Enhancing control systems of higher plant culture chambers via multilevel structural mechanistic modelling

Modelling higher plant growth is of strategic interest for modern agriculture as well as for the development of bioregenerative life support systems for space applications, where crop growth is expected to play an essential role. The capability of constraint-based metabolic models to cope the diel dynamics of plants growth is integrated into a multilevel modelling approach including mass and energy transfer and enzyme kinetics. Lactuca sativa is used as an exemplary crop to validate, with experimental data, the approach presented as well as to design a novel model-based predictive control strategy embedding metabolic information. The proposed modelling strategy predicts with high accuracy the dynamics of gas exchange and the distribution of fluxes in the metabolic network whereas the control architecture presented can be useful to manage higher plants chambers and open new ways of merging metabolome and control algorithms.

A structured model for simulation of cultures of the cyanobacterium Spirulina platensis in photobioreactors: II. Identification of kinetic parameters under light and mineral limitations

A structured model for the culture of cyanobacteria in photobioreactors is developed on the basis of Schuster's approximations for radiative light transfer. This model is therefore limited to monodimensional geometries and kinetic aspects.Light-harvesting pigments play a crucial role in defining the profile of radiative transfer inside the culture medium and in controlling the metabolism, particularly the metabolic deviations induced by mineral limitations. Modeling therefore requires the biomass to be divided into several compartments, among which the light-harvesting compartment allows a working illuminated volume to be defined within the photobioreactor. This volume may change during batch cultures, largely decreasing as pigment concentration increases during growth but increasing as pigments are consumed during mineral limitation. This approach enables, in photobioreactors of simple parallelepipedic, geometries, kinetic parameters to be determined with high accuracy; this may then be extended to vessels of more complex geometries, such as cylindrical photobioreactors.The model is applied to controlled batch cultures of the cyanobacterium Spirulina platensis in parallelepipedic photobioreactors to assess its ability to predict the behavior of these microorganisms in conditions of light and mineral limitations. Results allowed the study of optimal operating condition for continuous cultures to be approached.

Combined physico-chemical and water transfer modelling to predict bacterial growth during food processes

The quality and safety of food products depend on the microorganisms, the food characteristics and the process. The prediction of conditions that prevent growth in complex situations due to the characteristics of the process and of the food cannot be obtained by predictive models of bacterial growth only. Thus, a combined modelling approach was developed by integrating three models, which were selected in a first step: (1) a bacterial model that predicts the bacterial growth from the physico-chemical properties of the media; (2) a water transfer model that predicts the effects of the drying process variables on the medium characteristics; and (3) a thermodynamic model that predicts the water activity aw and the pH of the media from its composition. A second step consisted in separately validating each selected model in which all of the physical, chemical or biological parameters appearing in the equations were previously measured. The third step combined the three knowledge models. The global model was validated on the basis of experimental results concerning the growth of Listeria innocua on the surface of a gelatine gel, the surface of which was submitted to a drying process (changes in relative humidity and air velocity). It was shown that bacterial growth models had to be modified: a specific model was set up to predict the maximum growth rate and another for the lag. Additionally, growth models set up in broth could not be applied in gelatine, leading to the development of a specific growth model on a solid surface. The thermodynamic model accurately predicted the pH and aw of bacterial broth in which high concentrations of solutes were added, and those of the solid media, the gelatine. The water transfer model was applied on gelatine data to predict the evolution of its surface aw during the drying process. The three models-bacterial, water transfer and thermodynamic, separately validated-were combined according to an integrated modelling strategy. The water transfer model coupled with the thermodynamic model predicted the aw on the gel surface. The predicted surface aw explained why growth inhibition was observed. Indeed, growth stopped at a predicted surface aw <0.94, corresponding to L. innocua minimum aw during the drying process. The global model satisfactorily predicted L. innocua growth on the surface of the gel. This study proves the validity of the approach and shows that the combination of the water transfer and thermodynamic models compensates for the lack of aw measurement techniques.

A fully predictive model for one-dimensional light attenuation byChlamydomonas reinhardtii in a torus photobioreactor

The light attenuation in a photobioreactor is determined using a fully predictive model. The optical properties were first calculated, using a data bank of the literature, from only the knowledge of pigments content, shape, and size distributions of cultivated cells which are a function of the physiology of the current species. The radiative properties of the biological turbid medium were then deduced using the exact Lorenz-Mie theory. This method is experimentally validated using a large-size integrating sphere photometer. The radiative properties are then used in a rectangular, one-dimensional two-flux model to predict radiant light attenuation in a photobioreactor, considering a quasi-collimated field of irradiance. Combination of this radiative model with the predictive determination of optical properties is finally validated by in situ measurement of attenuation profiles in a torus photobioreactor cultivating the microalgae Chlamydomonas reinhardtii, after a complete and proper characterization of the incident light flux provided by the experimental set-up.

