Taking into account the worrying scenario of water scarcity, it is essential to enable more efficient technologies for wastewater treatment. Wastewater may be treated by conventional biological processes that remove pathogenic organisms, particulate and soluble organic compounds, and other components. However, secondary effluents from treatment plants may still contain toxic elements or high concentrations of inorganic nutrients (mainly nitrogen and phosphorus), which enable the growth of photosynthetic microorganisms in water bodies, resulting in eutrophication. In this context, cultivation of photosynthetic microorganisms in secondary wastewater from sewage treatment allows the removal of nutrients from such wastewater, reducing the possibility of eutrophication. Moreover, microalgal biomass, produced in this tertiary wastewater treatment, may be harvested by different methods with the potential for different applications, such as fertilizer and biofuel.

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.