Temporal dynamics of lake microbiota under nanoplastic and enrofloxacin stress

Nanoplastics (NPs) and antibiotics co-occur in freshwaters, yet their time-resolved, joint impacts on bacterial communities remain poorly constrained. We exposed lake microbiota to polystyrene NPs (∼420 nm; 10³-10⁹ particles L^-1) and enrofloxacin (ENR; 10 or 100 ng L^-1), tracking abundance, per-cell respiration, community-level physiological profiles (CLPP), and taxonomy for 14 days. By day 7, both stressors reduced abundance, with stronger ENR effects; CLPP was depressed mainly by ENR, whereas per-cell respiration rose in several NP × ENR combinations, and substrate use shifted transiently from complex carbons toward simpler substrates (notably amines). By day 14, population-level responses attenuated, but community structure diverged: α-diversity remained depressed only under high ENR with NPs, β-diversity resolved into three assemblages (elevated Sporichthyaceae and Comamonadaceae; reduced Burkholderiaceae and Verrucomicrobiaceae), and Actinobacteriota remained dominant. A Mantel test indicated that taxonomic restructuring increasingly explained physiological profiles at day 14. A complementary fate experiment showed dissolved ENR declined strongly over days, faster in bacteria-present (unfiltered) water and further with NPs, with several NP-amended, unfiltered treatments approaching the analytical LOD; a 4 h assay detected no treatment differences, indicating negligible immediate photolysis or rapid sorption. Together, results show that biological sinks, augmented by NPs over multi-day timescales, govern ENR removal, explaining the time-dependent decoupling between early physiological stress and later partial functional recovery alongside persistent compositional shifts. Overall, NP-antibiotic interactions were time-dependent and sometimes antagonistic at the population level, yet produced lasting community reassembly. Time-resolved, multi-level assessments are essential for ecological risk appraisal of contaminant mixtures in freshwater microbiomes.