152 research outputs found
Évaluation de la production primaire et de la chlorophylle à partir des données hydrologiques. Application au Golfe de Guinée
Quantitative relationships between nitrate distribution, chlorophyll and primary integrated values have been used to evaluate the phytoplankton abundance in the Gulf of Guinea. Data of Guinee I cruise (May-July 1968) of the R.V. Jean Charcot have been used. They show a large oligotrophic convergence area (< 250 mg C/m2/J). A relationship previously found between the depth of the nitracline (first level where the nitrate appears) and the depth of the thermal maximum gradient is confirmed. From a practical point of view, this relationship is very useful since it allows, when the biological or chemical data are not available, a rough estimation of phytoplankton integrated biomass and production in the water column, from a temperature profile
Facilitated diffusion of proteins on chromatin
We present a theoretical model of facilitated diffusion of proteins in the
cell nucleus. This model, which takes into account the successive
binding/unbinding events of proteins to DNA, relies on a fractal description of
the chromatin which has been recently evidenced experimentally. Facilitated
diffusion is shown quantitatively to be favorable for a fast localization of a
target locus by a transcription factor, and even to enable the minimization of
the search time by tuning the affinity of the transcription factor with DNA.
This study shows the robustness of the facilitated diffusion mechanism, invoked
so far only for linear conformations of DNA.Comment: 4 pages, 4 figures, accepted versio
Variabilité, circulation et chlorophylle dans la région du Dôme d'Angola en février-mars 1971
In February-March 1971 the hydrological conditions off Angola did not display the thermal dome mapped by Mazeika's averages (1967). Cold water cells observed are connected at the surface to a sinuous boundary between low-salinity coastal waters and high-salinity tropical oceanic waters. That boundary coincides rather regularly with an area where trades and SW winds alternate; photosynthesis growths rapidly in a thermoclinal layer that rises until 10 m of the surface but never outcrops. Below a poor and permanent homogeneous surface layer, chlorophyll concentrations show a distribution which is typical of divergence areas. Geostrophical and measured currents show off a transient process in horizontal and vertical movements, however the general curvature of the circulation is propitious to upwelling. Oxygen oversaturations of about 110%, suggest a moderate potential primary production which confirms slowness and alternation of movements. Also, the regular range of the various chemical and biological levels and moderate chlorophyll concentrations suggest an ecosystem where nutrients supply rapidly equilibrate phytoplankton consumption and not at all a 'phytoplankton bloom' area as that which exists in coastal upwelling. Values of Richardson's number show that instability becomes visible at the bottom of the euphotic layer. An evaluation of the vertical motion is inferred by the peculiar distribution and diurnal alternance of the winds shows that 'doming' structures may be sustained by local meteorological events
Substrate rigidity deforms and polarizes active gels
We present a continuum model of the coupling between cells and substrate that
accounts for some of the observed substrate-stiffness dependence of cell
properties. The cell is modeled as an elastic active gel, adapting recently
developed continuum theories of active viscoelastic fluids. The coupling to the
substrate enters as a boundary condition that relates the cell's deformation
field to local stress gradients. In the presence of activity, the coupling to
the substrate yields spatially inhomogeneous contractile stresses and
deformations in the cell and can enhance polarization, breaking the cell's
front-rear symmetry.Comment: 6 pages, 4 figures, EPL forma
Production primaire, secondaire et régéneration des sels nutritifs dans l'upwelling de Mauritanie
Primary and secondary productions and nutrient regeneration in the Mauritanian upwelling area were studied by following a drogue for 9 days, from the point of upwelling till the water mass dives under offshore waters. The lag between phytoplanktonic bloom, zooplanktonic peak and bacterial activity is very short and may be explained by a well-settled biological cycle connected with an undercurrent. Organic production was estimated in two ways: (1) from chlorophyll 'a' values, considering a C/Chla ratio of 25 during the 5.5 day phytoplankton growth period, primary production computed by this method reaches 13.5 g C/m2; (2) from 14C values net primary production calculated for the same period reaches 10.5 g C/m2 and total organic production (net production + organic excretion) reaches 19.5 g C/m2. Organic production computed ratios, delta O/ delta C/ delta N/ delta Si/ delta P are equal to 130/43/11/7.4/1. Secondary production and 'grazing' are estimated from mesozooplankton respiration values and have a huge increase during the bloom. Net secondary production is assessed to be 1.0-4.2 g C/m2 for 6 days. Evidence of nutrient regeneration as ammonia, phosphate and silicate is given and regeneration rates are calculated. Zooplankton excretion plays an important part in nitrogen and phosphorus regeneration. Bacterial activity is induced by zooplankton organic excretion, then increased by phytoplankton decomposition at the end of the bloom
