1,048 research outputs found
Inhibition of nitrogenase by oxygen in marine cyanobacteria controls the global nitrogen and oxygen cycles
International audienceCyanobacterial N2-fixation supplies the vast majority of biologically accessible inorganic nitrogen to nutrient-poor aquatic ecosystems. The process, catalyzed by the heterodimeric protein complex, nitrogenase, is thought to predate that of oxygenic photosynthesis. Remarkably, while the enzyme plays such a critical role in Earth's biogeochemical cycles, the activity of nitrogenase in cyanobacteria is markedly inhibited in vivo at a post-translational level by the concentration of O2 in the contemporary atmosphere leading to metabolic and biogeochemical inefficiency in N2 fixation. We illustrate this crippling effect with data from Trichodesmium spp. an important contributor of "new nitrogen" to the world's subtropical and tropical oceans. The enzymatic inefficiency of nitrogenase imposes a major elemental taxation on diazotrophic cyanobacteria both in the costs of protein synthesis and for scarce trace elements, such as iron. This restriction has, in turn, led to a global limitation of fixed nitrogen in the contemporary oceans and provides a strong biological control on the upper bound of oxygen concentration in Earth's atmosphere
Holography, Pade Approximants and Deconstruction
We investigate the relation between holographic calculations in 5D and the
Migdal approach to correlation functions in large N theories. The latter
employs Pade approximation to extrapolate short distance correlation functions
to large distances. We make the Migdal/5D relation more precise by quantifying
the correspondence between Pade approximation and the background and boundary
conditions in 5D. We also establish a connection between the Migdal approach
and the models of deconstructed dimensions.Comment: 28 page
Neutron-Electron EDM Correlations in Supersymmetry and Prospects for EDM Searches
Motivated by recent progress in experimental techniques of electric dipole
moment (EDM) measurements, we study correlations between the neutron and
electron EDMs in common supersymmetric models. These include minimal
supergravity (mSUGRA) with small CP phases, mSUGRA with a heavy SUSY spectrum,
the decoupling scenario and split SUSY. In most cases, the electron and neutron
EDMs are found to be observable in the next round of EDM experiments. They
exhibit certain correlation patterns. For example, if d_n ~ 10^{-27} e cm is
found, d_e is predicted to lie in the range 10^{-28}-10^{-29} e cm.Comment: 16 pages,12 figures. To appear in JHEP. A note on stability of the
correlations added in Conclusions; refs. and footnotes adde
Moduli stabilization with positive vacuum energy
We study the effect of anomalous U(1) gauge groups in string theory
compactification with fluxes. We find that, in a gauge invariant formulation,
consistent AdS vacua appear breaking spontaneously supergravity. Non vanishing
D-terms from the anomalous symmetry act as an uplifting potential and could
allow for de Sitter vacua. However, we show that in this case the gravitino is
generically (but not always) much heavier than the electroweak scale. We show
that alternative uplifting scheme based on corrections to the Kahler potential
can be compatible with a gravitino mass in the TeV range.Comment: 20 pages, 1 figur
Supersymmetric Boundaries and Junctions in Four Dimensions
We make a comprehensive study of (rigid) N=1 supersymmetric sigma-models with
general K\"ahler potentials K and superpotentials w on four-dimensional
space-times with boundaries. We determine the minimal (non-supersymmetric)
boundary terms one must add to the standard bulk action to make it off-shell
invariant under half the supersymmetries without imposing any boundary
conditions. Susy boundary conditions do arise from the variational principle
when studying the dynamics. Upon including an additional boundary action that
depends on an arbitrary real boundary potential B one can generate very general
susy boundary conditions. We show that for any set of susy boundary conditions
that define a Lagrangian submanifold of the K\"ahler manifold, an appropriate
boundary potential B can be found. Thus the non-linear sigma-model on a
manifold with boundary is characterised by the tripel (K,B,w). We also discuss
the susy coupling to new boundary superfields and generalize our results to
supersymmetric junctions between completely different susy sigma-models, living
on adjacent domains and interacting through a "permeable" wall. We obtain the
supersymmetric matching conditions that allow us to couple models with
different K\"ahler potentials and superpotentials on each side of the wall.Comment: 38 pages, 1 figur
Interesting consequences of brane cosmology
We discuss cosmology in four dimensions within a context of brane-world
scenario.Such models can predict chaotic inflation with very low reheat
temperature depending on the brane tension. We notice that the gravitino
abundance is different in the brane-world cosmology and by tuning the brane
tension it is possible to get extremely low abundance. We also study
Affleck-Dine baryogenesis in our toy model.Comment: 5 pages, Trivial changes to match the published versio
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Molecular bases and photobiological consequences of light intensity adaptation in photosynthetic organisms
By applying a combination of light transitions, uncouplers, and inhibitors of photosynthetic electron transport inhibitors we modulate the redox poise of many components in the plastid and examine the pattern of expression of cab1 gene. This gene encodes the major light harvesting protein that services photosystem II. While our results have confirmed our own previous finding that light intensity regulation of cab1 gene expression is signaled by the redox state of the PQ pool, we have also identified additional sensor(s) located in the PET chain
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