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17 results on '"Iversen, J.J.L."'

1. Relationship between specific dynamic action and food quality in the solitary ascidian Ciona intestinalis

Author

Sigsgaard, S.J., Petersen, J.K., and Iversen, J.J.L.

Subjects
Ascidiacea -- Research, Biological sciences
Abstract

This study aims to determine the influence of the quality of food on specific dynamic action (SDA) in the ascidian Ciona intestinalis and thereby if respiratory measurements of SDA can be used to monitor the quality of a potential food source. Weight specific rates of filtration, ingestion and respiration of C. intestinalis were examined using two different experimental methods. Two food sources were selected, the flagellate Rhodomonas sp. representing a high-quality food source and the eelgrass Zostera marina representing a potentially low-quality food source. In spite of insignificant differences in weight specific rates of filtration and ingestion of Rhodomonas sp. and Z. marina, significant differences in weight specific respiration rates and weight specific ingestion rates of C. intestinalis at different particle concentrations of the two food sources were found. Ingestion of Rhodomonas sp. resulted in a SDA coefficient of 20%, whereas the respiratory increase after ingestion of Z. marina was insignificant. From the experimental results it is concluded that SDA of C. intestinalis is strongly dependent on the quality of the food ingested.

Published
2003

2. On-line determination of respiration rates of aquatic organisms in a mono-phase oxystat at steady state dissolved oxygen tensions

Author

Eriksen, N.T. and Iversen, J.J.L.

Subjects
Respiration -- Measurement, Oxygen in the body -- Measurement, Aquatic animals -- Physiological aspects, Biological sciences
Abstract

A new approach for determining respiration rates of aquatic organisms at steady-state dissolved oxygen tension was developed. The procedure uses a measuring chamber called a mono-phase oxystat equipped with a galvanic electrode and computer-controlled oxygen addition to exclude gas bubbles from the liquid phase. The performance of the system was comparable with other open systems in terms of versatility, long-term stability and accuracy but superior in terms of response time and sensitivity.

Published
1997

4. Increased NADPH concentration obtained by metabolic engineering of the pentose phosphate pathway in Aspergillus niger

Author

Poulsen, B.R., Nohr, J., Douthwaite, S., Hansen, L.V., Iversen, J.J.L., Visser, J., and Ruijter, G.J.G.

Subjects
AFSG Stafafdelingen (WUATV), filamentous fungi, phosphoglucose isomerase, AFSG Staff Departments (WUATV), flux, Departement Agrotechnologie en Voedingswetenschappen, escherichia-coli, penicillium-chrysogenum, saccharomyces-cerevisiae, Department of Agrotechnology and Food Sciences, ralstonia-eutropha, genes, acid production, overexpression
Abstract

Many biosynthetic reactions and bioconversions are limited by low availability of NADPH. With the purpose of increasing the NADPH concentration and/or the flux through the pentose phosphate pathway in Aspergillus niger, the genes encoding glucose 6-phosphate dehydrogenase (gsdA), 6-phosphogluconate dehydrogenase (gndA) and transketolase (tktA) were cloned and overexpressed in separate strains. Intracellular NADPH concentration was increased two- to ninefold as a result of 13-fold overproduction of 6-phosphogluconate dehydrogenase. Although overproduction of glucose 6-phosphate dehydrogenase and transketolase changed the concentration of several metabolites it did not result in increased NADPH concentration. To establish the effects of overexpression of the three genes, wild-type and overexpressing strains were characterized in detail in exponential and stationary phase of bioreactor cultures containing minimal media, with glucose as the carbon source and ammonium or nitrate as the nitrogen source and final cell density limiting substrate. Enzymes, intermediary metabolites, polyol pools (intra- and extracellular), organic acids, growth rates and rate constant of induction of acid production in postexponential phase were measured. None of the modified strains had a changed growth rate. Partial least square regressions showed the correlations between NADPH and up to 40 other variables (concentration of enzymes and metabolites) and it was possible to predict the intracellular NADPH concentration from relatively easily obtainable data (the concentration of enzymes, polyols and oxalate). This prediction might be used in screening for high NADPH levels in engineered strains or mutants of other organisms.

Published
2005

5. Determination of first order rate constants by natural logarithm of the slope plot exemplified by analysis of Aspergillus niger in batch culture

Author

Poulsen, B.R., Ruiter, G., Visser, J., and Iversen, J.J.L.

Subjects
atmospheric co2, AFSG Stafafdelingen (WUATV), past 2 centuries, growth, Astrophysics::Cosmology and Extragalactic Astrophysics, AFSG Staff Departments (WUATV)
Abstract

Finding rate constants from experimental data is often difficult because of offset and noise. A computer program was developed to average experimental data points, reducing the effect of noise, and to produce a loge of slope plot - a plot of the natural logarithm of the slope of a curve - eliminating the effect of any offset. If y-values depend exponentially on x-values the loge of slope plot is rectilinear and the slope is equal to the first order rate constant. Therefore the loge of slope plot provides easy identification of exponential sections of any experimental or calculated data, corresponding rate constants, and small changes in the rate constant as exemplified by analysis of titrant added to a batch culture of Aspergillus niger. The loge of slope plot was easily applicable and superior to conventional methods of analysis of exponential decreasing or increasing data.

Published
2003

11. Glucose uptake and growth of glucose-limited chemostat cultures of Aspergillus niger and a disruptant lacking MstA, a high-affinity glucose transporter

Author

Jorgensen, T.R., vanKuyk, P.A., Poulsen, B.R., Ruijter, G.J.G., Visser, J., Iversen, J.J.L., Jorgensen, T.R., vanKuyk, P.A., Poulsen, B.R., Ruijter, G.J.G., Visser, J., and Iversen, J.J.L.

