3,543 research outputs found
On the Formation of Multiple-Shells Around Asymptotic Giant Branch Stars
Two types of models for the formation of semi-periodic concentric multiple
shells (M-shells) around asymptotic giant branch (AGB) stars and in planetary
nebulae are compared against observations. Models that attribute the M-shells
to processes in an extended wind acceleration zone around AGB stars result in
an optically thick acceleration zone, which reduces the acceleration efficiency
in outer parts of the extended acceleration zone. This makes such models an
unlikely explanation for the formation of M-shells. Models which attribute the
M-shell to semi-periodic variation in one or more stellar properties are most
compatible with observations. The only stellar variation models on time scales
of 50-1500 years that have been suggested are based on an assumed solar-like
magnetic cycle. Although ad-hoc, the magnetic cycle assumption fits naturally
into the increasingly popular view that magnetic activity plays a role in
shaping the wind from upper AGB stars.Comment: 8 pages, Submitted to Ap
Quantitative Electron Probe X-Ray Microanalysis of Lead-Sequestering Organelles in Earthworms: Technical Appraisal of Air-Dried Smears and Freeze-Dried Cryosections
Much of the current knowledge of Pb sequestration in earthworm chloragosome granules stems from quantitative and semi-quantitative Electron Probe X-ray Microanalysis (EPXMA) of unfixed, air-dried tissue smears. Fully quantitative EPXMA was used in this study to assess the validity of smearing by comparison with unfixed, freeze-dried cryosections. Samples were obtained from two earthworm species, Lumbricus rubellus (L.r.) and Dendrobaena rubida (D.r.) collected from a Pb and Zn polluted soil (confirmed by XRF). A number of conclusions emerged :- (a) the element compositions of whole worms, determined by EPXMA of sprayed microdroplets of wet-digested tissues, were different (D.r. : Pb = 29.9, Zn= 33.9, Ca = 207.3, S = 387.9, P = 258.0; L.r. : Pb = 12.3, Zn = 44.4, Ca = 288.1, S = 311.8, P = 354. 9 mM/kg dr.w.); (b) the major proportions of the body burdens of Pb and Zn are located in the chloragogenous tissue; (c) cryosectioned chloragosomes possess a concentric sub-structure, and the chemical composition of individual granules and granule populations is very heterogeneous; (d) smeared and cryosectioned chloragosome preparations revealed significant species-differences in compositional chemistry (e. g., higher Pb and lower Zn in D.r., c.f. with L.r), which reflected their body burdens; (e) the absolute conc. values presented for L.r. cryosectioned granules are systematically higher than equivalent smear data; the overall patterns of the quantitative data obtained from smeared and cryosectioned chloragosomes were substantially similar, suggesting that the simpler technique can be recommended for certain applications, particularly where monitoring a large granule population is paramount
The Contribution of Electron Probe X-Ray Microanalysis (EPXMA) to Pollution Studies
This paper surveys the literature on the use of EPXMA in conjunction with (heavy metal) pollution studies.
Metal-accumulating compartments are usually granules or vacuoles of 6 main types (i) extracellular, soluble and fairly pure calcium carbonate granules; (ii) insoluble, calcium and phosphorus-rich granules, of ten containing several metal contaminants; (iii) multifunctional, calcium and phosphorus granules containing a high proportion of rather complex organic molecules; (iv) copper-(and sulphur) rich granules; (v) iron-(and phosphorus) rich granules; (vi) Cd, Zn, Cu and Hg -containing vacuoles, where the metals are presumably complexed with cysteine-rich metallothionein proteins, or their derivatives. With a few exceptions, types (ii)-(vi) are intracellular. Many of these inclusions may be associated with the lysosomal system, and the metals are sequestered by the various compartments due to their ligand-binding properties (i.e., whether they belong to Class A or Class B) and on the availability of pre-existing or inducible ligands within those compartments. Future biochemical analysis may change the categories presented here; for example, some type (iv) granules may well be found to contain metallothionein, and thus they may be more appropriately described as type (vi) inclusions.
