5,670 research outputs found
Understanding Marine Microbes, the Driving Engines of the Ocean
When you hear the word microbes, what comes to your mind? Something much too small to see and that makes you fall ill? Just because some microbes cause diseases that does not mean they are all evil. For example, in the marine (ocean) environment, the vast majority of microbes are good ones. They are the âdriving enginesâ of the ocean and are essential for the health of our whole planet. Unfortunately, most of the marine microbes and their interactions with the marine environment are poorly understood. So, it is important to get an idea of which microbes are helping us and how they are doing this. These data will provide scientists with the knowledge to fight against big global challenges, such as climate change and environmental pollution. Unfortunately, it is very hard to study marine microbes due to their microscopic size, huge diversity, and their big home â the ocean. Therefore, we would like to engage âcitizen scientistsâ in this project to help us to sample marine microbes so that we can identify them
Hydration and the true water content of swellable clay minerals
Water affects biological, chemical and transportation processes as well as mechanical properties of soils. Thereby, clay mineral content determines the moisture balance of soils. In-situ moisture measurements depend on reliable calibration based on the true water content. Drying the soil at 105 °C is the most common procedure to determine the water content although it is known, swellable clay minerals retain hydration water up to much higher temperatures.
The amplified water uptake and retention by swellable clay minerals results from hydration of interlayer cations. Thereby, the water binding mechanisms are complex due to structural heterogeneity and are determined by layer charge density and location of substitutions. While several experimental studies deal with the maximum water uptake of selected smectites and heating conditions for full dehydration a comprehensive understanding of the relation between the structure of smectites and water uptake/release is still missing.
The Na-saturated smectite / water interface for the montmorillonite-beidellite series is investigated in the present work within the density functional theory (DFT). Layer charge is varied between 0.125 and 0.5 per formula unit [O10(OH)2] by substitution of Al3+ by Mg2+ in the octahedral sheet (montmorillonites) and by substitution of Si4+ by Al3+ in the tetrahedral sheets (beidellites). Starting from the water free supercells (with integer molar ratios), the number of water molecules is increased discretely. Stable hydration states (1H to 3H) do not necessarily correspond to the formation of water layers (1W to 3W) in the interlayer, which is deduced from the development of the basal spaces during hydration. With the help of ab initio thermodynamics, the energy states are related to temperature, and partial pressure of H2O and the resulting phase diagrams revealed hydration state in dependence of relative humidity (RH) as well as necessary temperatures for full dehydration to determine the true water content. Thereby it was shown that 2:1 layer silicates with a layer charge of 0.125 are swellable but reach only the 1H state even at 100% RH, but the removal of water molecules from the interlayer requires temperatures >110 °C and partial pressures of water <100 Pa. In contrast water uptake of smectites with layer charge 0.375 requires RH of >11% at room temperature, but dehydration occurs at moderate heating
Phenomenological analysis of K+ meson production in proton-nucleus collisions
Total and differential cross sections from literature, on the production of
K+ mesons in pA interactions at projectile energies between T=0.8 and 2.9 GeV,
covering the transition across the free nucleon-nucleon threshold at 1.58 GeV,
have been investigated. From the target-mass dependence of the production cross
sections no evidence for the expected change of the dominant reaction mechanism
from two-step to direct kaon production was found. At T=1.0 GeV the A
dependences of the total cross sections and of the most recent data from
COSY-Juelich, differential cross sections measured under forward angles, are
strongly different. The invariant K+ production cross sections show an overall
exponential scaling behavior with the squared four-momentum transfer between
the beam proton and the produced K+ meson for t< -0.05 GeV^2 independent of the
beam energy and emission angle. The data from COSY-Juelich reveal a strongly
different t dependence in the region of t>0 GeV^2. Further data at forward
angles and different beam energies should be taken in order to explore this
region of kinematically extreme conditions.Comment: 9 Pages, 11 Figure
Hadron Production in Heavy Ion Collisions
We review hadron production in heavy ion collisions with emphasis on pion and
kaon production at energies below 2 AGeV and on partonic collectivity at RHIC
energies.Comment: 31 pages, 26 figures, accepted for publication in Landolt-Boernstein
Volume 1-23
Short- and long-term conditioning of a temperate marine diatom community to acidification and warming
Ocean acidification and greenhouse warming will interactively influence competitive success of key phytoplankton groups such as diatoms, but how long-term responses to global change will affect community structure is unknown. We incubated a mixed natural diatom community from coastal New Zealand waters in a short-term (two-week) incubation experiment using a factorial matrix of warming and/or elevated pCO2 and measured effects on community structure. We then isolated the dominant diatoms in clonal cultures and conditioned them for 1 year under the same temperature and pCO2 conditions from which they were isolated, in order to allow for extended selection or acclimation by these abiotic environmental change factors in the absence of interspecific interactions. These conditioned isolates were then recombined into âartificialâ communities modelled after the original natural assemblage and allowed to compete under conditions identical to those in the short-term natural community experiment. In general, the resulting structure of both the unconditioned natural community and conditioned âartificialâ community experiments was similar, despite differences such as the loss of two species in the latter. pCO2 and temperature had both individual and interactive effects on community structure, but temperature was more influential, as warming significantly reduced species richness. In this case, our short-term manipulative experiment with a mixed natural assemblage spanning weeks served as a reasonable proxy to predict the effects of global change forcing on diatom community structure after the component species were conditioned in isolation over an extended timescale. Future studies will be required to assess whether or not this is also the case for other types of algal communities from other marine regimes
