Selective plankton feeding by the coregonid ”bondella” (Coregonus sp.) of Lake Maggiore [Translation from: Translation from: Memorie dell'Istituto Italiano di Idrobiologia Dott.Marco de Marchi 31 181-203, 1974]

In recent times, some specialists have begun to study the subject of food selection of fish in an organized way, thus highlighting its importance. In the present work, the author intends to evaluate the impact of predation on the eupelagic zooplanktonic biocenosis of Lake Maggiore, producing, in this way, as far as possible, a basis for a better understanding of the population dynamics of the species of zooplankton directly involved. Another aspect which has been studied is that related to the mechanism of selective capture set in action by the predator. To this end the study tries to bear in mind that the subject should be interpreted as a function of numerous factors acting contemporaneously, that is as the interaction of characters peculiar to the predator and to the prey. The species studied, locally called ”bondella”, belongs to the family Salmonidae, subfamily Coregoninae and was introduced into Lake Maggiore in 1950

Effective Intermolecular Pair Potentials for Sulphur Dioxide

New effective intermolecular pair potentials for sulphur dioxide, obtained by molecular dynamics simulation, are proposed. Potentials found in the literature, fitted to the gas and crystal properties, are also described. Their comparison points to the conclusion that the new ones are more realistic

A tetrameric iron superoxide dismutase from the eucaryote Tetrahymena pyriformis.

Abstract An iron-containing superoxide dismutase has been purified from the protozoan Tetrahymena pyriformis. It has a molecular weight of 85,000 and is composed of four subunits of equal size. The tetramer contains 2.5 g atoms of ferric iron. Visible absorption and electron spin resonance spectra closely resemble those of other iron-containing superoxide dismutases. The amino acid sequence of the iron superoxide dismutase was determined. Each subunit is made up of 196 residues, corresponding to a molecular weight of 22,711. Comparison of the primary structure with the known sequences of other iron-containing superoxide dismutases reveals a relatively low degree of identity (33-34%). However, a higher percentage identity is found with mammalian manganese-containing superoxide dismutases (41-42%). The amino acid sequence is discussed in consideration of residues that may distinguish iron from manganese or dimeric from tetrameric superoxide dismutases

Percolation transition of hydration water at hydrophilic surfaces

An analysis of water clustering is used to study the quasi-2D percolation transition of water adsorbed at planar hydrophilic surfaces. Above the critical temperature of the layering transition (quasi-2D liquid-vapor phase transition of adsorbed molecules) a percolation transition occurs at some threshold surface coverage, which increases with increasing temperature. The location of the percolation line is consistent with the existence of a percolation transition at the critical point. The percolation threshold at a planar surface is weakly sensitive to the size of the system when its lateral dimension increases from 80 to 150 A. The size distribution of the largest water cluster shows a specific two-peaks structure in a wide range of surface coverage : the lower- and higher-size peaks represent contributions from non-spanning and spanning clusters, respectively. The ratio of the average sizes of spanning and non-spanning largest clusters is about 1.8 for all studied planes. The two-peak structure becomes more pronounced with decreasing size of the planar surface and strongly enhances at spherical surfaces.Comment: 17 pages, 11 figure

Cooperative motions in a finite size model of liquid silica: an anomalous behavior

Finite size effects on dynamical heterogeneity are studied in liquid silica with Molecular Dynamics simulations using the BKS potential model. When the system size decreases relaxation times are found to increase in accordance with previous results in finite-size simulations and confined liquids. It has been suggested that this increase may be related to a modification of the cooperative motions in confined liquids. In agreement with this hypothesis we observe a decrease of the dynamical heterogeneities associated to the most and the least mobile atoms when the size L decreases. However we find that the decrease of the dynamical aggregation associated to the least mobile atoms is much more important than the decrease associated to the most mobile atoms. This result is surprising as the liquid is slowed down. The decrease of the heterogeneous behavior is also in contradiction with the increase of the heterogeneities observed in liquids confined in nanopores. However an increase of the non-Gaussian parameter appears both in nanopores and in the finite size simulations. As the non-Gaussian parameter is usually associated with dynamical heterogeneities, the increase of the non-Gaussian parameter together with a decrease of dynamical heterogeneity is also surprising.Comment: 18 pages,4 figure

On the transferability of three water models developed by adaptive force matching

