Effet de la température sur l'installation et la croissance des plantes annuelles de marais temporaires méditerranéens

L'abondance des plantes annuelles des marais temporaires est soumise à de fortes fluctuations dans le temps. Celles-ci peuvent être dues à la variabilité du climat méditerranéen. Nous avons étudié expérimentalement la relation entre ces variations d'abondance et la température de l'eau. La phase d'installation et la phase de croissance des espèces ont été étudiées dans trois gammes de températures. Chaque espèce possède son propre patron de germination, lequel varie avec la température. La vitesse d'émergence des plantules de toutes les espèces change en relation avec la température. Le taux de germination n'est modifié significativement que pour les charophytes, avec une forte diminution en conditions froides. L'abondance des réserves de semences de #Chara sp. et de #Zannichellia spp. explique en grande partie leur dynamique de germination. Le recouvrement à la fin de la phase d'installation est maximal en conditions chaudes et le #Callitriche truncata et les #Zannichellia spp. présentent plus fortes valeurs. En fin de période de croissance, le #Ranunculus baudotii domine généralement la communauté en terme de biomasse tandis que le #C. truncata est dominé. La biomasse des espèces est modifiée par la température de la phase d'installation et (ou) de croissance, sauf pour les #Zannichellia$ spp. Il n'existe pas d'effet de "préemption" apparent : les stratégies de vie contrastées des espèces aboutissent à modifier les relations de dominance au cours du cycle de vie. (Résumé d'auteur

Basic studies on dispersion hardening

Transmission electron microscopy and X-ray diffraction observations of substructure and elastic strains in cold worked and annealed dispersion strengthened alloy

Effect of a Grazing Intensity Gradient on Primary Production and Soil Nitrogen Mineralisation in a Humid Grassland of Western France

Large herbivores have a major influence on the structure and the functions of humid grasslands and especially on primary production. Earlier work on the study site showed that grazing intensity was spatially varied and created a diversity of vegetation patches in the grassland (Loucougaray, 2003). The first objective of this study was to determine whether the variation in grazing intensity led to variation of primary production within the three plant communities located at three topographic levels in the grassland. The second objective was to determine whether a relationship linked primary production variation and net soil nitrogen (N) mineralisation

Ultrasensitive Displacement Noise Measurement of Carbon Nanotube Mechanical Resonators

Mechanical resonators based on a single carbon nanotube are exceptional sensors of mass and force. The force sensitivity in these ultra-light resonators is often limited by the noise in the detection of the vibrations. Here, we report on an ultra-sensitive scheme based on a RLC resonator and a low-temperature amplifier to detect nanotube vibrations. We also show a new fabrication process of electromechanical nanotube resonators to reduce the separation between the suspended nanotube and the gate electrode down to $\sim 150$~nm. These advances in detection and fabrication allow us to reach $0.5~\mathrm{pm}/\sqrt{\mathrm{Hz}}$ displacement sensitivity. Thermal vibrations cooled cryogenically at 300~mK are detected with a signal-to-noise ratio as high as 17~dB. We demonstrate $4.3~\mathrm{zN}/\sqrt{\mathrm{Hz}}$ force sensitivity, which is the best force sensitivity achieved thus far with a mechanical resonator. Our work is an important step towards imaging individual nuclear spins and studying the coupling between mechanical vibrations and electrons in different quantum electron transport regimes.Comment: 9 pages, 5 figure

Superselectors: Efficient Constructions and Applications

We introduce a new combinatorial structure: the superselector. We show that superselectors subsume several important combinatorial structures used in the past few years to solve problems in group testing, compressed sensing, multi-channel conflict resolution and data security. We prove close upper and lower bounds on the size of superselectors and we provide efficient algorithms for their constructions. Albeit our bounds are very general, when they are instantiated on the combinatorial structures that are particular cases of superselectors (e.g., (p,k,n)-selectors, (d,\ell)-list-disjunct matrices, MUT_k(r)-families, FUT(k, a)-families, etc.) they match the best known bounds in terms of size of the structures (the relevant parameter in the applications). For appropriate values of parameters, our results also provide the first efficient deterministic algorithms for the construction of such structures

Imaging striatal dopamine release using a nongenetically encoded near infrared fluorescent catecholamine nanosensor.

Neuromodulation plays a critical role in brain function in both health and disease, and new tools that capture neuromodulation with high spatial and temporal resolution are needed. Here, we introduce a synthetic catecholamine nanosensor with fluorescent emission in the near infrared range (1000-1300 nm), near infrared catecholamine nanosensor (nIRCat). We demonstrate that nIRCats can be used to measure electrically and optogenetically evoked dopamine release in brain tissue, revealing hotspots with a median size of 2 µm. We also demonstrated that nIRCats are compatible with dopamine pharmacology and show D2 autoreceptor modulation of evoked dopamine release, which varied as a function of initial release magnitude at different hotspots. Together, our data demonstrate that nIRCats and other nanosensors of this class can serve as versatile synthetic optical tools to monitor neuromodulatory neurotransmitter release with high spatial resolution

Improving pH Prediction for High Pressure and High Temperature Applications in Oil and Gas Production

International audiencepH prediction represents a crucial step before selecting materials for use in sour oil and gas wells as regards weight loss corrosion and H 2 S cracking. Among the numerous parameters which determine the equilibrium pH, important ones are CO 2 and H 2 S partial pressures (P CO2 and P H2S respectively), the total pressure, the ionic strength and the chemical composition of the solution, and the temperature. Most models used by oil and gas operators present a too narrow range of validity for these parameters, which makes them inappropriate for high pressure and high temperature (HPHT) fields or for CO 2 reinjection. This paper presents modeling improvements which allows extending the prediction validity in temperature and pressure to respectively 200 °C and 1,000 bar of total pressure, and for an ionic strength up to 5 mol.kg-1. These improvements take into account the fugacities in gas phase of CO 2 and H 2 S as determined by the Soreide and Whitson formalism. The influence of water and CH 4 pressure is also taken into account up to several hundred bars. Activity coefficients in the water phase are calculated using the Pitzer model. The consistency of the model is verified by comparison with experimental measurements of pH under high pressure. It is then applied to oil and gas applications at high pressure and high temperature. The impact of the new calculation method is discussed both for pH evaluation and also for H 2 S activity, with strong implications for the evaluation of SSC risks

High flux polarized gamma rays production: first measurements with a four-mirror cavity at the ATF

The next generation of e+/e- colliders will require a very intense flux of gamma rays to allow high current polarized positrons to be produced. This can be achieved by converting polarized high energy photons in polarized pairs into a target. In that context, an optical system consisting of a laser and a four-mirror passive Fabry-Perot cavity has recently been installed at the Accelerator Test Facility (ATF) at KEK to produce a high flux of polarized gamma rays by inverse Compton scattering. In this contribution, we describe the experimental system and present preliminary results. An ultra-stable four-mirror non planar geometry has been implemented to ensure the polarization of the gamma rays produced. A fiber amplifier is used to inject about 10W in the high finesse cavity with a gain of 1000. A digital feedback system is used to keep the cavity at the length required for the optimal power enhancement. Preliminary measurements show that a flux of about $4\times10^6 \gamma$/s with an average energy of about 24 MeV was generated. Several upgrades currently in progress are also described