Influence of pH on complexing of model beta-d-glucans with zearalenone

Previous studies have shown that isolated beta-(1,3 and 1,6)-D-glucans and related alkali-extracted fractions from the cell wall of Saccharomyces cerevisiae are able to complex with zearalenone in vitro (affinity up to 50%) and thus may reduce the bioavailability of toxins in the digestive tract. The complexation mechanisms involve cooperative interaction between the two chemical entities that can be computed by Hill's model. Various linear or branched soluble or insoluble beta-D-glucans were evaluated to elucidate their roles in the adsorption mechanisms under three pH conditions (3.0, 6.0, and 8.0) found in the digestive tract. A constant quantity of each beta-D-glucans (1 mg/ml) was mixed at 39 degrees C with increasing amounts of zearalenone (2 to 100 microg/ml), and the amount of bound toxin was measured. Acidic and neutral conditions gave the highest affinity rates (64 to 77%) by beta-(1,3)-D-glucans, whereas alkaline conditions decreased adsorption except when beta-(1,6)-D-glucan side chains were branched on beta-(1,3)-D-glucans. Alkaline conditions appear to impede the active three dimensional conformation of beta-D-glucans and favor single helix and/or random coil structures. Study of the equilibrium between beta-D-glucan-bound and free toxins revealed that two types of chemical interactions occur during toxin complexation with beta-D-glucans, identified as weak chemical linkages such as hydrogen and van der Waals bonds.

Effect of aw, controlled by the addition of solutes or by water content, on the growth of Listeria innocua in broth and in a gelatine model

The effect of a(w) on the growth of Listeria innocua was investigated in broth and on the surface of a gelatine food model. In broth, a(w) was controlled from 0.91 to 0.99 by the addition of solutes such as NaCl, KCl, glucose, sucrose and glycerol. In the gelatine food model, a(w) was controlled by removal of water. In the a(w) range, 0.92-0.99, the generation times observed in broth in the presence of NaCl, KCl, sucrose and glucose were similar but were longer than those in glycerol. For lag times, the inhibition of L. innocua growth followed the order: NaCl = KCl = sucrose>glucose>glycerol. When comparing growth at a(w) 0.95 for the three media--broth + NaCl, gelatine gel (a(w) controlled by removal of water) and gelatine gel with NaCl (gel + NaCl, a(w) controlled by NaCl)--the shortest generation time was observed in broth + NaCl, followed by gel + NaCl and, finally, on gel with a larger gap between the last two. The generation time on gel was five times greater than the generation time in broth + NaCl and 2.5 times greater on gel + NaCl. It was concluded that not only the structure of the media (solid or liquid) had an effect on Listeria inhibition but also and mainly the way the a(w) was adjusted. Removal of water was more stressful to Listeria than the addition of NaCl.

Adsorption of Zearalenone by beta-D-glucans in the Saccharomyces cerevisiae cell wall

Cell walls of yeasts and bacteria are able to complex with mycotoxins and limit their bioavailability in the digestive tract when these yeasts and bacteria are given as feed additives to animals. To identify the component(s) of the yeast cell wall and the chemical interaction(s) involved in complex formation with zearalenone, four strains of Saccharomyces cerevisiae differing in their cell wall glucan and mannan content were tested. Laboratory strains wt292, fks1, and mnn9 were compared with industrial S. cerevisiae strain sc1026. The complex-forming capacity of the yeast cell walls was determined in vitro by modelling the plots of amount of toxin bound versus amount of toxin added using Hill's model. A cooperative relationship between toxin and adsorbent was shown, and a correlation between the amount of beta-D-glucans in cell walls and complex-forming efficacy was revealed (R2 = 0.889). Cell walls of strains wt292 and mnn9, which have higher levels of beta-D-glucans, were able to complex larger amounts of zearalenone, with higher association constants and higher affinity rates than those of the fks1 and sc1026 strains. The high chitin content in strains mnn9 and fks1 increased the alkali insolubility of beta-D-glucans from isolated cell walls and decreased the flexibility of these cell walls, which restricted access of zearalenone to the chemical sites of the beta-D-glucans involved in complex formation. The strains with high chitin content thus had a lower complex-forming capacity than expected based on their beta-D-glucans content. Cooperativity and the three-dimensional structure of beta-D-glucans indicate that weak noncovalent bonds are involved in the complex-forming mechanisms associated with zearalenone. The chemical interactions between beta-D-glucans and zearalenone are therefore more of an adsorption type than a binding type.

k L a determination: comparative study for a gas mass balance method

The determination of k(L) a by a gas balance method coupled with sulphite oxidation is compared for three kinds of processes (stirred tank, bubble column and fixed-bed column reactors) with a gassing-in and with a classical chemical sulphite oxidation method. The mathematical relations required for the determination of the k(L) a value are detailed. In coalescing gas-liquid conditions, the values calculated by the three methods are shown to be comparable. The gas balance method is more rapid than either the steady-state gassing-in or the chemical sulphite reaction rate measurement methods. It is also well adapted for three-phase systems (gas-liquid-solid) in which the non-coalescing effects of sulphite solution are reduced by solid interferences.

