Aquatic organisms are inevitably exposed to metallic nanoparticles (NPs) in natural environments, leading to potential harm, ecological disruption, and environmental pollution concerns. Importantly, the size of NPs plays a critical role in influencing their uptake by these organisms. Utilizing mass cytometry, we investigated the internalization characteristics of different-sized gold NPs (AuNPs) in an unicellular ciliate Tetrahymena thermophila, under a low exposure concentration of 1 ngmL