629 research outputs found
Does environmental flow speed affect the local relative abundance of Vorticella convallaria?
Vorticella convallaria are microscopic sessile suspension feeders which live attached to substrates and are ubiquitous in aquatic environments. They depend on a self-generated current to feed and help maintain the health of aquatic ecosystems by consuming bacteria and detritus. As highly prevalent filter feeders, they serve as biological indicators of ecosystem health and are heavily involved in nutrient and carbon cycling. Furthermore, they are essential to wastewater treatment and benefit bioremediation efforts. However, the conditions in which they collectively thrive and feed most effectively are not well known. We exposed organisms to a circulating flume containing four distinct unidirectional flow regimes of different speeds. Initial results suggest that organisms are significantly more abundant in the slow speeds, in which they are less pushed over and may feed more effectively. Vorticella remained more abundant in the slowest flow speed over time, and became more abundant when the order of flow regimes was reversed (transforming a fast flow regime into one with a slower speed), indicative of selective preference
Improving Lifelong Learning by Fostering Students' Learning Strategies at University
The foundation of how students usually learn is laid early in their academic lives. However, many or even most students do not primarily rely on those learning strategies that are most favorable from a scientific point of view. To change students' learning behavior when they start their university education, we developed a computer-based adaptive learning environment to train favorable learning strategies and change students' habits using them. This learning environment pursues three main goals: acquiring declarative and conditional knowledge about learning strategies, consolidating that knowledge, and applying these learning strategies in practice. In this report, we describe four experimental studies conducted to optimize this learning environment (n = 336). With those studies, we improved the learning environment with respect to how motivating it is, investigated an efficient way to consolidate knowledge, and explored how to facilitate the formation of effective implementation intentions for applying learning strategies and changing learning habits. Our strategy-training module is implemented in the curriculum for freshman students at the Department of Psychology, University of Freiburg (Germany). Around 120 students take part in our program every year. An open version of this training intervention is freely available to everyone
Frequency-dependent (ac) Conduction in Disordered Composites: a Percolative Study
In a recent paper [Phys. Rev. B{\bf57}, 3375 (1998)], we examined in detail
the nonlinear (electrical) dc response of a random resistor cum tunneling bond
network (, introduced by us elsewhere to explain nonlinear response of
metal-insulator type mixtures). In this work which is a sequel to that paper,
we consider the ac response of the -based correlated () model.
Numerical solutions of the Kirchoff's laws for the model give a power-law
exponent (= 0.7 near ) of the modulus of the complex ac conductance at
moderately low frequencies, in conformity with experiments on various types of
disordered systems. But, at very low frequencies, it gives a simple quadratic
or linear dependence on the frequency depending upon whether the system is
percolating or not. We do also discuss the effective medium approximation
() of our and the traditional random network model, and discuss
their comparative successes and shortcomings.Comment: Revised and reduced version with 17 LaTeX pages plus 8 JPEG figure
Open Connect Everywhere: A Glimpse at the Internet Ecosystem through the Lens of the Netflix CDN
The importance of IXPs to interconnect different networks and exchange
traffic locally has been well studied over the last few years. However, far
less is known about the role IXPs play as a platform to enable large-scale
content delivery and to reach a world-wide customer base. In this paper, we
study the infrastructure deployment of a content hypergiant, Netflix, and show
that the combined worldwide IXP substrate is the major corner stone of its
Content Delivery Network. To meet its worldwide demand for high-quality video
delivery, Netflix has built a dedicated CDN. Its scale allows us to study a
major part of the Internet ecosystem, by observing how Netflix takes advantage
of the combined capabilities of IXPs and ISPs present in different regions. We
find wide disparities in the regional Netflix deployment and traffic levels at
IXPs and ISPs across various local ecosystems. This highlights the complexity
of large-scale content delivery as well as differences in the capabilities of
IXPs in specific regions. On a global scale we find that the footprint provided
by IXPs allows Netflix to deliver most of its traffic directly from them. This
highlights the additional role that IXPs play in the Internet ecosystem, not
just in terms of interconnection, but also allowing players such as Netflix to
deliver significant amounts of traffic
Percolation Systems away from the Critical Point
This article reviews some effects of disorder in percolation systems even
away from the critical density p_c. For densities below p_c, the statistics of
large clusters defines the animals problem. Its relation to the directed
animals problem and the Lee-Yang edge singularity problem is described. Rare
compact clusters give rise to Griffiths singuraties in the free energy of
diluted ferromagnets, and lead to a very slow relaxation of magnetization. In
biassed diffusion on percolation clusters, trapping in dead-end branches leads
to asymptotic drift velocity becoming zero for strong bias, and very slow
relaxation of velocity near the critical bias field.Comment: Minor typos fixed. Submitted to Praman
An Empirical Study of the Cost of DNS-over-HTTPS
DNS is a vital component for almost every networked application. Originally
it was designed as an unencrypted protocol, making user security a concern.
DNS-over-HTTPS (DoH) is the latest proposal to make name resolution more
secure. In this paper we study the current DNS-over-HTTPS ecosystem, especially
the cost of the additional security. We start by surveying the current DoH
landscape by assessing standard compliance and supported features of public DoH
servers. We then compare different transports for secure DNS, to highlight the
improvements DoH makes over its predecessor, DNS-over-TLS (DoT). These
improvements explain in part the significantly larger take-up of DoH in
comparison to DoT. Finally, we quantify the overhead incurred by the additional
layers of the DoH transport and their impact on web page load times. We find
that these overheads only have limited impact on page load times, suggesting
that it is possible to obtain the improved security of DoH with only marginal
performance impact
mRNA Display Selection of an Optimized MDM2-Binding Peptide That Potently Inhibits MDM2-p53 Interaction
p53 is a tumor suppressor protein that prevents tumorigenesis through cell cycle arrest or apoptosis of cells in response to cellular stress such as DNA damage. Because the oncoprotein MDM2 interacts with p53 and inhibits its activity, MDM2-p53 interaction has been a major target for the development of anticancer drugs. While previous studies have used phage display to identify peptides (such as DI) that inhibit the MDM2-p53 interaction, these peptides were not sufficiently optimized because the size of the phage-displayed random peptide libraries did not cover all of the possible sequences. In this study, we performed selection of MDM2-binding peptides from large random peptide libraries in two stages using mRNA display. We identified an optimal peptide named MIP that inhibited the MDM2-p53 and MDMX-p53 interactions 29- and 13-fold more effectively than DI, respectively. Expression of MIP fused to the thioredoxin scaffold protein in living cells by adenovirus caused stabilization of p53 through its interaction with MDM2, resulting in activation of the p53 pathway. Furthermore, expression of MIP also inhibited tumor cell proliferation in a p53-dependent manner more potently than DI. These results show that two-stage, mRNA-displayed peptide selection is useful for the rapid identification of potent peptides that target oncoproteins
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