225 research outputs found
Blooms of a key grazer in the Southern Ocean – An individual-based model of Salpa thompsoni
The Southern Ocean near the Western Antarctic Peninsula (WAP) is strongly affected by climate change resulting in warmer air temperature, accompanied with reduced sea ice coverage, increased sea water temperature and potential changes in the abundances of two key grazer species Salpa thompsoni (salp) and Euphausia superba (Antarctic krill). While salp abundance is hypothesized to increase, krill abundance is hypothesized to decline with dramatic consequences for the entire food web of the Southern Ocean. A better understanding of the biotic interaction between krill and salps and their population dynamics is thus crucial. However, the life cycle of salps is complicated and barely understood. Therefore, we have developed an individual-based model describing the whole life cycle to better understand the population dynamics of salps and the conditions for blooms. The model has been used to explore if and under what conditions the empirical pattern of large variability in observed salp abundances at the WAP, generated by the long-term data of the US Antarctic Marine Living Resources Program (AMLR) can emerge from a small seeding population. The model reproduced this empirical pattern if daily growth rates of oozoids were higher than previously reported for the WAP (mean growth rate for oozoids ~ 1 mm d−1) and if growth rates of blastozooids were lower (mean growth rate ~ 0.2 mm d−1). The model suggests that a prerequisite for local salp blooms requires a small founding population in early spring. With climate change it has been suggested that more frequent and earlier transport of salps into the WAP or winter survival will occur. Hence, the risk of salp blooms in the WAP is likely to substantially increase. These findings highlight the importance for an
improved quantitative understanding of how primary production and the southward advection of salps will be impacted by climate change
Nematic Ordering of Rigid Rods in a Gravitational Field
The isotropic-to-nematic transition in an athermal solution of long rigid
rods subject to a gravitational (or centrifugal) field is theoretically
considered in the Onsager approximation. The new feature emerging in the
presence of gravity is a concentration gradient which coupled with the nematic
ordering. For rodlike molecules this effect becomes noticeable at centrifugal
acceleration g ~ 10^3--10^4 m/s^2, while for biological rodlike objects, such
as tobacco mosaic virus, TMV, the effect is important even for normal
gravitational acceleration conditions. Rods are concentrated near the bottom of
the vessel which sometimes leads to gravity induced nematic ordering. The
concentration range corresponding to phase separation increases with increasing
g. In the region of phase separation the local rod concentration, as well as
the order parameter, follow a step function with height.Comment: Full article http://prola.aps.org/abstract/PRE/v60/i3/p2973_
Isotropic-nematic phase transition in suspensions of filamentous virus and the neutral polymer Dextran
We present an experimental study of the isotropic-nematic phase transition in
an aqueous mixture of charged semi-flexible rods (fd virus) and neutral polymer
(Dextran). A complete phase diagram is measured as a function of ionic strength
and polymer molecular weight. At high ionic strength we find that adding
polymer widens the isotropic-nematic coexistence region with polymers
preferentially partitioning into the isotropic phase, while at low ionic
strength the added polymer has no effect on the phase transition. The nematic
order parameter is determined from birefringence measurements and is found to
be independent of polymer concentration (or equivalently the strength of
attraction). The experimental results are compared with the existing
theoretical predictions for the isotropic-nematic transition in rods with
attractive interactions.Comment: 8 Figures. To be published in Phys. Rev. E. For more information see
http://www.elsie.brandeis.ed
Conformational Mechanics of Polymer Adsorption Transitions at Attractive Substrates
Conformational phases of a semiflexible off-lattice homopolymer model near an
attractive substrate are investigated by means of multicanonical computer
simulations. In our polymer-substrate model, nonbonded pairs of monomers as
well as monomers and the substrate interact via attractive van der Waals
forces. To characterize conformational phases of this hybrid system, we analyze
thermal fluctuations of energetic and structural quantities, as well as
adequate docking parameters. Introducing a solvent parameter related to the
strength of the surface attraction, we construct and discuss the
solubility-temperature phase diagram. Apart from the main phases of adsorbed
and desorbed conformations, we identify several other phase transitions such as
the freezing transition between energy-dominated crystalline low-temperature
structures and globular entropy-dominated conformations.Comment: 13 pages, 15 figure
The Extremophile Endolithella mcmurdoensis gen. et sp. nov. (Trebouxiophyceae, Chlorellaceae), a new Chlorella-like Endolithic alga from Antarctica
The McMurdo Dry Valleys constitute the largest ice-free region of Antarctica and one of the most extreme deserts on Earth. Despite the low temperatures, dry and poor soils and katabatic winds, some microbes are able to take advantage of endolithic microenvironments, inhabiting the pore spaces of soil and constituting photosynthesis-based communities. We isolated a green microalga, Endolithella mcmurdoensis gen. et sp. nov, from an endolithic sandstone sample collected in the McMurdo Dry Valleys (Victoria Land, East Antarctica) during the K020 expedition, in January 2013. The single non-axenic isolate (E. mcmurdoensis LEGE Z-009) exhibits cup-shaped chloroplasts, electron-dense bodies, and polyphosphate granules but our analysis did not reveal any diagnostic morphological characters. On the basis of phylogenetic analysis of the 18S rRNA (SSU) gene, the isolate was found to represent a new genus within the family Chlorellaceae.We are sincerely grateful to Antarctica New Zealand for providing
