Effect of Mg, Si, and Sr on growth and antioxidant activity of the marine microalga Tetraselmis suecica

Efforts to increase the productivity of microalgal cultures have been focused on the improvement of photobioreactors, but little attention has been paid to the nutritional requirements of microalgae in order to improve culture media formulation. In this study, the main goal was obtaining a high productivity for Tetraselmis suecica (Chlorophyta) in semicontinuous culture by adding magnesium (Mg), silicon (Si), and strontium (Sr) at concentrations from 0.01 to 10 mM; at the time, the effect on steady-state cell density, biochemical composition, and antioxidant activity of T. suecica was evaluated. Because productivity is higher in high-density cultures, the work was focused many times to cell density. Mg (3 mM) and Sr (0.1 mM) added separately reached the highest steady-state cell density (7.0 × 106 ± 0.4 cells mL−1) in comparison to control (4.2 ± 0.1 cells mL−1), but simultaneous addition had a synergic effect, achieving 8.7 × 106 ± 0.6 cells mL−1. Silicon (3 mM) significantly affected the steady-state cell density, reaching 6.0 ± 0.3 cells mL−1 and increased the cell ash-free dry weight, reaching 127 ± 7.9 pg cell−1 in comparison to control (102.7 ± 5.0 pg cell−1), resulting in an ash-free dry weight productivity of 0.75 ± 0.07 g L−1 day−1. The highest fatty acids content and antioxidant activity, measured by 2, 2-diphenyl-1-picrylhydrazyl (DPPH) method were obtained with Sr 10 mM. Sr treatments showed a high correlation (R2 = 0.98) between DPPH inhibition and polyphenolic content, explaining its high antioxidant activity. Therefore, the addition of Mg, Si, and Sr to culture medium of T. suecica is recommended to achieve high steady-state cell density in semicontinuous cultures.