Magnesium Uptake by the Green Microalga Chlorella vulgaris in Batch Cultures

International audienceThe accumulation (internal and superficial distribution) of magnesium ions (Mg(2+)) by the green freshwater microalga Chlorella vulgaris (C. vulgaris) was investigated under autotrophic culture in a stirred photobioreactor. The concentrations of the three forms of Mg(2+) (dissolved, extracellular, and intracellular) were determined with atomic absorption spectroscopy during the course of C. vulgaris growth. The proportions of adsorbed (extracellular) and absorbed (intracellular) Mg(2+) were quantified. The concentration of the most important pigment in algal cells, chlorophyll a, increased over time in proportion to the increase in the biomass concentration, indicating a constant chlorophyll/biomass ratio during the linear growth phase. The mean-average rate of Mg(2+) uptake by C. vulgaris grown in a culture medium starting with 16 mg/l of Mg(2+) concentration was measured. A clear relationship between the biomass concentration and the proportion of the Mg(2+) removal from the medium was observed. Of the total Mg(2+) present in the culture medium, 18% was adsorbed on the cell wall and 51% was absorbed by the biomass by the end of the experiment (765 h). Overall, 69% of the initial Mg(2+) were found to be removed from the medium. This study supported the kinetic model based on a reversible first-order reaction for Mg(2+) bioaccumulation in C. vulgaris, which was consistent with the experimental data

The Use of Chlorella Vulgaris to Accumulate Magnesium under Different Culture Conditions

International audienceWe 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

Effect of magnesium ion concentration in autotrophic cultures of Chlorella vulgaris

International audienceChlorella vulgaris was grown autotrophically in shake-flask cultures in the presence of different initial Mg2 + ions concentrations in the range of 8.9–465.0 mg/L. No inhibition of growth was observed even at the highest concentration of magnesium tested. An existing extraction method was modified and applied to the cultures in order to measure the Mg2 + ions associated with the biomass. A distinction was made between the Mg2 + ions adsorbed onto the cells and the absorbed Mg2 + ions which were more strongly associated with the biomass. The cells became saturated with absorbed ions at approximately 46.1 mg Mg2 +/g DW biomass but the amount of adsorbed Mg2 + ions associated with the biomass increased in proportion with the initial concentration of these ions in the medium. The authors propose that the saturation level of the adsorbed Mg2 + ions was not reached in these experiments even at the highest concentration tested. The experimental data of all Mg2 + ions concentrations tested fitted well with an exponential model for the metal removal from the medium