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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
Summary
Tiny plastic particles in water can act like a "Trojan Horse," carrying antibiotics into algae and causing more cell damage than either pollutant alone. This study found that humic acid, a natural substance common in soil and water, can block this effect by preventing the antibiotics from sticking to the plastic in the first place. This is good news because it suggests that in real-world waters rich in natural organic matter, nanoplastic pollution may be less harmful to aquatic ecosystems, and the food chain we depend on, than lab studies alone might suggest.
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) modulates the combined effects of amino-modified polystyrene nanoplastics (NHPS, 25 mg L) and tetracycline (TC, 5-15 mg L) on microalga Chlorella vulgaris following 96 h exposure. Results showed that exposure to TC and NHPS, 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 NHPS 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 NHPS 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.