Biomedical subjects

R H Wijffels

Publications and source records attributed to R H Wijffels.

12 recordsLinked to original sources

PubMed2003

Primmorphs from seven marine sponges: formation and structure.

Primmorphs were obtained from seven different marine sponges: Stylissa massa, Suberites domuncula, Pseudosuberites aff. andrewsi, Geodia cydonium, Axinella polypoides, Halichondria panicea and Haliclona oculata. The formation process and the ultra structure of primmorphs were studied. A positive correlation was found between the initial sponge-cell concentration and the size of the primmorphs. By scanning electron microscopy (SEM) it was observed that the primmorphs are very densely packed sphere-shaped aggregates with a continuous pinacoderm (skin cell layer) covered by a smooth, cuticle-like structure. In the presence of amphotericin, or a cocktail of antibiotics (kanamycin, gentamycin, tylosin and tetracyclin), no primmorphs were formed, while gentamycin or a mixture of penicillin and streptomycin did not influence the formation of primmorphs. The addition of penicillin and streptomycin was, in most cases, sufficient to prevent bacterial contamination, while fungal growth was unaffected.

Aging↗

PubMed2001

Estimation of steady-state culture characteristics during acceleration-stats with yeasts.

Steady-state culture characteristics are usually determined in chemostat cultivations, which are very time-consuming. In contrast, acceleration-stat (A-stat) cultivations in which the dilution rate is continuously changed with a constant acceleration rate are not so time-consuming, especially at high acceleration rates. Therefore, the A-stat could be advantageous to use instead of the chemostat. However, the highest acceleration rate, meaning the fastest A-stat that can be applied for estimating steady-state culture characteristics, is not known yet. Experimental results obtained with Zygosaccharomyces rouxii, an important yeast in soy sauce processes, showed that the culture characteristics during the A-stat with an acceleration rate of 0.001 h(-2) were roughly comparable to those of the chemostat. For higher acceleration rates the deviation between the culture characteristics in the A-stat and those in the chemostat obtained at the same dilution rate generally started to increase. The source of these deviations was examined by simulation for Saccharomyces cerevisiae. The simulations demonstrated that this deviation was not only dependent on the metabolic adaptation rate of the yeast, but also on the rate of change in environmental substrate concentrations during A-stats. From this work, it was concluded that an A-stat with an acceleration rate of 0.001 h(-2) is attractive to be used instead of chemostat whenever a rough estimation of steady-state culture characteristics is acceptable.

Adaptation, Physiological↗

PubMed2001

Immobilized salt-tolerant yeasts: application of a new polyethylene-oxide support in a continuous stirred-tank reactor for flavour production.

Immobilization of salt-tolerant yeasts considerably decreases the total time required for the flavour development in soy-sauce processes. For immobilization of cells, alginate gel is mostly used as support material. However, alginate is not very suitable for use in soy-sauce processes because alginate is sensitive to abrasion and chemically unstable towards the high salt content of the soy-sauce medium. In contrast, a newly developed polyethylene-oxide gel seems to be more suitable, but this gel has not been used so far for flavour production in a bioreactor with a high salt content. Therefore, this gel was applied with immobilized salt-tolerant yeasts in a continuous stirred-tank reactor, containing more than 12.5% (w/v) salt. In this reactor, the polyethylene-oxide gel particles did not show any abrasion for several days, while alginate gel beads were already destroyed within 1 day. In addition, the polyethylene-oxide gel particles with immobilized salt-tolerant yeasts Candida versatilis and Zygosaccharomyces rouxii showed a good flavour production. From this work, it was concluded that the application of polyethylene-oxide gel in long-term soy-sauce processes is attractive in the case the sticking together of polyethylene-oxide gel particles can be controlled.

Alginates↗

PubMed2001

Acetate as a carbon source for hydrogen production by photosynthetic bacteria.

