Influence possible des protozoaires sur le taux de mortalité des bactéries autotrophes nitrifiantes

Le modèle de l'IAWQ du processus de boues activées représente les mécanismes endogènes de la biomasse nitrifiante par le décès des micro-organismes (équation d'ordre 1 par rapport à la biomasse). La constante de décès, ou taux de mortalité bA, est aujourd'hui encore mal connue, et en particulier les facteurs influants sur sa valeur. De récentes études ont montré que la prédation par la microfaune pourrait être un facteur déterminant sur la valeur de bA. Cette étude se propose donc de quantifier l'effet de la prédation sur la valeur de bA. Deux réacteurs maintenus sans alimentation en substrat ont été caractérisés en parallèle: l'un a reçu une dose d'antibiotique spécifique aux eucaryotes (cycloheximide) afin de diminuer la quantité d'organismes de la microfaune, alors que l'autre n'a reçu aucun antibiotique (témoin). Les résultats obtenus montrent que le cycloheximide inhibe la plupart des organismes de la microfaune sauf les amibes; celles-ci semblent plutôt stimulées par cet antibiotique. En ce qui concerne la nitrification, un ralentissement de la production de nitrate dans le réacteur traité à l'antibiotique est observé à partir du sixième jour. Cette diminution de production de nitrate est probablement causée par une réduction de l'azote nitrifiable (qui est mobilisé par les amibes) couplée à une prédation des organismes nitrifiants par les amibes. D'ailleurs, l'augmentation de la prédation par les amibes à partir du jour 6 a diminué l'activité nitrifiante également mesurée par respirométrie (rO2 Nmax). Cette diminution du taux de respiration indique une augmentation du taux de mortalité (bA) des organismes nitrifiants. En effet, la valeur du taux de mortalité mesurée dans le réacteur témoin est de 0.08 d-1 alors que selon la microfaune présente dans le réacteur inhibé au cycloheximide, la valeur de ce taux de mortalité a varié entre 0.05 d-1 et 0.15 d-1.Designing biological wastewater treatment plants with the aid of the model developed by the IAWQ requires the knowledge of biological kinetic parameters. For nitrifying activated sludge, these parameters are related to nitrifying bacteria: maximum autotrophic growth rate µAmax, yield coefficient YA and the autotrophic decay rate bA. Although variables influencing µAmax and YA values are well known, this is not the case for bA. MARTINAGE and PAUL (2000) have recently shown that the bA value is strongly influenced by the influent quality, leading to the assumption that influent quality has a strong effect on microfauna composition, and consequently on the grazing rate of microfauna on nitrifying bacteria. In fixed-film processes, protozoan grazing reduces the bacterial population considerably (NATUSCKA and WELANDER, 1994). However, although many data are available concerning the grazing rates of different protozoa, the effect of microfaunal grazing on nitrification is still a matter of debate (RATSAK et al., 1994) and its effect on the bA value is still unknown. These two topics are investigated here.Nitrifying activated sludges were grown in two identical batch reactors, but in one, cycloheximide was added to inhibit eucaryotic growth (MAURINES CARBONEILL et al., 1998). Microfauna organism numbers were quantified in both reactors by microscopic observations of flagellated protozoa (>8 µm), amoebae, ciliates, rotifers and higher invertebrates (Fig. 3). Microbial counts were then correlated with the bA value. The latter was determined using the procedure proposed by SALZER (1992) which consists of characterising the time behaviour of the maximum nitrification rate measured by respirometry of activated sludge under substrate starvation. Under these conditions bacteria die and organic nitrogen is released into the bulk phase. This nitrogen is ammonified, and nitrifying bacteria use this substrate to produce nitrate, and then autotrophic bacterial growth occurs. This method takes this growth into account by characterising nitrate production during the experiment (Fig. 2).The effect of cycloheximide on nitrification was first determined to make sure that this compound is not inhibitory toward nitrifiers. Results obtained (Table 1) show that cycloheximide was not inhibitory toward nitrate production or the maximum nitrification oxygen uptake rate (rO2 N) after 4 hours of contact with nitrifying biomass. Cycloheximide addition in the activated sludge had an important impact on rotifers and flagellates but no effect on ciliates; it also seemed to stimulate amoebae growth. In both reactors, flagellates were mainly Peranama, attached ciliates were mainly Opercularia and Epistylis and a few Vorticella. Free ciliates like Aspidisca and Euplotes were found in both reactors.Variation with time of the abundance of microfauna organisms is shown in Figures 4 and 5 for both reactors. In the reference reactor the number of microfauna organisms decreased with time (Fig. 4) probably due to substrate starvation. Microfauna composition remained however diversified. For the inhibited reactor (Fig. 5), three periods were observed. During period I, the microfauna was mainly composed of ciliates and the number of microfaunal organisms decreased rapidly. During period II, an important growth of amoebae was observed. Cycloheximide was then added during this period to reduce their number. This growth of amoebae seems to be caused by the resistance of these micro-organisms toward inhibiting compounds (SRIKANTH et BERK, 1993). During period III, the number of microfaunal organisms was lower than during period II, and microfauna was mainly composed of ciliates.Nitrate concentration behaviour, necessary for bA calculation, is shown on Figure 6. In the reference reactor, nitrate concentrations varied linearly. For the inhibited reactor, the linear pattern was not observed during period II. This result was probably caused by an important nitrogen assimilation need of amoebae (ELDRIGE and JACKSON, 1993). Because organic nitrogen released by bacterial decay is consumed by amoebae assimilation, less nitrogen is available for the ammonification process and therefore for nitrification. Ammonia concentrations remained below 0.2 mg N·l-1 during all the experiment for both reactors. When amoebae disappeared from the inhibited reactor (period III) nitrate concentration varied linearly again.Variations of the maximum nitrification oxygen uptake rate (rO2 Nmax) with time are presented in Figure 7 (A&B) for both reactors. Two curves are plotted on each figure. Empty squares represent the measured rO2 N and black points represent the maximum nitrification rate that would have been measured if no growth on ammonification products had occurred. For the reference reactor (Fig. 7A), a value for bA of 0.08 d-1 can be calculated and can be considered constant for a constant microfauna composition.Three bA values can be estimated for the reactor inhibited with cycloheximide (Fig. 7B), corresponding to the three periods observed for microfauna composition. During period I, the bA value is 0.05 d-1 : a decrease in the microfaunal organism numbers implies a decrease of the bA value. During period II, when a development of amoebae is observed, the bA value increases and reaches 0.15 d-1. During period III with reduced grazing, the bA value is 0.13 d-1. Since during periods I and III the microfauna is mainly composed of ciliates, this difference between bA values is likely due to the observed difference in floc size between periods I and III.The results obtained during this study tend to prove (1) that the use of cycloheximide reduces microfaunal populations but can lead to a development of amoebae, and (2) that microfauna grazing seems to have an influence on the bA value, which can vary from 0.05 to 0.15 d-1 depending on microfaunal composition and abundance

