Hétérotrophie algale : effets de la gentamycine et de la cycloheximide sur les activités hétérotrophes et photosynthétiques des bacteries et des algues

Afin de tenter de déterminer les parts respectives des activités bactérienne et algale, nous avons testé d'une part, l'action d'une substance antibactérienne (la gentamycine), et d'autre part, l'action d'un inhibiteur métabolique des cellules eucaryotes (la cycloheximide) sur des cultures d'algues et de bactéries et sur des échantillons provenant du milieu naturel et soumis à des filtrations différentielles. Les effets de ces inhibiteurs ont été testés au niveau des activités hétérotrophe et photosynthétique globales, mais également au niveau de l'incorporation des marqueurs radioactifs lors de la synthèse des macromolécules et des composés de faible poids moléculaire.Les résultats obtenus laissent apparaître que l'inhibition de l'activité bactérienne par la gentamyclne est significative mais non complète (pourcentage d'inhibition moyen = 67 %). De plus, l'efficacité de la gentamycine augmente avec la durée d'incubation. Par ailleurs, les effets secondaires de la gentamycine sur les activités hétérotrophe et photosynthétique d'une culture de Melosira italica subsp. subarctica sont acceptables seulement pour des incubations de courte durée (< 4 heures).En revanche, l'emploi de la cycloheximide s'est révélé sans aucun effet significatif sur les activités photosynthétique et hérérotrophe de la culture de Melosira, même après 24 h d'incubation.A partir des échantillons prélevés en milieu naturel, l'emploi de la gentamycine a permis de réduire l'interférence bactérienne dans les mesures d'activité hétérotrophe algale. Enfin, nous avons pu constater que la gentamycine modifie l'allocation des marqueurs radioactifs dans les macromolécules.The ability of many planktonic algae to use particulate and/or dissolved organic carbon directly by phagotrophy or osmotrophy in laboratory cultures is well documented (DROOP, 1974; NEILSON and LEWIN, 1974; HELLEBUST and LEWIN, 1977; BIRD and KALFF, 1986). In axenic cultures, numerous microalgae grow in the dark with micromolar concentrations of diverse organic nutrients as their sole sources of carbon and energy (RIVKIN and PUTT, 1987). However, to demonstrate algal heterotrophy in the field, it is necessary to differentiate between bacterial and algal activities. In the course of this study, we tested the effect of an antibacterial substance (gentamycin) and of a metabolic inhibitor of eukaryotic cells (cycloheximide) on algal and bacterial cultures, and also on lake water samples submitted to differential filtration. The effect of these inhibitors was tested both at the overall heterotrophic and photosynthetic activities level and the level of the incorporation of radiolabeled tracers in macromolecules and low molecular weight compounds.Gentamycin was tested on bacteria and on an axenic culture of the diatom Melosira italica subsp. subarctica, the dominant species of the spring phyto-planktonic bloom of many temperate lakes. Bacterial culture was obtained by filtration of a senescent culture of Melosira through a 0.45 µm pore-size membrane. During exponential growth, gentamycin (40 µg.ml-1) was added to different flasks containing 100 ml of culture, 30 mn after gentamycin addition, Na H14CO3 (12µCi/100 ml) was introduced into the flasks. In each case, two replicates were incubated in the light and two in the dark for 0.5, 1, 2, 4, 6, 8, 12 and 24 hours. After incubation, aliquots were collected on a filtration membrane (0.45 µm) and the radioactivity was measured using an LKB liquid scintillation counter to estimate photosynthetic activity. Incorporation of inorganic carbon into macromolecules was measured using the procedure described by LANCELOT and MATHOT (1985), which allows the separation of lipids, polysaccharides, proteins and low molecular weight compounds (e.g. amino acids, organic acids and monosaccharides) by virtue of their relative solubilities in different extraction solvents :- lipids were extracted with a 2/1 (v/v) chloroform-methanol mixture;- low molecular weight compounds were extracted with hot ethanol;- proteins were precipated with TCA at elevated temperature; this also separated them from polysaccharides. Nucleic acids were recovered with the polysaccharides fraction. Results are given as a percentage of total radioactivity. The same method, as that described for photosynthetic incorporation, was used to measure incorporation of glucose 3H (19 nmoles.l-1) and for the allocation of radioactive marker in macromolecules. To assess the effect of cycloheximide, the same procedure as that described for gentamycin was used. However, due to the lack of references, three cycloheximide concentrations were tested = 50, 100 and 150 µg.ml-1, and aliquots were collected on a 0.2 µm pore-size membrane.Field samples were taken in Lake Pavin, an oligomesotrophic French lake, during the sedimentation of Melosira italica subsp. subarctica, the prevailing species of the spring bloom. The effects of gentamycin on photosynthetic and heterotrophic incorporations were tested with the came procedure as that described for cultures. However, radioactivity was measured for different size tractions : 0.2-0.45 µm, 0.45-5 µm and 5-160 µm, after incubations for 0.5, 1, 3, 6, 12 and 24 hours. Phytoplanktonic cells were counted on a Wild M40 inverted microscope and bacterial enumeration was realized in epifluorescence microscopy after staining with acridine orange (HOBBIE el al., 1977).Results show that gentamycin used at 40 µg.ml-1 inhibits significantly but not completely the activity of the bacterial culture (mean inhibition percent = 67 %). Gentamycin efficiency increases with incubation time, the inhibition reaching 81 % after 24 h. At the same time, the secondary effects of gentamycin on heterotrophic and photosynthetic activities of Melosira italica subsp. subarctica in culture were only tolerable with short incubation times (< 4 h), when the percents of inhibition were respectively 13.6 and 12.2%. On the other band, cycloheximide produced no significant effect on photosynthetic and heterotrophic activities of Melosira italica in culture, the percent of inhibition always remaining below 6.5 %.The use of gentamycin in natural samples reduced bacterial interference with algal heterotrophic activily measurements. The percent of inhibition caused by gentamycin was high (< 76 %) In the small-size fraction where the bacterial biomass predominate on the phytoplanktonic one.Lastly, in all samples, we could demonstrate that gentamycin modifies the allocation of inorganic carbon and radioactivity in macromolecules. The incorporation in proteins is significantly reduced essentially to benefit of the incorporation in low molecular weight compounds

