Spherical homogeneous spaces of minimal rank

Let $G$ be a complex connected reductive algebraic group and $G/B$ denote the flag variety of $G$. A $G$-homogeneous space $G/H$ is said to be {\it spherical} if $H$ acts on $G/B$ with finitely many orbits. A class of spherical homogeneous spaces containing the tori, the complete homogeneous spaces and the group $G$ (viewed as a $G\times G$-homogeneous space) has particularly nice proterties. Namely, the pair $(G,H)$ is called a {\it spherical pair of minimal rank} if there exists $x$ in $G/B$ such that the orbit $H.x$ of $x$ by $H$ is open in $G/B$ and the stabilizer $H_x$ of $x$ in $H$ contains a maximal torus of $H$. In this article, we study and classify the spherical pairs of minimal rank.Comment: Document produced in 200

Une réévaluation de la methode d'incorporation de H14C03- pour mesurer la nitrification autotrophe et son application pour estimer des biomasses de bactéries nitrifiantes

Le processus de nitrification joue un rôle essentiel dans le cycle de l'azote dans les milieux aquatiques naturels. La mesure de l'activité nitrifiante est une étape obligée pour bien comprendre et quantifier les flux d'azote dans ces milieux. Ce travail présente une réévaluation de la méthode de mesure de l'activité nitrifiante autotrophe par la méthode d'incorporation de bicarbonate marqué au 14C et son application pour estimer des biomasses de bactéries nitrifiantes. La validité générale de la méthode a été démontrée par des tests menés sur des inhibiteurs de nitrification qui ont montré que l'utilisation combinée de N-serve (5 ppm) et de chlorate (10 mM) inhibait de manière complète et spécifique l'oxydation d'azote et l'incorporation de carbone des deux groupes de bactéries nitrifiantes. Un facteur de rendement (carbone incorporé par azote oxydé) de 0,1 mole C/mole N a également été déterminé sur des cultures pures de bactéries nitrosantes et nitratantes. Pour l'activité potentielle, en particulier, les conditions optimales pour la mesure d'activité nitrifiante ont également été établis: un pH entre 7 et 8, une température entre 20 et 30°C, une concentration en ammonium d'au moins 1 mmol/l et en oxygène d'au moins 6 mg/l. Une relation entre les mesures d'activité nitrifiante potentielle et la biomasse des bactéries nitrifiantes a été établie sur culture pure. Elle montre que dans les conditions de mesures de l'activité potentielle, 1 µg C de bactéries nitrifiantes oxyde 0,04 µmol N/hBy regenerating oxidised forms of nitrogen (nitrate), the nitrification process plays an important role in the nitrogen cycle of aquatic environments. The measurement of the activity and biomass of nitrifying bacteria is thus essential to understand and quantify the general nitrogen fluxes in those environments. Different methods of measuring the nitrifying activity exist. The first methods developed were based on the use of specific nitrification inhibitors: N-serve, allyl thio-urea, acetylene, methylfluoride and dimethyl ether, as most used. They consist in measuring differences of ammonium, nitrite and nitrate dynamics in an inhibited and control sample during time. These methods can be applied as long as the inhibitors are specific for nitrifying bacteria, and activities are high enough to allow the measurement of concentration variations during incubation times which are not too long. At the present time, the most used methods are dealing with isotopic tracers: 14C or 15N. 15N methods allow the direct measurement of the nitrifying activity, while 14C methods represent the measurement of a biomass production which can be converted into a substrate oxidation rate by the use of a yield factor. This factor is considered to be constant in the standard incubation conditions. The most frequently used enumeration methods of nitrifying bacteria are not very satisfactory. Classical culture techniques (most probable number) and immunofluorescence techniques are known to greatly underestimate the numbers of active organisms. Recently developed gene-probes techniques work well for the identification of particular strains, but are not yet useful for the numeration. A good alternative to these methods consists in the measurement of potential nitrifying activity which is correlated to the nitrifying biomass. This work presents a reassessment of the autotrophic nitrifying activity measurement by the 14C-bicarbonate incorporation method and its use to estimate the biomass of nitrifying bacteria. Several methods were used for our study: Continuous enrichment cultures of nitrifying bacteria were obtained from an inoculum coming from the Seine estuary (freshwater section). Pure cultures of Nitrosomonas europaea and Nitrobacter winogradskyi were obtained from the National Collection of Industrial and Marine Bacteria (Aberdeen, Scotland) and a continuous enrichment culture of mixed heterotrophic bacteria, without