Biofilm Source Shapes the Tolerance to Short-Term Ciprofloxacin Exposure

Periphytic biofilms are key mediators of freshwater ecosystem processes but are increasingly exposed to anthropogenic stressors. We compared biofilms sampled upstream and downstream of a wastewater treatment plant (WWTP) effluent, two source communities presumed to differ in prior exposure to wastewater-associated press disturbance, to examine differences in tolerance to ciprofloxacin (CIP) exposure. We assessed biomass, photosynthetic pigments and fatty acid (FA) profiles as well as microbial community structure using qPCR and metabarcoding. Downstream-sourced biofilms exhibited higher biomass stability, reduced pigment and FA losses and fewer bacterial and eukaryotic community shifts under CIP exposure than upstream-sourced biofilms. Upstream-sourced communities, despite higher baseline pigment and FA contents, showed pronounced sensitivity to CIP, including biomass declines and reduced prokaryotic diversity. Taxon-specific responses revealed stress-tolerant Chlorophyceae, cyanobacteria and heterotrophic taxa, which may have contributed to higher functional stability under the tested conditions. Our findings show that biofilm responses to CIP varied with source site and community composition. While not a direct test of environmental filtering, the observed patterns are consistent with its predictions. The results further suggest that legacy effects and spatial heterogeneity may influence biofilm vulnerability to antibiotic exposure, although this requires validation across multiple WWTP-impacted and reference sites.