47,031 research outputs found
Temperature and light requirements for growth of two diatom species (Bacillariophyceae) isolated from an Arctic macroalga
In the present study, two abundant epiphyticdiatom taxa were isolated from the assimilation hairs ofthe brown macroalga Chordaria flagelliformis collected inthe Arctic Kongsfjorden (Spitsbergen, Norway), establishedas unialgal cultures and their growth rates determinedunder controlled photon fluence rate andtemperature conditions. Using morphological (light andscanning electron microscopy) and SSU rRNA gene databoth isolates (ROS D99 and ROS D125) were identifiedas members of a FragilariaSynedropsis clade. Themolecular data of ROS D99 and ROS D125 were notidentical to any other published sequence. While ROSD99 has been identified as Fragilaria barbararum mainlydue to the SEM characteristics, ROS D125 could not bedefinitely identified although morphological data speakfor Fragilaria striatula. Both diatom species showedsimilar growth rates at all temperatures and photon fluencerates tested. They grew well between 0 and 15Cwithoptimum temperatures of 1214C, but did not survive20C. Therefore, compared to Antarctic diatoms bothtaxa from Kongsfjorden can be characterised as eurythermalorganisms. Increasing photon fluence rates between2 and 15 lmol m2 s1 were accompanied with analmost twofold increase in growth rates, but photon fluencerates >15 lmol m2 s1 did not further enhancegrowth pointing to low light requirements. From thesedata optimum, minimum and maximum photon fluencerates and temperatures for growth can be assessed indicatingthat both diatoms are well acclimated to the fluctuatingenvironmental conditions in the Arctic habitat
Competitively tight graphs
The competition graph of a digraph is a (simple undirected) graph which
has the same vertex set as and has an edge between two distinct vertices
and if and only if there exists a vertex in such that
and are arcs of . For any graph , together with sufficiently
many isolated vertices is the competition graph of some acyclic digraph. The
competition number of a graph is the smallest number of such
isolated vertices. Computing the competition number of a graph is an NP-hard
problem in general and has been one of the important research problems in the
study of competition graphs. Opsut [1982] showed that the competition number of
a graph is related to the edge clique cover number of the
graph via . We first show
that for any positive integer satisfying , there
exists a graph with and characterize a graph
satisfying . We then focus on what we call
\emph{competitively tight graphs} which satisfy the lower bound, i.e.,
. We completely characterize the competitively tight
graphs having at most two triangles. In addition, we provide a new upper bound
for the competition number of a graph from which we derive a sufficient
condition and a necessary condition for a graph to be competitively tight.Comment: 10 pages, 2 figure
Strong electron correlations in cobalt valence tautomers
We have examined cobalt based valence tautomer molecules such as
Co(SQ)(phen) using density functional theory (DFT) and variational
configuration interaction (VCI) approaches based upon a model Hamiltonian. Our
DFT results extend earlier work by finding a reduced total energy gap (order
0.6 eV) between high temperature and low temperature states when we fully relax
the coordinates (relative to experimental ones). Futhermore we demonstrate that
the charge transfer picture based upon formal valence arguments succeeds
qualitatively while failing quantitatively due to strong covalency between the
Co 3 orbitals and ligand orbitals. With the VCI approach, we argue that
the high temperature, high spin phase is strongly mixed valent, with about 30 %
admixture of Co(III) into the predominantly Co(II) ground state. We confirm
this mixed valence through a fit to the XANES spectra. Moreover, the strong
electron correlations of the mixed valent phase provide an energy lowering of
about 0.2-0.3 eV of the high temperature phase relative to the low temperature
one. Finally, we use the domain model to account for the extraordinarily large
entropy and enthalpy values associated with the transition.Comment: 10 pages, 4 figures, submitted to J. Chem. Phy
Development and application of a self-referencing glucose microsensor for the measurement of glucose consumption by pancreatic ?-cells
