Humic acid enhances adsorption of antibiotic ciprofloxacin on polylactic acid microplastics, leading to reproductive and mitochondrial toxicity in Daphnia magna: Quantitative analysis.

Biodegradable microplastics (BMPs), which undergo incomplete degradation in freshwater environments, have emerged as potential vectors of micropollutants. However, their adsorption behavior and combined toxicity with coexisting micropollutants in the presence of natural organic matter (NOM) remain poorly understood. This study investigated the interaction of polylactic acid (PLA) microplastics (MPs)-humic acid (HA), adsorption of ciprofloxacin (CIP), and the combined toxicity to Daphnia magna exposed to PLA MPs, HA, and CIP for 17 days. PLA MPs interacted with HA to form heteroaggregates featuring an HA corona and a bridge-like structure, increasing the surface O/C ratio. HA initially suppressed CIP adsorption onto PLA MPs during the first hour, but significantly enhanced it after three hours, reaching 55.46 % equilibrium adsorption. Furthermore, combined exposure with HA exacerbated mitochondrial DNA damage in D. magna, while ATP levels remained stable due to a compensatory response. At the individual level, this exposure impaired reproduction, embryonic development, and somatic growth. Our findings suggest that BMPs, in the presence of NOM, enhance ecological toxicity through interactions with environmental pollutants, highlighting their emerging risks in freshwater ecosystems.