1,052 research outputs found

    Grands chantiers et matériaux à l’ère du béton

    Get PDF
    Les problèmes liés au choix des matériaux et à l’approvisionnement des chantiers continuent de se poser presque dans les mêmes termes après la fin du XIXe siècle. Certes, le développement de la construction métallique et l’apparition du béton armé ont quelque peu modifié la donne. En effet, les poutrelles métalliques, les armatures, les ciments sont des produits industriels normalisés pour lesquels il n’y a pas d’autres choix possibles que des choix techniques. Néanmoins, tous les constituants du béton (granulats : sable, gravillons, et cailloux) ne sont pas des produits standard. Les parements ne le sont pas non plus. Au début du XXe siècle, le métal et le béton armé ne sont utilisés que comme des éléments structurels dépourvus d’esthétique, qu’il convient donc de masquer par des parements. Au premier tiers du XXe siècle, de nouvelles technologies apparaissent qui vont permettre au béton de devenir un matériau à part entière. On fait alors appel à des ciments spéciaux et à des adjuvants, mais surtout à des granulats sophistiqués qui ne sont pas nécessairement d’extraction locale. Qu’ils soient d’extraction, de fabrication de proximité ou qu’ils viennent de carrières, d’usines plus lointaines, les matériaux de construction sont des produits pondéreux et leur transport pose, pour l’organisation des chantiers, des problèmes de même nature qu’aux siècles précédents. Le chantier doit être régulièrement approvisionné pour éviter d’avoir à constituer sur place des stocks trop importants. Si les chantiers plus modestes sont approvisionnés en béton prêt à l’emploi, les grands chantiers ont leur propre centrale à béton. Les constituants arrivent alors par camions bennes et sont stockés sur place dans des silos avant malaxage. Le chantier de l’hôpital Beaujon (1932-1935) et celui de la reconstruction du Havre (1945-1964) ont été choisis pour illustrer ces propos.After the end of the XIXth century, the choice and supply of materials still raise the same problems as in previous centuries. Undoubtedly, the development of metal and concrete building somehow changed the deal, as metal girders, reinforcement devices and cements are normalised industrial products for which there is no other possible choice than technical choices. However, not all the constituents of concrete are standardized products: this is the case of granulates or sands and even more so of the fine gravel and the pebbles/stones. This is also the case of the facing. At the beginning of the XXth century, metal and concrete are only used as scaffolding elements devoid of aesthetics and which must be hidden: hence, the outer walls in freestone, bricks and precious materials like marble or porphyry, for which the aesthetic choices, the sources, the transportation and the cost raise the same problems as ever. However, in so far as resistance is no longer the major criterion, new materials appear for the exclusive use of facing: sandstones, ceramics, thin facings of stone or marble. Conversely, within the buildings, special attention is paid to decoration, with the most precious materials: precious woods, glass, metals fit well the concrete. By the thirties, new technologies develop that will allow concrete to become a materiel in itself which no longer needs to be hidden. One then turns to special cements and additives but more importantly to elaborate granulates which are not always of local origin. Whether extracted or prepared locally or transported from distant quarries or factories, building materials are heavy and, in the organization of the building sites, their transportation raises the same problems as in previous centuries. The contracting authorities are no longer princes, but large building sites still require much money and besides the big firms, it is most often the public orders which play the driving role. The building site needs continuous supply in order to avoid important storage. When present in the vicinity, waterways provide the best means of transportation, but river traffic remains weak in France and most materials are delivered by trucks. While smaller building sites are supplied in ready-to-use concrete, large ones have their own concrete station: the constituents are delivered by dumpers and stored in silos before mixing. Two examples will illustrate this XXth century technology: the building of the Hôpital Beaujon (1932-1935) and the reconstruction of Le Havre (1945-1964).Fragen nach der Auswahl der Baumaterialien und deren Lieferung auf Baustellen blieben nach der Jahrhundertwende fast unverändert, sie behandelten kaum die Entwicklung des Metallbaus und die Erscheinung des Stahlbetons. Damals waren Metallbalken, Betonausrüstungen, Zemente industrielle Produkte, die nur für ihre technischen Eigenschaften ausgewählt wurden. Im Gegenteil waren die Bestandteile des Betons – Sand, Splitt und Kiesel – keineswegs normalisierte Produkte. Zu Beginn des 20.Jahrhunderts wurden also Metall und Stahlbeton nur als strukturelle Bauelemente ohne ästhetische Eigenschaft angewandt, die einfach zu verkleiden waren. Durch die in dem ersten Drittel des Jahrhunderts entwickelten neuen Technologien wurde Beton als Baustoff für sich verwendet. Spezielle Zemente wurden also hergestellt, mit besonderen Zusätzen und auserwählten Granulaten, die nicht unbedingt aus lokaler Herkunft stammten.. Egal ob sie aus näheren oder weiteren Steinbrüchen und Werken herkommen, sind die Baumaterialien ganz schwergewichtige Produkte, die die gleichen Probleme wie in den vorigen Jahrhunderten an die Beförderung und an die Organisation der Baustellen stellen. Da keine bedeutenden Vorräte auf dem Bauplatz gehalten werden können, müssen Materialien regelmäßig geliefert werden : auf kleineren Baustellen wird Fertigbeton benutzt, große Baustellen werden mit eigenem Betonmischer ausgestattet. Die Betonbestandteile werden auf Kippladern befördert und auf der Baustelle in Silos gelagert, bevor sie gemischt werden. Zwei große Bauwerke stehen als Beispiel : das Krankenhaus Beaujon (1932-1935) und der Wiederaufbau der Stadt Le Havre (1945-1964

