Impact of riverbed renaturalization on the attenuation of antibiotics and antimicrobial resistance in wastewater effluent-dominated streams.

Mediterranean streams contain substantial proportions of wastewater treatment plant effluent, occasionally constituting the entire water flow. Here, we analysed the seasonal occurrence of 23 antibiotics (AB) and antimicrobial resistance (AMR) by tracking 3 marker genes and bacterial community dynamics in two wastewater effluent-dominated streams. One stream was renaturalized with meanders and vegetation, while the other was linear and had a low vegetation density. The concentration of ABs in the effluents ranged from 33 to 1,313 ng·L during summer and 4 to 2,337 ng·L during winter. The attenuation of ABs 3.5 km downstream varied depending on the compound, ranging from 42 to 88%. The half-lives of ABs obtained for the streams were 0.2 to 4.1 h in summer and 0.6 to 12.6 h in winter. Most ABs had a half-life of < 5 h, except sulfamethoxazole, acetyl-sulfamethoxazole, and trimethoprim. The vegetated stream exhibited a higher attenuation of ABs than the unaltered stream (88% vs. 67% on average), while also showing lower half-life values (on average 1.3 vs. 3.8 h). The bacterial community profiles in both streams were typical of effluents, with greater longitudinal dynamics in the vegetated stream during summer than in the other samplings. Similarly, AMR indicator genes decreased most in the vegetated stream during summer (0.8-1.1 log units). The ecotoxicological risk and the potential microbial risk selection values downstream at 3.5 km were reduced by > 45%. Overall, the results suggest that vegetation and meanders play an important role in the in-stream attenuation of ABs and AMRs.