L'accumulation et l'élimination de cadmium par deux mousses aquatiques, Fontinalis dalecarlica et Platyphypnidium ripariodes : Influence de la concentration de Cd, du temps d'exposition, de la dureté de l'eau et de l'espèce de mousses

Cette étude en laboratoire traite de l'accumulation et de l'élimination du Cd réalisées par deux mousses aquatiques indigènes du Québec, Fontinalis dalecarlica et Platyhypnidium riparioides. Les expositions au Cd étaient de 0 (témoin), 0,8, 2 et 10 µg·L-1, concentrations retrouvées en milieu naturel (non contaminé) et contaminé. Les expériences ont été réalisées à trois niveaux de dureté de l'eau (10 à 15, 40 à 50, 80 à 100 mg·L-1 de CaCO3), à alcalinité constante (80 à 100 mg·L-1 de CaCO3) et à pH stable (7,30) durant une période de 28 jours. Les facteurs d'augmentation des concentrations (FAC) ont démontré une diminution de l'accumulation totale de Cd dans les mousses dans 75% des cas lorsque la dureté de l'eau passe de très douce à dure. Les facteurs de contamination résiduelle (FCR) démontrent la lenteur de l'élimination du Cd par les mousses, et ce, indépendamment de la dureté de l'eau ou de la contamination préalablement subie. Deux équations de régression multiple par étape (Stepwise) ont été établies pour expliquer les facteurs influençant l'accumulation et l'élimination de Cd réalisées par les mousses. Les variables indépendantes incluses dans les équations linéaires de prédiction pour l'accumulation et l'élimination étaient la concentration de Cd dans l'eau, le temps d'exposition, la dureté de l'eau, l'espèce de mousses utilisée et/ou les interactions de ces variables. Les équations linéaires de prédiction pour l'accumulation et l'élimination ont permis d'expliquer respectivement 92% et 71% de la variance observée. Cette identification des principaux facteurs influençant l'accumulation et l'élimination du Cd dans les mousses est d'une grande importance pour la compréhension des processus complexes dirigeant l'absortion des métaux lourds par des organismes vivants. Les équations permettent également de mieux expliquer les interactions engendrées par la variation de divers paramètres sur l'accumulation et l'élimination du Cd par les mousses aquatiques.Aquatic mosses have played a large part in the assessment of toxic elements in water. The advantage of mosses over direct water sampling is that the use of the former lessens spatial and temporal variations, enhances the level of contaminant identification by concentrating toxic elements, and provides information relative to the bioavailable portion. However, the concentration of metals that can be measured in mosses is not a reliable indicator of the concentration of toxic elements in the water, which is why we need to model the bioaccumulation phenomenon.The present laboratory study deals with the accumulation and elimination of Cd by two indigenous Quebec aquatic mosses: Fontinalis dalecarlica and Platyhypnidium riparioides. The previously acclimatized mosses were treated with different concentrations of Cd, three different levels of water hardness, a constant alkalinity and constant pH level for a period of 28 days, in order to establish their bioaccumulative capacity. Cd exposure concentrations were 0 (control), 0.8, 2 and 10 mg·L-1, with a replication at 10 mg·L-1. The experiments were carried out at three levels of water hardness (10 to 15, 40 to 50, 80 to 100 mg·L-1 of CaCO3), with a constant degree of alkalinity (80 to 100 mg·L-1 of CaCO3) and stable pH (7.30). The mosses subsequently went through an elimination period (Cd-free water) of 28 days. The triplicate moss samples were mineralized using nitric acid and all Cd measurements were made by atomic absorption spectrophotometry. The results indicate that the water chemistry conditions remained stable and were properly controlled. The aquatic mosses demonstrated a considerable ability to absorb and adsorb Cd: the measured Cd water concentrations were less than the nominal concentrations. Nonetheless, moss uptake of Cd improves with an increase in Cd contamination and the concentration factors (CF) range from 6 to 122. For the same exposure concentration, the CF drops in some 63% of those instances where water hardness rises from very soft, through soft, to hard. In 75% of the cases there is a drop in CF when water hardness increases directly from very soft to hard. With a stable concentration (e.g. 2 mg·L-1), F. dalecarlica has respective CFs of 26.3, 22.2 and 18, which demonstrates the negative gradation of Cd accumulation as water hardness increases. The residual contamination factors (RCF) bear witness to the slow rate of Cd elimination by the mosses, irrespective of the level of water hardness or of any previous contamination. The elimination factor for RCF is never greater than 2. Mosses take up metals faster than they can eliminate them and have a memory of past contaminations, which is an advantage when it comes to studying ad hoc and/or sporadic contamination phenomena.Two stepwise multiple regression equations have been set up to explain the factors that impact on accumulation and elimination of Cd by mosses. The variables included in the equations were: level of Cd concentration in the water; exposure time; water hardness; the moss species involved, and/or the interactions between these variables. The predictive linear equations for the accumulation and elimination provided explanations for 92% and 71% respectively of the observed variances. The predictive linear equation for accumulation establishes that the length of exposure is the principal parameter responsible for the uptake of Cd by the aquatic mosses. It shows that the accumulation of Cd by the mosses is initially influenced by the level of Cd concentration in the water, but also depends on the length of time over which the bryophytes are exposed to this concentration. Thus, the higher the Cd concentration, the shorter the exposure time for the moss contamination, and vice versa. The second variable is the effect of water hardness taken together with the exposure time. This is a negative variable: the greater the increase in water hardness, the greater the exposure time required to obtain the same degree of moss contamination. This is indicative of the impact of Ca++ and Mg++ on moss uptake. The impact of water hardness is probably the consequence of the availability of or preference of plant-binding sites for Ca++ and Mg++ ions, thus reducing the number of available locations for Cd accumulation. Water hardness and Cd concentration levels are the third variable in this equation and are probably linked to the effect of water hardness on the bioavailability of Cd for the mosses. This variable may also explain why the increase in Cd concentration levels in the water lessens the impact of water hardness on the total accumulation of Cd in the mosses. Finally, the equation identifies a greater level of accumulation in the P. riparoides.Release linear regression shows that the absence of Cd in the water is the major parameter in the elimination of Cd by aquatic mosses. We should remember that the bryophytes are seeking to achieve a steady state condition with their environment, since the Cd is an element that is neither regulated or essential. Its elimination has little to do with water hardness, but is caused by the inversion of a diffusion gradient when the environment is no longer Cd contaminated. During the elimination process, the Ca++ and Mg++ ions have no real impact on the release of Cd by the mosses. The length of prior exposure does affect elimination: the greater it is, the longer the release period required for moss decontamination. Exposure time is less important during elimination than during accumulation. Elimination is a very slow process, and a longer study would probably have shown that this is a major factor in the elimination of moss-accumulated Cd.The present identification of the major factors impacting on the accumulation and elimination of Cd in mosses is extremely important if we are to understand the complex processes that determine the absorption of heavy metals by living organisms. The equations also allow us to better explain the interactions caused by variations in the different parameters with respect to the accumulation and elimination of Cd by aquatic mosses