Water is perhaps the most simulated liquid. Recently three water models have been developed following the adaptive force matching (AFM) method that provides excellent predictions of water properties with only electronic structure information as a reference. Compared to many other electronic structure based force fields that rely on fairly sophisticated energy expressions, the AFM water models use point-charge based energy expressions that are supported by most popular molecular dynamics packages. An outstanding question regarding simple force fields is whether such force fields provide reasonable transferability outside of their conditions of parameterization. A survey of three AFM water models, B3LYPD-4F, BLYPSP-4F, and WAIL are provided for simulations under conditions ranging from the melting point up to the critical point. By including ice-Ih configurations in the training set, the WAIL potential predicts the melting temperate, TM, of ice-Ih correctly. Without training for ice, BLYPSP-4F underestimates TM by about 15 K. Interestingly, the B3LYPD-4F model gives a TM 14 K too high. The overestimation of TM by B3LYPD-4F mostly likely reflects a deficiency of the B3LYP reference. The BLYPSP-4F model gives the best estimate of the boiling temperature TB and is arguably the best potential for simulating water in the temperature range from TM to TB. None of the three AFM potentials provides a good description of the critical point. Although the B3LYPD-4F model gives the correct critical temperature TC and critical density, there are good reasons to believe the agreement is reached fortuitously. Links to Gromacs input files for the three water models are provided at the end of the paper.Comment: 25 pages, 2 figure

Excitation and relaxation in atom-cluster collisions

Electronic and vibrational degrees of freedom in atom-cluster collisions are treated simultaneously and self-consistently by combining time-dependent density functional theory with classical molecular dynamics. The gradual change of the excitation mechanisms (electronic and vibrational) as well as the related relaxation phenomena (phase transitions and fragmentation) are studied in a common framework as a function of the impact energy (eV...MeV). Cluster "transparency" characterized by practically undisturbed atom-cluster penetration is predicted to be an important reaction mechanism within a particular window of impact energies.Comment: RevTeX (4 pages, 4 figures included with epsf

Molecular structural order and anomalies in liquid silica

The present investigation examines the relationship between structural order, diffusivity anomalies, and density anomalies in liquid silica by means of molecular dynamics simulations. We use previously defined orientational and translational order parameters to quantify local structural order in atomic configurations. Extensive simulations are performed at different state points to measure structural order, diffusivity, and thermodynamic properties. It is found that silica shares many trends recently reported for water [J. R. Errington and P. G. Debenedetti, Nature 409, 318 (2001)]. At intermediate densities, the distribution of local orientational order is bimodal. At fixed temperature, order parameter extrema occur upon compression: a maximum in orientational order followed by a minimum in translational order. Unlike water, however, silica's translational order parameter minimum is broad, and there is no range of thermodynamic conditions where both parameters are strictly coupled. Furthermore, the temperature-density regime where both structural order parameters decrease upon isothermal compression (the structurally anomalous regime) does not encompass the region of diffusivity anomalies, as was the case for water.Comment: 30 pages, 8 figure

Static and Dynamic Properties of a Viscous Silica Melt Molecular Dynamics Computer Simulations

We present the results of a large scale molecular dynamics computer simulation in which we investigated the static and dynamic properties of a silica melt in the temperature range in which the viscosity of the system changes from O(10^-2) Poise to O(10^2) Poise. We show that even at temperatures as high as 4000 K the structure of this system is very similar to the random tetrahedral network found in silica at lower temperatures. The temperature dependence of the concentration of the defects in this network shows an Arrhenius law. From the partial structure factors we calculate the neutron scattering function and find that it agrees very well with experimental neutron scattering data. At low temperatures the temperature dependence of the diffusion constants $D$ shows an Arrhenius law with activation energies which are in very good agreement with the experimental values. With increasing temperature we find that this dependence shows a cross-over to one which can be described well by a power-law, D\propto (T-T_c)^gamma. The critical temperature T_c is 3330 K and the exponent gamma is close to 2.1. Since we find a similar cross-over in the viscosity we have evidence that the relaxation dynamics of the system changes from a flow-like motion of the particles, as described by the ideal version of mode-coupling theory, to a hopping like motion. We show that such a change of the transport mechanism is also observed in the product of the diffusion constant and the life time of a Si-O bond, or the space and time dependence of the van Hove correlation functions.Comment: 30 pages of Latex, 14 figure

A New Method for the Generation of Realistic Atomistic Models of Siliceous MCM-41

A new method is outlined for constructing realistic models of the mesoporous amorphous silica adsorbent, MCM-41. The procedure uses the melt-quench molecular dynamics technique. Previous methods are either computationally expensive or overly simplified, missing key details necessary for agreement with experimental data. Our approach enables a whole family of models spanning a range of pore widths and wall thicknesses to be efficiently developed and yet sophisticated enough to allow functionalisation of the surface – necessary for modelling systems such as self-assembled monolayers on mesoporous supports (SAMMS), used in nuclear effluent clean-up. The models were validated in two ways. The first method involved the construction of adsorption isotherms from grand canonical Monte Carlo simulations, which were in line with experimental data. The second method involved computing isosteric heats at zero coverage and Henry law coefficients for small adsorbate molecules. The values obtained for carbon dioxide gave good agreement with experimental values. We use the new method to explore the effect of increasing the preparation quench rate, pore diameter and wall thickness on low pressure adsorption. Our results show that tailoring a material to have a narrow pore diameter can enhance the physisorption of gas species to MCM-41 at low pressure