Glucose gradients generated by an artificial source and ?-cells were measured using an enzyme-based glucose microsensor, 8-?m tip diameter, as a self-referencing electrode. The technique is based on a difference measurement between two locations in a gradient and thus allows us to obtain real-time flux values with minimal impact of sensor drift or noise. Flux values were derived by incorporation of the measured differential current into Fick's first equation. In an artificial glucose gradient, a flux detection limit of 8.2 ± 0.4 pmol·cm-2·s-1 (mean ± SEM, n = 7) with a sensor sensitivity of 7.0 ± 0.4 pA/mM (mean ± SEM, n = 16) was demonstrated. Under biological conditions, the glucose sensor showed no oxygen dependence with 5 mM glucose in the bulk medium. The addition of catalase to the bulk medium was shown to ameliorate surface-dependent flux distortion close to specimens, suggesting an underlying local accumulation of hydrogen peroxide. Glucose flux from ?-cell clusters, measured in the presence of 5 mM glucose, was 61.7 ± 9.5 fmol·nL-1·s-1 (mean ± SEM, n = 9) and could be pharmacologically modulated. Glucose consumption in response to FCCP (1 ?M) transiently increased, subsequently decreasing to below basal by 93 ± 16 and 56 ± 6%, respectively (mean ± SEM, n = 5). Consumption was decreased after the application of 10 ?M rotenone by 74 ± 5% (mean ± SEM, n = 4). These results demonstrate that an enzyme-based amperometric microsensor can be applied in the self-referencing mode. Further, in obtaining glucose flux measurements from small clusters of cells, these are the first recordings of the real-time dynamic of glucose movements in a biological microenvironment. <br/
New heat treatment to prepare high quality polycrystalline and single crystal MgB2 in single process
We report here on a new heat treatment to prepare both dense polycrystalline
and single crystal MgB2 high quality samples in one single process. Resistivity
measurements for polycrystalline part of the sample gives a residual
resistivity ratio RRR=16.6 and a very low normal state resistivity rho(40K)=
0.28 microOhmcm. Both SEM and SQUID study on polycrystals reveal the high
quality, dense character and well coupling of grain boundaries. On the other
hand, the high quality single crystals have a unique shape that resembles the
hexagonal crystal structure. SQUID measurements reveals very weak flux pinning
character implying our single crystals to be very clean. In this study, we
conclude that heat treatment is playing a major rule on the characteristics of
both polycrystalline and single crystal MgB2. Samples are thoroughly
characterized by x-ray, resistivity, dc SQUID and SEM
Quasi-particle scattering and protected nature of topological states in a parent topological insulator BiSe
We report on angle resolved photoemission spectroscopic studies on a parent
topological insulator (TI), BiSe. The line width of the spectral
function (inverse of the quasi-particle lifetime) of the topological metallic
(TM) states shows an anomalous behavior. This behavior can be reasonably
accounted for by assuming decay of the quasi-particles predominantly into bulk
electronic states through electron-electron interaction and defect scattering.
Studies on aged surfaces reveal that topological metallic states are very much
unaffected by the potentials created by adsorbed atoms or molecules on the
surface, indicating that topological states could be indeed protected against
weak perturbations.Comment: accepted for publication in Phys. Rev. B(R
Scanning tunneling spectroscopic studies of the pairing state of cuprate superconductors
Quasiparticle tunneling spectra of both hole-doped (p-type) and electron-doped (n-type) cuprates are studied using a low-temperature scanning tunneling microscope. The results reveal that neither the pairing symmetry nor the pseudogap phenomenon is universal among all cuprates, and that the response of n-type cuprates to quantum impurities is drastically different from that of the p-type cuprates. The only ubiquitous features among all cuprates appear to be the strong electronic correlation and the nearest-neighbor antiferromagnetic Cu2+-Cu2+ coupling in the CuO2 planes
Sequential Flavour Symmetry Breaking
The gauge sector of the Standard Model (SM) exhibits a flavour symmetry which
allows for independent unitary transformations of the fermion multiplets. In
the SM the flavour symmetry is broken by the Yukawa couplings to the Higgs
boson, and the resulting fermion masses and mixing angles show a pronounced
hierarchy. In this work we connect the observed hierarchy to a sequence of
intermediate effective theories, where the flavour symmetries are broken in a
step-wise fashion by vacuum expectation values of suitably constructed spurion
fields. We identify the possible scenarios in the quark sector and discuss some
implications of this approach.Comment: 22 pages latex, no figure
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