Compartment-specific effect of sulfamethoxazole at low μg/L concentrations on microbial nitrogen assimilation in a river system

Sulfamethoxazole (SMX) is one of the most frequently detected antibiotics in rivers, with concentrations occasionally exceeding the predicted no-effect concentration (PNEC). The impact of such concentrations on the microbial activity of riverine microbial communities remains poorly studied. Here, we investigated the effect of SMX concentrations at the upper end of reported PNEC values (12.5 µg/L) on microbial communities in flume systems with either near-pristine or wastewater-impacted river water. Using a combination of microbiological and chemical methods, we found that SMX was persistent in both near-pristine and wastewater-impacted river water over a time course of 63 days, and had no significant impact on the planktonic bacterial community composition. However, there was an increase in microbial activity after SMX addition. Tracking 15N incorporation in both sample types using Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) and an Elemental Analyser - Isotope Ratio Mass Spectrometer (EA-IRMS) revealed that SMX concentrations in the test range (10, 100 and 1000 µg/L) enhanced nitrogen assimilation from ammonium up to 64 %. The highest increase was found almost always at 10 µg/L SMX. ... mehrThe response was stronger in samples from the near-pristine site compared to the wastewater-impacted site, and in planktonic biomass compared to biofilms. Overall, our findings reveal a transient increase in microbial nitrogen assimilation with environmentally relevant concentrations of SMX in a habitat-specific manner, but not of SMX degradation, which could be of significance for nutrient dynamics and primary productivity in impacted rivers.