Abstract

This is a study of high-affinity glucose uptake in Aspergillus niger and the effect of disruption of a high-affinity monosaccharide-transporter gene, mstA. The substrate saturation constant (K-s) of a reference strain was about 15 mu M in glucose-limited chemostat culture. Disruption of mstA resulted in a two- to fivefold reduction in affinity for glucose and led to expression of a low-affinity glucose transport gene, mstC, at high dilution rate. The effect of mstA disruption was more subtle at low and intermediate dilution rates, pointing to some degree of functional redundancy in the high-affinity uptake system of A. niger. The mstA disruptant and a reference strain were cultivated in glucose-limited chemostat cultures at low, intermediate and high dilution rate (D=0.07 h(-1), 0.14 h(-1) and 0.20 h(-1)). Mycelium harvested from steady-state cultures was subjected to glucose uptake assays, and analysed for expression of mstA and two other transporter genes, mstC and mstF The capacity for glucose uptake (V-max) of both strains was significantly reduced at low dilution rate. The glucose uptake assays revealed complex uptake kinetics. This impeded accurate determination of maximum specific uptake rates (V-max) and apparent affinity constants (K-m(app)) at intermediate and high dilution rate. Two high-affinity glucose transporter genes, mstA and mstF, were expressed at all three dilution rates in chemostat cultures, in contrast to batch culture, where only mstC was expressed. Expression patterns of the three transporter genes suggested differential regulation and functionality of their products.

Published
2007

13. Characterization of pathway engineered strains of filamentous fungi in submerged cultures

Author

Tramper, Hans, Iversen, J.J.L., Ruijter, G.J.G., Visser, J., Poulsen, B.R., Tramper, Hans, Iversen, J.J.L., Ruijter, G.J.G., Visser, J., and Poulsen, B.R.

Abstract

Filamentous fungi are used by the industry for making a variety of products, e.g. penicillin, high-protein food, washing powder and soft drinks. Synthesis of many of these products requires reducing equivalents in the form of NADPH, which is a potential limiting factor. The pentose phosphate pathway (PPP) is believed to be the major source of NADPH. Our main goal has been to make strains ofAspergillus nigerwith increased NADPH availability and/or increased flux through the PPP by overexpression of enzymes in this pathway. Cloning of genes encoding for these enzymes enabled transformation to obtain multicopy transformants. To measure the effect(s) of overexpression, comparison with wild type in well-defined cultures is necessary. A bioreactor was designed especially for cultures of filamentous fungi and techniques were developed for automatic sampling of culture filtrate, rate analysis of culture data, morphology profiling and continuous culturing. NADPH availability was increased in multicopy transformants of the gene encoding for 6-phosphogluconate dehydrogenase.We succeeded to reach our main goal: to increase the availability of NADPH in A. niger. This was obtained by overexpression of 6-phosphogluconate dehydrogenase, but also overexpression of glucose 6-phosphate dehydrogenase and transketolase showed interesting effects and a combination of these overexpressions might result in even higher availability of NADPH and/or flux through the PPP. It has been confirmed by NMR studies that the flux through the PPP is increased in the 6-phosphogluconate dehydrogenase overexpressing strain [Hesse SJA, Dijkema C, de Graaf AA, Ruijter GJG, Visser J, unpublished results].

Published
2005

16. Influence of Different Nitrogen Sources on Composting Of Miscanthusin Open and Closed Systems

Author

Leth, M., Jensen, H.E.K., and Iversen, J.J.L.

Abstract

Composting of Miscanthusstraw was compared in two different, insulated systems, one constructed from four open boxes and one of four closed reactor vessels. Composting of Miscanthussupplemented with either ammonium sulfate, urea, liquid pig manure or brown sap from grass was studied. Hygienization of composts was evaluated by integrations of heat developments. Mixtures of Miscanthusand ammonium sulfate or liquid pig manure lead to high heat development in beginning of composting period while composting of mixtures of Miscanthusand brown sap or urea progressed later in composting period. The period of high microbial activity during composting may therefore be extended using mixtures of manure and brown sap or ammonium sulfate and urea as nitrogen source.

Published
2001
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17. C and N Turnover and Lignocellulose Degradation During Composting of MiscanthusStraw And Liquid Pig Manure

Author

Eiland, F., Leth, M., Klamer, M., Lind, A.-M., Jensen, H.E.K., and Iversen, J.J.L.

Abstract

To obtain a peat substitute for pot plants, Miscanthusstraw and liquid pig manure were composted in two different systems (open box and closed reactor) and examined for changes in pH, water content and chemical composition (nitrogen, carbon, hemicellulose, cellulose and lignin). Temperature maxima of 65-70°C were achieved within a few days in both systems. Composting and sampling were continued for 190 days in the box system and 150 days in the reactor. Major loss of nitrogen was found in a period of eight days after temperature maximum followed by stabilization. Accumulation of nitrate was observed during final weeks of composting in both systems. A marked decrease in content of hemicellulose was seen just after temperature maximum followed by slow, constant decrease throughout remaining composting period. The same pattern was observed for degradation of cellulose, though degradation was initiated later in the experiment. Degradation of hemicellulose was nearly 100% in both systems, whereas the cellulose decrease was 64 and 70% in the box and reactor, respectively. Lignin was not degraded during the experiments. Though initial C/N ratio was different in the two systems (25 in box and 16 in reactor), final ratios were the same (C/N=13). In spite of the similarities, the more complicated closed system is preferred if removal of ammonia from exit gas is considered.

Published
2001
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