Sulfamethazine biodegradation in sediments is driven by chronic exposure concentrations

Sulfonamide antibiotics like sulfamethazine (SMZ) can accumulate in river surface sediments and promote the development of sulfonamide-degrading microorganisms. However, it remains unclear whether the capacity of natural microbial communities to biodegrade sulfonamide is significantly stimulated above specific exposure thresholds. To investigate this, we performed a 47-day microcosm study in laboratory channels, exposing natural sediment communities to six SMZ concentrations (from C0 to C5) using an exponential dose-response design with a maximal nominal concentration of 10 mg.kg^-1 DW. At the end of the experiment, ¹⁴C-SMZ mineralization kinetics revealed a strong microbial adaptation to SMZ biodegradation at the two highest concentrations tested. Indeed, percentage mineralization was about 50 % in treatment C4 and 62 % in treatment C5 but remained lower than 2 % in all other treatments (C0 to C3). The relative abundance of sadA genes increased in C4 and C5, suggesting sadA as a key functional gene controlling potential to biodegrade SMZ within the community. The adaptation to biodegradation of SMZ was associated with changes in microbial community structure, diversity and composition. Cyanobacteria and Bacteroidota were the phyla most negatively affected by treatments C4 and C5 whereas the relative abundance of Proteobacteria increased, suggesting that at least some of the bacteria involved in SMZ biodegradation belonged to this class. Changes in diversity and the increase in the biodegradation potential of SMZ had no influence on the community tolerance to this pharmaceutical, as assessed through short-term toxicity tests on β-glucosidase activity.