Abstract : We investigated magnesium (Mg2+) ion uptake by Chlorella vulgaris under mixotrophic growth conditions (10 g/L of glucose) in a stirred photo-bioreactor. The culture nitrate and glucose consumptions were followed and analyzed during the experiments. The cellular chlorophyll α concentration decreased during exponential growth, indicating an adaptation to heterotrophic metabolism. The partition of magnesium partitioned between the culture medium, the cell surface and within the cells was determined throughout the experiment. A clear relationship between the microalgal concentration and Mg2+ ion removal-extent from the medium was observed. The removal rate was faster during mixotrophic growth than autotrophic growth and was related to the higher biomass production under the latter culture conditions. The Mg2+ concentration per gram of dry biomass was 3.44 mg/g under heterotrophic conditions and 6.0 mg/g under autotrophic growth. At the end of the experiment (330 h), 90 % of the initial magnesium (17.7 mg/L) in the medium was associated with the biomass, of which 4% was adsorbed and 86% absorbed by cells. This study was consistent with the kinetic model based on a reversible first-order reaction for Mg2+ bioaccumulation in C. vulgaris. Mixotrophic growth conditions can be used at industrial scale to increase both the speed and the extent of Mg2+ uptake by the microalgal.