Smooth tight frame wavelets and image microanalyis in the fourier domain

AbstractGeneral results on microlocal analysis and tight frames in R2 are summarized. To perform microlocal analysis of tempered distributions, orthogonal multiwavelets, whose Fourier transforms consist of characteristic functions of squares or sectors of annuli, are constructed in the Fourier domain and are shown to satisfy a multiresolution analysis with several choices of scaling functions. To have good localization in both the x and Fourier domains, redundant smooth tight wavelet frames, with frame bounds equal to one, called Parseval wavelet frames, are obtained in the Fourier domain by properly tapering the above characteristic functions. These nonorthogonal frame wavelets can be generated by two-scale equations from a multiresolution analysis. A natural formulation of the problem is by means of pseudodifferential operators. Singularities, which are added to smooth images, can be localized in position and direction by means of the frame coefficients of the filtered images computed in the Fourier domain. Using Plancherel's theorem, the frame expansion of the filtered images is obtained in the x domain. Subtracting this expansion from the scarred images restores the original images

Origins of the 1/4 keV Soft X-Ray Background

Snowden and coworkers have presented a model for the 1/4 keV soft X-ray diffuse background in which the observed flux is dominated by a ~ 10^6 K thermal plasma located in a 100-300 pc diameter bubble surrounding the Sun, but has significant contributions from a very patchy Galactic halo. Halo emission provides about 11% of the total observed flux and is responsible for half of the H I anticorrelation. The remainder of the anticorrelation is presumably produced by displacement of disk H I by the varying extent of the local hot bubble (LHB). The ROSAT R1 and R2 bands used for this work had the unique spatial resolution and statistical precision required for separating the halo and local components, but provide little spectral information. Some consistency checks had been made with older observations at lower X-ray energies, but we have made a careful investigation of the extent to which the model is supported by existing sounding rocket data in the Be (73-111 eV) and B bands (115-188 eV) where the sensitivities to the model are qualitatively different from the ROSAT bands. We conclude that the two-component model is well supported by the low-energy data. We find that these combined observations of the local component may be consistent with single-temperature thermal emission models in collisional ionization equilibrium if depleted abundances are assumed. However, different model implementations give significantly different results, offering little support for the conclusion that the astrophysical situation is so simple.Comment: 17 pages, 6 figures, accepted by the Astrophysical Journa

