Size-dependent aquatic toxicity of TiO2 and ZnO nanoparticles in Chlorella vulgaris: antagonistic interactions at high concentrations vs. synergistic effects at environmental levels

The widespread use of titanium dioxide (TiO 2 NPs) and zinc oxide nanoparticles (ZnONPs) in sunscreens necessitates a deeper understanding of their size-dependent aquatic toxicity and interaction mechanisms. This study systematically evaluates the individual and combined effects of TiO 2 NPs (5-10, 40, 100   nm) and ZnONPs (30, 50, 90   nm) on Chlorella vulgaris , integrating physicochemical behavior, oxidative stress responses, and interaction effects. ZnONPs exhibited 5.0–12.6 times higher toxicity than TiO 2 NPs, with EC 50 values of 55.9–77.0   mg   L -1 for ZnONPs versus 385.8–704.3   mg   L -1 for TiO 2 NPs. Smaller TiO 2 NPs (5–10   nm) showed greater toxicity ( EC 50 = 385.8   mg   L -1 ) than larger ones (704.3   mg   L -1 for 100   nm), while larger ZnONPs (90   nm) released 40% more Zn 2+ ions than smaller ones (20% for 30   nm). Joint toxicity presented was antagonistic at high concentrations but synergistic at low ( EC 5 ) and environmental levels ( p < 0.05). Small-sized mixtures (10   nm TiO 2 NPs + 30   nm ZnONPs) and large-sized mixtures (100   nm TiO 2 NPs + 90   nm ZnONPs) elevated malondialdehyde (MDA) levels by 1.8-fold compared to individual treatments. After 48   h depuration, reductions in MDA and superoxide dismutase (SOD) activities suggested cellular self-recovery, except the medium-sized mixtures remained persistent reactive oxygen species (ROS) depletion and impaired enzymatic recovery at specific concentration levels. These results provide new insights into nanoparticle toxicity mechanisms, underscoring that synergistic effects at environmentally realistic concentrations (ng L -1 to μg L -1 ) may pose unanticipated ecological risks to primary producers and disrupt aquatic food webs.