255 research outputs found
Microbial ecology of extreme environments: Antarctic dry valley yeasts and growth in substrate-limited habitats
The success of the Antarctic Dry Valley yeasts presumeably results from adaptations to multiple stresses, to low temperatures and substrate-limitation as well as prolonged resting periods enforced by low water availability. Previous investigations have suggested that the crucial stress is substrate limitation. Specific adaptations may be pinpointed by comparing the physiology of the Cryptococcus vishniacii complex, the yeasts of the Tyrol Valley, with their congeners from other habitats. Progress was made in methods of isolation and definition of ecological niches, in the design of experiments in competition for limited substrate, and in establishing the relationships of the Cryptococcus vishniacii complex with other yeasts. In the course of investigating relationships, a new method for 25SrRNA homology was developed. For the first time it appears that 25SrRNA homology may reflect parallel or convergent evolution
Microbial ecology of extreme environments: Antarctic dry valley yeasts and growth in substrate limited habitats
The multiple stresses temperature, moisture, and for chemoheterotrophs, sources of carbon and energy of the Dry Valley Antarctica soils allow at best depauperate communities, low in species diversity and population density. The nature of community structure, the operation of biogeochemical cycles, the evolution and mechanisms of adaptation to this habitat are of interest in informing speculations upon life on other planets as well as in modeling the limits of gene life. Yeasts of the Cryptococcus vishniacil complex (Basidiobiastomycetes) are investigated, as the only known indigenes of the most hostile, lichen free, parts of the Dry Valleys. Methods were developed for isolating these yeasts (methods which do not exclude the recovery of other microbiota). The definition of the complex was refined and the importance of nitrogen sources was established as well as substrate competition in fitness to the Dry Valley habitats
Microbial ecology of extreme environments: Antarctic yeasts and growth in substrate-limited habitats
An extreme environment is by definition one with a depauperate biota. While the Ross Desert is by no means homogeneous, the most exposed and arid habitats, soils in the unglaciated high valleys, do indeed contain a very sparse biota of low diversity. So sparse that the natives could easily be outnumbered by airborne exogenous microbes. Native biota must be capable of overwintering as well as growing in the high valley summer. Tourists may undergo a few divisions before contributing their enzymes and, ultimately, elements to the soil - or may die before landing. The simplest way to demonstrate the indigenicity of a particular microbe is therefore to establish unique distribution; occurrence only in the habitat in question precludes foreign origin
Reconnection in a Weakly Stochastic Field
We examine the effect of weak, small scale magnetic field structure on the
rate of reconnection in a strongly magnetized plasma. This affects the rate of
reconnection by reducing the transverse scale for reconnection flows, and by
allowing many independent flux reconnection events to occur simultaneously.
Allowing only for the first effect and using Goldreich and Sridhar's model of
strong turbulence in a magnetized plasma with negligible intermittency, we find
that the lower limit for the reconnection speed is the Alfven speed times the
Lundquist number to the power (-3/16). The upper limit on the reconnection
speed is typically a large fraction of Alfven speed. We argue that generic
reconnection in turbulent plasmas will normally occur at close to this upper
limit. The fraction of magnetic energy that goes directly into electron heating
scales as Lundquist number to the power (-2/5) and the thickness of the current
sheet scales as the Lundquist number to the power (-3/5). A significant
fraction of the magnetic energy goes into high frequency Alfven waves. We claim
that the qualitative sense of these conclusions, that reconnection is fast even
though current sheets are narrow, is almost independent of the local physics of
reconnection and the nature of the turbulent cascade. As the consequence of
this the Galactic and Solar dynamos are generically fast, i.e. do not depend on
the plasma resistivity.Comment: Extended version accepted to ApJ, 44pages, 2 figure
Simulations of MHD Turbulence in a Strongly Magnetized Medium
We analyze 3D numerical simulations of driven incompressible
magnetohydrodynamic (MHD) turbulence in a periodic box threaded by a moderately
strong external magnetic field. We sum over nonlinear interactions within
Fourier wavebands and find that the time scale for the energy cascade is
consistent with the Goldreich-Sridhar model of strong MHD turbulence. Using
higher order longitudinal structure functions we show that the turbulent
motions in the plane perpendicular to the local mean magnetic field are similar
to ordinary hydrodynamic turbulence while motions parallel to the field are
consistent with a scaling correction which arises from the eddy anisotropy. We
present the structure tensor describing velocity statistics of Alfvenic and
pseudo-Alfvenic turbulence. Finally, we confirm that an imbalance of energy
moving up and down magnetic field lines leads to a slow decay of turbulent
motions and speculate that this imbalance is common in the interstellar medium
where injection of energy is intermittent both in time and space.Comment: ApJ accepted, 29 pages, 10 figures; some revisions, new figure
Self-gravitating fluid shells and their non-spherical oscillations in Newtonian theory
We summarize the general formalism describing surface flows in
three-dimensional space in a form which is suitable for various astrophysical
applications. We then apply the formalism to the analysis of non-radial
perturbations of self-gravitating spherical fluid shells.
