DYNAMTQUE HAUTE FRkQUENCE DES POPULATIONS PICOPLANCTONIQUES

International audienceMot clefs : Plchantillonnage, haute frkquence, cytomktrie en flux, picoplancton RCsumC : En raison de l'importante variabilitk enregistrke par les paramktres cellulaires et physiologiques (abondance, taille, fluorescence pigmentaire, cycle cellulaire,. . .) du picoplancton sur des khelles de temps trks courtes, l'ktude des populations en culture comme dans le milieu nature1 nkessite des moyens de pr&vement et d'analyse i haute frbquence (de l'ordre de l'heure). La cytomktrie en flux est une technique d'analyse et d'acquisition de don&es qui rkpond parfaitement B cette problkmatique. L'kchantillonnage haute frbquence des populations reste un probkme souvent dif'ficile B rksoudre en raison de nombreuses contraintes logistiques ou humaines. Face & ce besoin, nous avons mis au point des appareils automatiques de prklkvement, capables d'kchantillonner de faibles volumes de culture ou d'eau de mer et de les prkserver avant leur analyse ou fixation. La description succincte d'un prototype et l'utilisation de la cytomktrie en flux appliquke h l'ktude de la dynamique des populations picoplanctoniques de la mer Mkditerranke (rade de Villefranche-surmer, mer d'Alboran et le bassin tout entier) sont pr&ent&

DYNAMTQUE HAUTE FRkQUENCE DES POPULATIONS PICOPLANCTONIQUES

International audienceMot clefs : Plchantillonnage, haute frkquence, cytomktrie en flux, picoplancton RCsumC : En raison de l'importante variabilitk enregistrke par les paramktres cellulaires et physiologiques (abondance, taille, fluorescence pigmentaire, cycle cellulaire,. . .) du picoplancton sur des khelles de temps trks courtes, l'ktude des populations en culture comme dans le milieu nature1 nkessite des moyens de pr&vement et d'analyse i haute frbquence (de l'ordre de l'heure). La cytomktrie en flux est une technique d'analyse et d'acquisition de don&es qui rkpond parfaitement B cette problkmatique. L'kchantillonnage haute frbquence des populations reste un probkme souvent dif'ficile B rksoudre en raison de nombreuses contraintes logistiques ou humaines. Face & ce besoin, nous avons mis au point des appareils automatiques de prklkvement, capables d'kchantillonner de faibles volumes de culture ou d'eau de mer et de les prkserver avant leur analyse ou fixation. La description succincte d'un prototype et l'utilisation de la cytomktrie en flux appliquke h l'ktude de la dynamique des populations picoplanctoniques de la mer Mkditerranke (rade de Villefranche-surmer, mer d'Alboran et le bassin tout entier) sont pr&ent&

SPB Academic Publishing bv, Amsterdam Effects of fire frequency on plant communities and landscape pattern in the Massif des Aspres (southern France)

Fire frequency can affect pattern and diversity in plant communities and landscapes. We had the opportunity to study changes due to recurring wildfires on the same sites over a period of 50 years in the “Massif des Aspres ” (southern France). The study was carried out in areas occupied by Quercus suber and Q. ilex series. A comparison of historical and cartographical documents (vegetation maps covering a 50 year interval and an accurate map of major wildfires during this period) allowed us to determine the changes occurring over time with or without fire action. Plant communities were grouped into three main vegetation types: forests, treed shrublands and shrublands. The passage of three successive wildfires on the same site led to a decrease in for-est areas and an increase in shrublands; however, shrublands were already present before the first fire of the period under consideration. Less frequent fire occurrence induced more complex heterogeneity and greater landscape diversity. In the study region as a whole, with or without fire action, a significant decrease in forest surfaces was recorded, whereas there was an increase of unforested communities such as treed shrublands and shrublands. In some parts of the Massif fires increased the homogeneity of the landscape, in other parts they created a greater heterogeneity and diversity of plant communities

Observation of nonlinear interactions in large-scale density enhancements of the high-latitude ionosphere

International audienceSimultaneous measurements of electron density and ELF electric field by the AUREOL-3 satellite have allowed Cerisier et al. [1985] to point out the existence of large-scale density irregularities in the high-latitude topside F region and to establish the role of the gradient drift instability in the plasma structuring. High-resolution spectra of the density and field fluctuations show the presence of low-frequency peaks linked by relationships such that fm q-fn = fm+n. Bispectral analyses point out nonlinear interactions between waves at 7.8 and 15.6 Hz and density irregularities at 5.85 Hz. The consistency of the results with regard to instability models is discussed. The existence of an interaction process involving Schumann resonances is suggested

Binding constants determined from Ca2+ current responses to rapid applications and washouts of nifedipine in frog cardiac myocytes

International audienceA fast perfusion system was used to analyse the kinetics of the response of L-type calcium current (ICa) to rapid applications and washouts of the dihydropyridine antagonist nifedipine in whole-cell patch-clamped frog ventricular myocytes. Both the inhibition of ICa induced by nifedipine and the recovery from inhibition upon washout of the drug behaved as mono-exponential functions of time. During application or washout of 100 nM nifedipine, only the peak amplitude of ICa varied but not its time course of activation or inactivation. The rate constant of the onset of ICa inhibition increased with the concentration of nifedipine. However, the time course of the recovery from inhibition was independent of drug concentration. Both rate constants were strongly sensitive to the holding potential but insensitive to the test potential. Using simple rate equations and a one-binding-site analysis it was possible to determine the rate constants for association (k1) and dissociation (k-1) and the equilibrium dissociation constant (KD) of the reaction between nifedipine and Ca2+ channels. KD values for nifedipine were identical to IC50 values obtained from classical steady-state experiments. With depolarized holding potentials, KD decreased strongly due to a large reduction in k-1 and a modest increase in k1. Assuming that these changes result from the distribution of Ca2+ channels between resting and inactivated states, a low-affinity binding to the resting state (R) and a high-affinity binding to the inactivated state (I) were obtained with the binding constants: k1R = 1.0 x 10(6) M-1 S-1, k-1R = 0.077 S-1, and KDR = 77 nM for the resting state; k1I = 4.47 x 10(6) M-1 S-1, k-1I = 7.7 x 10(-4) S-1, and KDI = 0.17 nM for the inactivated state. Rapid application/washout experiments provide a unique way to determine, in an intact cell and in a relatively short period (2-4 min), the binding rate constants and the KD value of the reaction between a dihydropyridine antagonist and the Ca2+ channels