Dissolved inorganic carbon uptake by Rhodomonas sp. and Isochrysis aff. galbana determined by a potentiometric technique

Abstract Dissolved inorganic carbon uptake (DIC) was determined in batch culture of the marine microalgae Rhodomonas sp. and Isochrysis aff. galbana (Clone T-ISO) for seven days using a simple potentiometric technique. The system used allowed the determination of small variations of the three independent parameters: pH, dissolved inorganic carbon and total alkalinity ( A t ). The aerated cultures began with 13 μmol kg −1 of CO 2 (calculated using C t –pH), but after the third day, the concentration was almost zero. Cultures of Rhodomonas sp. continued growing in the absence of CO 2 with a total DIC depletion of 1330.5 μmol kg −1 and a final pH of ∼10. Since HCO 3 − is the predominant form of inorganic carbon at alkaline pH, Rhodomonas sp. shows potential for direct HCO 3 − uptake. On the other hand, the cultures of I. aff. galbana only removed DIC during the first three days while CO 2 was available, and also when the pH from culture was below 9. Nutrients (PO 4 3− , NO 3 − , NO 2 − and NH 4 + ) uptakes were also analyzed for the two cultures. The nutrient uptake in both microalgae did not show dependency on any specific DIC form. An extra addition of CO 2 during cultivation is recommended for enhanced growth mainly in I. galbana .