Biomass and coccolith productions in Chrysotila sp. in high density cultivation system

Coccolithophores are unicellular marine microalgae capable of producing calcium carbonate exoskeletons composed of micrometric scales called coccoliths. Since they require CO₂ for both calcification and biomass production, they are expected to exhibit a potentially higher CO₂ fixation, making them promising candidates for applications in biological carbon capture systems. In addition, coccoliths themselves are considered high-value products due to their potential applications in many fields such as nanobiotechnology. In this study, a high cell density culture of Chrysotila sp. was tested using the CellDEG^® technology, with the objective of maximizing both biomass and coccolith production as a function of light intensity, nitrogen and carbon supply, thanks to a design of experiments (DoE) approach. The optimal conditions identified for biomass production were 600 mg L⁻¹ of nitrogen and a light intensity of 375 µmol m⁻² s⁻¹, which allowed the system to reach a final biomass productivity of 0.96 g L⁻¹ d⁻¹. However, coccolith production was found to be very limited under these conditions. Additional experiments assessed the effect of calcium addition and light regime, which did not remarkably affect the PIC:POC ratio. On the other hand, when Chrysotila sp. was cultivated in a semicontinuous system with controlled pH (to maintain the carbonate structure of the coccosphere) with a light bubbling as mixing, a remarkable particulate inorganic carbon versus particulate organic carbon ratio (PIC:POC) of 0.3 was obtained. Semicontinuous feeding was selected for its ability to maintain relatively stable concentrations of nutrients and cells over time. Therefore, semicontinuous cultivation under these conditions is recommended for coccolith production, whereas the CellDEG^® system is more suitable for biomass generation. This study allowed us to obtain data on the growth of Chrysotila sp. using semicontinuous and high cell density culture systems, which is innovative given the limited information available in literature regarding the cultivation of this species. KEY POINTS: CellDEG^® technology allows high biomass production, but low PIC:POC Changes in calcium concentration or photoperiod cannot increase CellDEG^®'s PIC:POC Semicontinuous cultivation yields high PIC:POC but reduces biomass levels.