1,389 research outputs found
Factors Affecting Development of Peroxisomes and Glycolate Metabolism among Algae of Different Evolutionary Lines of the Prasinophyceae
Microstructure of nanocrystalline diamond powders studied by powder diffractometry
High resolution x-ray diffraction peaks of diamond nanosize powders of nominal sizes ranging from 5 to 250 nm were analyzed and provided information on grain structure, average size of crystallites, and concentration of dislocations. Selected samples were heat treated at 1670 K at pressures 2.0 and 5.5 GPa or had surface modified by outgassing, heat treatment at vacuum conditions, and by controlled adsorption of gases. The apparent lattice parameter method was applied to characterize the structure of a shell-core model of nanosize particles. The multiple whole profile fitting provided information on crystallite sizes and density of dislocations. Population of dislocations increased with applied pressure, while strain and interplanar distances in the surface layers decreased. Adsorption of foreign gases on the grain surface modified the structure of the surface layers but did not affect dislocations near the center of the grains
The Role of a Hot Gas Environment on the Evolution of Galaxies
Most spiral galaxies are found in galaxy groups with low velocity
dispersions; most E/S0 galaxies are found in galaxy groups with relatively high
velocity dispersions. The mass of the hot gas we can observe in the E/S0 groups
via their thermal X-ray emission is, on average, as much as the baryonic mass
of the galaxies in these groups. By comparison, galaxy clusters have as much or
more hot gas than stellar mass. Hot gas in S-rich groups, however, is of low
enough temperature for its X-ray emission to suffer heavy absorption due to
Galactic HI and related observational effects, and hence is hard to detect. We
postulate that such lower temperature hot gas does exist in low velocity
dispersion, S-rich groups, and explore the consequences of this assumption. For
a wide range of metallicity and density, hot gas in S-rich groups can cool in
far less than a Hubble time. If such gas exists and can cool, especially when
interacting with HI in existing galaxies, then it can help link together a
number of disparate observations, both Galactic and extragalactic, that are
otherwise difficult to understand.Comment: 16 pages with one figure. ApJ Letters, in pres
State Transformations and Ice Nucleation in Glassy or (Semi-) Solid Amorphous Organic Aerosol
Glassy or amorphous (semi-)solid organic aerosol particles have the potential to serve as surfaces for heterogeneous ice nucleation in cirrus clouds. Raman spectroscopy and optical microscopy have been used in conjunction with a cold stage to examine water uptake and ice nucleation on individual aqueous organic glass particles at atmospherically relevant temperatures (200–273 K). Three organic compounds considered proxies for atmospheric secondary organic aerosol (SOA) were used in this investigation: sucrose, citric acid and glucose. Internally mixed particles consisting of each organic species and ammonium sulfate were also investigated.
Results from water uptake experiments were used to construct glass transition curves and state diagrams for each organic and corresponding mixture. A unique glass transition point on each state diagram, Tg\u27, was used to quantify and compare results from this study to previous works. Values of Tg\u27 determined for aqueous sucrose, glucose and citric acid glasses were 236 K, 230 K and 220 K, respectively. Values of Tg\u27 for internally mixed organic/sulfate particles were always significantly lower; 210 K, 207K and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate, respectively.
All investigated organic species were observed to serve as heterogeneous ice nuclei at tropospheric temperatures. Heterogeneous ice nucleation on pure organic particles occurred at Sice = 1.1–1.4 for temperatures between 235K and 200 K. Particles consisting of 1 : 1 organic-sulfate mixtures remained liquid over a greater range of conditions but were in some cases also observed to depositionally nucleate ice at temperatures below 202 K (Sice = 1.25–1.38).
Glass transition curves constructed from experimental data were incorporated into the Community Aerosol Radiation Model for Atmospheres (CARMA) along with the predicted range of glass transition temperatures for atmospheric SOA from Koop et al. (2011). Model results suggest that organic and organic/sulfate aerosol will be glassy more than 60% of the time in the midlatitude upper troposphere and more than 40% of the time in the tropical tropopause region (TTL). At conditions favorable for ice formation (Sice \u3e 1), particles in the TTL are expected to be glassy more than 50% of the time for temperatures below 200 K. Combined with the low saturation ratios measured for ice nucleation, this work suggests heterogeneous ice formation on glassy substances may play an important role in cirrus cloud formation
Reproductive toxins and alligator abnormalities at Lake Apopka, Florida.
The alligator population at Lake Apopka in central Florida declined dramatically between 1980 and 1987. Endocrine-disrupting chemicals and specifically DDT metabolites have been implicated in the alligators' reproductive failure. The DDT metabolite hypothesis is based largely on the observation of elevated concentrations of p,p-DDE and p,p-DDD in alligator eggs obtained from Lake Apopka in 1984 and 1985. In the following commentary, we draw attention to two nematocides that are established reproductive toxins in humans, dibromochloropropane (DBCP) and ethylene dibromide (EDB), which could also have played a role in the reproductive failure observed in alligators from Lake Apopka in the early 1980s
State Transformations and Ice Nucleation in Amorphous (Semi-) Solid Organic Aerosol
Amorphous (semi-)solid organic aerosol particles have the potential to serve as surfaces for heterogeneous ice nucleation in cirrus clouds. Raman spectroscopy and optical microscopy have been used in conjunction with a cold stage to examine water uptake and ice nucleation on individual amorphous (semi-)solid particles at atmospherically relevant temperatures (200–273 K). Three organic compounds considered proxies for atmospheric secondary organic aerosol (SOA) were used in this investigation: sucrose, citric acid and glucose. Internally mixed particles consisting of each organic and ammonium sulfate were also investigated.
