Mariana Titica

Kinetic modeling of light limitation and sulfur deprivation effects in the induction of hydrogen production with Chlamydomonas reinhardtii: Part I. Model development and parameter identification

Chlamydomonas reinhardtii is a green microalga capable of turning its metabolism towards H2 production under specific conditions. However this H2 production, narrowly linked to the photosynthetic process, results from complex metabolic reactions highly dependent on the environmental conditions of the cells. A kinetic model has been developed to relate culture evolution from standard photosynthetic growth to H2 producing cells. It represents transition in sulfur‐deprived conditions, known to lead to H2 production in Chlamydomonas reinhardtii, and the two main processes then induced which are an over‐accumulation of intracellular starch and a progressive reduction of PSII activity for anoxia achievement. Because these phenomena are directly linked to the photosynthetic growth, two kinetic models were associated, the first (one) introducing light dependency (Haldane type model associated to a radiative light transfer model), the second (one) making growth a function of available sulfur amount under extracellular and intracellular forms (Droop formulation). The model parameters identification was realized from experimental data obtained with especially designed experiments and a sensitivity analysis of the model to its parameters was also conducted. Model behavior was finally studied showing interdependency between light transfer conditions, photosynthetic growth, sulfate uptake, photosynthetic activity and O2 release, during transition from oxygenic growth to anoxic H2 production conditions. Biotechnol. Bioeng. 2009;102: 232–245.

A model-based method for investigating bioenergetic processes in autotrophically growing eukaryotic microalgae: Application to the green algae Chlamydomonas reinhardtii

A constraint‐based modeling approach was developed to investigate the metabolic response of the eukaryotic microalgae Chlamydomonas reinhardtii under photoautotrophic conditions. The model explicitly includes thermodynamic and energetic constraints on the functioning metabolism. A mixed integer linear programming method was used to determine the optimal flux distributions with regard to this set of constraints. It enabled us, in particular, to highlight the existence of a light‐driven respiration depending on the incident photon flux density in photobioreactors functioning in physical light limitation.

Kinetic modeling of light limitation and sulfur deprivation effects in the induction of hydrogen production with Chlamydomonas reinhardtii. Part II: Definition of model-based protocols and experimental validation.

Photosynthetic hydrogen production under light by the green microalga Chlamydomonas reinhardtii was investigated in a torus‐shaped PBR in sulfur‐deprived conditions. Culture conditions, represented by the dry biomass concentration of the inoculum, sulfate concentration, and incident photon flux density (PFD), were optimized based on a previously published model (Fouchard et al., 2009. Biotechnol Bioeng 102:232–245). This allowed a strictly autotrophic production, whereas the sulfur‐deprived protocol is usually applied in photoheterotrophic conditions. Experimental results combined with additional information from kinetic simulations emphasize effects of sulfur deprivation and light attenuation in the PBR in inducing anoxia and hydrogen production. A broad range of PFD was tested (up to 500 µmol photons m−2 s−1). Maximum hydrogen productivities were 1.0 ± 0.2 mL H2/h/L (or 25 ± 5 mL H2/m2 h) and 3.1 mL ± 0.4 H2/h L (or 77.5 ± 10 mL H2/m2 h), at 110 and 500 µmol photons m−2 s−1, respectively. These values approached a maximum specific productivity of approximately 1.9 mL ± 0.4 H2/h/g of biomass dry weight, clearly indicative of a limitation in cell capacity to produce hydrogen. The efficiency of the process and further optimizations are discussed. Biotechnol. Bioeng. 2011;108: 2288–2299.

Model-based versus model-free control designs for improving microalgae growth in a closed photobioreactor: Some preliminary comparisons

Controlling microalgae cultivation, i.e., a crucial industrial topic today, is a challenging task since the corresponding modeling is complex, highly uncertain and time-varying. A model-free control setting is therefore introduced in order to ensure a high growth of microalgae in a continuous closed photobioreactor. Computer simulations are displayed in order to compare this design to an input-output feedback linearizing control strategy, which is widely used in the academic literature on photobioreactors. They assess the superiority of the model-free standpoint both in terms of performances and implementation simplicity.

