2,004 research outputs found
Effet du débit sur la dynamique temporelle des algues périphytiques dans une rivière influencée par les activités agricoles
Le périphyton de la rivière Boyer Nord, une rivière affectée par les activités agricoles dans le sud du Québec (Canada), a été échantillonné toutes les deux semaines de la mi-mai à la fin septembre 1999 afin d'évaluer son évolution temporelle et d'identifier les variables potentielles qui le contrôlent. Les résultats montrent la grande variabilité temporelle de la biomasse périphytique (poids sec organique et chlorophylle a) et de la structure de la communauté de diatomées. La communauté d'algues benthiques dans la rivière Boyer était principalement composée de diatomées (Nitzschia, Cocconeis, Cymbella, Cyclotella), d'algues vertes (Scenedesmus, Pediastrum, Cosmarium, Closterium) et de cyanobactéries (Phormidium, Oscillatoria, Merismopedia). La pointe de débit observée durant la semaine précédant l'échantillonnage était fortement corrélée à plusieurs variables physico-chimiques (N-total, NH3-N, NO3-N, P-total, turbidité) et était le plus fortement corrélée aux changements temporels de la biomasse. La biomasse (chlorophylle a et poids sec organique) était négativement corrélée au phosphore total, ce qui reflète la relation avec le débit. Les changements temporels dans la composition spécifique des diatomées étaient régis par différentes variables physico-chimiques, selon les limites de tolérances et la valence écologique des espèces. Les algues périphytiques de cette communauté ont réagi aux variations de l'environnement à l'intérieur d'une période de 2 semaines puisque des changements majeurs dans la structure de l'assemblage de diatomées ont été observés lors de chaque échantillonnage. Ces observations montrent la forte variabilité de la biomasse et de la structure de la communauté périphytique dans les rivières enrichies par les éléments nutritifs et souligne l'influence majeure du débit dans ce type d'environnement.Periphyton in an agriculturally enriched river (Boyer River, Québec, Canada) was sampled from mid-May to the end of September 1999 to evaluate the temporal succession of periphyton and to identify potential controlling variables. The river is located on the south shore of the St. Lawrence River and discharges into it approximately 30 kilometres downstream (east) of Québec City. Land-use in the Boyer River watershed is 60% farmland and 40% forests. The site was chosen for its intense farming, accessibility and proximity to an automatic sampling station for water quality operated by the Québec Ministry of the Environment that continuously recorded pH, temperature, dissolved oxygen and discharge. Event water samples were collected for nutrients, turbidity, conductivity and suspended solids according to discharge. Periphyton growth was scraped every two weeks from rocks over a 10 meter reach, between mid-May and the end of September using a template, blade and toothbrush. Samples for chlorophyll a (Chl a) and ash-free-dry-weight (AFDW) were filtered on to Whatman GF/C glass fiber filters the same day and additional samples were preserved with paraformaldehyde-glutaraldehyde for taxonomic analysis. Chl a was extracted in 95% ethanol and quantified by spectrophotometry. AFDW was determined by drying the samples for 24 hours at 80ºC followed by combustion in a muffle furnace at 500ºC for 2 hours. Samples for diatom analysis were cleaned using a mixture of 1:1 sulphuric and nitric acid at 60°C and mounted on slides with Naphrax mounting medium. Diatoms were then identified and counted with a Zeiss Axiovert 10 inverted microscope at 1000X magnification. A minimum of 400 valves were enumerated for each sample. The presence of other algal constituents was also determined. Statistical analyses included Pearson correlations, stepwise regression analysis and analysis of variance.The chemical and physical properties of the river fluctuated substantially during the sampling season. Nutrient levels were consistently high with total N in the range of 1,2-7,2 mg/l and total P in the range of 0,07-0,37 mg/l, confirming the strong agricultural enrichment of the Boyer River. All measured nitrogen components (total-N, NH3-N, NO3-N) showed a decreasing trend during the sampling season while soluble reactive phosphorus (SRP), total dissolved P, conductivity, pH and temperature showed a general increase over the season. Oxygen levels were often well above or below saturation (29-176% of air equilibrium) indicating a strong biological influence on oxygen dynamics.Periphytic biomass (Chl a and AFDW) and diatom community structure showed major fluctuations over time. The temporal changes in biomass were most strongly correlated (negatively) with peak discharge during the preceding week (AFDW : r=-0,748, p < 0,05; and Chl a : r=-0,649, p < 0,05). This discharge variable was strongly correlated (positively) with several physico-chemical variables (total-N, NH3-N, NO3-N, total-P, turbidity). The negative correlation between biomass and total phosphorus suggested that algae in the Boyer River were not nutrient-limited. However, stepwise regressions showed that variations in diatom-specific composition over time were regulated by various physico-chemical variables linked to environmental preferences and tolerance of each species. Navicula seminulum, Navicula cf. subminusculus and Navicula saprophila were strongly influenced by the peak discharge during the week preceding the sampling (R2 =0,76, F(1,8) =25,7, p 0,001; R2 =0,66, F(1,8) =15,82, p < 0,05 and R2=0,55, F(1,8) =9,85, p < 0,05, respectively). Cymbella silesiaca, Cocconeis placentula, Cyclotella meneghiniana and Navicula saprophila were strongly correlated with temperature (R2 =0,58, F(1,8) =11,08, p < 0,05; R2 =0,61, F(1,8) =28,15, p 0,001; R2 =0,41, F(1,8) =5,65, p < 0,05 and R2 =0,59, F(1,8) =11,38, p < 0,005, respectively). Diatoma vulgaris was mostly influenced by suspended solids plus discharge (R2 =0,4, F(1,8) =5,38, p < 0,05; and R2 =0,86, F(1,7) =20,72, p < 0,005, respectively). The abundance of Navicula lanceolata was strongly correlated with conductivity (R2 =45, F(1,8) =6,55, p < 0,05) while that of Nitzschia spp. correlated with total dissolved phosphorus (R2 =49, F(1,8) =7,63, p < 0,05). No significant influence of the physico-chemical environment was observed on Navicula cryptocephala or Surirella brebissonii. Benthic algae in this nutrient-rich ecosystem responded to environmental variations within 2 weeks since major changes in community composition were observed between all sampling dates. Although diatom community structure changed markedly during the sampling season, most of the observed species are indicative of nutrient-enriched rivers and streams.The results of this study show the dynamic nature of periphyton communities in nutrient-enriched rivers and streams and underscore the importance of discharge as a regulator of biomass. The rapid shifts in community structure also imply that benthic algae can be used as a sensitive measure of environmental conditions in agriculturally impacted ecosystems
Erratum to: Comparative hydrolysis of P2 receptor agonists by NTPDases 1, 2, 3 and 8. Purinergic Signalling
Comparative hydrolysis of P2 receptor agonists by NTPDases 1, 2, 3 and 8
Nucleoside triphosphate diphosphohydrolases 1, 2, 3 and 8 (NTPDases 1, 2, 3 and 8) are the dominant ectonucleotidases and thereby expected to play important roles in nucleotide signaling. Distinct biochemical characteristics of individual NTPDases should allow them to regulate P2 receptor activation differentially. Therefore, the biochemical and kinetic properties of these enzymes were compared. NTPDases 1, 2, 3 and 8 efficiently hydrolyzed ATP and UTP with Km values in the micromolar range, indicating that they should terminate the effects exerted by these nucleotide agonists at P2X1- and P2Y2,4,11 receptors. Since NTPDase1 does not allow accumulation of ADP, it should terminate the activation of P2Y1,12,13 receptors far more efficiently than the other NTPDases. In contrast, NTPDases 2, 3 and 8 are expected to promote the activation of ADP specific receptors, because in the presence of ATP they produce a sustained (NTPDase2) or transient (NTPDases 3 and 8) accumulation of ADP. Interestingly, all plasma membrane NTPDases dephosphorylate UTP with a significant accumulation of UDP, favoring P2Y6 receptor activation. NTPDases differ in divalent cation and pH dependence, although all are active in the pH range of 7.0-.5. Various NTPDases may also distinctly affect formation of extracellular adenosine and therefore adenosine receptor-mediated responses, since they generate different amounts of the substrate (AMP) and inhibitor (ADP) of ecto-5-nucleotidase, the rate limiting enzyme in the production of adenosine. Taken together, these data indicate that plasma membrane NTPDases hydrolyze nucleotides in a distinctive manner and may therefore differentially regulate P2 and adenosine receptor signaling
Optical one-way quantum computing with a simulated valence-bond solid
One-way quantum computation proceeds by sequentially measuring individual
spins (qubits) in an entangled many-spin resource state. It remains a
challenge, however, to efficiently produce such resource states. Is it possible
to reduce the task of generating these states to simply cooling a quantum
many-body system to its ground state? Cluster states, the canonical resource
for one-way quantum computing, do not naturally occur as ground states of
physical systems. This led to a significant effort to identify alternative
resource states that appear as ground states in spin lattices. An appealing