Tunneling Anisotropic Magnetoresistance in Co/AlOx/Au Tunnel Junctions

We observe spin-valve-like effects in nano-scaled thermally evaporated Co/AlOx/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and depends on the relative orientation of the magnetization direction of the Co electrode with respect to the current direction. We attribute this effect to a two-step magnetization reversal and an anisotropic density of states resulting from spin-orbit interaction. The results of this study points to future applications of novel spintronics devices involving only one ferromagnetic layer.Comment: 11 pages, 5 figures. Accpted for publishing on Nano Letters, 200

Ferromagnetism and Temperature-Driven Reorientation Transition in Thin Itinerant-Electron Films

The temperature-driven reorientation transition which, up to now, has been studied by use of Heisenberg-type models only, is investigated within an itinerant-electron model. We consider the Hubbard model for a thin fcc(100) film together with the dipole interaction and a layer-dependent anisotropy field. The isotropic part of the model is treated by use of a generalization of the spectral-density approach to the film geometry. The magnetic properties of the film are investigated as a function of temperature and film thickness and are analyzed in detail with help of the spin- and layer-dependent quasiparticle density of states. By calculating the temperature dependence of the second-order anisotropy constants we find that both types of reorientation transitions, from out-of-plane to in-plane (``Fe-type'') and from in-plane to out-of-plane (``Ni-type'') magnetization are possible within our model. In the latter case the inclusion of a positive volume anisotropy is vital. The reorientation transition is mediated by a strong reduction of the surface magnetization with respect to the inner layers as a function of temperature and is found to depend significantly on the total band occupation.Comment: 10 pages, 8 figures included (eps), Phys Rev B in pres

Evolutionary Games with Affine Fitness Functions: Applications to Cancer

We analyze the dynamics of evolutionary games in which fitness is defined as an affine function of the expected payoff and a constant contribution. The resulting inhomogeneous replicator equation has an homogeneous equivalent with modified payoffs. The affine terms also influence the stochastic dynamics of a two-strategy Moran model of a finite population. We then apply the affine fitness function in a model for tumor-normal cell interactions to determine which are the most successful tumor strategies. In order to analyze the dynamics of concurrent strategies within a tumor population, we extend the model to a three-strategy game involving distinct tumor cell types as well as normal cells. In this model, interaction with normal cells, in combination with an increased constant fitness, is the most effective way of establishing a population of tumor cells in normal tissue.Comment: The final publication is available at http://www.springerlink.com, http://dx.doi.org/10.1007/s13235-011-0029-

Measurement of the top quark-pair production cross section with ATLAS in pp collisions at \sqrt{s}=7\TeV

