Blocking the "Trojan Horse" effect: Humic acid alleviates the combined toxicity of amino-modified nanoplastics and tetracycline to microalgae by inhibiting heteroaggregation and oxidative stress.

Given the ever-increasing accumulation of nanoplastics in aquatic environments worldwide, there is concern that plastic pollution will lead to ecological disasters due to the transfer of man-made pollutants, such as antibiotics, into aquatic life by nanoplastics. However, the intensity of this "Trojan Horse" effect may be influenced by natural organic matter that can interact with nanoplastics and antibiotics, possibly affecting their biological impacts. Thus, this study assessed how humic acid (HA, 10 mg L-1) modulates the combined effects of amino-modified polystyrene nanoplastics (NH2PS, 25 mg L-1) and tetracycline (TC, 5-15 mg L-1) on microalga Chlorella vulgaris following 96 h exposure. Results showed that exposure to TC and NH2PS, especially in combination, elicited various adverse effects on microalgae (e.g., membrane disintegration, mitochondrial dysfunction, growth retardation and impaired synthesis of photosynthetic pigments) due to elevated oxidative stress resulting from heteroaggregation between pollutants and algal cells. This adsorption of TC onto NH2PS created a carrier-enhanced delivery system that amplified cellular internalization of both pollutants, leading to persistent disruption of cellular homeostasis. Importantly, HA could ameliorate these toxic effects on microalgae as physicochemical characterization revealed that HA competitively reduced TC adsorption onto NH2PS surfaces and enhanced pollutant dispersion stability, thereby reducing the oxidative and membrane damage caused by direct contacts with pollutants. Overall, this study provides a mechanistic understanding of how natural organic matter mitigates the "Trojan Horse" effect of nanoplastics with antibiotics, suggesting that the ecological impacts of nanoplastics may be less severe than anticipated under realistic conditions. Such insights are crucial for refining ecological risk assessment and guiding nanoplastic pollution management.