Lemna minor ’s accumulation and physiological response to common over-the-counter pharmaceuticals: paracetamol, pseudoephedrine, and their combination

Pharmaceutical contamination of aquatic environments is an emerging concern, particularly for over-the-counter drugs such as paracetamol (PA) and pseudoephedrine (PSE), which persist in surface waters due to incomplete wastewater treatment. This study investigated the bioaccumulation and physiological responses of Lemna minor exposed to PA, PSE, and their mixtures at environmentally relevant concentrations. Accumulation in fronds was quantified, and plant responses were assessed using biomass, chlorophyll content, and phenolic compounds. PSE showed strong, concentration-dependent accumulation, increasing significantly (p < 0.05) from 6.3 ± 1 µg/g d.w. at 0.001 mg/L to 219 ± 51 µg/g d.w. at 0.1 mg/L. In contrast, PA remained below detection limits at 0.01-0.1 mg/L and was detected only at 1 mg/L (12 ± 7 µg/g d.w.), suggesting limited uptake or rapid transformation. Under mixture exposure, PSE accumulation decreased by 39% at the lowest concentration but increased by 62% at the highest mixture level compared with single-compound treatments, whereas PA uptake remained largely unchanged. PA significantly stimulated biomass production, increasing fresh weight by up to 168% at 0.1 mg/L (p < 0.05), while PSE increased biomass by up to 119%. Low-dose exposures induced elevated phenolic levels, indicating activation of stress-related metabolic pathways, whereas higher PA concentrations enhanced chlorophyll a content, with the highest mixture treatment showing a 66% increase relative to control. PCA revealed a shift from photosynthetic investment toward secondary metabolism under pharmaceutical exposure. These asymmetric interactions demonstrate that mixture effects cannot be predicted from assays. Lemna minor exhibited sensitivity to pharmaceutical stress and a capacity to accumulate PSE, supporting its use as a bioindicator and phytoremediator.