5,909 research outputs found
Stability of Cross-Feeding Polymorphisms in Microbial Communities
This is the final version of the article. Available from Public Library of Science via the DOI in this record.Cross-feeding, a relationship wherein one organism consumes metabolites excreted by another, is a ubiquitous feature of natural and clinically-relevant microbial communities and could be a key factor promoting diversity in extreme and/or nutrient-poor environments. However, it remains unclear how readily cross-feeding interactions form, and therefore our ability to predict their emergence is limited. In this paper we developed a mathematical model parameterized using data from the biochemistry and ecology of an E. coli cross-feeding laboratory system. The model accurately captures short-term dynamics of the two competitors that have been observed empirically and we use it to systematically explore the stability of cross-feeding interactions for a range of environmental conditions. We find that our simple system can display complex dynamics including multi-stable behavior separated by a critical point. Therefore whether cross-feeding interactions form depends on the complex interplay between density and frequency of the competitors as well as on the concentration of resources in the environment. Moreover, we find that subtly different environmental conditions can lead to dramatically different results regarding the establishment of cross-feeding, which could explain the apparently unpredictable between-population differences in experimental outcomes. We argue that mathematical models are essential tools for disentangling the complexities of cross-feeding interactions.IG was supported by a Natural Environment Research Council (NERC) Advanced Fellowship NE/E013007/3 and a European Research Council (ERC) Consolidator grant MathModExp 647292, MK was funded by a National Aeronautics and Space Administration (NASA) NPP Fellowship and NASA NNX12AD87G, IG and PR were funded by a Biotechnology and Biological Sciences Research Council (BBSRC) grant BB/J010340/1, KS was supported by National Human Genome Research Institute (NHGRI) 2R01HG003328 - 07A1 and FR was supported by NASA NNX12AD87G. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
Numerical Study of the Lowest Energy Configurations for Global String-Antistring Pairs
We investigate the lowest energy configurations for string - antistring pairs
at fixed separations by numerically minimizing the energy. We show that for
separations smaller than a critical value, a region of false vacuum develops in
the middle due to large gradient energy density. Consequently, well defined
string - antistring pairs do not exist for such separations. We present an
example of vortex - antivortex production by vacuum bubbles where this effect
seems to play a dynamical role in the annihilation of the pair. We also study
the dependence of the energy of an string-antistring pair on their separation
and find deviations from a simple logarithmic dependence for small separations.Comment: 14 pages, in LATEX, 7 figures (not included
On the Theory of Relativistic Strong Plasma Waves
The influence of motion of ions and electron temperature on nonlinear
one-dimensional plasma waves with velocity close to the speed of light in
vacuum is investigated. It is shown that although the wavebreaking field weakly
depends on mass of ions, the nonlinear relativistic wavelength essentially
changes. The nonlinearity leads to the increase of the strong plasma
wavelength, while the motion of ions leads to the decrease of the wavelength.
Both hydrodynamic approach and kinetic one, based on Vlasov-Poisson equations,
are used to investigate the relativistic strong plasma waves in a warm plasma.
The existence of relativistic solitons in a thermal plasma is predicted.Comment: 13 pages, 8 figure
Stability of the Black Hole Horizon and the Landau Ghost
The stability of the black hole horizon is demanded by both cosmic censorship
and the generalized second law of thermodynamics. We test the consistency of
these principles by attempting to exceed the black hole extremality condition
in various process in which a U(1) charge is added to a nearly extreme
Reissner--Nordstr\"om black hole charged with a {\it different\/} type of U(1)
charge. For an infalling spherical charged shell the attempt is foiled by the
self--Coulomb repulsion of the shell. For an infalling classical charge it
fails because the required classical charge radius exceeds the size of the
black hole. For a quantum charge the horizon is saved because in order to avoid
the Landau ghost, the effective coupling constant cannot be large enough to
accomplish the removal.Comment: 12 pages, RevTe
Energy Loss of a High Charge Bunched Electron Beam in Plasma
There has been much interest in the blowout regime of plasma wakefield
acceleration (PWFA), which features ultra-high fields and nonlinear plasma
motion. Using an exact analysis, we examine here a fundamental limit of
nonlinear PWFA excitation, by an infinitesimally short, relativistic electron
beam. The beam energy loss in this case is shown to be linear in charge even
for nonlinear plasma response, where a normalized, unitless charge exceeds
unity. The physical basis for this effect is discussed, as are deviations from
linear behavior observed in simulations with finite length beams.Comment: Submitted to Physical Review Letter
Observation of Plasma Focusing of a 28.5 GeV Positron Beam
The observation of plasma focusing of a 28.5 GeV positron beam is reported.
The plasma was formed by ionizing a nitrogen jet only 3 mm thick. Simultaneous
focusing in both transverse dimensions was observed with effective focusing
strengths of order Tesla per micron. The minimum area of the beam spot was
reduced by a factor of 2.0 +/- 0.3 by the plasma. The longitudinal beam
envelope was measured and compared with numerical calculations
Relating Dynamic Protein Interactions Of Metallochaperones With Metal Transfer At The Single-Molecule Level
Metallochaperones undertake specific interactions with their target proteins to deliver metal ions inside cells. Understanding how these protein interactions are coupled with the underlying metal transfer process is important, but challenging because they are weak and dynamic. Here we use a nanovesicle trapping scheme to enable single-molecule FRET measurements of the weak, dynamic interactions between the copper chaperone Hahl and the fourth metal binding domain (MBD4) of WDP. By monitoring the behaviors of single interacting pairs, we visualize their interactions in real time in both the absence and the presence of various equivalents of Cu(1+). Regardless of the proteins\u27 metallation state, we observe multiple, interconverting interaction complexes between Hah1 and MBD4. Within our experimental limit, the overall interaction geometries of these complexes appear invariable, but their stabilities are dependent on the proteins\u27 metallation state. In apo-holo Hah1-MBD4 interactions, the complexes are stabilized relative to that observed in the apo-apo interactions. This stabilization is indiscernible when Hah1\u27s Cu(1+)-binding is eliminated or when both proteins have Cu(1+) loaded. The nature of this Cu(1+)-induced complex stabilization and of the interaction complexes are discussed. These Cu(1+)-induced effects on the Hah1-MBD4 interactions provide a step toward understanding how the dynamic protein interactions of copper chaperones are coupled with their metal transfer function
Novel Studies on the \eta' Effective Lagrangian
The effective Lagrangian for \eta' incorporating the effect of the QCD
\theta-angle has been developed previously. We revisit this Lagrangian and
carry out its canonical quantization with particular attention to the test
function spaces of constraints and the topology of the \eta'-field. In this
way, we discover a new chirally symmetric coupling of this field to chiral
multiplets which involves in particular fermions. This coupling violates P and
T symmetries. In a subsequent paper, we will evaluate its contribution to the
electric dipole moment (EDM) of fermions. Our motivation is to test whether the
use of mixed states restores P and T invariance, so that EDM vanishes. This
calculation will be shown to have striking new physical consequences.Comment: 14 pages, 1 figure; V2: NEW TITLE; revised version to be published in
JHEP; references adde
- …