candidate is a valence-bond-solid state described by Affleck, Kennedy, Lieb,
and Tasaki (AKLT). It is the unique, gapped ground state for a two-body
Hamiltonian on a spin-1 chain, and can be used as a resource for one-way
quantum computing. Here, we experimentally generate a photonic AKLT state and
use it to implement single-qubit quantum logic gates.Comment: 11 pages, 4 figures, 8 tables - added one referenc
The XXL Survey X: K-band luminosity - weak-lensing mass relation for groups and clusters of galaxies
We present the K-band luminosity-halo mass relation, ,
for a subsample of 20 of the 100 brightest clusters in the XXL Survey observed
with WIRCam at the Canada-France-Hawaii Telescope (CFHT). For the first time,
we have measured this relation via weak-lensing analysis down to . This allows us to investigate whether the slope
of the relation is different for groups and clusters, as seen in other
works. The clusters in our sample span a wide range in mass, , at . The K-band luminosity
scales as with and an
intrinsic scatter of . Combining our
sample with some clusters in the Local Cluster Substructure Survey (LoCuSS)
present in the literature, we obtain a slope of and an
intrinsic scatter of . The flattening in the seen
in previous works is not seen here and might be a result of a bias in the mass
measurement due to assumptions on the dynamical state of the systems. We also
study the richness-mass relation and find that group-sized halos have more
galaxies per unit halo mass than massive clusters. However, the brightest
cluster galaxy (BCG) in low-mass systems contributes a greater fraction to the
total cluster light than BCGs do in massive clusters; the luminosity gap
between the two brightest galaxies is more prominent for group-sized halos.
This result is a natural outcome of the hierarchical growth of structures,
where massive galaxies form and gain mass within low-mass groups and are
ultimately accreted into more massive clusters to become either part of the BCG
or one of the brighter galaxies. [Abridged]Comment: A&A, in pres
Low-dimensional quite noisy bound entanglement with cryptographic key
We provide a class of bound entangled states that have positive distillable
secure key rate. The smallest state of this kind is 4 \bigotimes 4. Our class
is a generalization of the class presented in [1] (IEEE Trans. Inf. Theory 54,
2621 (2008); arXiv:quant-ph/0506203). It is much wider, containing, in
particular, states from the boundary of PPT entangled states (all of the states
in the class in [1] were of this kind) but also states inside the set of PPT
entangled states, even, approaching the separable states. This generalization
comes with a price: for the wider class a positive key rate requires, in
general, apart from the one-way Devetak-Winter protocol (used in [1]) also the
recurrence preprocessing and thus effectively is a two-way protocol. We also
analyze the amount of noise that can be admixtured to the states of our class
without losing key distillability property which may be crucial for
experimental realization. The wider class contains key-distillable states with
higher entropy (up to 3.524, as opposed to 2.564 for the class in [1]).Comment: 10 pages, final version for J. Phys. A: Math. Theo
Quantum-inspired interferometry with chirped laser pulses
We introduce and implement an interferometric technique based on chirped
femtosecond laser pulses and nonlinear optics. The interference manifests as a
high-visibility (> 85%) phase-insensitive dip in the intensity of an optical
beam when the two interferometer arms are equal to within the coherence length
of the light. This signature is unique in classical interferometry, but is a
direct analogue to Hong-Ou-Mandel quantum interference. Our technique exhibits
all the metrological advantages of the quantum interferometer, but with signals
at least 10^7 times greater. In particular we demonstrate enhanced resolution,
robustness against loss, and automatic dispersion cancellation. Our
interferometer offers significant advantages over previous technologies, both
quantum and classical, in precision time delay measurements and biomedical
imaging.Comment: 6 pages, 4 figure
Spectral compression of single photons
Photons are critical to quantum technologies since they can be used for
virtually all quantum information tasks: in quantum metrology, as the
information carrier in photonic quantum computation, as a mediator in hybrid
systems, and to establish long distance networks. The physical characteristics
of photons in these applications differ drastically; spectral bandwidths span
12 orders of magnitude from 50 THz for quantum-optical coherence tomography to
50 Hz for certain quantum memories. Combining these technologies requires
coherent interfaces that reversibly map centre frequencies and bandwidths of