ALMA observations of molecular clouds in three group centered elliptical galaxies: NGC 5846, NGC 4636, and NGC 5044

We present new ALMA CO(2--1) observations of two well studied group-centered elliptical galaxies: NGC~4636 and NGC~5846. In addition, we include a revised analysis of Cycle 0 ALMA observations of the central galaxy in the NGC~5044 group that has been previously published. We find evidence that molecular gas, in the form of off-center orbiting clouds, is a common presence in bright group-centered galaxies (BGG). CO line widths are $\gtrsim 10$ times broader than Galactic molecular clouds, and using the reference Milky Way $X_{CO}$, the total molecular mass ranges from as low as $2.6\times 10^5 M_\odot$ in NGC~4636 to $6.1\times 10^7 M_\odot$ in NGC~5044. With these parameters the virial parameters of the molecular structures is $\gg 1$. Complementary observations of NGC~5846 and NGC~4636 using the ALMA Compact Array (ACA) do not exhibit any detection of a CO diffuse component at the sensitivity level achieved by current exposures. The origin of the detected molecular features is still uncertain, but these ALMA observations suggest that they are the end product of the hot gas cooling process and not the result of merger events. Some of the molecular clouds are associated with dust features as revealed by HST dust extinction maps suggesting that these clouds formed from dust-enhanced cooling. The global nonlinear condensation may be triggered via the chaotic turbulent field or buoyant uplift. The large virial parameter of the molecular structures and correlation with the warm ($10^3 - 10^5 K$)/hot ($\ge10^6$) phase velocity dispersion provide evidence that they are unbound giant molecular associations drifting in the turbulent field, consistently with numerical predictions of the chaotic cold accretion process. Alternatively, the observed large CO line widths may be generated by molecular gas flowing out from cloud surfaces due to heating by the local hot gas atmosphere.Comment: Revised version to be published in ApJ, 16 pages, 10 figures, 4 table

