Numerical study of substrate assimilation by a microorganism exposed to fluctuating concentration

In most modelling works on bioreactors, the substrate assimilation is computed from the volume average concentration. The possible occurrence of a competition between the transport of substrate towards the cell and the assimilation at the cell level is generally overlooked. In order to examine the consequences of such a competition, a diffusion equation for the substrate is coupled with a specific boundary condition defining the up take rate at the cell liquid interface. Two assimilation laws are investigated, whereas the concentration far from the cell is varied in order to mimic concentration fluctuations. Both steady and unsteady conditions are investigated. The actual uptake rate computed from the interfacial concentration is compared to the time-averaged uptake rate based on the mean far-field concentration. Whatever the assimilation law, it is found that the uptake rate can be correlated to the mean far-field concentration, but the actual values of the parameters are affected in case of transport limitation. Moreover, the structure of the far-field signal influences the substrate assimilation by the microorganism, and the mean interfacial uptake rate depends on the ratio between the characteristic time of the signal and the diffusional time scale, as well as on the amplitude of the fluctuations around the mean far-field concentration in substrate. The present work enlightens some experimental results and helps in understanding the differences between the concentration measured and that present in the microenvironment of the cells

Piv study of mixing characteristics in a stirred vessel with a non-Newtonian fluid

PIV is used to analyze the flow induced by a Rushton turbine in a shear-thinning fluid, at constant input power, constant impeller velocity but different concentrations. The rheology of each shear-thinning fluid is first addressed. The mean velocity fields are compared. POD methodology is applied to estimate coherent structures and turbulence levels. Finally, the heterogeneity of shear rate is estimated and the spatial distribution of dissipation rate of total kinetic energy is addressed

A photosynthetic rotating annular bioreactor (Taylor–Couette type flow) for phototrophic biofilm cultures

In their natural environment, the structure and functioning of microbial communities from river phototrophic biofilms are driven by biotic and abiotic factors. An understanding of the mechanisms that mediate the community structure, its dynamics and the biological succession processes during phototrophic biofilm development can be gained using laboratory-scale systems operating with controlled parameters. For this purpose, we present the design and description of a new prototype of a rotating annular bioreactor (RAB) (TayloreCouette type flow, liquid working volume of 5.04 L) specifically adapted for the cultivation and investigation of phototrophic biofilms. The innovation lies in the presence of a modular source of light inside of the system, with the biofilm colonization and development taking place on the stationary outer cylinder (onto 32 removable polyethylene plates). The biofilm cultures were investigated under controlled turbulent flowing conditions and nutrients were provided using a synthetic medium (tap water supplemented with nitrate, phosphate and silica) to favour the biofilm growth. The hydrodynamic features of the water flow were characterized using a tracer method, showing behaviour corresponding to a completely mixed reactor. Shear stress forces on the surface of plates were also quantified by computer simulations and correlated with the rotational speed of the inner cylinder. Two phototrophic biofilm development experiments were performed for periods of 6.7 and 7 weeks with different inoculation procedures and illumination intensities. For both experiments, biofilm biomasses exhibited linear growth kinetics and produced 4.2 and 2.4 mg cm-2 of ash-free dry matter. Algal and bacterial community structures were assessed by microscopy and T-RFLP, respectively, and the two experiments were different but revealed similar temporal dynamics. Our study confirmed the performance and multipurpose nature of such an innovative photosynthetic bioreactor for phototrophic biofilm investigations

Valeurs et perception stéréotypée des groupes

Les valeurs sont un élément central de la vie sociale, elles sont des guides, et permettent aussi de justifier les situations. Les deux études présentées ici s’intéressent à la question de l’impact de valeurs (d’après Schwartz) sur la perception stéréotypée (ici, la perception d’entitativité) des groupes. La première étude montre que l’impact des valeurs dépend de leur type : des valeurs d’Affirmation de Soi ont un impact différent des valeurs de Transcendance de Soi, et ce, probablement parce qu’elles mettent davantage la personne en position de juger. L’impact dépend aussi du groupe cible du jugement. La seconde étude montre que l’impact des valeurs dépend du fait qu’elles sont ou non consciemment activées. L’amorçage de valeurs (mais pas leur activation consciente) a un impact différent selon leur type, ce qui renvoie à la question du contrôle ou du non-contrôle des déterminants des jugements. Globalement ces études montrent que l’entitativité perçue et les valeurs d’Affirmation de soi sont liées. En outre elles confirment expérimentalement que les valeurs, quand elles sont activées, sont un des déterminants de la perception stéréotypée des groupes humains.Social values are one core element of social life. They are guides, and also allow justifying different social situations. The aim of the two studies reported here was to test the effects of an activation of values (following Schwartz) on stereotypic perception (here, entitativity) of a group. In the first study, it is shown that the impact of values depends on their type (Self-Enhancement Values vs. Self-Transcendence Values), probably because the former refer to the ability to judge. Moreover, their impact depends on the nature of the target group. In the second study, it is shown that this impact differs according to the level of consciousness of the activation. Priming values (but not their conscious activation) has a variable impact according to the type of values, which is relevant to the question of the (un)consciousness of the determinants of the judgements. Taken together, these studies show that Self-Enhancement Values lead to a more entitative perception of a group. Moreover, they experimentally confirm that the activation of values is one of the determinants of stereotypic perception of human groups

Climate change adaptation practice in semi-arid regions : views and insights by practitioners

This report elaborates recommendations aimed at ASSAR’s Regional Research Teams (RRT) as they enter the transition between the Regional Diagnostic Studies (RDS) and the Regional Research Programmes(RRP), and as such, recommendations hope to influence the design and refining of research questions. The Oxfam team is committed to supporting the RRTs in this transition process by offering them tailored support.Funded by IDRC and DFID, and implemented by a consortium of five core partners: The University of Cape Town, the University of East Anglia, START, Oxfam GB and the Indian Institute for Human Settlements.: http://www.assar.uct.ac.za

