Optimization of Chrysotila sp. Growth in Continuous Photobioreactor Systems for Improved CO₂ Fixation and Coccolith Production

Coccolithophores are marine haptophytes capable of partitioning fixed inorganic carbon between particulate organic carbon, via photosynthesis, and particulate inorganic carbon, via the intracellular precipitation of calcium carbonate coccoliths. This dual carbon-fixing capacity makes them candidates for integrated Carbon Capture and Utilization platforms. The physiological determinants of carbon partitioning in Chrysotila sp. under controlled cultivation remain, however, poorly defined. This thesis addressed that gap through two experimental lines. The first characterized the steady-state response of Chrysotila sp. to incident light intensity and hydraulic residence time in continuous culture. The second explored a two-step process in which cells grown at steady state were transferred to a batch stage designed to stimulate calcification. In the continuous experiments, a photosaturation threshold near 210 µmol m-2 s-1 was identified, where volumetric productivity peaked at 0.410 g L-1 d-1 and the PIC:POC ratio was at its minimum. Above this irradiance, organic carbon fixation saturated while inorganic carbon continued to increase up to 400 µmol m-2 s-1, revealing partially non-overlapping intensity ranges for the two carbon-fixing pathways. At fixed irradiance, the highest volumetric productivity (0.882 g L-1 d-1) occurred at the shortest residence time tested (τ = 0.85 d), while the longest residence time (τ = 3.0 d) shifted carbon allocation toward the inorganic fraction, raising PIC:POC to 0.163. In the batch experiments, CO₂ enrichment alone promoted organic growth but suppressed the PIC:POC ratio; the addition of Ca2+ and Trizma buffer restored calcification, while nutrient limitation yielded the highest IC concentration (0.098 g L-1) and PIC:POC ratio (0.355) recorded in the study. These results provide a quantitative basis for the rational design of bioprocesses with Chrysotila sp., targeting both CO2 biofixation and biogenic calcium carbonate production.