Degradation of plant wastes by anaerobic process using rumen bacteria

An operational reactor has been designed for the fermentation of a pure culture of Fibrobacter succinogenes with the constraints of strict anaerobic condition. The process is controlled by measurements of pH, redox, temperature and CO2 pressure; it allows an efficient degradation (67%) of lignocellulosic wastes such as a mixture of wheat straw, soya bean cake and green cabbage.

HUMEX, a study on the survivability and adaptation of humans to long-duration exploratory missions, part I: lunar missions

The European Space Agency has recently initiated a study of the human responses, limits and needs with regard to the stress environments of interplanetary and planetary missions. Emphasis has been laid on human health and performance care as well as advanced life support developments including bioregenerative life support systems and environmental monitoring. The overall study goals were as follows: (i) to define reference scenarios for a European participation in human exploration and to estimate their influence on the life sciences and life support requirements; (ii) for selected mission scenarios, to critically assess the limiting factors for human health, wellbeing, and performance and to recommend relevant countermeasures; (iii) for selected mission scenarios, to critically assess the potential of advanced life support developments and to propose a European strategy including terrestrial applications; (iv) to critically assess the feasibility of existing facilities and technologies on ground and in space as testbeds in preparation for human exploratory missions and to develop a test plan for ground and space campaigns; (v) to develop a roadmap for a future European strategy towards human exploratory missions, including preparatory activities and terrestrial applications and benefits. This paper covers the part of the HUMEX study dealing with lunar missions. A lunar base at the south pole where long-time sunlight and potential water ice deposits could be assumed was selected as the Moon reference scenario. The impact on human health, performance and well being has been investigated from the view point of the effects of microgravity (during space travel), reduced gravity (on the Moon) and abrupt gravity changes (during launch and landing), of the effects of cosmic radiation including solar particle events, of psychological issues as well as general health care. Countermeasures as well as necessary research using ground-based test beds and/or the International Space Station have been defined. Likewise advanced life support systems with a high degree of autonomy and regenerative capacity and synergy effects were considered where bioregenerative life support systems and biodiagnostic systems become essential. Finally, a European strategy leading to a potential European participation in future human exploratory missions has been recommended.

Energy model and metabolic flux analysis for autotrophic nitrifiers

The behavior of pure cultures of nitrifying microorganisms under autotrophic growth operating conditions was investigated and the relations between their energy metabolism and their anabolism analyzed by means of metabolic network computation. The description of the metabolism of the nitrifiers is extended to their energy metabolism by introducing compartmentalization (cytoplasmic and periplasmic sides) and studying coupling between the electron transport chain and the proton gradient generation. The energy model of Nitrosomonas and Nitrobacter was developed based on the oxidoreduction reactions known to be involved. The electron transport chains and the associated proton translocation for these models are described. Several possible hypotheses are analyzed and discussed concerning the thermodynamic consistency of all the oxidoreduction reactions. For Nitrosomonas, the most delicate point is the second step of hydroxylamine oxidation. For Nitrobacter a new energy model is proposed in which NO plays an important role as node in the distribution of electrons from NO(2)(-) oxidation to the membrane electron transport chain. The compartmentalization enables us to consider a proton gradient dissipation flux as the expression of the overall energy loss in metabolic analysis (the so-called maintenance phenomena). The energy model (electron transport chain, proton gradient) is associated with an overall description of the metabolism of Nitrosomonas and Nitrobacter in terms of metabolic flux calculation. This representation demonstrates that a maintenance in nitrifiers expressed as a proton leak is no higher than for other aerobes. The yields calculated from the energy models integrated with the metabolic models of nitrifiers are consistent with the experimental yields in the literature.

Characterization of water distribution in cell pellets using nonlabeled sodium thiosulfate as an interstitial space marker

A procedure for determination of the intracellular water content of cells using a single, nonlabeled solute as an interstitial space marker is proposed. Sodium thiosulfate, which can be accurately assayed by a tritrimetric method, is found to be a good compound for this purpose. Cells are recovered both by filtration and centrifugation; the two techniques gave the same value for internal water, i.e., 650 mg of H2O/g of wet matter for Corynebacterium melassecola and 390 mg of H2O/g of wet matter for Penicillium roquefortii spores. The methodology of data handling, based on a regression technique, is also described. It allows one to obtain very reliable results and should be useful for any marker.