406 research outputs found
Acidification increases microbial polysaccharide degradation in the ocean
© The Authors, 2010. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Biogeosciences 7 (2010): 1615–1624, doi:10.5194/bg-7-1615-2010.With the accumulation of anthropogenic carbon dioxide (CO2), a proceeding decline in seawater pH has been induced that is referred to as ocean acidification. The ocean's capacity for CO2 storage is strongly affected by biological processes, whose feedback potential is difficult to evaluate. The main source of CO2 in the ocean is the decomposition and subsequent respiration of organic molecules by heterotrophic bacteria. However, very little is known about potential effects of ocean acidification on bacterial degradation activity. This study reveals that the degradation of polysaccharides, a major component of marine organic matter, by bacterial extracellular enzymes was significantly accelerated during experimental simulation of ocean acidification. Results were obtained from pH perturbation experiments, where rates of extracellular α- and β-glucosidase were measured and the loss of neutral and acidic sugars from phytoplankton-derived polysaccharides was determined. Our study suggests that a faster bacterial turnover of polysaccharides at lowered ocean pH has the potential to reduce carbon export and to enhance the respiratory CO2 production in the future ocean.This study was supported by the Helmholtz
Association (HZ-NG-102) and the Belgian Science Policy
(SD/CS/03)
Effect of CO2 enrichment on bacterial metabolism in an Arctic fjord
he anthropogenic increase of carbon dioxide (CO2) alters the seawater carbonate chemistry, with a decline of pH and an increase in the partial pressure of CO2 (pCO2). Although bacteria play a major role in carbon cycling, little is known about the impact of rising pCO2 on bacterial carbon metabolism, especially for natural bacterial communities. In this study, we investigated the effect of rising pCO2 on bacterial production (BP), bacterial respiration (BR) and bacterial carbon metabolism during a mesocosm experiment performed in Kongsfjorden (Svalbard) in 2010. Nine mesocosms with pCO2 levels ranging from ca. 180 to 1400 μatm were deployed in the fjord and monitored for 30 days. Generally BP gradually decreased in all mesocosms in an initial phase, showed a large (3.6-fold average) but temporary increase on day 10, and increased slightly after inorganic nutrient addition. Over the wide range of pCO2 investigated, the patterns in BP and growth rate of bulk and free-living communities were generally similar over time. However, BP of the bulk community significantly decreased with increasing pCO2 after nutrient addition (day 14). In addition, increasing pCO2 enhanced the leucine to thymidine (Leu : TdR) ratio at the end of experiment, suggesting that pCO2 may alter the growth balance of bacteria. Stepwise multiple regression analysis suggests that multiple factors, including pCO2, explained the changes of BP, growth rate and Leu : TdR ratio at the end of the experiment. In contrast to BP, no clear trend and effect of changes of pCO2 was observed for BR, bacterial carbon demand and bacterial growth efficiency. Overall, the results suggest that changes in pCO2 potentially influence bacterial production, growth rate and growth balance rather than the conversion of dissolved organic matter into CO2
Thermal and Magnetorotational Instability in the ISM: Two-Dimensional Numerical Simulations
The structure and dynamics of diffuse gas in the Milky Way and other disk
galaxies may be strongly influenced by thermal and magnetorotational
instabilities (TI and MRI) on scales of about 1-100 pc. We initiate a study of
these processes, using two-dimensional numerical hydrodynamic and
magnetohydrodynamic (MHD) simulations with conditions appropriate for the
atomic interstellar medium (ISM). We demonstrate, consistent with previous
work, that nonlinear development of ``pure TI'' produces a network of filaments
that condense into cold clouds at their intersections, yielding a distinct
two-phase warm/cold medium within about 20 Myr. TI-driven turbulent motions of
the clouds saturate at subsonic amplitudes for uniform initial P/k=2000 K
cm^-3. MRI has previously been studied in near-uniform media; our simulations
include both TI+MRI models, which begin from uniform-density conditions, and
cloud+MRI models, which begin with a two-phase cloudy medium. Both the TI+MRI
and cloud+MRI models show that MRI develops within a few galactic orbital
times, just as for a uniform medium. The mean separation between clouds can
affect which MRI mode dominates the evolution. Provided intercloud separations
do not exceed half the MRI wavelength, we find the MRI growth rates are similar
to those for the corresponding uniform medium. This opens the possibility, if
low cloud volume filling factors increase MRI dissipation times compared to
those in a uniform medium, that MRI-driven motions in the ISM could reach
amplitudes comparable to observed HI turbulent linewidths.Comment: 42 pages, 15 figures, accepted for publication in ApJ; For better
postscript figures and mpeg animations, see
http://www.astro.umd.edu/~rpiontek/papers/ti_mri_2D.htm
GPI-anchor signal sequence influences PrPC sorting, shedding and signalling, and impacts on different pathomechanistic aspects of prion disease in mice
The cellular prion protein (PrPC) is a cell surface glycoprotein attached to the membrane by a glycosylphosphatidylinositol (GPI)-anchor and plays a critical role in transmissible, neurodegenerative and fatal prion diseases. Alterations in membrane attachment influence PrPC-associated signaling, and the development of prion disease, yet our knowledge of the role of the GPI-anchor in localization, processing, and function of PrPC in vivo is limited We exchanged the PrPC GPI-anchor signal sequence of for that of Thy-1 (PrPCGPIThy-1) in cells and mice. We show that this modifies the GPI-anchor composition, which then lacks sialic acid, and that PrPCGPIThy-1 is preferentially localized in axons and is less prone to proteolytic shedding when compared to PrPC. Interestingly, after prion infection, mice expressing PrPCGPIThy-1 show a significant delay to terminal disease, a decrease of microglia/astrocyte activation, and altered MAPK signaling when compared to wild-type mice. Our results are the first to demonstrate in vivo, that the GPI-anchor signal sequence plays a fundamental role in the GPI-anchor composition, dictating the subcellular localization of a given protein and, in the case of PrPC, influencing the development of prion disease
A distributed multiscale computation of a tightly coupled model using the Multiscale Modeling Language