Results from water uptake experiments followed the shape of a humidity-induced glass transition (Tg(RH)) curve and were used to construct state diagrams for each organic and corresponding mixture. Experimentally derived Tg(RH) curves are in good agreement with theoretical predictions of Tg(RH) following the approach of Koop et al. (2011). A unique humidity-induced glass transition point on each state diagram, Tg\u27(RH), was used to quantify and compare results from this study to previous works. Values of Tg\u27(RH) determined for sucrose, glucose and citric acid glasses were 236, 230 and 220 K, respectively. Values of Tg\u27(RH) for internally mixed organic/sulfate particles were always significantly lower; 210, 207 and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate, respectively.
All investigated SOA proxies were observed to act as heterogeneous ice nuclei at tropospheric temperatures. Heterogeneous ice nucleation on pure organic particles occurred at Sice = 1.1–1.4 for temperatures below 235 K. Particles consisting of 1:1 organic-sulfate mixtures took up water over a greater range of conditions but were in some cases also observed to heterogeneously nucleate ice at temperatures below 202 K (Sice= 1.25–1.38).
Polynomial curves were fitted to experimental water uptake data and then incorporated into the Community Aerosol Radiation Model for Atmospheres (CARMA) along with the predicted range of humidity-induced glass transition temperatures for atmospheric SOA from Koop et al. (2011). Model results suggest that organic and organic/sulfate aerosol could be glassy more than 60% of the time in the midlatitude upper troposphere and more than 40% of the time in the tropical tropopause region (TTL). At conditions favorable for ice formation (Sice \u3e 1), particles in the TTL are expected to be glassy more than 50% of the time for temperatures below 200 K. Results from this study suggests that amorphous (semi-)solid organic particles are often present in the upper troposphere and that heterogeneous ice formation on this type of particle may play an important role in cirrus cloud formation
Astrocytic glutamate transport regulates a Drosophila CNS synapse that lacks astrocyte ensheathment.
Anatomical, molecular, and physiological interactions between astrocytes and neuronal synapses regulate information processing in the brain. The fruit fly Drosophila melanogaster has become a valuable experimental system for genetic manipulation of the nervous system and has enormous potential for elucidating mechanisms that mediate neuron-glia interactions. Here, we show the first electrophysiological recordings from Drosophila astrocytes and characterize their spatial and physiological relationship with particular synapses. Astrocyte intrinsic properties were found to be strongly analogous to those of vertebrate astrocytes, including a passive current-voltage relationship, low membrane resistance, high capacitance, and dye-coupling to local astrocytes. Responses to optogenetic stimulation of glutamatergic premotor neurons were correlated directly with anatomy using serial electron microscopy reconstructions of homologous identified neurons and surrounding astrocytic processes. Robust bidirectional communication was present: neuronal activation triggered astrocytic glutamate transport via excitatory amino acid transporter 1 (Eaat1), and blocking Eaat1 extended glutamatergic interneuron-evoked inhibitory postsynaptic currents in motor neurons. The neuronal synapses were always located within 1 μm of an astrocytic process, but none were ensheathed by those processes. Thus, fly astrocytes can modulate fast synaptic transmission via neurotransmitter transport within these anatomical parameters. J. Comp. Neurol. 524:1979-1998, 2016. © 2016 Wiley Periodicals, Inc.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1002/cne.2401
Instrumental oscillations in RHESSI count rates during solar flares
Aims: We seek to illustrate the analysis problems posed by RHESSI spacecraft
motion by studying persistent instrumental oscillations found in the
lightcurves measured by RHESSI's X-ray detectors in the 6-12 keV and 12-25 keV
energy range during the decay phase of the flares of 2004 November 4 and 6.
Methods: The various motions of the RHESSI spacecraft which may contribute to
the manifestation of oscillations are studied. The response of each detector in
turn is also investigated. Results: We find that on 2004 November 6 the
observed oscillations correspond to the nutation period of the RHESSI
instrument. These oscillations are also of greatest amplitude for detector 5,
while in the lightcurves of many other detectors the oscillations are small or
undetectable. We also find that the variation in detector pointing is much
larger during this flare than the counterexample of 2004 November 4.
Conclusions: Sufficiently large nutation motions of the RHESSI spacecraft lead
to clearly observable oscillations in count rates, posing a significant hazard
for data analysis. This issue is particularly problematic for detector 5 due to
its design characteristics. Dynamic correction of the RHESSI counts, accounting
for the livetime, data gaps, and the transmission of the bi-grid collimator of
each detector, is required to overcome this issue. These corrections should be
applied to all future oscillation studies.Comment: 8 pages, 10 figure
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