Dose-Dependent Effects of ZnO Nanoparticles Towards the Microalgae Lobosphaera: Compensation of Salt Stress at Low Concentration and Toxicity at High Concentrations

This study investigated the concentration-dependent effects of zinc oxide nanoparticles (ZnO NPs, 30–70 nm) on the freshwater microalga Lobosphaera sp. under different salinity conditions (0–4 g L−1 NaCl). ZnO NPs demonstrated dual effects: low concentration (0.75 mg L−1) enhanced growth and alleviated salt stress, while higher concentrations (7.5–75 mg L−1) caused significant growth inhibition (up to 52%) and induced oxidative stress. Salinity did not significantly affect NPs aggregation patterns, and neither salinity nor aggregation degree influenced toxicity outcomes. NPs concentration plays a dominant role of toxicological effects. Dose-dependent increases in catalase activity and ROS-positive cells confirmed NPs-induced oxidative stress. Crucially, zinc bioaccumulation correlated with NPs concentration but dissociated from dissolved Zn2+ release, demonstrating particle-driven toxicity. Our findings challenge the ion-release paradigm and highlight the potential of low-dose ZnO nanoparticles as effective stress-protectors in algal biotechnology, offering new strategies for enhancing microalgal resilience under environmental stress.