    Role of delta-tubulin and the C-tubule in assembly of Paramecium basal bodies

    Get PDF
    BACKGROUND: A breakthrough in the understanding of centriole assembly was provided by the characterization of the UNI3 gene in Chlamydomonas. Deletion of this gene, found to encode a novel member of the tubulin superfamily, delta-tubulin, results in the loss of the C-tubule, in the nine microtubule triplets which are the hallmark of centrioles and basal bodies. Delta-tubulin homologs have been identified in the genomes of mammals and protozoa, but their phylogenetic relationships are unclear and their function is not yet known. RESULTS: Using the method of gene-specific silencing, we have inactivated the Paramecium delta-tubulin gene, which was recently identified. This inactivation leads to loss of the C-tubule in all basal bodies, without any effect on ciliogenesis. This deficiency does not directly affect basal body duplication, but perturbs the cortical cytoskeleton, progressively leading to mislocalization and loss of basal bodies and to altered cell size and shape. Furthermore, additional loss of B- and even A-tubules at one or more triplet sites are observed: around these incomplete cylinders, the remaining doublets are nevertheless positioned according to the native ninefold symmetry. CONCLUSIONS: The fact that in two distinct phyla, delta-tubulin plays a similar role provides a new basis for interpreting phylogenetic relationships among delta-tubulins. The role of delta-tubulin in C-tubule assembly reveals that tubulins contribute subtle specificities at microtubule nucleation sites. Our observations also demonstrate the existence of a prepattern for the ninefold symmetry of the organelle which is maintained even if less than 9 triplets develop

    Oil accumulation in the model green alga Chlamydomonas reinhardtii: characterization, variability between common laboratory strains and relationship with starch reserves