It is concluded that EPXMA provides unique multi-element information concerning the metabolism of essential and toxic heavy metals within cells. However, this information may be considerably enhanced by the use of complementary experimental methods
In Vivo Metal Substitutions in Metal Sequestering Subcellular Compartments: X-Ray Mapping in Cryosections
Qualitative digital X-ray mapping techniques were employed to determine the distributions of essential and non-essential elements in three invertebrate models : (1) Pb, Zn, Cd, Cu, Fe in thin cryosections of the hepatopancreas of the terrestrial isopod, Oniscus asellus; (2) Pb, Zn, Cd, Ca in thin cryosections of the chloragogenous tissue of the earthworm, Lumbricus rubellus; and (3) As in air-dried smears and thin cryosections of chloragogen in L. rubellus. Four general conclusions were drawn from the results of these studies: (a) non-essential elements can accumulate, distribute and be compartmentalized because they, or the organo-complexes that they form, act as mimics of essential elements with which they share to a greater or lesser extent certain chemical affinities; (b) thermodynamic considerations notwithstanding, the influence of biological factors on the sequestration and fates of certain elements (e.g., arsenic) is profound through modifications of redox states and organo-compound formation; (c) X-ray mapping, combined with anhydrous preparative procedures, yields unbiased information concerning the relative spatial distributions of several elements in structurally heterogeneous sampling fields , although the morphological characterization of (occasionally unsuspected) subcellular compartments may be constrained by the intrinsic quality of the preparation; and (d) X-ray microanalysis yields codistribution data, when integrated with biochemical information from other sources, which give strong pointers to the identity of binding ligands and of the valence state of sequestered cations
Alterações nos estoques de carbono do solo após conversão de pastagem em plantios de eucalipto.
Resumo
Undisturbed Posidonia oceanica meadows maintain the epiphytic bacterial community in different environments
Seagrasses harbour different and rich epiphytic bacterial communities. These microbes may establish intimate and
symbiotic relationships with the seagrass plants and change according to host species, environmental conditions, and/or
ecophysiological status of their seagrass host. Although Posidonia oceanica is one of the most studied seagrasses in the world, and bacteria associated with seagrasses have been studied for over a decade, P. oceanica’s microbiome remains
hitherto little explored. Here, we applied 16S rRNA amplicon sequencing to explore the microbiome associated with the
leaves of P. oceanica growing in two geomorphologically different meadows (e.g. depth, substrate, and turbidity) within the Limassol Bay (Cyprus). The morphometric (leaf area, meadow density) and biochemical (pigments, total phenols) descriptors highlighted the healthy conditions of both meadows. The leaf-associated bacterial communities showed similar structure and composition in the two sites; core microbiota members were dominated by bacteria belonging to the Thalassospiraceae, Microtrichaceae, Enterobacteriaceae, Saprospiraceae, and Hyphomonadaceae families. This analogy, even under different geomorphological conditions, suggest that in the absence of disturbances, P. oceanica maintains characteristic-associated bacterial communities. This study provides a baseline for the knowledge of the P. oceanica microbiome and further supports its use as a putative seagrass descriptor
Analysis of nanopore detector measurements using Machine-Learning methods, with application to single-molecule kinetic analysis
<p>Abstract</p> <p>Background</p> <p>A nanopore detector has a nanometer-scale trans-membrane channel across which a potential difference is established, resulting in an ionic current through the channel in the pA-nA range. A distinctive channel current blockade signal is created as individually "captured" DNA molecules interact with the channel and modulate the channel's ionic current. The nanopore detector is sensitive enough that nearly identical DNA molecules can be classified with very high accuracy using machine learning techniques such as Hidden Markov Models (HMMs) and Support Vector Machines (SVMs).</p> <p>Results</p> <p>A non-standard implementation of an HMM, emission inversion, is used for improved classification. Additional features are considered for the feature vector employed by the SVM for classification as well: The addition of a single feature representing spike density is shown to notably improve classification results. Another, much larger, feature set expansion was studied (2500 additional features instead of 1), deriving from including all the HMM's transition probabilities. The expanded features can introduce redundant, noisy information (as well as diagnostic information) into the current feature set, and thus degrade classification performance. A hybrid Adaptive Boosting approach was used for feature selection to alleviate this problem.</p> <p>Conclusion</p> <p>The methods shown here, for more informed feature extraction, improve both classification and provide biologists and chemists with tools for obtaining a better understanding of the kinetic properties of molecules of interest.</p
Novel designs for Penning ion traps
We present a number of alternative designs for Penning ion traps suitable for
quantum information processing (QIP) applications with atomic ions. The first
trap design is a simple array of long straight wires which allows easy optical
access. A prototype of this trap has been built to trap Ca+ and a simple
electronic detection scheme has been employed to demonstrate the operation of
the trap. Another trap design consists of a conducting plate with a hole in it
situated above a continuous conducting plane. The final trap design is based on
an array of pad electrodes. Although this trap design lacks the open geometry
of the traps described above, the pad design may prove useful in a hybrid
scheme in which information processing and qubit storage take place in
different types of trap. The behaviour of the pad traps is simulated
numerically and techniques for moving ions rapidly between traps are discussed.