Cosmic Reionization and the 21-cm signal: Comparison between an analytical model and a simulation

We measure several properties of the reionization process and the corresponding low-frequency 21-cm signal associated with the neutral hydrogen distribution, using a large volume, high resolution simulation of cosmic reionization. The brightness temperature of the 21-cm signal is derived by post-processing this numerical simulation with a semi-analytical prescription. Our study extends to high redshifts (z ~ 25) where, in addition to collisional coupling, our post-processed simulations take into account the inhomogeneities in the heating of the neutral gas by X-rays and the effect of an inhomogeneous Lya radiation field. Unlike the well-studied case where spin temperature is assumed to be significantly greater than the temperature of the cosmic microwave background due to uniform heating of the gas by X-rays, spatial fluctuations in both the Lya radiation field and X-ray intensity impact predictions related to the brightness temperature at z > 10, during the early stages of reionization and gas heating. The statistics of the 21-cm signal from our simulation are then compared to existing analytical models in the literature and we find that these analytical models provide a reasonably accurate description of the 21-cm power spectrum at z < 10. Such an agreement is useful since analytical models are better suited to quickly explore the full astrophysical and cosmological parameter space relevant for future 21-cm surveys. We find, nevertheless, non-negligible differences that can be attributed to differences in the inhomogeneous X-ray heating and Lya coupling at z > 10 and, with upcoming interferometric data, these differences in return can provide a way to better understand the astrophysical processes during reionization.Comment: Major paper revision to match version accepted for publication in ApJ. Simulation now fully includes fluctuations in the X-ray heating and the Lya radiation field. 18 pages, 13 figure

Nonequilibrium phase transition in the coevolution of networks and opinions

Models of the convergence of opinion in social systems have been the subject of a considerable amount of recent attention in the physics literature. These models divide into two classes, those in which individuals form their beliefs based on the opinions of their neighbors in a social network of personal acquaintances, and those in which, conversely, network connections form between individuals of similar beliefs. While both of these processes can give rise to realistic levels of agreement between acquaintances, practical experience suggests that opinion formation in the real world is not a result of one process or the other, but a combination of the two. Here we present a simple model of this combination, with a single parameter controlling the balance of the two processes. We find that the model undergoes a continuous phase transition as this parameter is varied, from a regime in which opinions are arbitrarily diverse to one in which most individuals hold the same opinion. We characterize the static and dynamical properties of this transition

Variation of Mid and Far-IR Luminosities among Early-Type Galaxies: Relation to Stellar Metallicity and Cold Dust

The Hubble morphological sequence from early to late galaxies corresponds to an increasing rate of specific star formation. The Hubble sequence also follows a banana-shaped correlation between 24 and 70 micron luminosities, both normalized with the K-band luminosity. We show that this correlation is significantly tightened if galaxies with central AGN emission are removed, but the cosmic scatter of elliptical galaxies in both 24 and 70 micron luminosities remains significant along the correlation. We find that the 24 micron variation among ellipticals correlates with stellar metallicity, reflecting emission from hot dust in winds from asymptotic giant branch stars of varying metallicity. Infrared surface brightness variations in elliptical galaxies indicate that the K - 24 color profile is U-shaped for reasons that are unclear. In some elliptical galaxies cold interstellar dust emitting at 70 and 160 microns may arise from recent gas-rich mergers. However, we argue that most of the large range of 70 micron luminosity in elliptical galaxies is due to dust transported from galactic cores by feedback events in (currently IR-quiet) active galactic nuclei. Cooler dusty gas naturally accumulates in the cores of elliptical galaxies due to dust-cooled local stellar mass loss and may accrete onto the central black hole, releasing energy. AGN-heated gas can transport dust in cores 5-10 kpc out into the hot gas atmospheres where it radiates extended 70 micron emission but is eventually destroyed by sputtering. This, and some modest star formation, defines a cycle of dust creation and destruction. Elliptical galaxies evidently undergo large transient excursions in the banana plot in times comparable to the sputtering time or AGN duty cycle, 10 Myrs. Normally regarded as passive, elliptical galaxies are the most active galaxies in the IR color-color correlation.Comment: 17 pages, 8 figures, 2 tables, accepted for publication in Ap

