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In the framework of environment preservation, microalgae biotechnology appears as a promising alternative for CO₂ mitigation. Advanced control strategies can be further developed to maximize biomass productivity, by maintaining these microorganisms in bioreactors at optimal operating conditions. This article proposes the implementation of Nonlinear Predictive Control combined with an on-line estimation of the biomass concentration, using dissolved carbon dioxide concentration measurements. First, optimal culture conditions are determined so that biomass productivity is maximized. To cope with the lack of on-line biomass concentration measurements, an interval observer for biomass concentration estimation is built and described. This estimator provides a stable accurate interval for the state trajectory and is further included in a nonlinear model predictive control framework that regulates the biomass concentration at its optimal value. The proposed methodology is applied to cultures of the microalgae Chlorella vulgaris in a laboratory-scale continuous photobioreactor. Performance and robustness of the proposed control strategy are assessed through experimental results.

Microalgae are photosynthetic microorganisms that have generated increasing interest in recent years due to their potential applications. Their biological capacity to grow faster than higher plants and their ability to convert solar energy into biomass and other bioactive molecules, has led to the development of various culture systems in order to produce different high-value products with commercial interest. The industrialization of the microalgae cultivation process requires the introduction of standardized quality parameters. In order to obtain bioactive compounds with high added value at a commercial level, it is necessary to sustainably produce biomass at a large scale. Such a process would imply specific stress conditions, such as variation in temperature, light or pH. These environmental conditions would make it more difficult to maintain the viability of the culture and protect the yield and condition of the target molecules. The physiological and biochemical impact of these stress factors on the microalgae biomass can be potentially measured by the presence and activity of various biochemical indicators called biomarkers. This review presents an overview of the main techniques that exist for assessing the "quality" of microalgae cultures through quantification of cell viability and vitality by monitoring specific markers indicative of the status of the culture.