Plant-Derived Aqueous Extracts as Bio-Based Modulators of Dunaliella salina Growth for Sustainable Cultivation

This study assessed the effects of aqueous extracts from turmeric (Curcuma longa), mint (Mentha sp.), grape peel (Vitis sp.), and mango peel (Mangifera indica) on the growth and total carotenoid content of the microalga Dunaliella salina. A two-stage cultivation strategy was employed, in which aqueous extracts prepared by decoction were added at 0.5% (v/v) either at the inoculation or after the exponential growth phase. Microalgal growth was monitored by direct cell counting, and total carotenoid content was determined spectrophotometrically in the harvested biomass. The chemical profiles of the extracts were characterised by HPLC-MS/MS, enabling the annotation of key phenolic and flavonoid compounds. Specific orange turmeric and mint formulations enhanced microalgal cell proliferation by up to 31.88% and 28.95%, respectively, compared to the control. However, despite the higher biomass, the total carotenoid content did not increase correspondingly; orange turmeric extract even led to reductions of 90.84 to 92.86% in carotenoid content. These findings suggest that exogenous antioxidants may suppress the oxidative stress signals required to trigger carotenoid biosynthesis in D. salina. Additionally, grape peel extracts prepared at higher temperatures promoted better algal growth than those obtained at lower temperatures. These findings highlight the dual role of plant extracts in modulating microalgal growth and secondary metabolism, demonstrating that antioxidant-rich extracts may be used to enhance biomass accumulation before the subsequent induction of stress-mediated carotenogenesis, thereby potentially improving biotechnological yields.