Mobility and Social Network Effects on Extremist Opinions

Understanding the emergence of extreme opinions and in what kind of environment they might become less extreme is a central theme in our modern globalized society. A model combining continuous opinions and observed discrete actions (CODA) capable of addressing the important issue of measuring how extreme opinions might be has been recently proposed. In this paper I show extreme opinions to arise in a ubiquitous manner in the CODA model for a multitude of social network structures. Depending on network details reducing extremism seems to be possible. However, a large number agents with extreme opinions is always observed. A significant decrease in the number of extremists can be observed by allowing agents to change their positions in the network.Comment: 7 pages, 8 figures, discussion expanded, new references, new figure

Spectral Analysis of Multi-dimensional Self-similar Markov Processes

In this paper we consider a discrete scale invariant (DSI) process $\{X(t), t\in {\bf R^+}\}$ with scale $l>1$. We consider to have some fix number of observations in every scale, say $T$, and to get our samples at discrete points $\alpha^k, k\in {\bf W}$ where $\alpha$ is obtained by the equality $l=\alpha^T$ and ${\bf W}=\{0, 1,...\}$. So we provide a discrete time scale invariant (DT-SI) process $X(\cdot)$ with parameter space $\{\alpha^k, k\in {\bf W}\}$. We find the spectral representation of the covariance function of such DT-SI process. By providing harmonic like representation of multi-dimensional self-similar processes, spectral density function of them are presented. We assume that the process $\{X(t), t\in {\bf R^+}\}$ is also Markov in the wide sense and provide a discrete time scale invariant Markov (DT-SIM) process with the above scheme of sampling. We present an example of DT-SIM process, simple Brownian motion, by the above sampling scheme and verify our results. Finally we find the spectral density matrix of such DT-SIM process and show that its associated $T$-dimensional self-similar Markov process is fully specified by $\{R_{j}^H(1),R_{j}^H(0),j=0, 1,..., T-1\}$ where $R_j^H(\tau)$ is the covariance function of $j$th and $(j+\tau)$th observations of the process.Comment: 16 page

Variations à court terme des compartiments planctoniques d'un lac humique du Bouclier canadien

