Effects of stream acidification on fungal biomass in decaying beech leaves and leaf palatability

We examined the effect of surface water acidification on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and palatability to shredders of common beech leaves (Fagus sylvatica L.) in five mountain streams (pH 4.7-7.1). Leaf decomposition was significantly faster in the circumneutral streams (pH 6.4-7.1; k > or = 0.00175 d(-1)), when compared to acidic streams (pH 4.7-4.9; k < or = 0.00100 d(-1)). Fungal biomass showed no particular trend along the acidification gradient except that it peaked earlier in the stream closest to neutrality. Leaf palatability, measured as the feeding activity of the leaf-shredding amphipod Gammarus fossarum Koch, varied with the exposure time in the streams. Except for the higher palatability of leaves exposed during 6 weeks at the highest pH, patterns among streams were mostly similar. These results suggest that reduced processing rates in the most acidic streams were not related to differences in fungal biomass associated with decomposing leaves and that microbial conditioning was only slightly delayed by acidification. Possible effects of low pH and related variables (Ca, Al) on microbial decomposition and detritivorous macroinvertebrates are discussed to clarify the inhibition of beech leaf decomposition in the studied systems

Impacts of stream acidification on litter breakdown: implications for assessing ecosystem functioning

1. Scientific understanding of acidification in aquatic ecosystems relies on effective assessment, which at present is mostly limited to chemical and sometimes structural biological variables. Effects on ecosystem functioning are, in contrast, largely neglected. Litter breakdown is a potentially useful, highly integrative and crucial process that could enhance such assessment programmes. 2. Breakdown rates of beech Fagus sylvatica leaves were determined in 25 woodland headwater streams along an acidification gradient in the Vosges Mountains, France. Additional data relating to micro-organisms (microbial respiration, fungal biomass and degree of conditioning measured as leaf palatability) and macroinvertebrates (shredder diversity, abundance and biomass) associated with decomposing leaves were collected to elucidate the mechanisms underlying leaf breakdown. 3. Breakdown rates varied more than 20-fold between the most acidified and circum- neutral sites (k = 0·0002–0·0055 day−1). Stream water alkalinity and total Al concen- tration together accounted for 88% of the variation in litter breakdown rates among streams. Microbial factors associated with decaying leaves, particularly microbial respiration, declined with increasing stream acidity and were significantly related to Ca2+ and total Al concentrations. 4. Total abundance, biomass and richness of leaf-shredding invertebrates associated with decomposing leaves were not related to stream acidity. However, the abundance and biomass of the amphipod Gammarus fossarum, an acid-sensitive and particularly efficient leaf-shredder, showed a strong positive relationship with leaf breakdown rate. Gammarus abundance and microbial respiration together accounted for 85% of the variation in litter breakdown rates among streams. 5. Synthesis and applications. These results indicate that leaf-litter breakdown responds strongly to stream acidification, with both microbial decomposers and invertebrate detritivores markedly affected. Measuring leaf breakdown rate may be developed into a simple, powerful and low-cost tool for assessing a critical component of ecosystem functioning. We advocate further investigation of this approach for the routine bio- monitoring of freshwaters affected by, or recovering from, other anthropogenic stresses

Recirculation cells in a wide channel

International audienceSecondary flow cells are commonly observed in straight laboratory channels, where they are often associated with duct corners. Here, we present velocity measurements acquired with an acoustic Doppler current profiler in a straight reach of the Seine river (France). We show that a remarkably regular series of stationary flow cells spans across the entire channel. They are arranged in pairs of counter-rotating vortices aligned with the primary flow. Their existence away from the river banks contradicts the usual interpretation of these secondary flow structures, which invokes the influence of boundaries. Based on these measurements, we use a depth-averaged model to evaluate the momentum transfer by these structures, and find that it is comparable with the classical turbulent transfer

Inclination not force is sensed by plants during shoot gravitropism

International audienceGravity perception plays a key role in how plants develop and adapt to environmental changes. However, more than a century after the pioneering work of Darwin, little is known on the sensing mechanism. Using a centrifugal device combined with growth kinematics imaging, we show that shoot gravitropic responses to steady levels of gravity in four representative angiosperm species is independent of gravity intensity. All gravitropic responses tested are dependent only on the angle of inclination from the direction of gravity. We thus demonstrate that shoot gravitropism is stimulated by sensing inclination not gravitational force or acceleration as previously believed. This contrasts with the otolith system in the internal ear of vertebrates and explains the robustness of the control of growth direction by plants despite perturbations like wind shaking. Our results will help retarget the search for the molecular mechanism linking shifting statoliths to signal transduction