Hydrogen is a clean energy alternative to fossil fuels. Photosynthetic bacteria produce hydrogen from organic compounds by an anaerobic light-dependent electron transfer process. In the present study hydrogen production by three photosynthetic bacterial strains (Rhodopseudomonas sp., Rhodopseudomonas palustris and a non-identified strain), from four different short-chain organic acids (lactate, malate, acetate and butyrate) was investigated. The effect of light intensity on hydrogen production was also studied by supplying two different light intensities, using acetate as the electron donor. Hydrogen production rates and light efficiencies were compared. Rhodopseudomonas sp. produced the highest volume of H2. This strain reached a maximum H2 production rate of 25 ml H2 l(-1) h(-1), under a light intensity of 680 micromol photons m(-2) s(-1), and a maximum light efficiency of 6.2% under a light intensity of 43 micromol photons m(-2) s(-1). Furthermore, a decrease in acetate concentration from 22 to 11 mM resulted in a decrease in the hydrogen evolved from 214 to 27 ml H2 per vessel.

Acetates↗

PubMed2000

Rheological properties and mechanical stability of new gel-entrapment systems applied in bioreactors.

The mechanical stability of gels applied for entrapment and retention of biocatalysts in bioreactors is of crucial importance for successful scale-up applications. Gel abrasion in agitated reactors will depend on liquid shear, bubble shear, and wall shear, as well as collisions between the gel particles. As a simplified standardized model system, abrasion of gel beads was studied in 1-m-high bubble columns with controlled aeration, and quantified by measuring the loss of gel material into solution. Gel beads were also taken out to measure stress-strain response during controlled compression. More general rheological properties of different gels were studied by applying a variety of regimes of controlled compression of standardized gel cylinders: Gel strength was measured by recording the fracture properties and the Young's modulus. Viscoelastic properties were revealed by recording creep during compression as well as recovery after compression. Oscillation tests up to 1000 cyclic compressions were applied to compare the fatigue of different gels. Results obtained for Ca-alginate gels, gels of chemically modified polyvinyl alcohol with stilbazolium groups (PVA-SbQ) as well as mixed gels of Ca-alginate and PVA-SbQ are compared with previously published data for kappa-carrageenan, agar, and polyethylene glycol (PEG) gels. It is concluded that material fatigue rather than mechanical properties such as stiffness or fracture stress should be considered when selecting a suitable gel material on the basis of abrasion resistance. The very soft and superelastic PVA-SbQ gel showed no significant fatigue in mechanical tests and no abrasion was detected in the standardized model system used. Ca-alginate gels, however, showed severe irreversible changes due to fatigue at oscillating loads and creep at constant load. Due to their similarities with kappa-carrageenan gels in mechanical tests, it is likely that Ca-alginate would also be sensitive to abrasion. Mixed gels of Ca-alginate and PVA-SbQ represent a complex system with intermediate properties, showing significant fatigue and creep, but elastic properties from the PVA-SbQ gel make it less sensitive than the pure Ca-alginate gel.

Alginates↗

PubMed2000

Modeling solid-to-solid biocatalysis: integration of six consecutive steps.

A quantitative model for the conversion of a solid-substrate salt to a solid-product salt in a batch bioreactor seeded with product crystals is presented. The overall process consists of six serial steps (with dissolution and crystallization each in themselves complex multistep processes): solid-salt dissolution, salt dissociation into an ionic substrate and a counter-ion, bioconversion accompanied by biocatalyst inactivation, complexation of the ionic product with the counter-ion, and salt crystal growth. In the model, the consecutive steps are integrated, including biocatalyst inactivation and assuming that salt dissociation and complexation of ions are at equilibrium. Model parameters were determined previously in separate independent experiments. To validate the model, either dissolved or solid Ca-maleate was converted to solid Ca-D-malate by permeabilized Pseudomonas pseudoalcaligenes in a batch bioreactor seeded with Ca-D-malate crystals. The model very well predicted the concentrations of all components in the liquid phase (Ca-maleate, Ca(2+), maleate(2-), D-malate(2-), and Ca-D-malate) and the amounts of the solid phases (Ca-maleate. H(2)O and Ca-D-malate. 3H(2)O), especially when high initial amounts of Ca-maleate. H(2)O and Ca-D-malate. 3H(2)O were present.

