20,542 research outputs found
CO2 perturbation experiments: similarities and differences between dissolved inorganic carbon and total alkalinity manipulations
Increasing atmospheric carbon dioxide (CO2) through human activities and invasion of anthropogenic CO2 into the surface ocean alters the seawater carbonate chemistry, increasing CO2 and bicarbonate (HCO3−) at the expense of carbonate ion (CO32−) concentrations. This redistribution in the dissolved inorganic carbon (DIC) pool decreases pH and carbonate saturation state (Ω). Several components of the carbonate system are considered potential key variables influencing for instance calcium carbonate precipitation in marine calcifiers such as coccolithophores, foraminifera, corals, mollusks and echinoderms. Unravelling the sensitivities of marine organisms and ecosystems to CO2 induced ocean acidification (OA) requires well-controlled experimental setups and accurate carbonate system manipulations. Here we describe and analyse the chemical changes involved in the two basic approaches for carbonate chemistry manipulation, i.e. changing DIC at constant total alkalinity (TA) and changing TA at constant DIC. Furthermore, we briefly introduce several methods to experimentally manipulate DIC and TA. Finally, we examine responses obtained with both approaches using published results for the coccolithophore Emiliania huxleyi. We conclude that under most experimental conditions in the context of ocean acidification DIC and TA manipulations yield similar changes in all parameters of the carbonate system, which implies direct comparability of data obtained with the two basic approaches for CO2 perturbation
Elastic and plastic effects on heterogeneous nucleation and nanowire formation
We investigate theoretically the effects of elastic and plastic deformations
on heterogeneous nucleation and nanowire formation. In the first case, the
influence of the confinement of the critical nucleus between two parallel
misfitting substrates is investigated using scaling arguments. We present phase
diagrams giving the nature of the nucleation regime as a function of the
driving force and the degree of confinement. We complement this analytical
study by amplitude equations simulations. In the second case, the influence of
a screw dislocation inside a nanowire on the development of the morphological
surface stability of the wire, related to the Rayleigh-Plateau instability, is
examined. Here the screw dislocation provokes a torsion of the wire known as
Eshelby twist. Numerical calculations using the finite element method and the
amplitude equations are performed to support analytical investigations. It is
shown that the screw dislocation promotes the Rayleigh-Plateau instability.Comment: 16 page
Mapping the neutral atomic hydrogen gas outflow in the restarted radio galaxy 3C 236
The energetic feedback that is generated by radio jets in active galactic
nuclei (AGNs) has been suggested to be able to produce fast outflows of atomic
hydrogen (HI) gas that can be studied in absorption at high spatial resolution.
We have used the Very Large Array (VLA) and a global
very-long-baseline-interferometry (VLBI) array to locate and study in detail
the HI outflow discovered with the Westerbork Synthesis Radio Telescope (WSRT)
in the re-started radio galaxy 3C 236. We confirm, from the VLA data, the
presence of a blue-shifted wing of the HI with a width of
. This HI outflow is partially recovered by the
VLBI observation. In particular, we detect four clouds with masses of
with VLBI that do not follow the regular
rotation of most of the HI. Three of these clouds are located, in projection,
against the nuclear region on scales of , while the
fourth is co-spatial to the south-east lobe at a projected distance of
. Their velocities are between and
blue-shifted with respect to the velocity of the
disk-related HI. These findings suggest that the outflow is at least partly
formed by clouds, as predicted by some numerical simulations and originates
already in the inner (few tens of pc) region of the radio galaxy. Our results
indicate that all of the outflow could consist of many clouds with perhaps
comparable properties as the ones detected, distributed also at larger radii
from the nucleus where the lower brightness of the lobe does not allow us to
detect them. However, we cannot rule out the presence of a diffuse component of
the outflow. The fact that 3C 236 is a low excitation radio galaxy, makes it
less likely that the optical AGN is able to produce strong radiative winds
leaving the radio jet as the main driver for the HI outflow.Comment: 13 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Phase diagram of the one dimensional Hubbard-Holstein Model at 1/2 and 1/4 filling