photons to avoid excessive loss. Here we demonstrate bandwidth compression of
single photons by a factor 40 and tunability over a range 70 times that
bandwidth via sum-frequency generation with chirped laser pulses. This
constitutes a time-to-frequency interface for light capable of converting
time-bin to colour entanglement and enables ultrafast timing measurements. It
is a step toward arbitrary waveform generation for single and entangled
photons.Comment: 6 pages (4 figures) + 6 pages (3 figures
Structure modeling hints at a granular organization of the Golgi ribbon
Funding Information: This work was funded by the Medical Research Council (grants MC_UU_12018/2 and MC_UU_00012/2 to D.F.C.) and by the British Heart Foundation (grant PG/14/76/31087 to D.F.C.). Publisher Copyright: © 2022, The Author(s).Background: In vertebrate cells, the Golgi functional subunits, mini-stacks, are linked into a tri-dimensional network. How this “ribbon” architecture relates to Golgi functions remains unclear. Are all connections between mini-stacks equal? Is the local structure of the ribbon of functional importance? These are difficult questions to address, without a quantifiable readout of the output of ribbon-embedded mini-stacks. Endothelial cells produce secretory granules, the Weibel-Palade bodies (WPB), whose von Willebrand Factor (VWF) cargo is central to hemostasis. The Golgi apparatus controls WPB size at both mini-stack and ribbon levels. Mini-stack dimensions delimit the size of VWF "boluses” whilst the ribbon architecture allows their linear co-packaging, thereby generating WPBs of different lengths. This Golgi/WPB size relationship suits mathematical analysis. Results: WPB lengths were quantized as multiples of the bolus size and mathematical modeling simulated the effects of different Golgi ribbon organizations on WPB size, to be compared with the ground truth of experimental data. An initial simple model, with the Golgi as a single long ribbon composed of linearly interlinked mini-stacks, was refined to a collection of mini-ribbons and then to a mixture of mini-stack dimers plus long ribbon segments. Complementing these models with cell culture experiments led to novel findings. Firstly, one-bolus sized WPBs are secreted faster than larger secretory granules. Secondly, microtubule depolymerization unlinks the Golgi into equal proportions of mini-stack monomers and dimers. Kinetics of binding/unbinding of mini-stack monomers underpinning the presence of stable dimers was then simulated. Assuming that stable mini-stack dimers and monomers persist within the ribbon resulted in a final model that predicts a “breathing” arrangement of the Golgi, where monomer and dimer mini-stacks within longer structures undergo continuous linking/unlinking, consistent with experimentally observed WPB size distributions. Conclusions: Hypothetical Golgi organizations were validated against a quantifiable secretory output. The best-fitting Golgi model, accounting for stable mini-stack dimers, is consistent with a highly dynamic ribbon structure, capable of rapid rearrangement. Our modeling exercise therefore predicts that at the fine-grained level the Golgi ribbon is more complex than generally thought. Future experiments will confirm whether such a ribbon organization is endothelial-specific or a general feature of vertebrate cells.publishersversionpublishe
Separation between coherent and turbulent fluctuations. What can we learn from the Empirical Mode Decomposition?
The performances of a new data processing technique, namely the Empirical
Mode Decomposition, are evaluated on a fully developed turbulent velocity
signal perturbed by a numerical forcing which mimics a long-period flapping.
First, we introduce a "resemblance" criterion to discriminate between the
polluted and the unpolluted modes extracted from the perturbed velocity signal
by means of the Empirical Mode Decomposition algorithm. A rejection procedure,
playing, somehow, the role of a high-pass filter, is then designed in order to
infer the original velocity signal from the perturbed one. The quality of this
recovering procedure is extensively evaluated in the case of a "mono-component"
perturbation (sine wave) by varying both the amplitude and the frequency of the
perturbation. An excellent agreement between the recovered and the reference
velocity signals is found, even though some discrepancies are observed when the
perturbation frequency overlaps the frequency range corresponding to the
energy-containing eddies as emphasized by both the energy spectrum and the
structure functions. Finally, our recovering procedure is successfully
performed on a time-dependent perturbation (linear chirp) covering a broad
range of frequencies.Comment: 23 pages, 13 figures, submitted to Experiments in Fluid
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