Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Protistan microbial observatory in the Cariaco Basin, Caribbean. I. Pyrosequencing vs Sanger insights into species richness

Abstract: Microbial diversity and distribution are topics of intensive research. In two companion papers in this issue, we describe the results of the Cariaco Microbial Observatory (Caribbean Sea, Venezuela). The Basin contains the largest body of marine anoxic water, and presents an opportunity to study protistan communities across biogeochemical gradients. In the first paper, we survey 18S ribosomal RNA (rRNA) gene sequence diversity using both Sanger- and pyrosequencing-based approaches, employing multiple PCR primers, and state-of-the-art statistical analyses to estimate microbial richness missed by the survey. Sampling the Basin at three stations, in two seasons, and at four depths with distinct biogeochemical regimes, we obtained the largest, and arguably the least biased collection of over 6000 nearly full-length protistan rRNA gene sequences from a given oceanographic regime to date, and over 80,000 pyrosequencing tags. These represent all major and many minor protistan taxa, at frequencies globally similar between the two sequence collections. This large data set provided, via the recently developed parametric modeling, the first statistically sound prediction of the total size of protistan richness in a large and varied environment, such as the Cariaco Basin: over 36,000 species, defined as almost full-length 18S rRNA gene sequence clusters sharing over 99% sequence homology. This richness is a small fraction of the grand total of known protists (over 100,000-500,000 species), suggesting a degree of protistan endemism.

Dynamic models for spatiotemporal data

Abstract: We propose a model for non-stationary spatiotemporal data. To account for spatial variability, we model the mean function at each time period as a locally weighted mixture of linear regressions. To incorporate temporal variation, we allow the regression coefficients to change through time. The model is cast in a Gaussian state space framework, which allows us to include temporal components such as trends, seasonal effects and autoregressions, and permits a fast implementation and full probabilistic inference for the parameters, interpolations and forecasts. To illustrate the model, we apply it to two large environmental data sets: tropical rainfall levels and Atlantic Ocean temperatures.

Improving packaged food quality and safety. Part 2: nanocomposites.

Abstract: This paper gathers a number of significant results where nanotechnology was satisfactorily applied to improve packaged food quality and safety by increasing the barrier properties to oxygen of an ethylene-vinyl alcohol copolymer (EVOH) in dry and under humid conditions and of a poly(lactic acid) (PLA) biopolymer. The nanodispersion in the polymer matrix of modified monolayers of clays included in positive lists for food-contact applications is an adequate methodology to increase packaged food shelf-life. In spite of the fact that, in principle, there is no reason to believe that 'adequately' modified nanocomposites making use of substances in positive lists can impose any immediate risk threat for food-contact applications, further studies concerning potential migration issues and life-cycle analysis have to still emerge within the overall field of nanotechnology to corroborate the fact.

Synthesis of antenna arrays using genetic algorithms

Abstract: In this article, two particular methods for the synthesis of the complex radiation pattern of a linear and a planar array, based on genetic algorithms (GAs), are discussed. First, we present a combination of Schelkunoff's method and GAs for linear arrays with arbitrary radiation patterns; then, we consider the synthesis of planar arrays with rectangular cells. The results show the flexibility of GAs to solve complex problems related to antenna synthesis, subject to many restrictions which are difficult to treat by classical optimization techniques. There is good agreement between the desired and calculated radiation patterns.

Tolman IV solution in the Randall-Sundrum Braneworld

Abstract: In the context of the Randall-Sundrum braneworld, the minimal geometric deformation approach is used to generate an analytic interior solution to four-dimensional effective Einstein field equations for a spherically symmetric compact distribution. This solution represents the braneworld version of the well-known Tolman IV solution in general relativity. By using this analytic solution, an exhaustive analysis of the braneworld effects on realistic stellar interiors is developed, finding strong evidence in favor of the hypothesis that compactness is reduced due to bulk effects on stellar configurations.