Variabilités spatiales et temporelles de la qualité physico-chimique et des invertébrés pélagiques des eaux de la retenue hydroélectrique de Petit Saut (Guyane française).

Le bassin versant du fleuve Sinnamary est situé en Guyane française, au nord du continent sud américain, en zone néo-tropicale. Il s'étend entre 4 et 5°de latitude Nord et entre 52°50' et 53°30' de longitude Ouest. En 1994, la mise en eau du barrage hydroélectrique construit sur le site de Petit Saut occasionne l'ennoiement de 365 km2 de forêt primaire. Le remplissage s'effectue en 18 mois, et la demande en oxygène dissous nécessaire à la dégradation de la matière organique est telle qu'en quelques jours la masse d'eau se stratifie en un épilimnion oxygéné et un hypolimnion anoxique. Représentant quelques centimètres en 1994, l'épilimnion s'épaissit progressivement jusqu'en 1998. Depuis, cette progression s'est nettement ralentie et l'épilimnion oscille autour d'une valeur moyenne de 5 à 6 mètres. Il abrite la plus grande partie du zooplancton pélagique. L'hypolimnion est anoxique et riche en éléments réduits. Le zooplancton s'est rapidement installé avec des rotifères, des cladocères et des copépodes. Des ostracodes et des Chaoboridae y sont désormais associés. La retenue présente une zonation longitudinale, de la tête de la retenue vers la queue, aussi bien en termes de densités que de composition des peuplements. De même, il existe une zonation transversale, notamment marquée par un gradient croissant de la conductivité de l'axe vers les berges. Ces ressources endogènes ainsi que des apports complémentaires (végétaux, invertébrés terrestres, détritus…) sont utilisées par la faune ichtyque qui a su s'adapter au changement du milieu. Le temps de rétention des eaux qui fluctue saisonnièrement en fonction des pluies semble être le facteur prépondérant de variabilité pour la qualité physico-chimique des eaux et les communautés biologiques.The Sinnamary River is located in a neotropical zone, in French Guiana, in the northern part of South America. Its flood basin extends between 4° and 5° N latitude and 52°50' and 53°30' W longitude. The river is found in an equatorial climate, and is influenced by its proximity to the ocean. The temperature varies little, with an average of 25°C for the least hot month (January) and 27°C for the hottest month (October). The humidity is always high and averages more than 90%. The Sinnamary River bed is broad (> 50 m), and runs from south to north for approximately 240 km with a very weak slope (0.0003%). Petit Saut, the site of the hydroelectric dam, is located approximately 60 km from the mouth of the river, with a flood basin area of 5927 km2 and an average flow rate 260 m3 /s.The construction of the hydroelectric dam at Petit Saut in 1994 resulted in the flooding of 365 km2 of primary forest in a neotropical zone (the average depth of the reservoir is 11 m). The dam reservoir was filled over a period of 18 months and the amount of dissolved oxygen necessary for the organic matter to decompose was such that, in a matter of days, the body of water became stratified into an oxygenated epilimnion and an anoxic hypolimnion. Only centimeters thick in 1994, by 1998 the epilimnion had progressively expanded. Since that time it has varied between an average thickness of 5 to 6 m and is home to various biological communities. The hypolimnion is anoxic and rich in reduced compounds (nitrogen and phosphorus minerals, dissolved organic matter, particles, metals and gas). The products of the degradation of the flooded organic matter, which still prevail at the bottom, are gradually relayed towards surface by phenomena of dilution, hydrolysis and oxidation.The river fauna disappeared from the reservoir and was replaced after a few weeks by various pelagic fauna. The zooplanktonic communities, primarily confined to the epilimnion, were quickly settled with rotifera, cladocera and copepoda. Initially dominated by detritivores (Bosminidae) and Cyclopidae from February to May 1994, the settlements diversified with phytophagous zooplankton (Calanidae and Daphniidae) when the phytoplankton developed. This procession was quickly accompanied by Sididae, which has dominated the cladocera since mid-1996 and by ostracoda, which were particularly abundant from 1995 to 1997. Carnivores such as Chaoboridae larvae were also present in Petit Saut reservoir. In contrast to the other zooplankton species, Chaoboridae larvae occupy the entire water column. As a result of great quantities of organic matter available at the time the reservoir was filled, the biomasses were initially large and have gradually decreased with decreasing nutrient concentrations.The reservoir was not homogeneous in terms of zooplankton density, with a longitudinal zonation from the dam towards the upstream tail. In the tail, fauna were identical to those of the river. In the lake zone, phytophagous Calanidae dominated. In the intermediate zone, known as the zone of transition, the zooplankton biomass normalized to the volume of oxygenated water was less important than in the lake zone. Cyclopidae, well represented at the time of the reservoir colonization, dominated the communities in the transition zone. This distribution of zooplankton was likely connected with the dissolved oxygen concentrations, as well as with the quantities of phytoplankton and bacterio-chlorophyll. Similarly, there was a transverse zonation, marked by an increase in conductivity towards the reservoir banks. The density of the communities varied seasonally. The highest biomasses were measured along the central axis at the time of high waters and close to the banks at the time of low waters.The time of water retention, which fluctuates according to the rainy season, seemed to be the factor dominating the physicochemical water quality variability and the biological communities. The rainy season is characterized by the dilution of various elements (e.g., conductivity is reduced). Conversely, during the dry season, the elements concentrate again in the water mass. Nevertheless, the peaks of pelagic invertebrate density were observed during times of high waters, when the hydraulic conditions favour the exchanges between the hypolimnion (anoxic and rich in nutritive elements) and the epilimnion (oxygenated and very low in nutrients).These endogenous resources, as well as complementary contributions (vegetation, invertebrate, terrestrial, refuse), are used by the fish fauna, which adapted to the change in their environment. Predation contributes to the seasonal evolution of the zooplankton. Predation pressure was more important in the dry season than in the rainy season

