816 research outputs found
Dependence of the fluctuation-dissipation temperature on the choice of observable
On general grounds, a nonequilibrium temperature can be consistently defined
from generalized fluctuation-dissipation relations only if it is independent of
the observable considered. We argue that the dependence on the choice of
observable generically occurs when the phase-space probability distribution is
non-uniform on constant energy shells. We relate quantitatively this observable
dependence to a fundamental characteristics of nonequilibrium systems, namely
the Shannon entropy difference with respect to the equilibrium state with the
same energy. This relation is illustrated on a mean-field model in contact with
two heat baths at different temperatures.Comment: 4 pages, 2 figures, final versio
Criticality at finite strain rate in fluidized soft glassy materials
We study the emergence of critical dynamics in the steady shear rheology of
fluidized soft glassy materials. Within a mesoscale elasto-plastic model
accounting for a shear band instability, we show how an additional noise can
induce a transition from phase separated to homogeneous flow, accompanied by
critical-like fluctuations of the macroscopic shear rate. Both macroscopic
quantities and fluctuations exhibit power law behaviors in the vicinity of this
transition, consistent with previous experimental findings on vibrated granular
media. Altogether, our results suggest a generic scenario for the emergence of
criticality when shear weakening mechanisms compete with a fluidizing noise.Comment: 7 pages, 7 figure
Intensive thermodynamic parameters in nonequilibrium systems
Considering a broad class of steady-state nonequilibrium systems for which
some additive quantities are conserved by the dynamics, we introduce from a
statistical approach intensive thermodynamic parameters (ITPs) conjugated to
the conserved quantities. This definition does not require any detailed balance
relation to be fulfilled. Rather, the system has to satisfy a general
additivity property, which holds in most of the models usually considered in
the literature, including those described by a matrix product ansatz with
finite matrices. The main property of these ITPs is to take equal values in two
subsystems, making them a powerful tool to describe nonequilibrium phase
coexistence, as illustrated on different models. We finally discuss the issue
of the equalization of ITPs when two different systems are put into contact.
This issue is closely related to the possibility of measuring the ITPs using a
small auxiliary system, in the same way as temperature is measured with a
thermometer, and points at one of the major difficulties of nonequilibrium
statistical mechanics. In addition, an efficient alternative determination,
based on the measure of fluctuations, is also proposed and illustrated.Comment: 17 pages, 5 figures; final version, with minor change
On the relevance of disorder in athermal amorphous materials under shear
We show that, at least at a mean-field level, the effect of structural
disorder in sheared amorphous media is very dissimilar depending on the thermal
or athermal nature of their underlying dynamics. We first introduce a toy
model, including explicitly two types of noise (thermal versus athermal).
Within this interpretation framework, we argue that mean-field athermal
dynamics can be accounted for by the so-called H{\'e}braud-Lequeux (HL) model,
in which the mechanical noise stems explicitly from the plastic activity in the
sheared medium. Then, we show that the inclusion of structural disorder, by
means of a distribution of yield energy barriers, has no qualitative effect in
the HL model, while such a disorder is known to be one of the key ingredients
leading kinematically to a finite macroscopic yield stress in other mean-field
descriptions, such as the Soft-Glassy-Rheology model. We conclude that the
statistical mechanisms at play in the emergence of a macroscopic yield stress,
and a complex stationary dynamics at low shear rate, are different in thermal
and athermal amorphous systems
Influence of flux balance on the generalized chemical potential in mass transport models
In equilibrium systems, the conservation of the number of particles (or mass)
leads to the equalization of the chemical potential throughout the system.
Using a non-equilibrium generalization of the notion of chemical potential, we
investigate the influence of disorder and of the balance of mass fluxes on the
generalized chemical potential in the framework of stochastic mass transport
models. We focus specifically on the issue of local mesurements of the chemical
potential. We find that while local dynamical disorder does not affect the
measurement process, the presence of large-scale geometrical heterogeneities
(branching geometry) leads to unequal local measurement results in different
points of the system. We interpret these results in terms of mass flux balance,
and argue that the conditions for the global definition of the chemical
potential still hold, but that local measurements fail to capture the global
theoretical value.Comment: 14 pages, 7 figure
Paths to Fisheries Subsidies Reform: Creating Sustainable Fisheries Through Trade and Economics
The world depends on the oceans for food and livelihood. More than a billion people worldwide depend on fish as a source of protein, including some of the poorest populations on earth. According to the United Nations Food and Agriculture Organization (FAO), the world must produce 70 percent more food to meet coming hunger needs.Fishing activities support coastal communities and hundreds of millions of people who depend on fishing for all or part of their income. Of the world's fishers, more than 95 percent engage in small-scale and artisanal activity and catch nearly the same amount of fish for human consumption as the highly capitalized industrial sector. Small-scale and artisanal fishing produces a greater return than industrial operations by unit of input, investment in catch, and number of people employed.Today, overfishing and other destructive fishing practices have severely decreased the world's fish populations. The FAO estimates that 90 percent of marine fisheries worldwide are now overexploited, fully exploited, significantly depleted, or recovering from overexploitation
Entropy-based characterizations of the observable-dependence of the fluctuation-dissipation temperature
The definition of a nonequilibrium temperature through generalized
fluctuation-dissipation relations relies on the independence of the
fluctuation-dissipation temperature from the observable considered. We argue
that this observable independence is deeply related to the uniformity of the
phase-space probability distribution on the hypersurfaces of constant energy.
