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.

Influence of hydrological seasonality on bacterioplankton in two neotropical floodplain lakes

Abstract: Seasonal fluctuation in river stage strongly affects the ecological functioning of tropical floodplain lakes. This study was conducted to assess the influence of hydrological seasonality on bacterial production and abundance in two floodplain lakes of the Autana River, a blackwater river in the Middle Orinoco basin, Venezuela. Water samples for nutrient chemistry, chlorophyll a, and microbiological determinations were collected in two floodplain lakes and in the mainstem of the river during 1997–98. DOC and chlorophyll a concentrations were similar between mainstem and lake sites during high water when river and lakes were well connected but became different during the period of low water when the interaction was minimal. Higher values of bacterial production were observed in the floodplain lakes (0.62–1.03 μg C l−1 h−1) compared to the mainstem sites (0.17–0.19 μg C l−1 h−1) during the period of low water, while during the period of high water river and lake sites showed similar levels (0.04 μg C l−1 h−1). Bacterial numbers followed bacterial production in the floodplain lakes, reaching higher numbers during the period of low water (1.41–2.40 × 106 cells ml−1). Availability of substrate and inorganic nutrients, pH, and inputs and losses of bacterial cells could be determining the observed seasonal patterns in bacterial production and abundance. The Autana lakes exhibited a strong seasonal pattern in the chemical and biological conditions, showing higher productivity during the lentic phase that lasted between 5 and 6 months.

Leaf Longevity, Construction, and Maintenance Costs of Three Mangrove Species Under Field Conditions

Abstract: This study assessed the effect of leaf age on construction cost (CC) in the mangrove species Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle growing in their natural habitat. Leaf osmolality values were species-specific, the highest in A. germinans (1 693 mmol kg−1) and the lowest in L. racemosa (1 270 mmol kg−1). In the three species, contents of chlorophyll (a+b) (Chla+b) and nitrogen (N) per unit of leaf area were maximal in adult leaves and tended to decline with age. Leaf mass to leaf area ratio (LMA) and ash content increased during leaf ageing. Similarly, as leaves aged, a significant increase in leaf construction cost per leaf area (CCa) was observed, while per leaf mass (CCm) it remained almost constant, suggesting a sustained production of leaf compounds as leaves became older. CC was positively correlated with LMA and heat of combustion (Hc) per leaf area, suggesting differences among species in the quantity and composition of expensive compounds. Leaf half lifetime (t0.5) showed contrasting values in the three mangrove species (60, 111, and 160 d in L. racemosa, R. mangle, and A. germinans, respectively). Overall, L. racemosa was the species with less expensive leaves to construct while leaves of A. germinans and R. mangle had the highest CCm and CCa, respectively. Leaf longevity was positively correlated with the ratio between CC and maximum photosynthetic rate (P max), clearly showing the existence of a balance between leaf costs and benefits.

Value of a tomato byproduct as a source of dietary fiber in rats

Abstract: A residue consisting of tomato peel and seeds is a byproduct of the tomato paste industry. This product had, after drying and grinding, 101.4 g water, 175.6 g protein, 95.9 g lipids, 36.4 g ash and 590.7 g total carbohydrates per kilogram of residue. The carbohydrates in the residue were mainly dietary fiber (495.3 g) from which 405.4 g were insoluble fiber. Additionally, the residue had ascorbic acid 0.25 g, P 3 g, Ca 1.7 g, K 13.1 g, Mg 2.4 g, Cu 0.013 g, Fe 0.248 g, Mn 0.018 g and Zn 0.174 g per kilogram of residue. The residue fed to rats at increasing concentrations in the diet (0, 134, 263 and 387 g/kg diet) over 18 days had no effect on body weight gain; caused an increase in food intake and slightly reduced feed efficiency. Incorporation of the residue in the diet caused a substantial increase in fecal mass, which was proportional to the dietary fiber provided by the residue (r = 0.89); apparent absorption of protein, energy, Ca, Mg, Fe, Zn and Cu present in the diets was reduced. These reductions were proportional to the amount of dietary fiber provided by the residue and became substantial when the residue was included at 263 or 387 g/kg. However, when the residue was included at 134 g/kg, little effect on the apparent absorption of the dietary nutrients was noted. Because this concentration provided the rats with approximately 10 times more fiber than that recommended for humans, one can expect that if this tomato residue were consumed at concentrations recommended for humans, it should have no effect on nutrient availability. These results indicate that this tomato residue represents an attractive source of fiber, which in rats had a low apparent absorption (52-56%). In addition, it was very effective in increasing fecal mass with no negative effects on growth performance.

Residual alignment and its effect on weld strength in material-extrusion 3D-printing of polylactic acid

Abstract: Gaining a molecular understanding of material extrusion (MatEx) 3D printing is crucial to predicting and controlling part properties. Here we report the direct observation of distinct birefringence localised to the weld regions between the printed filaments, indicating the presence of molecular orientation that is absent from the bulk of the filament. The value of birefringence at the weld increases at higher prints speeds and lower nozzle temperatures, and is found to be detrimental to the weld strength measured by tensile testing perpendicular to the print direction. We employ a molecularly-aware non-isothermal model of the MatEx flow and cooling process to predict the degree of alignment trapped in the weld at the glass transition. We find that the predicted residual alignment factor, A ¯ , is linearly related to the extent of birefringence, Δn. Thus, by combining experiments and molecular modelling, we show that weld strength is not limited by inter-diffusion, as commonly expected, but instead by the configuration of the entangled polymer network. We adapt the classic molecular interpretation of glassy polymer fracture to explain how the measured weld strength decreases with increasing print speed and decreasing nozzle temperature.

Hydroxamic acid glucosides in honeydew of aphids feeding on wheat

Abstract: DIMBOA glucoside (2-O-/gb-D-glucopyranosyl-4-hydroxy-7-meth-oxy-1,4-benzoxazin-3-one), the main hydroxamic acid (Hx) in intact wheat plants, was detected in the honey dew ofRhopalosiphum padi feeding on seedlings of six wheat cultivars that differed in their concentration of Hx, suggesting that the chemical circulates in the phloem. Neither the aglucone (DIMBOA) nor its main breakdown product were found in any of the honeydew samples. Honey dew production by aphids caged on seedlings of the wheat cultivars and DIMBOA glucoside concentrations in the honeydew followed biphasic curves when plotted against Hx concentration, suggesting passive ingestion of the chemical from the phloem at low Hx concentrations and limited ingestion due to feeding deterrency by Hx in mesophyll cells at high Hx concentrations. The presence of plant toxins such as Hx glucosides in the phloem sap, the main ingesta of aphids, and in the mesophyll cells, has major implications for plant defense, through a feeding deterrent effect during stylet penetration, and deterrency (antixenosis) along with antibiosis during feeding.