The Hubbard-Holstein model is one of the simplest to incorporate both
electron-electron and electron-phonon interactions. In one dimension at half
filling the Holstein electron-phonon coupling promotes onsite pairs of
electrons and a Peierls charge density wave while the Hubbard onsite Coulomb
repulsion U promotes antiferromagnetic correlations and a Mott insulating
state. Recent numerical studies have found a possible third intermediate phase
between Peierls and Mott states. From direct calculations of charge and spin
susceptibilities, we show that (i) As the electron-phonon coupling is
increased, first a spin gap opens, followed by the Peierls transition. Between
these two transitions the metallic intermediate phase has a spin gap, no charge
gap, and properties similar to the negative-U Hubbard model. (ii) The
transitions between Mott/intermediate and intermediate/Peierls states are of
the Kosterlitz-Thouless form. (iii) For larger U the two transitions merge at a
tritical point into a single first order Mott/Peierls transition. In addition
we show that an intermediate phase also occurs in the quarter-filled model.Comment: 10 pages, 10 eps figure
On the metal-insulator transition in the two-chain model of correlated fermions
The doping-induced metal-insulator transition in two-chain systems of
correlated fermions is studied using a solvable limit of the t-J model and the
fact that various strong- and weak-coupling limits of the two-chain model are
in the same phase, i.e. have the same low-energy properties. It is shown that
the Luttinger-liquid parameter K_\rho takes the universal value unity as the
insulating state (half-filling) is approached, implying dominant d-type
superconducting fluctuations, independently of the interaction strength. The
crossover to insulating behavior of correlations as the transition is
approached is discussed.Comment: 7 pages, 1 figur
Image processing applied to gravity and topography data covering the continental United States
The applicability of fairly standard image processing techniques to processing and analyzing large geologic data sets in addressed. Image filtering techniques were used to interpolate between gravity station locations to produce a regularly spaced data array that preserves detail in areas with good coverage, and that produces a continuous tone image rather than a contour map. Standard image processing techniques were used to digitally register and overlay topographic and gravity data, and the data were displayed in ways that emphasize subtle but pervasive structural features. The potential of the methods is illustrated through a discussion of linear structures that appear in the processed data between the midcontinent gravity high and the Appalachians
CasTuner: a degron and CRISPR/Cas-based toolkit for analog tuning of endogenous gene expression
Certain cellular processes are dose-dependent, requiring a specific quantity of gene products or a defined stoichiometry between them. This is exemplified by haploinsufficiency or by the need for dosage compensation for X-linked genes between the sexes in many species. Understanding dosage-sensitive processes requires the ability to perturb endogenous gene products in a quantitative manner. Here we present CasTuner, a CRISPR-based toolkit that allows analog tuning of endogenous gene expression. In the CasTuner system, activity of Cas-derived repressors is controlled through a FKBP12F36V degron domain and can thereby be quantitatively tuned by titrating the small molecule degrader dTAG-13. The toolkit can be applied at the transcriptional level, using the histone deacetylase hHDAC4 fused to dCas9, or at the post-transcriptional level, using the RNA-targeting CasRx. To optimise efficiency, inducibility and homogeneity of repression we target a fluorescently tagged endogenous gene, Esrrb, in mouse embryonic stem cells. Through flow cytometry, we show that CasTuner allows analog tuning of the target gene in a homogeneous manner across cells, as opposed to the widely used KRAB repressor domain, which exhibits a digital mode of action. We quantify repression and derepression dynamics for CasTuner and use it to measure dose-response curves between the pluripotency factor NANOG and several of its target genes, providing evidence for target-specific dose dependencies. CasTuner thus provides an easy-to-implement tool to perturb gene expression in an inducible, tunable and reversible manner and will be useful to study dose-responsive processes within their physiological contex
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