Size-dependent internalization of gold nanoparticles in individual Tetrahymena thermophila characterized by single-cell mass cytometry

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⁻¹. This investigation, conducted at both the population and single-cell levels, revealed that the size of AuNPs significantly affected their uptake by T. thermophila cells. The average mass of intracellular AuNPs peaked at 0.5 h and subsequently decreased, attributed to the efflux of AuNPs or cell proliferation. Larger AuNPs resulted in a lower average intracellular AuNPs mass and a smaller proportion of T. thermophila cells accumulating AuNPs (Au-positive (Au^P) T. thermophila). However, when exposed to larger AuNPs, the Au^P T. thermophila cells had a higher AuNPs mass and volumetric concentration factors compared to their exposure to smaller AuNPs. After exposure, while most Au^P T. thermophila cells had intracellular Au content below 2.41 × 10⁻¹⁵ g cell⁻¹, the small groups of T. thermophila cells that accumulated higher mass of AuNPs may be the ones more susceptible to the effects of AuNPs exposure. Additionally, we developed a three-dimensional fitting surface model to depict the relationship among exposure time, AuNP size, and intracellular AuNPs mass in individual T. thermophila cells. This study enhances our understanding of size-specific NPs accumulation in unicellular organisms and provides valuable insights for ecological risk assessment of different sized NPs.