Dynamique de photoacclimatation des microalgues vertes : expériences et modélisation

Climate change and its consequences are the century's most important issues. To try to limit their impact on our environment, drastic changes in our modes of consumption and production are needed. Biotechnologies have emerged over the last decades as a key to shifting some production sectors to more environmentally friendly solutions. In this context, the culture of microalgae for biotechnological purposes has undergone an important development. Microalgae possess important potential in terms of high-added value molecules and ecosystemic benefits accompanying their growth.However, despite promising laboratory results, photoautotrophic processes struggle to overcome their limitations. The main constraint in these cultures is undoubtedly light availability, a complex parameter to control both regarding the quantity of photons and their temporal fluctuations. In industrial-scale bioreactors, a high concentration of cells causes an important light gradient, which, coupled with the agitation, leads to intermittent light patterns perceived by the cells. In addition, the outdoor culture faces slowly changing irradiance from the evolving daylight and weather conditions. Yet, a knowledge gap persists regarding the impact of intermittent light patterns on growth and pigment expression. A better understanding of these phenomena could improve culture control and productivity.In this work, we propose to develop a model of microalgae photoacclimation inside an industrial photobioreactor based on experimental assays performed on the green microalgae Chlorella vulgaris.Cultures were grown in ultra-thin flat panels with low cell density under continuous and intermittent light ranging between 35 μmol Photon_PAR.m^(-2).s^(-1) and 600 μmol Photon_PAR.m^(-2).s^(-1). The dynamic of photoacclimation after a sudden light intensity change was evaluated through pigment quantification and fluorometric assays. The characteristic time of chlorophyll modulation as a response to the light change was found to be dependent on the level of light imposed on the microalgae, advocating for an energetical limitation in the photoacclimation mechanisms. The behavior of the microalgal cells was similar for continuous and intermittent light for the intensities and frequencies tested (f=1 Hz, duty cycle of 0.5). Lastly, no hysteresis effect was observable for the pigment and fluorometric indicators considering cycles of two successive light shifts.The experimental results were used to parameterize a macroscopic model combining the light field over a tubular photobioreactor and a growth model integrating photoacclimation. The simulation tool combined a ray tracing method to obtain the light field, a random walk based on hydrodynamic laws to represent the trajectories of the microalgal cells, and a three-state photosystem model. The tool accounted for the self-shading of the algae evolving with biomass accumulation and the pigment modulation as a result of photoacclimation. The impact of the parameters' uncertainty on the model's output was evaluated. The photoacclimation dynamic highlighted experimentally had only a limited impact on the simulation output. Taking it into account could have at more important interest in pigment production applications. Lastly, the model was used to showcase applications such as the dimensioning of a fed-batch outdoor culture or the use of complementary external illumination to compensate for cloud shading.