AbstractNature is observed at all scales; with multiscale modeling, scientists bring together several scales for a holistic analysis of a phenomenon. The models on these different scales may require significant but also heterogeneous computational resources, creating the need for distributed multiscale computing. A particularly demanding type of multiscale models, tightly coupled, brings with it a number of theoretical and practical issues. In this contribution, a tightly coupled model of in-stent restenosis is first theoretically examined for its multiscale merits using the Multiscale Modeling Language (MML); this is aided by a toolchain consisting of MAPPER Memory (MaMe), the Multiscale Application Designer (MAD), and Gridspace Experiment Workbench. It is implemented and executed with the general Multiscale Coupling Library and Environment (MUSCLE). Finally, it is scheduled amongst heterogeneous infrastructures using the QCG-Broker. This marks the first occasion that a tightly coupled application uses distributed multiscale computing in such a general way
A distributed multiscale computation of a tightly coupled model using the Multiscale Modeling Language
Recommended from our members
Climate impact research: Beyond patchwork
Despite significant progress in climate impact research, the narratives that science can presently piece together of a 2, 3, 4, or 5 °C warmer world remain fragmentary. Here we briefly review past undertakings to characterise comprehensively and quantify climate impacts based on multi-model approaches. We then report on the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP), a community-driven effort to compare impact models across sectors and scales systematically, and to quantify the uncertainties along the chain from greenhouse gas emissions and climate input data to the modelling of climate impacts themselves. We show how ISI-MIP and similar efforts can substantially advance the science relevant to impacts, adaptation and vulnerability, and we outline the steps that need to be taken in order to make the most of the available modelling tools. We discuss pertinent limitations of these methods and how they could be tackled. We argue that it is time to consolidate the current patchwork of impact knowledge through integrated cross-sectoral assessments, and that the climate impact community is now in a favourable position to do so
Recommended from our members
Climate impact research: Beyond patchwork
Despite significant progress in climate impact research, the narratives that science can presently piece together of a 2, 3, 4, or 5 degrees C warmer world remain fragmentary. Here we briefly review past undertakings to characterise comprehensively and quantify climate impacts based on multi-model approaches. We then report on the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP), a community-driven effort to compare impact models across sectors and scales systematically, and to quantify the uncertainties along the chain from greenhouse gas emissions and climate input data to the modelling of climate impacts themselves. We show how ISI-MIP and similar efforts can substantially advance the science relevant to impacts, adaptation and vulnerability, and we outline the steps that need to be taken in order to make the most of the available modelling tools. We discuss pertinent limitations of these methods and how they could be tackled. We argue that it is time to consolidate the current patchwork of impact knowledge through integrated cross-sectoral assessments, and that the climate impact community is now in a favourable position to do so
The {\eta}'-carbon potential at low meson momenta
The production of mesons in coincidence with forward-going
protons has been studied in photon-induced reactions on C and on a
liquid hydrogen (LH) target for incoming photon energies of 1.3-2.6 GeV at
the electron accelerator ELSA. The mesons have been identified
via the decay
registered with the CBELSA/TAPS detector system. Coincident protons have been
identified in the MiniTAPS BaF array at polar angles of . Under these kinematic constraints the
mesons are produced with relatively low kinetic energy (
150 MeV) since the coincident protons take over most of the momentum of the
incident-photon beam. For the C-target this allows the determination of the
real part of the -carbon potential at low meson momenta by
comparing with collision model calculations of the kinetic energy
distribution and excitation function. Fitting the latter data for
mesons going backwards in the center-of-mass system yields a potential depth of
V = (44 16(stat)15(syst)) MeV, consistent with earlier
determinations of the potential depth in inclusive measurements for average
momenta of 1.1 GeV/. Within the experimental
uncertainties, there is no indication of a momentum dependence of the
-carbon potential. The LH data, taken as a reference to check
the data analysis and the model calculations, provide differential and integral
cross sections in good agreement with previous results for
photoproduction off the free proton.Comment: 9 pages, 13 figures. arXiv admin note: text overlap with
arXiv:1608.0607
Angular Momentum Transport in Extended Galactic Disks
We demonstrate a significant difference in the angular momentum transport
properties of galactic disks between regions in which the interstellar medium
is single phase or two phase. Our study is motivated by observations of HI in
extended galactic disks which indicate velocity dispersions of nonthermal
origin, suggesting that turbulence in the gas may be contributing significantly
to the observed dispersion. To address this, we have implemented a shearing-box
framework within the FLASH code. The new code was used to perform local
simulations of galactic disks that incorporate differential rotation,
self-gravity, vertical stratification, hydrodynamics and cooling. These
simulations explore plausible mechanisms for driving turbulent motions via the
thermal and self-gravitational instabilities coupling to differential rotation.
Where a two-phase medium develops, gravitational angular momentum transporting
stresses are much greater, creating a possible mechanism for transferring
energy from galactic rotation to turbulence. In simulations where the disk
conditions do not trigger the formation of a two-phase medium, it is found that
perturbations to the flow damp without leading to a sustained mechanism for
driving turbulence. The differing angular momentum transport properties of the
single- and two-phase regimes of the disk suggest that a significant,
dynamically motivated division can be drawn between the two, even when this
division occurs far outside the star formation cutoff in a galactic disk.Comment: ApJ Letters accepted, 5 pages, 3 figur
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