This property is shown explicitly on three different stochastic models, where
observable-dependence of the fluctuation-dissipation temperature arises only
when the uniformity of the phase-space distribution is broken. The first model
is an energy transport model on a ring, with biased local transfer rules. In
the second model, defined on a fully connected geometry, energy is exchanged
with two heat baths at different temperatures, breaking the uniformity of the
phase-space distribution. Finally, in the last model, the system is connected
to a zero temperature reservoir, and preserves the uniformity of the
phase-space distribution in the relaxation regime, leading to an
observable-independent temperature.Comment: 15 pages, 7 figure
Symbiotic Human Gut Bacteria with Variable Metabolic Priorities for Host Mucosal Glycans.
UnlabelledMany symbiotic gut bacteria possess the ability to degrade multiple polysaccharides, thereby providing nutritional advantages to their hosts. Like microorganisms adapted to other complex nutrient environments, gut symbionts give different metabolic priorities to substrates present in mixtures. We investigated the responses of Bacteroides thetaiotaomicron, a common human intestinal bacterium that metabolizes more than a dozen different polysaccharides, including the O-linked glycans that are abundant in secreted mucin. Experiments in which mucin glycans were presented simultaneously with other carbohydrates show that degradation of these host carbohydrates is consistently repressed in the presence of alternative substrates, even by B. thetaiotaomicron previously acclimated to growth in pure mucin glycans. Experiments with media containing systematically varied carbohydrate cues and genetic mutants reveal that transcriptional repression of genes involved in mucin glycan metabolism is imposed by simple sugars and, in one example that was tested, is mediated through a small intergenic region in a transcript-autonomous fashion. Repression of mucin glycan-responsive gene clusters in two other human gut bacteria, Bacteroides massiliensis and Bacteroides fragilis, exhibited variable and sometimes reciprocal responses compared to those of B. thetaiotaomicron, revealing that these symbionts vary in their preference for mucin glycans and that these differences occur at the level of controlling individual gene clusters. Our results reveal that sensing and metabolic triaging of glycans are complex processes that vary among species, underscoring the idea that these phenomena are likely to be hidden drivers of microbiota community dynamics and may dictate which microorganisms preferentially commit to various niches in a constantly changing nutritional environment.ImportanceHuman intestinal microorganisms impact many aspects of health and disease, including digestion and the propensity to develop disorders such as inflammation and colon cancer. Complex carbohydrates are a major component of the intestinal habitat, and numerous species have evolved and refined strategies to compete for these coveted nutrients. Our findings reveal that individual bacteria exhibit different preferences for carbohydrates emanating from host diet and mucosal secretions and that some of these prioritization strategies are opposite to one another. Thus, we reveal new aspects of how individual bacteria, some with otherwise similar metabolic potential, partition to "preferred niches" in the complex gut ecosystem, which has important and immediate implications for understanding and predicting the behavioral dynamics of this community
Cloning and sequence analysis of brain cDNA encoding a Xenopus D2 dopamine receptor
AbstractA D2 dopamine receptor pharmacologically different from the mammalian D2 receptor has previously been characterized in the amphibian Xenopus laevis. Here we report the cloning of a Xenopus D2 receptor which revealed about 75% amino acid sequence identity with its mammalian counterpart and the presence of an additional 33 amino acid sequence in the 3rd cytoplasmic loop instead of the additional 29 residues in the large form of the mammalian D2 receptor, All 7 predicted transmembrane domains are highly conserved between the Xenopus and mammalian D2 receptors, as are the 1st and 2nd intracellular loop, the 1st and 3rd extracellular loop and the carboxy-terminal portion of the receptors. The amino-terminal portion, the 2nd extracellular loop and the middle portion of the 3rd intracellular loop of these receptors, however, differ considerably, Knowledge of the locations of these regions of conservation and divergence within the D2 receptors or Xenopus and mammals will help to delineate portions of the receptor molecule that are functionally important. Interestingly, the 5-untranslated region of the Xenopus D2 receptor mRNA contains 4 small open reading frames which may affect translational efficiency
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