K^+ production in the reaction at incident energies from 1 to 2 AGeV
Semi-inclusive triple differential multiplicity distributions of positively
charged kaons have been measured over a wide range in rapidity and transverse
mass for central collisions of Ni with Ni nuclei. The transverse
mass () spectra have been studied as a function of rapidity at a beam
energy 1.93 AGeV. The distributions of K^+ mesons are well described by a
single Boltzmann-type function. The spectral slopes are similar to that of the
protons indicating that rescattering plays a significant role in the
propagation of the kaon. Multiplicity densities have been obtained as a
function of rapidity by extrapolating the Boltzmann-type fits to the measured
distributions over the remaining phase space. The total K^+ meson yield has
been determined at beam energies of 1.06, 1.45, and 1.93 AGeV, and is presented
in comparison to existing data. The low total yield indicates that the K^+
meson can not be explained within a hadro-chemical equilibrium scenario,
therefore indicating that the yield does remain sensitive to effects related to
its production processes such as the equation of state of nuclear matter and/or
modifications to the K^+ dispersion relation.Comment: 24 pages Latex (elsart) 7 PS figures to be submitted to Nucl. Phys
A global perspective on marine photosynthetic picoeukaryote community structure
A central goal in ecology is to understand the factors affecting the temporal dynamics and spatial distribution of microorganisms and the underlying processes causing differences in community structure and composition. However, little is known in this respect for photosynthetic picoeukaryotes (PPEs), algae that are now recognised as major players in marine CO2 fixation. Here, we analysed dot blot hybridisation and cloningâsequencing data, using the plastid-encoded 16S rRNA gene, from seven research cruises that encompassed all four ocean biomes. We provide insights into global abundance, α- and ÎČ-diversity distribution and the environmental factors shaping PPE community structure and composition. At the class level, the most commonly encountered PPEs were Prymnesiophyceae and Chrysophyceae. These taxa displayed complementary distribution patterns, with peak abundances of Prymnesiophyceae and Chrysophyceae in waters of high (25:1) or low (12:1) nitrogen:phosphorus (N:P) ratio, respectively. Significant differences in phylogenetic composition of PPEs were demonstrated for higher taxonomic levels between ocean basins, using Unifrac analyses of clone library sequence data. Differences in composition were generally greater between basins (interbasins) than within a basin (intrabasin). These differences were primarily linked to taxonomic variation in the composition of Prymnesiophyceae and Prasinophyceae whereas Chrysophyceae were phylogenetically similar in all libraries. These data provide better knowledge of PPE community structure across the world ocean and are crucial in assessing their evolution and contribution to CO2 fixation, especially in the context of global climate change
An upper limit on the electron-neutrino flux from the HiRes detector
Air-fluorescence detectors such as the High Resolution Fly's Eye (HiRes)
detector are very sensitive to upward-going, Earth-skimming ultrahigh energy
electron-neutrino-induced showers. This is due to the relatively large
interaction cross sections of these high-energy neutrinos and to the
Landau-Pomeranchuk-Migdal (LPM) effect. The LPM effect causes a significant
decrease in the cross sections for bremsstrahlung and pair production, allowing
charged-current electron-neutrino-induced showers occurring deep in the Earth's
crust to be detectable as they exit the Earth into the atmosphere. A search for
upward-going neutrino-induced showers in the HiRes-II monocular dataset has
yielded a null result. From an LPM calculation of the energy spectrum of
charged particles as a function of primary energy and depth for
electron-induced showers in rock, we calculate the shape of the resulting
profile of these showers in air. We describe a full detector Monte Carlo
simulation to determine the detector response to upward-going
electron-neutrino-induced cascades and present an upper limit on the flux of
electron-neutrinos.Comment: 13 pages, 3 figures. submitted to Astrophysical Journa
Search for Correlations between HiRes Stereo Events and Active Galactic Nuclei
We have searched for correlations between the pointing directions of
ultrahigh energy cosmic rays observed by the High Resolution Fly's Eye
experiment and Active Galactic Nuclei (AGN) visible from its northern
hemisphere location. No correlations, other than random correlations, have been
found. We report our results using search parameters prescribed by the Pierre
Auger collaboration. Using these parameters, the Auger collaboration concludes
that a positive correlation exists for sources visible to their southern
hemisphere location. We also describe results using two methods for determining
the chance probability of correlations: one in which a hypothesis is formed
from scanning one half of the data and tested on the second half, and another
which involves a scan over the entire data set. The most significant
correlation found occurred with a chance probability of 24%.Comment: 13 pages, 1 table, 5 figure
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