nitrifying organisms, was obtained with a freshwater inoculum by imposing a residence time of 2 hours (less than the generation time of nitrifying bacteria). Nitrifying cell numbers and size in the pure cultures were determined by epifluorescence with a microscope, after DAPI staining. Biovolumes were estimated according to cell size and converted in biomasses according to a conversion factor determined experimentally with a carbon analyser. Ammonium was measured with the indophenol blue method, nitrate was reduced in nitrite on a cadmium bed and nitrite was measured with the sulfanilamide method. Bicarbonate was measured by acid titration in natural water samples, and with the evolution method for culture samples. C incorporation rates are measured by the incubation of samples with 14C-bicarbonate, the samples being filtered on 0.2 µm membranes, acidified and counted for radioactivity by liquid scintillation. The general validity of the method was demonstrated by experiments on nitrification inhibitors in enrichment cultures. These experiments consisted in measuring the effect of different combinations of N-serve, ethanol (the organic solvent of N-serve) and chlorate, on N-oxidation rates and C incorporation rates on samples of the two nitrifying enrichment cultures (ammonium- and nitrite-oxidising bacteria). The inhibitors effects were also determined on the C incorporation rates of heterotrophic bacteria. The results showed that the use of a combination of N-serve (5 mg/l, final concentration) and chlorate (10 mmol/l, final concentration) gave the best inhibition of ammonium- and nitrite-oxidation. However, the ethanolic solution of N-serve had an unwanted result on C incorporation. The organic solvent enhanced heterotrophic incorporation of C which totally masked out the autotrophic contribution of nitrifying bacteria. For this reason N-serve was added in the empty flask before the sample to allow the evaporation of the solvent. By acting this way, inhibition of autotrophic C incorporation by nitrifying bacteria was also complete, while heterotrophic incorporation was unaffected.To measure potential nitrifying activities, the optimal growth conditions of nitrifying bacteria were determined on enrichment cultures: a pH between 7 and 8, a temperature between 20 and 30 °C, an ammonium concentration over 1 mmol/l, and an oxygen concentration over 6 mg/l. An experience consisting in following N oxidation, C incorporation and cell growth in a pure culture of Nitrosomonas europaea and Nitrobacter winogradskyi in optimal conditions allowed us to determine a yield factor (incorporated C/oxidised N) of 0.09 and 0.02 molC/molN for the ammonia- and nitrite-oxidising bacteria respectively. The determined optimal growth rate was 0.05 h-1 for the two nitrifying species. The specific activity of nitrifying bacteria, which correspond to the maximum N-oxidation rate of 1 µg C of nitrifying bacteria, is given by the ratio between the growth yield and the growth rate of those organisms. This factors allowed us to establish a relationship between potential nitrifying activity measurements and nitrifying biomass: in optimal growth conditions, 1 µgC of ammonium-oxidising bacteria oxidised 0.05 µmolN/h and 1 µgC of nitrite-oxidising bacteria oxidised 0.21 µmolN/h.Our conclusion is that the results presented in this paper allow the validation of the 14C-bicarbonate incorporation method with and without inhibitors to measure the nitrifying activity. The main differences of our protocol to the original ones is that we propose the use of a combination of 2 inhibitors, N-serve and chlorate, and the elimination by evaporation of the organic solvent of N-serve (ethanol) to avoid any interference with the heterotrophic populations. The method can be used in in situ conditions, to allow real nitrifying activities measurements in samples. In this case, carbon incorporation rates can be converted in ammonium oxidation rates with the use of the conversion factor 0.11 µmoles incorporated C by µmoles oxidised N (0.09 molC/molN for ammonium oxidation and 0.02 for nitrite-oxidation). The method can also be used by placing the sample in optimal temperature, pH, oxygen and ammonium conditions for nitrifying bacteria, to allow potential nitrifying activity measurements. This potential activity can be used to estimate the nitrifying biomass by considering a conversion factor of 0.04 µmolN/h/µgC (0.05 µmolN/h/µgC for ammonium-oxidation and 0.21 µmolN/h/µgC for nitrite-oxidation). The rapidity of the method, itís sensitivity and the fact that no special equipment is needed, except the one for 14C detection, makes it a very useful method in aquatic ecology

Effective constructions in plethysms and Weintraub's conjecture

We give a short proof of Weintraub's conjecture by constructing explicit highest weight vectors in the symmetric power of an even exterior power

