University of Rennes

About: University of Rennes is a education organization based out in Rennes, France. It is known for research contribution in the topics: Population & Catalysis. The organization has 18404 authors who have published 40374 publications receiving 995327 citations.

Saving freshwater from salts

Abstract: Many human activities—like agriculture and resource extraction—are increasing the total concentration of dissolved inorganic salts (i.e., salinity) in freshwaters. Increasing salinity can have adverse effects on human health ( 1 ); increase the costs of water treatment for human consumption; and damage infrastructure [e.g., amounting to $700 million per year in the Border Rivers catchment, Australia ( 2 )]. It can also reduce freshwater biodiversity ( 3 ); alter ecosystem functions ( 4 ); and affect economic well-being by altering ecosystem goods and services (e.g., fisheries collapse). Yet water-quality legislation and regulations that target salinity typically focus on drinking water and irrigation water, which does not automatically protect biodiversity. For example, specific electrical conductivities (a proxy for salinity) of 2 mS/cm can be acceptable for drinking and irrigation but could extirpate many freshwater insect species ( 3 ). We argue that salinity standards for specific ions and ion mixtures, not just for total salinity, should be developed and legally enforced to protect freshwater life and ecosystem services. We identify barriers to setting such standards and recommend management guidelines.

The Molting Gland of Crustaceans: Localization, Activity, and Endocrine Control (a Review)

Abstract: Y-organs are paired cephalic endocrine organs of higher Crustacea (Malacostraca). In lower groups (e.g., Entomostraca), they are absent. They were demonstrated as molting glands by Echalier in 1954. They originate from epidermis, and they may either remain attached to epidermis (e.g., in crayfishes) or become fully independent organs (e.g., in crabs); this is taken as an example of the formation of an endocrine gland from part of the target tissue of neurohormones (Buckmann, 1984). Their anatomical features show very large variations among species; these are extensively discussed here. Their ultrastructural characteristics were recently the subject of two excellent reviews (Birkenbeil, 1990; Spaziani, 1990) and are not fully described here. Y-organs secrete three different ecdysteroids, identified as ecdysone (E), 25-deoxyecdysone (25dE), and 3-dehydroecdysone (3DE). Usually these organs produce either E + 25dE or E + 3DE. The significance of these variations is unclear at the moment. These ecdysteroids are derived from dietary cholesterol, and our knowledge of the biosynthetic pathway is far from complete. Apart from the first step (conversion of cholesterol to 7-dehydrocholesterol) and the three last steps (hydroxylations at positions 25, 22, and 2), the reactions are still unknown, as is also the case in insects. The regulation of Y-organ activity is rather unusual, as it seems to be mainly exerted by an inhibitory neuropeptide secreted from the X-organ-sinus gland complex, the so-called molt-inhibiting hormone or MIH. MIH, recently isolated and sequenced in two species, is a member of a new 7-9 kDa peptide family that appears unique to crustaceans. The inhibitory mechanism of action of MIH on Y-organs has been extensively investigated, but it still remains controversial concerning the transduction mechanism involved, and the regulated steps are unknown. More recently, it has been shown that xanthurenic acid (XA), a derivative of tryptophan isolated from eyestalks, displays also an inhibitory effect on ecdysteroid biosynthesis that would be caused by a direct inhibition of cytochrome P-450 monooxygenase(s). The elucidation of the regulatory mechanisms of Y-organ activity by MIH (and other factors) should contribute to a better understanding of fundamental aspects of the regulation of steroidogenesis in general.

Effects of species diversity on the primary productivity of ecosystems: extending our spatial and temporal scales of inference

Abstract: The number of studies examining how species diversity influences the productivity of ecosystems has increased dramatically in the past decade as concern about global loss of biodiversity has intensified. Research to date has greatly improved our understanding of how, when, and why species loss alters primary production in ecosystems. However, because experiments have been performed at rather small spatial and short temporal scales, it is unclear whether conclusions can be readily extrapolated to the broader scales at which natural communities are most likely to influence ecosystem functioning. Here we develop a simple patch-dynamics model to examine some of the scale-dependent and independent qualities of the diversity-productivity relationship. We first simulate a typical diversity-productivity experiment and show that the influence of species richness on productivity is temporally dynamic, growing stronger through successional time. This holds true irrespective of whether resource partitioning or a sampling effect is the underlying mechanism. We then increase the spatial scale of the simulation from individual patches to a region consisting of many patch types. Results suggest that the diversity-productivity relationship is not influenced by spatial scale per se, but that the mechanism producing the relationship can change from sampling effects within individual patches to resource partitioning across patch types composing the region. This change occurs even though model dynamics are the same at both scales, suggesting that sampling effects and resource partitioning can represent different descriptions of the same biological processes operating concurrently at differing scales of observation. Lastly, we incorporate regional processes of dispersal and disturbance into the model and show that these processes can amplify the effect of species richness on productivity, resulting in patterns not easily anticipated from experiments. We conclude that the relative control of community structure by local versus regional processes may be a primary determinant of the diversity-productivity relationship in natural ecosystems. Therefore, past experiments having focused only on local processes might not reflect patterns and processes underlying diversity-productivity relationships in communities where disturbance and dispersal regulate species biomasses.

Poisson's ratio and the densification of glass under high pressure.

Abstract: Because of a relatively low atomic packing density, (${C}_{g}$) glasses experience significant densification under high hydrostatic pressure. Poisson's ratio ($\ensuremath{ u}$) is correlated to ${C}_{g}$ and typically varies from 0.15 for glasses with low ${C}_{g}$ such as amorphous silica to 0.38 for close-packed atomic networks such as in bulk metallic glasses. Pressure experiments were conducted up to 25 GPa at 293 K on silica, soda-lime-silica, chalcogenide, and bulk metallic glasses. We show from these high-pressure data that there is a direct correlation between $\ensuremath{ u}$ and the maximum post-decompression density change.