Estimation of Chlamydomonas reinhardtii Growth in a Torus Photobioreactor

Microalgae culture is used in various biotechnological applications. Optimisation of the system productivity needs reliable sensors. However, physical sensors for biomass and dissolved dioxide carbon concentrations are expensive and not accurate, especially for online measurements. In this context, robust and efficient software sensors have to be developed. In this paper, an Unscented Kalman filter (UKF) methodology is proposed to estimate components concentrations in a photobioreactor. The microalgae Chlamydomonas reinhardtii is used as model organism. The aim of this paper is to develop an online software estimator that reconstructs the biomass, carbon dioxide and oxygen concentrations in the liquid phase, from online measurements of components molar fraction in the output gas provided by a mass spectrometer. The proposed estimator is validated through experimental data collected on a lab-scale photobioreactor.

Coupling biological and radiative models to describe microalgal growth in a photobioreactor

A new dynamical model has been developped to describe microalgal growth in a photobioreactor under light and nitrogen limitations. The strong interactions between irradiance and chlorophyll encouraged us to couple biological and radiative models. We assume that biomass growth is a function of light and nitrogen quota and we relate the chlorophyll content to the nitrogen quota, for a given photoadaptation light. The biomass and chlorophyll contents are used to compute the radiative properties for the medium from which we can deduce an irradiance distribution inside the photobioreactor. The resulting model is used to simulate Isochrysis affinis galbana growth under light/dark cycles and to study the dependence of biomass production on the dilution rate and the influent substrate concentration.

Extremum seeking control for an anaerobic digestion process

The paper deals with the optimal control (extremum seeking algorithm) of an anaerobic digestion process. It uses a simplified version of the anaerobic digester model. Three cases of optimization criteria are considered taking into account the quality variable of the process: 1. the methane quantity in gaseous form, 2. the level of pollutants (it is expressed as the difference between the influent substrate and the substrate quantities accumulated in the digester) and 3. an aggregated quality variable resulted from the combination of the first two criteria. The analysis of anaerobic digestion process static characteristics shows that each characteristic reaches a point of maximum. The extremum seeking algorithm aims to keep the operating point in this point of maximum. The simulation results confirm the good behaviour of the extremum seeking algorithm.

Optimal Operation of a Lumostatic Microalgae Cultivation Process

This chapter proposes the optimization of batch microalgae cultures in artificially lighted photobioreactors . The strategy consists in controlling the incident light intensity so that the microalgae growth rate is maximized. Two approaches were developed and compared. In the first one, the ratio between the incident light intensity and the cell concentration (light-to-microalgae ratio) is optimized, either offline or online, and then maintained at its optimal value. In the second approach, the cells growth rate is maintained at its optimal value by means of nonlinear model predictive controller (NMPC). The proposed control strategies are illustrated and their efficiency is assessed, in simulation, for Chlamydomonas reinhardtii batch cultures. The proposed lumostatic operation strategies are shown to lead to a higher cell productivity and to a more efficient light utilization in comparison to conventional constant light operation approach.

Control of the light-to-microalgae ratio in a photobioreactor

This paper deals with the design of a controller that maximizes the biomass production in an artificially lighted photobioreactor operated in batch mode, by maintaining the ratio between the incident light intensity and the biomass concentration (light-to-microalgae ratio) at targeted light regime. The controller acts on the incident light intensity applied on the photobioreactor surface. First, the optimal ratio trajectory is determined and then the obtained optimal trajectory is tracked using a Nonlinear Model Predictive Control law. The proposed control strategy performances were illustrated and assessed in simulation for a Chlamydomonas reinhardtii batch culture.

Feedback linearizing control of light-to-microalgae ratio in artificially lighted photobioreactors

Abstract The present paper describes the design and the validation of a lumostatic controller for artificially lighted photobioreactors operated in discontinuous mode. The ratio between the incident light intensity and the biomass concentration, termed light-to-microalgae ratio, was selected as output variable, while its control was provided by manipulating the power supply of a light source and consequently the incident light intensity. The biomass yield on light energy was introduced in order to properly compare the batches operated under constant light intensities with the lumostatic batches. The results obtained in simulation show that a lumostatic batch can yield at least 10% more biomass per mole of supplied photons. The nonlinear controller, synthesized on the feedback linearizing technique, was implemented and validated on a laboratory torus photobioreactor inoculated with Chlamydomonas reinhardtii cells.