A measurement of the production cross-section for top quark pairs(\ttbar) in $pp$ collisions at \sqrt{s}=7 \TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron $e$ or muon $\mu$) with large missing transverse energy and at least four jets, and dilepton ($ee$, $\mu\mu$ or $e\mu$) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb-1, 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-\ttbar Standard Model processes are estimated using data-driven methods and determined to be $12.2 \pm 3.9$ events and $2.5 \pm 0.6$ events, respectively. The kinematic properties of the selected events are consistent with SM \ttbar production. The inclusive top quark pair production cross-section is measured to be \sigmattbar=145 \pm 31 ^{+42}_{-27} pb where the first uncertainty is statistical and the second systematic. The measurement agrees with perturbative QCD calculations.Comment: 30 pages plus author list (50 pages total), 9 figures, 11 tables, CERN-PH number and final journal adde

Inclusive search for same-sign dilepton signatures in pp collisions at root s=7 TeV with the ATLAS detector

An inclusive search is presented for new physics in events with two isolated leptons (e or mu) having the same electric charge. The data are selected from events collected from p p collisions at root s = 7 TeV by the ATLAS detector and correspond to an integrated luminosity of 34 pb(-1). The spectra in dilepton invariant mass, missing transverse momentum and jet multiplicity are presented and compared to Standard Model predictions. In this event sample, no evidence is found for contributions beyond those of the Standard Model. Limits are set on the cross-section in a fiducial region for new sources of same-sign high-mass dilepton events in the ee, e mu and mu mu channels. Four models predicting same-sign dilepton signals are constrained: two descriptions of Majorana neutrinos, a cascade topology similar to supersymmetry or universal extra dimensions, and fourth generation d-type quarks. Assuming a new physics scale of 1 TeV, Majorana neutrinos produced by an effective operator V with masses below 460 GeV are excluded at 95% confidence level. A lower limit of 290 GeV is set at 95% confidence level on the mass of fourth generation d-type quarks

Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

The inclusive and dijet production cross-sections have been measured for jets containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass energy of sqrt(s) = 7 TeV, using the ATLAS detector at the LHC. The measurements use data corresponding to an integrated luminosity of 34 pb^-1. The b-jets are identified using either a lifetime-based method, where secondary decay vertices of b-hadrons in jets are reconstructed using information from the tracking detectors, or a muon-based method where the presence of a muon is used to identify semileptonic decays of b-hadrons inside jets. The inclusive b-jet cross-section is measured as a function of transverse momentum in the range 20 < pT < 400 GeV and rapidity in the range |y| < 2.1. The bbbar-dijet cross-section is measured as a function of the dijet invariant mass in the range 110 < m_jj < 760 GeV, the azimuthal angle difference between the two jets and the angular variable chi in two dijet mass regions. The results are compared with next-to-leading-order QCD predictions. Good agreement is observed between the measured cross-sections and the predictions obtained using POWHEG + Pythia. MC@NLO + Herwig shows good agreement with the measured bbbar-dijet cross-section. However, it does not reproduce the measured inclusive cross-section well, particularly for central b-jets with large transverse momenta.Comment: 10 pages plus author list (21 pages total), 8 figures, 1 table, final version published in European Physical Journal

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector

A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results

Single hadron response measurement and calorimeter jet energy scale uncertainty with the ATLAS detector at the LHC

The uncertainty on the calorimeter energy response to jets of particles is derived for the ATLAS experiment at the Large Hadron Collider (LHC). First, the calorimeter response to single isolated charged hadrons is measured and compared to the Monte Carlo simulation using proton-proton collisions at centre-of-mass energies of sqrt(s) = 900 GeV and 7 TeV collected during 2009 and 2010. Then, using the decay of K_s and Lambda particles, the calorimeter response to specific types of particles (positively and negatively charged pions, protons, and anti-protons) is measured and compared to the Monte Carlo predictions. Finally, the jet energy scale uncertainty is determined by propagating the response uncertainty for single charged and neutral particles to jets. The response uncertainty is 2-5% for central isolated hadrons and 1-3% for the final calorimeter jet energy scale.Comment: 24 pages plus author list (36 pages total), 23 figures, 1 table, submitted to European Physical Journal

Standalone vertex finding in the ATLAS muon spectrometer

A dedicated reconstruction algorithm to find decay vertices in the ATLAS muon spectrometer is presented. The algorithm searches the region just upstream of or inside the muon spectrometer volume for multi-particle vertices that originate from the decay of particles with long decay paths. The performance of the algorithm is evaluated using both a sample of simulated Higgs boson events, in which the Higgs boson decays to long-lived neutral particles that in turn decay to bbar b final states, and pp collision data at √s = 7 TeV collected with the ATLAS detector at the LHC during 2011