ROS regulation in Dunaliella salina by fulvic acid: induction of enzymes related to the ascorbate–glutathione pathway and antioxidant metabolites

Fulvic acid (FA) is an active humic substance and is found to improve the antioxidant defence system under unfavourable conditions. Dunaliella salina is a unicellular microalga and is a rich source of β-carotene. In this study the impact of fulvic acid (FA) was investigated on some physiological parameters, H₂O₂ production, antioxidant metabolites, and enzymatic responses related to the ascorbate (ASA)-glutathione (GSH) pathway in D. salina. Results showed that 100 mg FA L⁻¹ maintained the D. salina cell growth compared to control, and exhibited as a suitable treatment to induce carotenoids compared to 50 FA mg L⁻¹. However, 200 FA mg L⁻¹ significantly decreased the cell biomass. FA (100 mg L⁻¹) enhanced the total carotenoids (3.92 pg cell⁻¹), β-carotene (2.46 pg cell⁻¹), and lutein (0.81 pg cell⁻¹) after 7 days, which was accompanied by a reduction in the chlorophyll a and b. Cell number did not change significantly under FA and cells trended to be round in shape after 7 days. Protein content showed an insignificant change, while H₂O₂ level was promoted by FA treatment, which was related to the regulation of the ASA-GSH pathway. The activity of ascorbate peroxidase, superoxide dismutase, and dehydroascorbate reductase was more promoted in the FA-treated cells, while glutathione reductase activity and glutathione content were just induced under Sevier oxidative conditions. The findings of this research suggest new insights into the inducing impact of FA on the ASA-GSH pathway and carotenoid biosynthesis to regulate the ROS level in D. salina.