Les variations spatio-temporelles à court terme des compartiments planctoniques ont été étudiées simultanément du 30 juillet au 5 août 1986 dans un lac humique du Bouclier canadien. L'abondance du bactérioplancton fluctue de 1,4 à 1,7.106 cell. ml.-1 (coloration DAPI) ou de 2,7 à 7,7.106 cell. ml-1; (coloration Acridine Orange). La production du bactérioplancton estimée par incorporation de 3H méthyl thimidine, varie de 4 à 24.106 cell. l-1 h-1. Les valeurs d'activité hétérotrophe potentielle bactérienne estimée par assimilation de 14C glucose, s'échelonnent de 0,007 à 0,065 µg C.l-1. h-1. La biomasse pigmentaire (chlore. a et pheopigments) varie de 6,8 à 21,7 mg.m-3 . La production primaire est très faible (max. : 10 mg C. m-3 .h-1; 20 mg C.m-2 .h-1 ) et décroît très rapidement avec la profondeur (25 % à 82 % dans le premier mètre). Le microzooplancton (Rotifères, Bosmina, nauplies) représente plus de 90 % du peuplement zooplanctonique et les taux de broutage global du macrozooplancton sont faibles (25 % j-1). Les compartiments hétérotrophes prédominent dans le métabolisme du lac par rapport au compartiment autotrophe. Les patrons de variation spatiale reflètent la stratification verticale des compartiments et des processus autotrophes et hétérotrophes. Les maxima de photosynthèse, d'ATP et de production bactérienne se situent dans les eaux épilimnétiques tandis que les maxima d'abondance du bactérioptancton et des pigments se rencontrent dans l'hypolimnion. Le zooplancton est plus dense et broute d'avantage dans la strate 1-3 m. Ces variations spatiales semblent influencer l'activité hétérotrophe potentielle du bactérioptancton. Nos résultats montrent aussi des variations temporelles à court terme de la production primaire, de l'assimilation hétérotrophe et du broutage du macrozooplancton. Cette étude préliminaire met en lumière la nécessité de tenir compte des variations à court terme lors des études des relations trophodynamiques dans les écosystèmes planctoniques.Short term spatial and temporal variations in planktonic compartments were studied simultaneously, from July 30th to August 5th, 1986, in a humic lake on the Canadian Shield. Abundance of bacterioplankton ranged from 1,4 to 1,7 106 cell. ml-1 (DAP1) or from 2,7 to 7,7 106 cell. ml-1 (Acridine Orange). Bacterial production, as measured by incorporation of 3H methyl-thimidine, was estimated at 4-24 106 cells. l-1. h-1 and potential heterotrophic bacterial activity ranged from 0,007 to 0,065 µg C.l-1. h-1, as estimated by 14C glucose incorporation. Pigments biomass (chloro. a and phaeopigments) varied from 6,8 to 21,7 mg m-3. Primary production was low (max. : 10 mg C.m-3. h-1; 20 mg C.m-2. h-1) and decreased rapidly with depth (25-82 % in 1 m depth). Microzooplankton (rotifera, Bosmina, nauplii) accounted for more than 90 % of total numbers and macrozooplankton global grazing rates were low (25 % d-1). The heterotrophic compartments play a greater rate in the lake metabolism than the autotrophic compartment. Spatial variations reflect the stratification of the heterotrophic and autotrophic organisms and processes with depth. Maximum levels in primary production, ATP and bacterial production occur in epilimnetic waters, while maxima in bacterial numbers and algal pigments occur in the hypolimnion. Zooplancton density and macrozooplankton grazing rates were higher in the 1-3 m strata. These spatial patterns seem to influence the vertical profiles of the bacterial potential heterotrophic activity. Our results also show short term temporal variations in primary production, potential heterotrophic activity of bacterioplankton and macrozooplankton grazing rates. This preliminary study stresses the importance of short term variations in the assessment of the trophodynamics of the planktonic food wed

Effects of Destriping Errors on CMB Polarisation Power Spectra and and Pixel Noise Covariances

Low frequency detector noise in CMB experiments must be corrected to produce faithful maps of the temperature and polarization anisotropies. For a Planck-type experiment the low frequency noise corrections lead to residual stripes in the maps. Here I show that for a ring torus and idealised detector geometry it is possible to calculate analytically the effects of destriping errors on the temperature and polarization power spectra. It is also possible to compute the pixel-pixel noise covariances for maps of arbitrary resolution. The analytic model is compared to numerical simulations using a realistic detector and scanning geometries. We show that Planck polarization maps at 143 GHz should be signal dominated on large scales. Destriping errors are the dominant source of noise for the temperature and polarization power spectra at multipoles ell < 10. A fast Monte-Carlo method for characterising noise, including destriping errors, is described that can be applied to Planck. This Monte-Carlo method can be used to quantify pixel-pixel noise covariances and to remove noise biases in power spectrum estimates.Comment: 19 pages submitted to MNRA

Is a Classical Language Adequate in Assessing the Detectability of the Redshifted 21cm Signal from the Early Universe?

The classical radiometer equation is commonly used to calculate the detectability of the 21cm emission by diffuse cosmic hydrogen at high redshifts. However, the classical description is only valid in the regime where the occupation number of the photons in phase space is much larger than unity and they collectively behave as a classical electromagnetic field. At redshifts z<20, the spin temperature of the intergalactic gas is dictated by the radiation from galaxies and the brightness temperature of the emitting gas is in the range of mK, independently from the existence of the cosmic microwave background. In regions where the observed brightness temperature of the 21cm signal is smaller than the observed photon energy, of 68/(1+z) mK, the occupation number of the signal photons is smaller than unity. Neverethless, the radiometer equation can still be used in this regime because the weak signal is accompanied by a flood of foreground photons with a high occupation number (involving the synchrotron Galactic emission and the cosmic microwave background). As the signal photons are not individually distinguishable, the combined signal+foreground population of photons has a high occupation number, thus justifying the use of the radiometer equation.Comment: 4 pages, Accepted for publication in JCA