Pilot-Scale Oxygen-Balanced Mixotrophic Cultivation of Galdieria sulphuraria

Oxygen-balanced mixotrophy (OBM) is a particular type of microalgae mixotrophic cultivation, where the supply of an organic carbon substrate is adjusted to match heterotrophic oxygen consumption with photosynthetic production. In this way, the need for aeration is eliminated due to intracellular gas recycling during daytime. After implementing this process at lab scale, we sought to explore its scalability in a tubular photobioreactor (TPBR). In this study, OBM was implemented in a two-phase tubular photobioreactor of 1700 L placed in a greenhouse and exposed to sunlight. The process was run with the polyextremophilic species Galdieria sulphuraria, using glucose as a carbon source. The gas phase was continuously recirculated, and the oxygen concentration was monitored and utilized to manage the glucose supply through a proportional-integral controller. An excessive rate of night aeration, however, resulted in CO₂ limitation issues. Subsequent tuning and optimization of controller settings and the nighttime aeration rate effectively addressed the problem. The average biomass productivity reached 0.81 g·L^-1·day^-1, a significant improvement over autotrophic productivity in the same pilot system. On the other hand, the biomass yield on the substrate was 0.68 C-mol_x·C-mol_s^-1, indicating that considerable carbon recycling took place but to a lower extent than at lab scale. These results provide a solid foundation for the large-scale industrial implementation of OBM.