    Get PDF
    International audienceBackground: When cultivated under stress conditions, many microalgae species accumulate both starch and oil (triacylglycerols). The model green microalga Chlamydomonas reinhardtii has recently emerged as a model to test genetic engineering or cultivation strategies aiming at increasing lipid yields for biodiesel production. Blocking starch synthesis has been suggested as a way to boost oil accumulation. Here, we characterize the triacylglycerol (TAG) accumulation process in Chlamydomonas and quantify TAGs in various wild-type and starchless strains. Results: In response to nitrogen deficiency, Chlamydomonas reinhardtii produced TAGs enriched in palmitic, oleic and linoleic acids that accumulated in oil-bodies. Oil synthesis was maximal between 2 and 3 days following nitrogen depletion and reached a plateau around day 5. In the first 48 hours of oil deposition, a~80% reduction in the major plastidial membrane lipids occurred. Upon nitrogen re-supply, mobilization of TAGs started after starch degradation but was completed within 24 hours. Comparison of oil content in five common laboratory strains (CC124, CC125, cw15, CC1690 and 11-32A) revealed a high variability, from 2 μg TAG per million cell in CC124 to 11 μg in 11-32A. Quantification of TAGs on a cell basis in three mutants affected in starch synthesis (cw15sta1-2, cw15sta6 and cw15sta7-1) showed that blocking starch synthesis did not result in TAG over-accumulation compared to their direct progenitor, the arginine auxotroph strain 330. Moreover, no significant correlation was found between cellular oil and starch levels among the twenty wild-type, mutants and complemented strains tested. By contrast, cellular oil content was found to increase steeply with salt concentration in the growth medium. At 100 mM NaCl, oil level similar to nitrogen depletion conditions could be reached in CC124 strain. Conclusion: A reference basis for future genetic studies of oil metabolism in Chlamydomonas is provided. Results highlight the importance of using direct progenitors as control strains when assessing the effect of mutations on oil content. They also suggest the existence in Chlamydomonas of complex interplays between oil synthesis, genetic background and stress conditions. Optimization of such interactions is an alternative to targeted metabolic engineering strategies in the search for high oil yields

    Mutations in UDP-Glucose:Sterol Glucosyltransferase in Arabidopsis Cause Transparent Testa Phenotype and Suberization Defect in Seeds

    Get PDF
    In higher plants, the most abundant sterol derivatives are steryl glycosides (SGs) and acyl SGs. Arabidopsis (Arabidopsis thaliana) contains two genes, UGT80A2 and UGT80B1, that encode UDP-Glc:sterol glycosyltransferases, enzymes that catalyze the synthesis of SGs. Lines having mutations in UGT80A2, UGT80B1, or both UGT80A2 and UGT8B1 were identified and characterized. The ugt80A2 lines were viable and exhibited relatively minor effects on plant growth. Conversely, ugt80B1 mutants displayed an array of phenotypes that were pronounced in the embryo and seed. Most notable was the finding that ugt80B1 was allelic to transparent testa15 and displayed a transparent testa phenotype and a reduction in seed size. In addition to the role of UGT80B1 in the deposition of flavanoids, a loss of suberization of the seed was apparent in ugt80B1 by the lack of autofluorescence at the hilum region. Moreover, in ugt80B1, scanning and transmission electron microscopy reveals that the outer integument of the seed coat lost the electron-dense cuticle layer at its surface and displayed altered cell morphology. Gas chromatography coupled with mass spectrometry of lipid polyester monomers confirmed a drastic decrease in aliphatic suberin and cutin-like polymers that was associated with an inability to limit tetrazolium salt uptake. The findings suggest a membrane function for SGs and acyl SGs in trafficking of lipid polyester precursors. An ancillary observation was that cellulose biosynthesis was unaffected in the double mutant, inconsistent with a predicted role for SGs in priming cellulose synthesis

    Fatty acid export (FAX) proteins contribute to oil production in the green microalga Chlamydomonas reinhardtii

