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

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

CMB Constraints on Primordial non-Gaussianity from the Bispectrum (f_{NL}) and Trispectrum (g_{NL} and \tau_{NL}) and a New Consistency Test of Single-Field Inflation

We outline the expected constraints on non-Gaussianity from the cosmic microwave background (CMB) with current and future experiments, focusing on both the third (f_{NL}) and fourth-order (g_{NL} and \tau_{NL}) amplitudes of the local configuration or non-Gaussianity. The experimental focus is the skewness (two-to-one) and kurtosis (two-to-two and three-to-one) power spectra from weighted maps. In adition to a measurement of \tau_{NL} and g_{NL} with WMAP 5-year data, our study provides the first forecasts for future constraints on g_{NL}. We describe how these statistics can be corrected for the mask and cut-sky through a window function, bypassing the need to compute linear terms that were introduced for the previous-generation non-Gaussianity statistics, such as the skewness estimator. We discus the ratio A_{NL} = \tau_{NL}/(6f_{NL}/5)^2 as an additional test of single-field inflationary models and discuss the physical significance of each statistic. Using these estimators with WMAP 5-Year V+W-band data out to l_{max}=600 we constrain the cubic order non-Gaussianity parameters \tau_{NL}, and g_{NL} and find -7.4 < g_{NL}/10^5 < 8.2 and -0.6 < \tau_{NL}/10^4 < 3.3 improving the previous COBE-based limit on \tau_{NL} < 10^8 nearly four orders of magnitude with WMAP.Comment: 15 pages. 14 figure

CLC, a promising concept with challenging development issues

Chemical Looping Combustion (CLC) is a promising technique to achieve fuel combustion in a nitrogen free atmosphere, therefore giving the possibility to separate and store or use CO2. Several potential applications are considered in the field of power generation with gas, liquid and solid fuels. In the Carbon Capture, Storage and Utilization (CCSU) context, energy penalty is reduced compared to other routes. In addition, other applications of Chemical Looping are considered in the field of H2 production or gasification for instance. In the past years, a huge effort has been conducted worldwide to investigate CLC materials and process issues. In 2008, IFPEN and Total have started an ambitious collaboration to develop CLC applications. Nowadays, the CLC concept is well demonstrated at the pilot scale. The next step is to demonstrate the technology over time at larger scale. However, for further developments, the challenges are numerous and will be discussed, both on market and technical aspects. Short term market is limited. Uncertainties around CO2 emission market and storage issues are related to CO2 policy and public acceptance of storage which still must evolve in the right direction... Financing of demonstration units in this context is challenging and other applications of CLC have to be investigated. The industrial use of synthetic metal oxides or natural ores at large scale generates a lot of issues related to availability, price, waste disposal, health and safety, additionally to chemical and mechanical stability over time, reactivity, and oxygen transfer capacity. Chemical looping reactor and process technology concepts have to be explored, developed, modeled and scaled-up in order to ensure adequate power production together with good gas solid contact and reaction requirement, controlled circulation of mixtures of particle (oxygen carrier, ash, solid fuel for instance). All these points should be considered at very large scale for CCS applications in order to minimize energy penalty and cost in severe operating conditions (temperatures above 800°C and intense solid circulation). Technical challenges remain to be solved and proven with large demonstration over long periods of time. In this context, research in the field of fluidization technology is essential and we will address a couple of key points already investigated at IFPEN and related to control of solid circulation, oxygen carrier attrition, conceptual design of CLC reactors and process performance

Structure of microbial communities in Sphagnum peatlands and effect of atmospheric carbon dioxide enrichment

Little is known about the structure of microbial communities in Sphagnum peatlands, and the potential effects of the increasing atmospheric C02 concentration on these communities are not known. We analyzed the structure of microbial communities in five Sphagnum-dominated peatlands across Europe and their response to C02 enrichment using miniFACE systems. After three growing seasons, Sphagnum samples were analyzed for heterotrophic bacteria, cyanobacteria, microalgae, heterotrophic flagellates, ciliates, testate amoebae, fungi, nematodes, and rotifers. Heterotrophic organisms dominated the microbial communities and together represented 78% to 97% of the total microbial biomass. Testate amoebae dominated the protozoan biomass. A canonical correspondence analysis revealed a significant correlation between the microbial community data and four environmental variables (Na+, DOC, water table depth, and DIN), reflecting continentality, hydrology, and nitrogen deposition gradients. Carbon dioxide enrichment modified the structure of microbial communities, but total microbial biomass was unaffected. The biomass of heterotrophic bacteria increased by 48%, and the biomass of testate amoebae decreased by 13%. These results contrast with the absence of overall effect on methane production or on the vegetation, but are in line with an increased below-ground vascular plant biomass at the same sites. We interpret the increase in bacterial biomass as a response to a C02-induced enhancement of Sphagnum exudation. The causes for the decrease of testate amoebae are unclear but could indicate a top-down rather than a bottom-up control on their densit

Search for Gravitational Waves in the CMB After WMAP3: Foreground Confusion and The Optimal Frequency Coverage for Foreground Minimization

B-modes of Cosmic Microwave Background (CMB) polarization can be created by a primordial gravitational wave background. If this background was created by Inflation, then the amplitude of the polarization signal is proportional the energy density of the universe during inflation. The primordial signal will be contaminated by polarized foregrounds including dust and synchrotron emission within the galaxy. In light of the WMAP polarization maps, we consider the ability of several hypothetical CMB polarization experiments to separate primordial CMB B-mode signal from galactic foregrounds. We also study the optimization of a CMB experiment with a fixed number of detectors in the focal plane to determine how the detectors should be distributed in different frequency bands to minimize foreground confusion. We show that the optimal configuration requires observations in at least 5 channels spread over the frequency range between 30 GHz and 500 GHz with substantial coverage around 150 GHz. If a low-resolution space experiment using 1000 detectors to reach a noise level of about 1000 nK$^2$ concentrates on roughly 66% of the sky with the least foreground contamination the minimum detectable level of the tensor-to-scalar ratio would be about 0.002 at the 99% confidence level for an optical depth of 0.1.Comment: 13 pages, 8 figures, replaced to match the accepted version in PR