Oxygen-dependent dynamics of metformin dissipation at the sediment-water interface and their effects on prokaryotic communities

The major antidiabetic drug metformin is frequently detected in aquatic environments due to anthropic contamination by unmetabolized metformin and only partial subsequent removal by wastewater treatment. Here we examined the degradation and impact of metformin (17.6 µM, i.e., 2.3 mg L-1) on prokaryotic communities in laboratory microcosms mimicking the sediment-water interface under different oxygen conditions, including variations between oxic and anoxic conditions. Slow metformin dissipation and limited formation of transformation products were observed in abiotic experiments irrespectively of oxygen status. However, oxygenation and incubation time significantly impacted the composition of prokaryotic communities under biotic conditions, with increasing effect of metformin upon repeated exposure. Metformin degradation was complete within less than 13 days after an initial lag up to 28 days. Guanylurea was detected transiently as the unique transformation product. This suggests that metformin was degraded through pathways involving metformin hydrolase yielding guanylurea and dimethylamine as a potential carbon source for microbial growth. Changes in procaryotic communities indicated that the combined effects of metformin exposure and oxygen levels were mainly additive. However, synergistic or antagonistic effects were also observed for some taxa, enabling the identification of potential bioindicators of metformin exposure under varying oxygen conditions. Overall, this study underscores the importance of considering environmental factors, prokaryotic communities, and their interplay when evaluating pharmaceutical contamination and its effects at the sediment-water interface.