Regulatory effect of gibberellic acid (GA3) on the biomass productivity and some metabolites of a marine microalga, Isochrysis galbana

The application of phytohormones is an effective and economical way to accelerate biomass growth and the synthesis of valuable metabolites in the microalgae industry. In this study different levels of gibberellic acid (GA3), including 0 (control), 1, 2, 4, and 6 mg L−1, were added to the growth medium of Isochrysis galbana to evaluate the growth performance, metabolites (i.e., chlorophyll a, protein, lipid, carbohydrate, and ash), and fatty acids profile, compared with the control group (0 mg L−1 GA3) during a 14-day experiment. The highest cell number (14.2 × 106 cells mL−1) and the lowest doubling time (4.31 day) were obtained in 4 mg L−1 GA3 (P < 0.05) and the highest specific growth rate was recorded in the treatments with GA3 at 2 and 4 mg L−1. The production of chlorophyll a was elevated by increasing GA3 concentration up to 4 mg L−1, while the lowest value was recorded in 6 mg L−1 GA3 (P < 0.05). The highest level of protein was measured in 1, 2, and 4 mg L−1 GA3, while the lowest value was obtained in the control group (P < 0.05). In addition, the lipid content of the microalgae treated with different GA3 concentrations was significantly increased compared with the control group. The highest polyunsaturated fatty acid (PUFA) content was achieved using GA3 at 6 mg L−1, while the highest saturated and monounsaturated fatty acids were determined in 2 mg L−1 GA3. The findings showed that GA3 at 4 mg L−1 could effectively improve the growth and biosynthesis of most metabolites in I. galbana.