Microbiota modulate metformin phytoremediation and stress responses in Lemna minor

The phytoremediation of pharmaceuticals by aquatic plants is influenced by both plant physiology and microbial interactions. This study investigated how microbial symbiosis modulates the uptake, transformation, and physiological responses of Lemna minor to metformin. Plants were cultivated under axenic and non-axenic conditions and exposed to 10, 50, and 100 µg/L metformin for 7 days. Both systems removed > 99 % of metformin from water, but exhibited distinct accumulation patterns, stress biomarkers, and metabolic profiles. Axenic plants accumulated 2.1-fold more metformin and 1.7-fold more guanylurea, a key metformin metabolite, at 100 µg/L, along with increased oxidative stress (↑MDA) and elevated cytochrome P450 activity. Non-axenic systems exhibited extracellular guanylurea concentrations up to 0.9 µg/L, indicating a reliance on intrinsic detoxification pathways. Guanylurea was detected in both plant types, but appeared in water only under non-axenic conditions, suggesting microbial-mediated excretion. Principal component analysis revealed that guanylurea accumulation was correlated with elevated P450 activity, lipid peroxidation, and hormonal shifts, especially in axenic plants. These results confirmed that L. minor can biotransform metformin independently of microbes, albeit with a greater physiological burden. Microbial presence mitigates stress and enhances extracellular degradation. Overall, the data demonstrate complementary roles of plants and microbiota, with microbiota reducing internal contaminant load and protecting plant homeostasis.