    Get PDF
    In algae and land plants, transport of fatty acids (FAs) from their site of synthesis in the plastid stroma to the endoplasmic reticulum (ER) for assembly into acyl lipids is crucial for cellular lipid homeostasis, including the biosynthesis of triacylglycerol (TAG) for energy storage. In the unicellular green alga Chlamydomonas reinhardtii, understanding and engineering of these processes is of particular interest for microalga-based biofuel and biomaterial production. Whereas in the model plant Arabidopsis thaliana, FAX (fatty acid export) proteins have been associated with a function in plastid FA-export and hence TAG synthesis in the ER, the knowledge on the function and subcellular localization of this protein family in Chlamydomonas is still scarce. Among the four FAX proteins encoded in the Chlamydomonas genome, we found Cr-FAX1 and Cr-FAX5 to be involved in TAG production by functioning in chloroplast and ER membranes, respectively. By in situ immunolocalization, we show that Cr-FAX1 inserts into the chloroplast envelope, while Cr-FAX5 is located in ER membranes. Severe reduction of Cr-FAX1 or Cr-FAX5 proteins by an artificial microRNA approach results in a strong decrease of the TAG content in the mutant strains. Further, overexpression of chloroplast Cr-FAX1, but not of ER-intrinsic Cr-FAX5, doubled the content of TAG in Chlamydomonas cells. We therefore propose that Cr-FAX1 in chloroplast envelopes and Cr-FAX5 in ER membranes represent a basic set of FAX proteins to ensure shuttling of FAs from chloroplasts to the ER and are crucial for oil production in Chlamydomonas

    The small molecule fenpropimorph rapidly converts chloroplast membrane lipids to triacylglycerols in Chlamydomonas reinhardtii

    Get PDF
    Concern about global warming has prompted an intense interest in developing economical methods of producing biofuels. Microalgae provide a promising platform for biofuel production, because they accumulate high levels of lipids, and do not compete with food or feed sources. However, current methods of producing algal oil involve subjecting the microalgae to stress conditions, such as nitrogen deprivation, and are prohibitively expensive. Here, we report that the fungicide fenpropimorph rapidly causes high levels of neutral lipids to accumulate in Chlamydomonas reinhardtii cells. When treated with fenpropimorph (10 mu g mL(-1)) for 1 h, Chlamydomonas cells accumulated at least fourfold the amount of triacylglycerols (TAGs) present in the untreated control cells. Furthermore, the quantity of TAGs present after 1 h of fenpropimorph treatment was over twofold higher than that formed after 9 days of nitrogen starvation in medium with no acetate supplement. Biochemical analysis of lipids revealed that the accumulated TAGs were derived mainly from chloroplast polar membrane lipids. Such a conversion of chloroplast polar lipids to TAGs is desirable for biodiesel production, because polar lipids are usually removed during the biodiesel production process. Thus, our data exemplified that a cost and time effective method of producing TAGs is possible using fenpropimorph or similar drugs.open1110sciescopu

    The Phosphate Fast-Responsive Genes <i>PECP1</i> and <i>PPsPase1</i> Affect Phosphocholine and Phosphoethanolamine Content

    Get PDF
    International audiencePhosphate starvation-mediated induction of the HAD-type phosphatases PPsPase1 (AT1G73010) and PECP1 (AT1G17710) has been reported in Arabidopsis (Arabidopsis thaliana). However, little is known about their in vivo function or impact on plant responses to nutrient deficiency. The preferences of PPsPase1 and PECP1 for different substrates have been studied in vitro but require confirmation in planta. Here, we examined the in vivo function of both enzymes using a reverse genetics approach. We demonstrated that PPsPase1 and PECP1 affect plant phosphocholine and phosphoethanolamine content, but not the pyrophosphate-related phenotypes. These observations suggest that the enzymes play a similar role in planta related to the recycling of polar heads from membrane lipids that is triggered during phosphate starvation. Altering the expression of the genes encoding these enzymes had no effect on lipid composition, possibly due to compensation by other lipid recycling pathways triggered during phosphate starvation. Furthermore, our results indicated that PPsPase1 and PECP1 do not influence phosphate homeostasis, since the inactivation of these genes had no effect on phosphate content or on the induction of molecular markers related to phosphate starvation. A combination of transcriptomics and imaging analyses revealed that PPsPase1 and PECP1 display a highly dynamic expression pattern that closely mirrors the phosphate status. This temporal dynamism, combined with the wide range of induction levels, broad expression, and lack of a direct effect on Pi content and regulation, makes PPsPase1 and PECP1 useful molecular markers of the phosphate starvation response
    corecore