Future experiments with these various designs are discussed. All of the designs
lend themselves to the construction of multiple trap arrays, as required for
scalable ion trap QIP.Comment: 11 pages, 10 figure
Understanding and optimising the packing density of perylene bisimide layers on CVD-grown graphene
The non-covalent functionalisation of graphene is an attractive strategy to
alter the surface chemistry of graphene without damaging its superior
electrical and mechanical properties. Using the facile method of aqueous-phase
functionalisation on large-scale CVD-grown graphene, we investigated the
formation of different packing densities in self-assembled monolayers (SAMs) of
perylene bisimide derivatives and related this to the amount of substrate
contamination. We were able to directly observe wet-chemically deposited SAMs
in scanning tunnelling microscopy (STM) on transferred CVD graphene and
revealed that the densely packed perylene ad-layers adsorb with the conjugated
{\pi}-system of the core perpendicular to the graphene substrate. This
elucidation of the non-covalent functionalisation of graphene has major
implications on controlling its surface chemistry and opens new pathways for
adaptable functionalisation in ambient conditions and on the large scale.Comment: 27 pages (including SI), 10 figure
Meltwater Intrusions Reveal Mechanisms for Rapid Submarine Melt at a Tidewater Glacier
Submarine melting has been implicated as a driver of glacier retreat and sea level rise, but to date melting has been difficult to observe and quantify. As a result, melt rates have been estimated from parameterizations that are largely unconstrained by observations, particularly at the near-vertical termini of tidewater glaciers. With standard coefficients, these melt parameterizations predict that ambient
melting (the melt away from subglacial discharge outlets) is negligible compared to discharge-driven melting for typical tidewater glaciers. Here, we present new data from LeConte Glacier, Alaska, that challenges this paradigm. Using autonomous kayaks, we observe ambient meltwater intrusions that are ubiquitous within 400 m of the terminus, and we provide the first characterization of their properties, structure, and distribution. Our results suggest that ambient melt rates are substantially higher (Ă—100) than standard theory predicts and that ambient melting is a significant part of the total submarine melt flux. We explore modifications to the prevalent melt parameterization to provide a path forward for improved modeling of ocean-glacier interactions.This work was funded by NSF OPP Grants 1503910, 1504191, 1504288,
and 1504521 and National Geographic Grant CP4-171R-17. Additionally, this research was supported by the NOAA Climate and Global Change Postdoctoral Fellowship Program, administered by UCAR’s Cooperative Programs for the Advancement of Earth System Science (CPAESS) under award #NA18NWS4620043B. These observations would not be possible without the skilled engineering team who developed the autonomous kayaks—including Jasmine Nahorniak, June Marion, Nick McComb, Anthony Grana, and Corwin Perren—and also the Captain and crew of the M/V Amber Anne. We thank Donald Slater and an anonymous reviewer for valuable feedback that improved this manuscript. Data availability: All of the oceanographic data collected by ship and kayak have been archived with the National Centers for Environmental Information (Accession 0189574, https://accession.nodc.noaa.gov/ 0189574). The glacier data have been archived at the Arctic Data Center (https://doi.org/10.18739/A22G44).Ye
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