Continuous-Flow and Batch Cultures of Toxic Alexandrium catenella and A. minutum in Stirred Tank Bioreactors

Extrinsic environmental factors are known to exert profound effects on the general physiological status of toxicogenic microalgae. In order to study this relation and minimize natural variability, toxic Alexandrium strains were cultured in 2.3-L stirred-tank bioreactors equipped with various probes to measure light absorption, pH, temperature and gas flow. Specific supervisor software managed the calibration of these probes and the on-line recording of culture parameters. Chemostat cultivation of Alexandrium minutum originating from the N. Atlantic and Alexandrium catenella from the Thau Lagoon (W. Mediterranean coast) produced cultures with a relatively high division rate (0.5 d ‐1 ) and maximum cell concentrations of 80,000 and 60,000 cells/mL, respectively. Growth kinetics showed considerable variations relative to available nitrogen and phosphorus sources. Analysis of cell extracts by liquid chromatography/fluorescence detection (LC/FD) gave remarkably consistent toxin profiles, but the cell toxin quota exhibited substantial variations in relation to the physiological stage of the culture. As continuous-flow cultures could be maintained for several months in our experimental system, it was possible to investigate the capacity of these organisms to adapt to exogenous parameters such as salinity, temperature, nitrogen and phosphorus. The combination of all probes (light absorption, pH, temperature and gas flow) and the real-time monitoring and storage of data provided a historical record of each culture set, which was of considerable value for understanding growth kinetics and toxicogenesis.