Bioreactors↗

PubMed2000

Growth of Ca-D-malate crystals in a bioreactor.

To develop a bioreactor for solid-to-solid conversions, the conversion of solid Ca-maleate to solid Ca-D-malate by permeabilized Pseudomonas pseudoalcaligenes was studied. In a bioreactor seeded with product (Ca-D-malate) crystals, growth of Ca-D-malate crystals is the last step in the solid-to-solid conversion and is described here. Crystal growth is described as a transport process followed by surface processes. In contrast to the linear rate law obeyed by the transport process, the surface processes of a crystal-growth process can also obey a parabolic or exponential rate law. Growth of Ca-D-malate crystals from a supersaturated aqueous solution was found to be surface-controlled and obeyed an exponential rate law. Based on this rate law, a kinetic model was developed which describes the decrease in supersaturation due to Ca-D-malate crystal growth as a function of the constituent ions, Ca(2+) and D-malate(2-). The kinetic parameters depended on temperature, but, as expected (surface-controlled), they were hardly affected by the stirring speed.

Bioreactors↗

PubMed2000

Immobilized soy-sauce yeasts: development and characterization of a new polyethylene-oxide support.

Entrapment of cells in alginate gel is a widely used mild immobilization procedure. However, alginate gel is not very suitable for use in long-term continuous soy-sauce processes because alginate is sensitive to abrasion and chemically unstable towards the high salt content of soy-sauce medium. Therefore, a chemically crosslinked polyethylene-oxide gel was used instead. The disadvantage of this gel was that due to the crosslinking reaction, the viability of the cells after immobilization was poor. For this reason, a new mild procedure for immobilizing soy-sauce yeasts in polyethylene-oxide gel was developed, resulting in high survival percentages of the soy-sauce yeasts Zygosaccharomyces rouxii and Candida versatilis. This newly developed polyethylene-oxide gel, unlike alginate gel, appeared not to be sensitive to abrasion, even in the presence of high salt concentrations. Therefore, we concluded that this newly developed polyethylene-oxide gel is more suitable than alginate gel for use as immobilization material in long-term processes with a high salt content, like soy-sauce processes.

Candida↗

PubMed2000

D-malate production by permeabilized Pseudomonas pseudoalcaligenes; optimization of conversion and biocatalyst productivity.

For the development of a continuous process for the production of solid D-malate from a Ca-maleate suspension by permeabilized Pseudomonas pseudoalcaligenes, it is important to understand the effect of appropriate process parameters on the stability and activity of the biocatalyst. Previously, we quantified the effect of product (D-malate2 -) concentration on both the first-order biocatalyst inactivation rate and on the biocatalytic conversion rate. The effects of the remaining process parameters (ionic strength, and substrate and Ca2 + concentration) on biocatalyst activity are reported here. At (common) ionic strengths below 2 M, biocatalyst activity was unaffected. At high substrate concentrations, inhibition occurred. Ca2+ concentration did not affect biocatalyst activity. The kinetic parameters (both for conversion and inactivation) were determined as a function of temperature by fitting the complete kinetic model, featuring substrate inhibition, competitive product inhibition and first-order irreversible biocatalyst inactivation, at different temperatures simultaneously through three extended data sets of substrate concentration versus time. Temperature affected both the conversion and inactivation parameters. The final model was used to calculate the substrate and biocatalyst costs per mmol of product in a continuous system with biocatalyst replenishment and biocatalyst recycling. Despite the effect of temperature on each kinetic parameter separately, the overall effect of temperature on the costs was found to be negligible (between 293 and 308 K). Within pertinent ranges, the sum of the substrate and biocatalyst costs per mmol of product was calculated to decrease with the influent substrate concentration and the residence time. The sum of the costs showed a minimum as a function of the influent biocatalyst concentration.

