Magnesium Uptake by the Green Microalga Chlorella vulgaris in Batch Cultures

The 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.