Comparison of two models in the estimation of nitrogen uptake rates using data from 15-N incubation experiments

This paper compares two uptake rate models, Dugdale & Goering's (D&G) model and Elskens' model. The aim is to provide an insight into how estimates of uptake processes, i.e. regeneration and loss rates from both dissolved and particulate nitrogen pools, influence the total uptake rates when the two models are compared. The uptake rates of three nitrogenous nutrients (nitrate, ammonium and urea) from 15-N incubation experimental data were compared. The comparison indicated that the D&G model underestimated nitrate uptake rates by about 34%, implying a significant regeneration and loss rates of the nutrient. Elskens' model further showed that the loss rates from the dissolved phase were about 40% and 25% for the ammonium and urea pools, respectively, indicating that the D&G model underestimated the experimental uptake rates of the nutrients. On average, nitrification made up about 30% of the total ammonium uptake flux, whereas the sinks from particulate nitrogen and dissolved nitrogen were estimated at 36% and 56%, respectively. The D&G model sometimes overestimated the f-ratio values to about 60% and higher as a result of ammonium and urea uptake rates underestimation. This paper also shows that detritus adsorption, bacterial uptake and cell lysis are equally important processes

Distribution of nitrifying activity in the Seine River (France) from Paris to the estuary

The distribution of nitrification has been measured with the H14CO3- incorporation method in the Seine River and its estuary during summer conditions. The Seine River below Paris receives large amounts of ammonium through wastewater discharge. In the river itself, this ammonium is only slowly nitrified, while in the estuary nitrification is rapid and complete. We show that this contrasting behavior is related to the different hydrosedimentary conditions of the two systems, as nitrifying bacteria are associated with suspended particles. In the river, particles and their attached bacteria either rapidly settle or have a sestonic behavior. Because of the short residence times of the water masses, the dow growing nitrifying population has no time to develop sufficiently to nitrify the available ammonium. The estuary is characterized by strong tidal dynamics. Particles settle and are resuspended continuously with the strong current inversions of ebb and hood. As a result of these dynamics, particles and their attached nitrifying bacteria experience longer residence times in a temporary suspended state than the water masses themselves, providing to slow growing nitrifying bacteria the opportunity to develop a large population capable of nitrifying all the available ammonium

Zero kinetic energy-pulsed field ionization and resonance enhanced multiphoton ionization photoelectron spectroscopy: Ionization dynamics of Rydberg states in HBr

The results of rotationally resolved resonance enhanced multiphoton ionization photoelectron spectroscopy and zero kinetic energy‐pulsed field ionization studies on HBr via various rotational levels of the F^ 1Δ_2 and f^ 3Δ_2 Rydberg states are reported. These studies lead to an accurate determination of the lowest ionization threshold as 94 098.9±1 cm^(−1). Observed rotational and spin–orbit branching ratios are compared to the results of ab initio calculations. The differences between theory and experiment highlight the dominant role of rotational and spin–orbit interactions for the dynamic properties of the high‐n Rydberg states involved in the pulsed field ionization process

On spherical twisted conjugacy classes

Let G be a simple algebraic group over an algebraically closed field of good odd characteristic, and let theta be an automorphism of G arising from an involution of its Dynkin diagram. We show that the spherical theta-twisted conjugacy classes are precisely those intersecting only Bruhat cells corresponding to twisted involutions in the Weyl group. We show how the analogue of this statement fails in the triality case. We generalize to good odd characteristic J-H. Lu's dimension formula for spherical twisted conjugacy classes.Comment: proof of Lemma 6.4 polished. The journal version is available at http://www.springerlink.com/content/k573l88256753640

High-resolution Fourier-transform XUV photoabsorption spectroscopy of 14N15N

The first comprehensive high-resolution photoabsorption spectrum of 14N15N has been recorded using the Fourier-transform spectrometer attached to the Desirs beamline at the Soleil synchrotron. Observations are made in the extreme ultraviolet (XUV) and span 100,000-109,000 cm-1 (100-91.7 nm). The observed absorption lines have been assigned to 25 bands and reduced to a set of transition energies, f values, and linewidths. This analysis has verified the predictions of a theoretical model of N2 that simulates its photoabsorption and photodissociation cross section by solution of an isotopomer independent formulation of the coupled-channel Schroedinger equation. The mass dependence of predissociation linewidths and oscillator strengths is clearly evident and many local perturbations of transition energies, strengths, and widths within individual rotational series have been observed.Comment: 14 pages, 8 figures, one data archiv