Exact calculations of first-passage quantities on recursive networks
We present general methods to exactly calculate mean-first passage quantities
on self-similar networks defined recursively. In particular, we calculate the
mean first-passage time and the splitting probabilities associated to a source
and one or several targets; averaged quantities over a given set of sources
(e.g., same-connectivity nodes) are also derived. The exact estimate of such
quantities highlights the dependency of first-passage processes with respect to
the source-target distance, which has recently revealed to be a key parameter
to characterize transport in complex media. We explicitly perform calculations
for different classes of recursive networks (finitely ramified fractals,
scale-free (trans)fractals, non-fractals, mixtures between fractals and
non-fractals, non-decimable hierarchical graphs) of arbitrary size. Our
approach unifies and significantly extends the available results in the field.Comment: 16 pages, 10 figure
Reversible Diffusion-Limited Reactions: "Chemical Equilibrium" State and the Law of Mass Action Revisited
The validity of two fundamental concepts of classical chemical kinetics - the
notion of "Chemical Equilibrium" and the "Law of Mass Action" - are re-examined
for reversible \textit{diffusion-limited} reactions (DLR), as exemplified here
by association/dissociation reactions. We consider a
general model of long-ranged reactions, such that any pair of particles,
separated by distance , may react with probability , and
any may dissociate with probability into a geminate
pair of s separated by distance . Within an exact analytical
approach, we show that the asymptotic state attained by reversible DLR at is generally \textit{not a true thermodynamic equilibrium}, but rather
a non-equilibrium steady-state, and that the Law of Mass Action is invalid. The
classical picture holds \text{only} in physically unrealistic case when
for any value of .Comment: 4 page
Spontaneous flow states in active nematics: a unified picture
Continuum hydrodynamic models of active liquid crystals have been used to
describe dynamic self-organising systems such as bacterial swarms and
cytoskeletal gels. A key prediction of such models is the existence of
self-stabilising kink states that spontaneously generate fluid flow in
quasi-one dimensional channels. Using simple stability arguments and numerical
calculations we extend previous studies to give a complete characterisation of
the phase space for both contractile and extensile particles (ie pullers and
pushers) moving in a narrow channel as a function of their flow alignment
properties and initial orientation. This gives a framework for unifying many of
the results in the literature. We describe the response of the kink states to
an imposed shear, and investigate how allowing the system to be polar modifies
its dynamical behaviour.Comment: 6 pages, 6 figures; submitted to Europhysics Letter
Residual mean first-passage time for jump processes: theory and applications to L\'evy flights and fractional Brownian motion
We derive a functional equation for the mean first-passage time (MFPT) of a
generic self-similar Markovian continuous process to a target in a
one-dimensional domain and obtain its exact solution. We show that the obtained
expression of the MFPT for continuous processes is actually different from the
large system size limit of the MFPT for discrete jump processes allowing
leapovers. In the case considered here, the asymptotic MFPT admits
non-vanishing corrections, which we call residual MFPT. The case of L/'evy
flights with diverging variance of jump lengths is investigated in detail, in
particular, with respect to the associated leapover behaviour. We also show
numerically that our results apply with good accuracy to fractional Brownian
motion, despite its non-Markovian nature.Comment: 13 pages, 8 figure
Generic phase diagram of active polar films
We study theoretically the phase diagram of compressible active polar gels
such as the actin network of eukaryotic cells. Using generalized hydrodynamics
equations, we perform a linear stability analysis of the uniform states in the
case of an infinite bidimensional active gel to obtain the dynamic phase
diagram of active polar films. We predict in particular modulated flowing
phases, and a macroscopic phase separation at high activity. This qualitatively
accounts for experimental observations of various active systems, such as
acto-myosin gels, microtubules and kinesins in vitro solutions, or swimming
bacterial colonies.Comment: 4 pages, 1 figur
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