An assessment of methods of moments for the simulation of population dynamics in large-scale bioreactors

A predictive modelling for the simulation of bioreactors must account for both the biological and hydrodynamics complexities. Population balance models (PBM) are the best approach to conjointly describe these complexities, by accounting for the adaptation of inner metabolism for microorganisms that travel in a large-scale heterogeneous bioreactor. While being accurate for solving the PBM, the Class and Monte-Carlo methods are expensive in terms of calculation and memory use. Here, we apply Methods of Moments to solve a population balance equation describing the dynamic adaptation of a biological population to its environment. The use of quadrature methods (Maximum Entropy, QMOM or EQMOM) is required for a good integration of the metabolic behavior over the population. We then compare the accuracy provided by these methods against the class method which serves as a reference. We found that the use of 5 moments to describe a distribution of growth-rate over the population gives satisfactory accuracy against a simulation with a hundred classes. Thus, all methods of moments allow a significant decrease of memory usage in simulations. In terms of stability, QMOM and EQMOM performed far better than the Maximum Entropy method. The much lower memory impact of the methods of moments offers promising perspectives for the coupling of biological models with a fine hydrodynamics depiction

New developments of the Extended Quadrature Method of Moments to solve Population Balance Equations

Population Balance Models have a wide range of applications in many industrial fields as they allow accounting for heterogeneity among properties which are crucial for some system modelling. They actually describe the evolution of a Number Density Function (NDF) using a Population Balance Equation (PBE). For instance, they are applied to gas–liquid columns or stirred reactors, aerosol technology, crystallisation processes, fine particles or biological systems. There is a significant interest for fast, stable and accurate numerical methods in order to solve for PBEs, a class of such methods actually does not solve directly the NDF but resolves their moments. These methods of moments, and in particular quadrature-based methods of moments, have been successfully applied to a variety of systems. Point-wise values of the NDF are sometimes required but are not directly accessible from the moments. To address these issues, the Extended Quadrature Method of Moments (EQMOM) has been developed in the past few years and approximates the NDF, from its moments, as a convex mixture of Kernel Density Functions (KDFs) of the same parametric family. In the present work EQMOM is further developed on two aspects. The main one is a significant improvement of the core iterative procedure of that method, the corresponding reduction of its computational cost is estimated to range from 60% up to 95%. The second aspect is an extension of EQMOM to two new KDFs used for the approximation, the Weibull and the Laplace kernels. All MATLAB source codes used for this article are provided with this article

Vulnerability and Risk Assessment in Botswana's Bobirwa Sub - District: Fostering People - Centered Adaptation to Climate Change

In November 2015, ASSAR’s (Adaptation at Scale in Semi-Arid Regions) Southern Africa researchers from the University of Botswana (UB), University of Cape Town (UCT), University of Namibia (UNAM) and Oxfam, conducted a two-day Vulnerability Risk Assessment (VRA) in order to bring stakeholder groups closer to ASSAR’s work. Based on the findings, the aim was to reassess ASSAR’s priorities. The workshop was attended by various government officials, Village Development Committee (VDC) members, local community members, and representatives from farmer committees

Stakeholder and influence network mapping exercise with the government, development and research actors in Namibia

This report summarises the outcomes of the Stakeholder and Influence Network Mapping exercise were undertaken on the 23rd of July 2015 in Windhoek, Namibia. ASSAR southern African Team members: Margaret Angula, Nahas Angula, Nguza Siyambango (UNAM); Dian Spear, Salma Hegga (UCT); Hillary Masundire and Chandapiwa Molefe (UB) assisted and participated in the stakeholder mapping workshop facilitated by Daniel Morchain (Oxfam). This event brought together 11 national stakeholders from the government (Ministry of Environment and Tourism, Ministry of Industrialization, Trade and SME Development, Namibia Energy Institute), NGOs (NNF, IRDNC) and Researcher/ Academic institutions (SASSCAL, DRFN & UNAM) (see Annex 9.2for a list of participants). The Adaptation at Scale in Semi-Arid (ASSAR) a multi-institutional and multi-national study investigates the factors that restrict effective adaptation to climate change impacts in agriculture-dependent communities in north-central Namibia. A multi-method approach including literature review, household surveys and semi-structured interviews is used to: i) explore the drivers of vulnerabilities to floods and droughts; 2) identify adaptation strategies; and 3) identify the barriers that impede successful adaptation. This report contains five main sections. Section 2 introduces the aims and objectives of the mapping exercise to ASSAR research. The Third section describes the methodology and approaches used, while section 4 describes the framing and analysis procedures employed. Section 5 presents the findings of the Network Influence Mapping exercise from the three groups of stakeholders attending the workshop. The discussion of the results and implication of the results are presented in section 6 and 7, respectively

Finding ways together to build resilience the vulnerability and risk assessment methodology

The VRA in the Bobirwa Sub-district, Botswana, was conducted in collaboration with ASSAR partners University of Botswana, University of Cape Town and University of Namibia. ASSAR is one of five research programmes funded under the Collaborative Adaptation Research Initiative in Africa and Asia (CARIAA), with financial support from the UK Government’s Department for International Development (DfID) and the International Development Research Centre (IDRC), Canada. Oxfam is one of the consortium leads of ASSAR. The views expressed in this report are solely those of the authors. ASSAR is not responsible for its content.agriculture; food security; livelihoods; climate change adaptation; resilience; Vulnerability; risk; Social groups; Gender; Development; Landscape; Afghanistan; Armenia; Bangladesh; Botswana; Ghana; Myanmar; Philippines