CO2 mitigation by microalgae

Photosynthetic micro-organisms such as microalgae and cyanobacteria are gaining ground in many application sectors. Thanks to their rapid photosynthetic growth in aqueous media, these microorganisms also offer the possibility of combining their growth with the fixation of CO ₂ of industrial origin. However, photosynthetic microorganisms do not have the capacity to assimilate carbon in gaseous form (CO _2,g ). CO _2,g must first be transferred to the liquid phase in the form of dissolved inorganic carbon (DIC) before it can be assimilated and thus biofixed. This is a major difference compared with higher plants, and has a number of consequences which will be described in this article.

This applies in particular to the physico-chemical dissolution of CO _2,g , which is closely linked to the pH and physico-chemistry of the culture medium in general. Gas-liquid transfer in the culture system is also important, as the low dissolution rate of CO _2,g makes it difficult to implement significant purification of the CO _2,g injected. This has a major impact on the implementation strategy, but also on industrial integration. Thus, carbon biofixation, gas phase abatement and microalgal biomass production are closely linked.

The aim of this article is to present the essential elements involved in this process, as well as the main conclusions drawn from its practical implementation. In the first part, the general principles of photosynthetic growth and its link to carbon are presented. The biological mechanisms of assimilation and conversion are introduced, demonstrating the need to maintain sufficient concentrations of dissolved carbon in the culture medium to avoid the appearance of biological mechanisms leading to a loss of kinetic performance. In the second part, the various theoretical elements required to understand and model the phenomena involved in the physico-chemistry of carbon dissolution, as well as gas-liquid transfer in the reactor, are presented. These elements highlight the particularities of CO ₂ , such as the close coupling of dissolved carbon chemistry to culture pH, itself impacting on biological growth reactions. What emerges is a close coupling between various major quantities of the biological process. This is illustrated in a third section for different cases, leading to details of the main carbon supply strategies used in practice, with their respective advantages and disadvantages depending on the objective pursued, such as optimizing biofixation,...