisting of Silver birch (Betula pendula Roth.), Scots pine (Pinus sylvestris L.), Japanese larch (Larix kaempferi Carr.) and Douglas fir (Pseudotsuga menziesii Mirb. Franco) in the Netherlands. Results are evaluated within the framework of a trade-off between high-light growth and low-growth survival. Radial growth, measured at ground level, generally declined over time. In addition, a decreasing light availability further reduced growth in all species except Douglas fir. Trees died when radial growth was reduced to about 0.5 mm year¿1. Mortality in all species except Scots pine was significantly related to recent growth, but mortality curves were not different. The light-demanding Silver birch and Japanese larch differed from the shade-tolerant Douglas fir in both high-light growth and low-growth mortality, in line with a growth-survival trade-off. The light-demanding Scots pine did not fit this pattern as it was unable to transfer high radial growth into height gain, leaving it in suppressed canopy positions. This indicates the importance of height growth in the growth-survival trade-off. Differences in mortality probabilities affect the potential for coexistence, however, in all species also fast-growing individuals died suggesting additional factors causing mortality during self-thinning, other than direct competition for ligh

The influence of student characteristics on the use of adaptive e-learning material

Adaptive e-learning materials can help teachers to educate heterogeneous student groups. This study provides empirical data about the way academic students differ in their learning when using adaptive elearning materials. Ninety-four students participated in the study. We determined characteristics in a heterogeneous student group by collecting demographic data and measuring motivation and prior knowledge. We also measured the learning paths students followed and learning strategies they used when working with adaptive e-learning material in a molecular biology course. We then combined these data to study if and how student characteristics relate to the learning paths and strategies they used. We observed that students did follow different learning paths. Gender did not have an effect, but (mainly Dutch) BSc students differed from (international) MSc students in the intrinsic motivation they had and the learning paths and strategies they followed when using the adaptive e-learning materia

VLBA imaging of a periodic 12.2 GHz methanol maser flare in G9.62+0.20E

The class II methanol maser source G9.62+0.20E undergoes periodic flares at both 6.7 and 12.2 GHz. The flare starting in 2001 October was observed at seven epochs over three months using the VLBA at 12.2 GHz. High angular resolution images (beam size $\sim$ 1.7 x 0.6 mas) were obtained, enabling us to observe changes in 16 individual maser components. It was found that while existing maser spots increased in flux density, no new spots developed and no changes in morphology were observed. This rules out any mechanism which disturbs the masing region itself, implying that the flares are caused by a change in either the seed or pump photon levels. A time delay of 1--2 weeks was observed between groups of maser features. These delays can be explained by light travel time between maser groups. The regularity of the flares can possibly be explained by a binary system.Comment: 11 pages, accepted for publication in MNRA

Periodic class II methanol masers in G9.62+0.20E

We present the light curves of the 6.7 and 12.2 GHz methanol masers in the star forming region G9.62+0.20E for a time span of more than 2600 days. The earlier reported period of 244 days is confirmed. The results of monitoring the 107 GHz methanol maser for two flares are also presented. The results show that flaring occurs in all three masing transitions. It is shown that the average flare profiles of the three masing transitions are similar. The 12.2 GHz masers are the most variable of the three masers with the largest relative amplitude having a value of 2.4. The flux densities for the different masing transitions are found to return to the same level during the low phase of the masers, suggesting that the source of the periodic flaring is situated outside the masing region, and that the physical conditions in the masing region are relatively stable. On the basis of the shape of the light curve we excluded stellar pulsations as the underlying mechanism for the periodicity. It is argued that a colliding wind binary can account for the observed periodicity and provide a mechanism to qualitatively explain periodicity in the seed photon flux and/or the pumping radiation field. It is also argued that the dust cooling time is too short to explain the decay time of about 100 days of the maser flare. A further analysis has shown that for the intervals from days 48 to 66 and from days 67 to 135 the decay of the maser light curve can be interpreted as due to the recombination of a thermal hydrogen plasma with densities of approximately $1.6 \times 10^6 \mathrm{cm^{-3}}$ and $6.0 \times 10^5 \mathrm{cm^{-3}}$ respectively.Comment: 11 pages, 9 figuer