logistics support during K020 event. This campaign
was conducted as part of the New Zealand Terrestrial Antarctic
Biocomplexity Survey (nzTABS) through awards
(UOWX0710 and UOWX1401) from the New Zealand Foundation
for Research and Technology (FRST), Antarctica New
Zealand, and the New Zealand Ministry of Business, Innovation
and Employment to SCC. The work was also supported
by the Portuguese Science and Technology Foundation
(FCT) through grants UID/Multi/04423/2019 to CIIMAR,
IF/01358/2014 and PTDC/MAR-BIO/2818/2012 to PNL,
PTDC/CTA-AMB/30997/2017 to CM, and PhD scholarships
to AR (SFRH/BD/140567/2018).info:eu-repo/semantics/publishedVersio
Radius and chirality dependent conformation of polymer molecule at nanotube interface
Temperature dependent conformations of linear polymer molecules adsorbed at
carbon nanotube (CNT) interfaces are investigated through molecule dynamics
simulations. Model polyethylene (PE) molecules are shown to have selective
conformations on CNT surface, controlled by atomic structures of CNT lattice
and geometric coiling energy. PE molecules form entropy driven assembly
domains, and their preferred wrapping angles around large radius CNT (40, 40)
reflect the molecule configurations with energy minimums on a graphite plane.
While PE molecules prefer wrapping on small radius armchair CNT (5, 5)
predominantly at low temperatures, their configurations are shifted to larger
wrapping angle ones on a similar radius zigzag CNT (10, 0). A nematic
transformation around 280 K is identified through Landau-deGennes theory, with
molecule aligning along tube axis in extended conformationsComment: 19 pages, 7 figure2, submitted to journa
Open-source genomic analysis of Shiga-toxin–producing E. coli O104:H4
An outbreak caused by Shiga-toxin–producing Escherichia coli O104:H4 occurred in Germany in May and June of 2011, with more than 3000 persons infected. Here, we report a cluster of cases associated with a single family and describe an open-source genomic analysis of an isolate from one member of the family. This analysis involved the use of rapid, bench-top DNA sequencing technology, open-source data release, and prompt crowd-sourced analyses. In less than a week, these studies revealed that the outbreak strain belonged to an enteroaggregative E. coli lineage that had acquired genes for Shiga toxin 2 and for antibiotic resistance
A origem das parcerias público-privada na governança global da educação
Durante a última década, a globalização da governança educacional por meio de parcerias público-privadas (PPP) tem gerado considerável debate quanto ao seu significado, propósito, status e resultados. Este debate é particularmente aquecido no setor da educação por causa da ampla aceitação da educação como atividade complexa, social e polÃtica que deve permanecer, em grande parte, se não totalmente, no setor público, servindo a interesses públicos. O artigo analisa a rápida expansão das parcerias público-privadas em educação (PPPE) articulada à introdução de regras de mercado no setor. Neste estudo nos concentramos sobre o papel de uma rede de desenvolvimento global, fundamental na globalização de um tipo particular de PPPE, indicando que a ideia de PPP encaixa-se em um projeto mais amplo de reconstituição da educação pública no âmbito do setor de serviços, a ser governada como parte da construção de uma sociedade de mercado.Over the past decade, the globalization and governing of education through Public Private Partnerships (PPPs) have generated considerable debate as to their meaning, purpose, status and outcomes. This debate is particularly heated in the education sector because of the widely-held view that education is a complex social and political activity that should remain largely, if not wholly, in the public sector serving public interests. The article analyses the rapid expansion of Education Public Private Partnerships (EPPPs) and the associated introduction of market rules into the education sector. We focus on the role of a key global development network in globalizing a particular kind of ePPPs, and show that the EPPP idea fi ts into a wider project of reconstituting public education as an education services industry to be governed as part of the construction of a market society
Stimulus Dependence of Barrel Cortex Directional Selectivity
Neurons throughout the rat vibrissa somatosensory pathway are sensitive to the angular direction of whisker movement. Could this sensitivity help rats discriminate stimuli? Here we use a simple computational model of cortical neurons to analyze the robustness of directional selectivity. In the model, directional preference emerges from tuning of synaptic conductance amplitude and latency, as in recent experimental findings. We find that directional selectivity during stimulation with random deflection sequences is strongly dependent on the mean deflection frequency: Selectivity is weakened at high frequencies even when each individual deflection evokes strong directional tuning. This variability of directional selectivity is due to generic properties of synaptic integration by the neuronal membrane, and is therefore likely to hold under very general physiological conditions. Our results suggest that directional selectivity depends on stimulus context. It may participate in tasks involving brief whisker contact, such as detection of object position, but is likely to be weakened in tasks involving sustained whisker exploration (e.g., texture discrimination)
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