Spherically symmetric gravitating shells (or bubbles) have been used in
numerous model problems especially in general relativity and cosmology. A
radially oscillating shell was recently suggested as a model for a variable
cosmic object. Within Newtonian gravity we show that self-gravitating static
fluid shells are unstable with respect to linear non-radial perturbations. Only
shells (bubbles) with a negative mass (or with a charge the repulsion of which
is compensated by a tension) are stable.Comment: 20 pages, to be published in the Astrophysical Journal, typos
correcte
An Incoherent Dynamo in Accretion Disks
We use the mean-field dynamo equations to show that an incoherent alpha
effect in mirror-symmetric turbulence in a shearing flow can generate a large
scale, coherent magnetic field. We illustrate this effect with simulations of a
few simple systems. In accretion disks, this process can lead to axisymmetric
magnetic domains whose radial and vertical dimensions will be comparable to the
disk height. This process may be responsible for observations of dynamo
activity seen in simulations of dynamo-generated turbulence involving, for
example, the Balbus-Hawley instability. In this case the magnetic field
strength will saturate at times the ambient pressure in real
accretion disks. The resultant dimensionless viscosity will be of the same
order. In numerical simulations the azimuthal extent of the simulated annulus
should be substituted for . We compare the predictions of this model to
numerical simulations previously reported by Brandenburg et al. (1995). In a
radiation pressure dominated environment this estimate for viscosity should be
reduced by a factor of due to magnetic buoyancy.Comment: 23 pages, uses aaste
Tight coupling expansion and fully inhomogeneous magnetic fields
The tight coupling expansion, appropriately generalized to include
large-scale magnetic fields, allows the estimate of the brightness
perturbations of CMB anisotropies for typical wavelengths that are larger than
the Hubble radius after matter-radiation equality. After discussing the basic
features of the the pre-decoupling initial conditions in the presence of fully
inhomogeneous magnetic fields, the tight coupling expansion is studied both
analytically and numerically. From the requirement that the amplitudes and
phases of Sakharov oscillations are (predominantly) adiabatic and from the
inferred value of the plateau in the temperature autocorrelation, the effects
of the magnetized contribution can be systematically investigated and
constrained.Comment: 30 pages, 14 figure
Prediction of bioactive compounds activity against wood contaminant fungi using artificial neural networks
Biopesticides based on natural endophytic bacteria to control plant diseases are an ecological alternative to the chemical treatments. Bacillus species produce a wide variety of metabolites with biological activity like iturinic lipopeptides. This work addresses the production of biopesticides based on natural endophytic bacteria, isolated from Quercus suber. Artificial Neural Networks were used to maximize the percentage of inhibition triggered by antifungal activity of bioactive compounds produced by Bacillus amyloliquefaciens. The active compounds, produced in liquid cultures, inhibited the growth of fifteen fungi and exhibited a broader spectrum of antifungal activity against surface contaminant fungi, blue stain fungi and phytopathogenic fungi. A 19-7-6-1 neural network was selected to predict the percentage of inhibition produced by antifungal bioactive compounds. A good match among the observed and predicted values was obtained with the R2 values varying between 0.9965 â 0.9971 and 0.9974 â 0.9989 for training and test sets. The 19-7-6-1 neural network was used to establish the dilution rates that maximize the production of antifungal bioactive compounds, namely 0.25 h-1 for surface contaminant fungi, 0.45 h-1 for blue stain fungi and between 0.30 and 0.40 h-1 for phytopathogenic fungi. Artificial neural networks show great potential in the modelling and optimization of these bioprocesses.Les biopesticides Ă base de bactĂ©ries endophytes naturelles pour lutter contre les maladies des plantes constituent une
alternative écologique aux traitements chimiques. Les espÚces de Bacillus produisent une grande variété de métabolites biologiquement
actifs tels que les lipopeptides ituriniques. Cette étude porte sur la production de biopesticides par des bactéries
endophytes naturelles isolées du Quercus suber L. Des réseaux neuronaux artificiels ont été utilisés pour maximiser le pourcentage
dâinhibition provoquĂ©e par lâactivitĂ© antifongique des composĂ©s bioactifs produits par Bacillus amyloliquefaciens. Les composĂ©s
actifs, produits en culture liquide, ont inhibĂ© la croissance de 15 champignons et avaient un spectre dâactivĂ© antifongique plus
large contre les contaminants fongiques de surface, les champignons de bleuissement et les champignons phytopathogĂšnes. Un
rĂ©seau neuronal 19-7-6-1 a Ă©tĂ© choisi pour prĂ©dire le pourcentage dâinhibition produit par les composĂ©s bioactifs antifongiques.
Une bonne concordance entre les valeurs observées et prédites a été obtenue; les valeurs de R2 variaient de 0,9965 a` 0,9971 et de
0,9974 a` 0,9989 pour les bases dâapprentissage et de test. Le rĂ©seau neuronal 19-7-6-1 a Ă©tĂ© utilisĂ© pour Ă©tablir les taux de dilution
qui maximisent la production des composĂ©s bioactifs antifongiques, nommĂ©ment 0,25 hâ1 pour les contaminants fongiques de
surface, 0,45 hâ1 pour les champignons de bleuissement et entre 0,30 et 0,40 hâ1 pour les champignons phytopathogĂšnes. Les
réseaux neuronaux artificiels ont un potentiel élevé pour modéliser et optimiser ces processus biologiques
Next-to-leading resummation of cosmological perturbations via the Lagrangian picture: 2-loop correction in real and redshift spaces
We present an improved prediction of the nonlinear perturbation theory (PT)
via the Lagrangian picture, which was originally proposed by Matsubara (2008).
Based on the relations between the power spectrum in standard PT and that in
Lagrangian PT, we derive analytic expressions for the power spectrum in
Lagrangian PT up to 2-loop order in both real and redshift spaces. Comparing
the improved prediction of Lagrangian PT with -body simulations in real
space, we find that the 2-loop corrections can extend the valid range of wave
numbers where we can predict the power spectrum within 1% accuracy by a factor
of 1.0 (), 1.3 (1), 1.6 (2) and 1.8 (3) vied with 1-loop Lagrangian PT
results. On the other hand, in all redshift ranges, the higher-order
corrections are shown to be less significant on the two-point correlation
functions around the baryon acoustic peak, because the 1-loop Lagrangian PT is
already accurate enough to explain the nonlinearity on those scales in -body
simulations.Comment: 18pages, 4 figure
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