Oxygen-dependent dynamics of metformin biodegradation at the sediment–water interface and non-additive effects on prokaryotic communities

Metformin, a widely prescribed antidiabetic drug, is frequently detected in aquatic environments due to its limited removal during wastewater treatment. Yet its environmental behaviour and ecological effect on microbial communities at the biogeochemically active sediment-water interface remains poorly understood. We examined metformin degradation and its effect on prokaryotic communities under diverse controlled oxygenation conditions in laboratory microcosms mimicking the sediment-water interface. In autoclaved microcosms, metformin showed slow dissipation without significant formation of transformation products, irrespective of oxygenation conditions. In contrast, biotic microcosms showed metformin biodegradation within 13 days following a lag phase of up to 28 days. Degradation was faster under anoxic conditions. Guanylurea was the sole transformation product detected, suggesting enzymatic hydrolysis and potential use of dimethylamine and guanylurea as carbon and nitrogen sources for microbial growth. Prokaryotic community composition was significantly affected by oxygenation conditions and repeated metformin contamination. Metformin exposure and changes in oxygenation regime had predominantly additive effects. Nevertheless, non-additive effects on procaryotic community composition emerged over time, particularly after repeated metformin exposure. Twenty-one bacterial taxonomic biomarkers of metformin exposure were tentatively identified, including methylotrophic taxa potentially associated with the utilisation of metformin and its metabolites. Microbial activity was essential for metformin dissipation at the sediment-water interface, and oxygenation regime modulated the effect of metformin and its transformation on prokaryotic communities. Our study shows the importance of oxygenation conditions and microbial community responses in assessing the behaviour and associated risks of pharmaceuticals in dynamic aquatic environments such as the sediment-water interface.