Calcium↗

PubMed2000

Efficiency of light utilization of Chlamydomonas reinhardtii under medium-duration light/dark cycles.

The light regime inside a photobioreactor is characterized by a light gradient with full (sun)light at the light-exposed surface and darkness in the interior of the bioreactor. Consequently, depending on the mixing characteristics, algae will be exposed to certain light/dark cycles. In this study the green alga Chlamydomonas reinhardtii was cultivated under five different light regimes: (1) continuous illumination; (2) a square-wave light/dark cycle with a light fraction (epsilon) of 0.5 and a duration (t(c)) of 6.1 s; (3) epsilon=0.5, t(c)=14.5 s; (4) epsilon=0.5, t(c)=24.3 s and (5) epsilon=0.8, t(c)=15.2 s. The biomass yield on light energy, protein per photons, decreased under light/dark cycles (epsilon=0. 5) in comparison to continuous light (CL), from 0.207 (CL) to 0.117-0.153 g mol(-1) (epsilon=0.5). Concomitantly, the maximal specific photosynthetic activity, oxygen production per protein, decreased from 0.94 (CL) to 0.64-0.66 g g(-1) h(-1) (epsilon=0.5). Also the quantum yield of photochemistry, yield of the conversion of light energy into chemical energy, decreased from 0.47 (CL) to 0. 23 (epsilon=0.5, t(c)=24.3 s). Apparently, C. reinhardtii is not able to maintain a high photosynthetic capacity under medium-duration light/dark cycles and since specific light absorption did not change, light utilization efficiency decreased in comparison to continuous illumination.

Animals↗

PubMed1999

Kinetics and performance of a co-immobilised system of amyloglucosidase and Zymomonas mobilis.

High operational stability and productivity of co-immobilised systems are important aspects for their successful application in industrial processes. A dynamic model is required to describe artificially co-immobilised systems because the time needed to reach steady state normally exceeds the operational life span of these systems. Time dependent intraparticle concentration profiles and macroscopic conversion were modelled to study the operational stability and productivity of these systems theoretically. The model was used to describe experimental results of ethanol production from maltose by a co-immobilised system of amyloglucosidase and Zymomonas mobilis. Furthermore, the influence of the immobilisation procedure with glutaraldehyde and polyethyleneimine could also be studied with and incorporated in the model. From the model it could be derived that co-immobilised systems performing a consecutive reaction evolve towards a steady state, characterised by a constant concentration of the intermediate in the particle if product inhibition is neglected. Such a situation develops independently of the biomass concentration and the radial position, and has important consequences for co-immobilised systems. When the concentration of the intermediate in the bulk liquid is lower than this constant value in the biocatalyst particle, two regions may be distinguished in the particle: an inactive peripheral region without biomass and an active core with a biomass concentration depending on the substrate and immobilised enzyme concentration. Unlike immobilised single cell systems, it is possible to obtain a real steady state and therefore a stable situation for co-immobilised systems. However, a high operational life time could only be achieved at the expense of the productivity of the biocatalyst particle. A stability criterion is derived which agrees very well with the simulation results.

Biotechnology↗

PubMed1993

Simultaneous nitrification and denitrification using immobilized microorganisms.

Nitrogen removal from wastewaters is a multiple step process in which nitrification is often a problem due to the slow growth rates of the nitrifying bacteria. By immobilization of these bacteria, nitrification can be efficiently accomplished in compact reactors. In this paper, the possibilities of integrated nitrification and denitrification within a single reactor system are evaluated. Two main systems are studied: a) Nitrosamines europaea and Pseudomonas denitrificans co-immobilized in a a gel bead and operated in an air-lift reactor; b) the same bacteria separately immobilized and operated in different compartments of a multiple gas-lift reactor.

Ammonia↗