Influence of Thermal Treatment on Electrical and Physical Properties of Coated Ceramics

Technical dielectric materials and ceramics are used in many different high technology industrial areas and especially for spacecraft applications. On satellites, these materials are subjected to extreme conditions due to the space plasma environment. To survive, these ceramic insulators must have exceptional electrical and thermal properties. Boron Nitride (BN) and Aluminum Oxide (Al 2O3) are used in particular because they combine good electrical insulation and high thermal conductivity. However, BN and Al2O3 used in spacecraft interiors are exposed to critical radiation demands, where these insulators are irradiated by electrons with high energies and flux. Charged particles are trapped in the ceramics, producing high electric fields. Subsequently, internal disturbances and electrical breakdowns can occur. Over time, these phenomena may cause degradation or failure of various components and embedded systems. Consequently, this study endeavors to understand the physical mechanisms which occur in these ceramics materials under electron irradiation. These dielectrics materials have been characterized at ONERA Toulouse (DESP) in the CEDRE (Chambre d’Etude De Revêtement Electrisés) irradiation chamber. A parametric study was performed to assess the influence of incident energy and flux, temperature, coating s, annealing, and ionizing dose on the charging and relaxation kinetics of BN and Al2O3. Surface and thermal treatments were found to limit BN’s charging. Dedicated treatments enhanced charge transport. To identify the effect of thermal annealing on electrical behavior in these materials, a thorough study of electron trapping processes was performed using cathodoluminescence in the Electron Emission Test facility at Utah State University. These tests explored differences in the nature and density of defect states. Together, these investigations determined correlations between chemical, structural and physical properties for each insulator’s configurations. Further, we observed degradation of coatings and an evolution of the concentration of their chemical defects. Contamination and ageing effects were identified on the rough material surfaces of ceramics exposed under a critical electron flux. Therefore, treatments applied to optimize electrical properties were found to be ineffective, especially for long-term charging mitigation. We will discuss these results and compare them for each ceramic configuration. The goal of this investigation is to understand the predominant physical mechanisms and main structural and chemical differences between these ceramic configurations in order to perform an exhaustive correlation between the properties. In future studies, we propose to define a defect-based model which can be used to optimize a material to limit both its charging and degradation over the time

Ceramic Substrates for High-temperature Electronic Integration

One of the most attractive ways to increase power handling capacity in power modules is to increase the operating temperature using wide-band-gap semiconductors. Ceramics are ideal candidates for use as substrates in high-power high-temperature electronic devices. The present article aims to determine the most suitable ceramic material for this application

Investigation of the thermal expansion and heat capacity of the CaCu3Ti4O12 ceramics

The thermal expansion of the CaCu3Ti4O12 ceramics has been measured over a wide temperature range 120–1200 K. The high quality of the samples under study has been confirmed by good agreement of the results of measurements of the heat capacity in the range 2–300 K and in the vicinity of the phase transition of magnetic nature at 25 K with the data for the single crystal. No anomalies in the thermal expansion that can be associated with the phase transition at 726–732 K assumed by other investigators have been found. The influence exerted on the thermal expansion by the heat treatment of the sample in a helium atmosphere and in air has been investigated

Colossal dielectric permittivity of BaTiO3-based nanocrystalline ceramics sintered by spark plasma sintering