Submillimeter Polarization of Galactic Clouds: A Comparison of 350 micron and 850 micron Data

The Hertz and SCUBA polarimeters, working at 350 micron and 850 micron respectively, have measured the polarized emission in scores of Galactic clouds. Of the clouds in each dataset, 17 were mapped by both instruments with good polarization signal-to-noise ratios. We present maps of each of these 17 clouds comparing the dual-wavelength polarization amplitudes and position angles at the same spatial locations. In total number of clouds compared, this is a four-fold increase over previous work. Across the entire data-set real position angle differences are seen between wavelengths. While the distribution of \phi(850)-\phi(350) is centered near zero (near-equal angles), 64% of data points with high polarization signal-to-noise (P >= 3\sigma_p) have |\phi(850)-\phi(350)| > 10 degrees. Of those data with small changes in position angle (<= 10 degrees) the median ratio of the polarization amplitudes is P(850)/P(350) = 1.7 +/- 0.6. This value is consistent with previous work performed on smaller samples and models which require mixtures of different grain properties and polarization efficiencies. Along with the polarization data we have also compiled the intensity data at both wavelengths; we find a trend of decreasing polarization with increasing 850-to-350 micron intensity ratio. All the polarization and intensity data presented here (1699 points in total) are available in electronic format.Comment: 50 pages, 26 figures, one electronic data file. Submitted to ApJ

Statistical Assessment of Shapes and Magnetic Field Orientations in Molecular Clouds through Polarization Observations

We present a novel statistical analysis aimed at deriving the intrinsic shapes and magnetic field orientations of molecular clouds using dust emission and polarization observations by the Hertz polarimeter. Our observables are the aspect ratio of the projected plane-of-the-sky cloud image, and the angle between the mean direction of the plane-of-the-sky component of the magnetic field and the short axis of the cloud image. To overcome projection effects due to the unknown orientation of the line-of-sight, we combine observations from 24 clouds, assuming that line-of-sight orientations are random and all are equally probable. Through a weighted least-squares analysis, we find that the best-fit intrinsic cloud shape describing our sample is an oblate disk with only small degrees of triaxiality. The best-fit intrinsic magnetic field orientation is close to the direction of the shortest cloud axis, with small (~24 deg) deviations toward the long/middle cloud axes. However, due to the small number of observed clouds, the power of our analysis to reject alternative configurations is limited.Comment: 14 pages, 8 figures, accepted for publication in MNRA

Design and Initial Performance of SHARP, a Polarimeter for the SHARC-II Camera at the Caltech Submillimeter Observatory

We have developed a fore-optics module that converts the SHARC-II camera at the Caltech Submillimeter Observatory into a sensitive imaging polarimeter at wavelengths of 350 and 450 microns. We refer to this module as "SHARP". SHARP splits the incident radiation into two orthogonally polarized beams that are then re-imaged onto opposite ends of the 32 x 12 pixel detector array in SHARC-II. A rotating half-wave plate is used just upstream from the polarization-splitting optics. The effect of SHARP is to convert SHARC-II into a dual-beam 12 x 12 pixel polarimeter. A novel feature of SHARP's design is the use of a crossed grid in a submillimeter polarimeter. Here we describe the detailed optical design of SHARP and present results of tests carried out during our first few observing runs. At 350 microns, the beam size (9 arcseconds), throughput (75%), and instrumental polarization (< 1%) are all very close to our design goals.Comment: submitted to Applied Optic

Far-infrared polarimetry from the Stratospheric Observatory for Infrared Astronomy

Multi-wavelength imaging polarimetry at far-infrared wavelengths has proven to be an excellent tool for studying the physical properties of dust, molecular clouds, and magnetic fields in the interstellar medium. Although these wavelengths are only observable from airborne or space-based platforms, no first-generation instrument for the Stratospheric Observatory for Infrared Astronomy (SOFIA) is presently designed with polarimetric capabilities. We study several options for upgrading the High-resolution Airborne Wideband Camera (HAWC) to a sensitive FIR polarimeter. HAWC is a 12 x 32 pixel bolometer camera designed to cover the 53 - 215 micron spectral range in 4 colors, all at diffraction-limited resolution (5 - 21 arcsec). Upgrade options include: (1) an external set of optics which modulates the polarization state of the incoming radiation before entering the cryostat window; (2) internal polarizing optics; and (3) a replacement of the current detector array with two state-of-the-art superconducting bolometer arrays, an upgrade of the HAWC camera as well as polarimeter. We discuss a range of science studies which will be possible with these upgrades including magnetic fields in star-forming regions and galaxies and the wavelength-dependence of polarization.Comment: 12 pages, 5 figure