Occurrence of antibiotics and antibiotic resistance genes influenced by tides around a sewage outlet in the Yangtze Estuary, China.

The extensive use of antibiotics in anthropogenic activities enables their continuous discharge into estuarine ecosystems and thus causes the subsequent proliferation of antibiotic resistance genes (ARGs), which is also influenced by hydrodynamic conditions of the estuaries, especially tides. This study investigates the occurrences and variations of antibiotics and ARGs in the surface water and sediment around a sewage outlet in the Yangtze Estuary during a tidal cycle. A total of ten antibiotics and seven ARGs were detected. The absolute abundances of ARGs varied from 101 to 105 copies/mL in water and from 105 to 108 copies/g in sediment. Significant positive correlations between the relative abundances of sul1 (r = 0.403, p < 0.01), sul2 (r = 0.569, p < 0.01), and intI1 were revealed by Pearson analysis. The maximum concentrations of antibiotics reached 303 ng/L in water and 19212 ng/kg in sediment, respectively. Seasonal analysis showed that aqueous antibiotic diversity peaked in summer, while sedimentary antibiotic diversity dominated in winter. Risk Quotient (RQ) analysis revealed that approximately 50% of antibiotics showed measurable ecological risks with changing tidal levels. Sulfonamides (SAs) presented a higher risk than tetracyclines (TCs), notably sulfapyridine (SP). Environmental factors (except for pH) significantly positively correlated with some ARGs in water. Strong correlations between antibiotics and corresponding ARGs were observed in water, while insignificant correlations were found in sediment. During a tidal cycle, the complementary sediment-water dynamics of ARGs were observed in autumn. There was a significant positive association (p < 0.01) with tidal levels for doxycycline (DXC) and a significant negative correlation (p < 0.05) for chlortetracycline (CTC) in water.