Water type modulates particle state and toxicity of polyethylene terephthalate nanoplastics.

Nanoplastic toxicity is typically assessed using pristine particles under simplified laboratory conditions, despite growing evidence that environmental transformations can substantially alter particle behaviour and biological effects. Here, we investigated how water type influences the particle state and toxicity of polyethylene terephthalate nanoplastics (PET-NPs) in the freshwater zooplankton Daphnia magna. In a chronic life-history assay, exposure to 0.5 mg L-1 PET-NPs caused no detectable effects on survival, growth or reproduction, whereas exposure to 10 mg L-1 resulted in complete mortality. We then incubated PET-NPs for four days in reverse osmosis water, ASTM artificial freshwater or natural freshwater to examine short-term particle transformations prior to exposure. Toxicity remained high following incubation in reverse osmosis water (80% mortality) and ASTM water (60% mortality), with ASTM-transformed particles additionally reducing growth and reproduction. In contrast, PET-NPs transformed in natural freshwater caused no detectable lethal or sub-lethal effects under the conditions tested. Particle-size measurements showed extensive aggregation in ASTM water, whereas PET-NPs remained largely dispersed in both reverse osmosis water and natural freshwater. Natural freshwater also altered particle zeta potential, indicating changes in surface properties. The contrasting toxicity of similarly dispersed particles in reverse osmosis and natural freshwater indicates that aggregation alone cannot explain the biological responses. Together, these findings demonstrate that water type can rapidly transform PET-NP particle state and substantially modify chronic biological effects. They highlight the importance of considering environmentally induced particle transformations when assessing nanoplastic toxicity.