In pursuit of high permittivity materials for electronic application, there has been a considerable interest recently in the dielectric properties of various perovskite oxides like calcium copper titanate or lanthanum doped barium titanate. When processed in a particular way, this later material present at ambient temperature and at f=1 kHz unusual interesting dielectric properties, a so called “colossal” permittivity value up to several 106 with relatively low dielectric losses. Moreover and contrary to what is classically expected and evidenced for this type of materials, no temperature dependence is observed. This behavior is observed in nanopowders based ceramics. An assumption to explain the observed properties is proposed. These results have important technological applications, since these nanoceramics open a new route to the fabrication of very thin dielectric films

Investigation of the reactivity of AlCl3 and CoCl2 toward molten alkali-metal nitrates in order to synthesize CoAl2O4

Cobalt aluminate CoAl2O4 powder, constituted of nano-sized crystallites, is prepared, involving the reactivity of AlCl3 and CoCl2 with molten alkali-metal nitrates. The reaction at 450 °C for 2 h leads to a mixture of spinel oxide Co3O4 and amorphous γ-Al2O3. It is transformed into the spinel oxide CoAl2O4 by heating at 1000 °C. The powders are mainly characterized by XRD, FTIR, ICP, electron microscopy and diffraction, X-EDS and diffuse reflection. Their properties are compared to those of powders obtained by solid state reactions of a mechanical mixture of chlorides or oxides submitted to the same thermal treatment

Cation distribution in manganese cobaltite spinels Co3−xMnxO4 (0 ≤ x ≤ 1) determined by thermal analysis

Thermogravimetric analysis was used in order to study the reduction in air of submicronic powders of Co3−x Mn x O4 spinels, with 0 ≤ x ≤ 1. For x = 0 (i.e. Co3O4), cation reduction occurred in a single step. It involved the CoIII ions at the octahedral sites, which were reduced to Co2+ on producing CoO. For 0 < x ≤ 1, the reduction occurred in two stages at increasing temperature with increasing amounts of manganese. The first step corresponded to the reduction of octahedral CoIII ions and the second was attributed to the reduction of octahedral Mn4+ ions to Mn3+. From the individual weight losses and the electrical neutrality of the lattice, the CoIII and Mn4+ ion concentrations were calculated. The distribution of cobalt and manganese ions present on each crystallographic site of the spinel was determined. In contrast to most previous studies that took into account either CoIII and Mn3+ or Co2+, CoIII and Mn4+ only, our thermal analysis study showed that Co2+/CoIII and Mn3+/Mn4+ pairs occupy the octahedral sites. These results were used to explain the resistivity measurements carried out on dense ceramics prepared from our powders sintered at low temperature (700–750 °C) in a Spark Plasma Sintering apparatus

Trypan Blue Dye Enters Viable Cells Incubated with the Pore-Forming Toxin HlyII of Bacillus cereus

Trypan blue is a dye that has been widely used for selective staining of dead tissues or cells. Here, we show that the pore-forming toxin HlyII of Bacillus cereus allows trypan blue staining of macrophage cells, despite the cells remaining viable and metabolically active. These findings suggest that the dye enters viable cells through the pores. To our knowledge, this is the first demonstration that trypan blue may enter viable cells. Consequently, the use of trypan blue staining as a marker of vital status should be interpreted with caution. The blue coloration does not necessarily indicate cell lysis, but may rather indicate pore formation in the cell membranes and more generally increased membrane permeability

Bacillus anthracis Protease InhA Increases Blood-Brain Barrier Permeability and Contributes to Cerebral Hemorrhages

Hemorrhagic meningitis is a fatal complication of anthrax, but its pathogenesis remains poorly understood. The present study examined the role of B. anthracis-secreted metalloprotease InhA on monolayer integrity and permeability of human brain microvasculature endothelial cells (HBMECs) which constitute the blood-brain barrier (BBB). Treatment of HBMECs with purified InhA resulted in a time-dependent decrease in trans-endothelial electrical resistance (TEER) accompanied by zonula occluden-1 (ZO-1) degradation. An InhA-expressing B. subtilis exhibited increased permeability of HBMECs, which did not occur with the isogenic inhA deletion mutant (ΔinhA) of B. anthracis, compared with the corresponding wild-type strain. Mice intravenously administered with purified InhA or nanoparticles-conjugated to InhA demonstrated a time-dependent Evans Blue dye extravasation, leptomeningeal thickening, leukocyte infiltration, and brain parenchymal distribution of InhA indicating BBB leakage and cerebral hemorrhage. Mice challenged with vegetative bacteria of the ΔinhA strain of B. anthracis exhibited a significant decrease in leptomeningeal thickening compared to the wildtype strain. Cumulatively, these findings indicate that InhA contributes to BBB disruption associated with anthrax meningitis through proteolytic attack on the endothelial tight junctional protein zonula occluden (ZO)-1