Rare earth elements as proxies of supergene alteration processes from the giant Imiter silver deposit (Morocco)

International audienceThe giant Imiter silver mine located at the northern edge of the West African craton in Morocco is assumed to be a late Neoproterozoic epithermal deposit mainly characterized by a hypogene paragenesis of Agrich sulphides and sulfosalts, and Ag-Hg alloys occuring preferentially in quartz-rich veins. The secondary enrichment zone at Imiter reaches a thickness of 50 to 150 m below ground surface. The upper levels, famous because of giant native silver crystals, grade up to 300 kg/t. Metallographic observations, SEM-EDS and XRD analyses reveal the presence of a quite complex secondary paragenesis made of acanthite, cinnabar, imiterite, perroudite, cerussite, mimetite, iron oxyhydroxides, synchisite and coronadite. Supergene alteration processes of the giant Imiter silver mine deposit consist of the remobilisation of the primary hypogene paragenesis by (i) deep and old basinal brines and (ii) downward infiltrations of surficial waters becoming progressively more reduced and F-enriched in response to fluid-rock interactions. Development of such a supergene mineralization strongly suggests prevalence of arid to semiarid conditions

Magnetoelastic polarons in the hole-doped quasi-one dimensional model system Y2-xCaxBaNiO5

October 4th, 2004Charge transport in the hole-doped quasi-1D model system Y$_{2-x}$Ca$_x$BaNiO$_5$ (x $\\leq$ 0.15) is investigated in the 50-300 K temperature range. The resistivity temperature dependence is characterized by a constant activation energy $E_{a}/k_{B}\\sim$ 1830 K at room temperature while $E_{a}$ decreases upon cooling. We suggest that $E_{a}$ measures the binding energy of the doped holes which form magneto-acoustic polarons when polarizing the neighboring Ni spins. A semi-classical model is proposed which allows to relate the electrical measurements and the bulk magnetic susceptibility. This model gives a picture of the spin-charge-lattice relation in this inhomogeneously doped quasi-1D system and explains its unusual one-particle charge excitation spectrum close to the Fermi level

Magnetoelastic polarons in the hole-doped quasi-one dimensional model system Y2-xCaxBaNiO5

Charge transport in the hole-doped quasi-1D model system Y$_{2-x}$Ca$_x$BaNiO$_5$ (x $leq$ 0.15) is investigated in the 50-300 K temperature range. The resistivity temperature dependence is characterized by a constant activation energy $E_{a}/k_{B} sim$ 1830 K at room temperature while $E_{a}$ decreases upon cooling. We suggest that $E_{a}$ measures the binding energy of the doped holes which form magneto-acoustic polarons when polarizing the neighboring Ni spins. A semi-classical model is proposed which allows to relate the electrical measurements and the bulk magnetic susceptibility. This model gives a picture of the spin-charge-lattice relation in this inhomogeneously doped quasi-1D system and explains its unusual one-particle charge excitation spectrum close to the Fermi level.Comment: October 4th, 200

Structure, nonlinear properties, and photosensitivity of (GeSe2)​100-​x(Sb2Se3)​x glasses

International audienceChalcogenide glasses from (GeSe2)​100-​x(Sb2Se3)​x system were synthesized, with x varying from 5 to 70, in order to evaluate the influence of antimony selenide addn. on nonlinear optical properties and photosensitivity. Nonlinear refractive index and two photon absorption coeffs. were measured both at 1064 nm in picosecond regime using the Z-​scan technique and at 1.55 μm in femtosecond regime using an original method based on direct anal. of beam profile change while propagating in the chalcogenide glasses. The study of their photosensitivity at 1.55 μm revealed highly glass compn. dependent behavior and quasi-​photostable compns. have been identified in femtosecond regime. To better understand these characteristics, the evolution of the glass transition temp., d. and structure with the chem. compn. were detd