Trophic transfer and biological effects of azithromycin across a freshwater food chain.

Antibiotics in the aquatic environment can bioaccumulate in aquatic organisms and transfer to higher trophic levels. While the direct toxic effects of antibiotics on aquatic organisms have been extensively studied, research on their transfer through aquatic food chains and their indirect effects on ecosystems remains limited. This study investigated the transfer and toxicity of the antibiotic azithromycin within a simplified food chain comprising the primary producer Scenedesmus obliquus, the primary consumer Daphnia magna, and the predator Chaoborus obscuripes. Toxicokinetic analyses revealed a high bioconcentration potential of azithromycin in S. obliquus, with a bioconcentration factor (BCF) of 1017 L kg⁻¹ , whereas D. magna exhibited a lower BCF of 372 L kg⁻¹ , indicating interspecific differences in uptake and elimination kinetics. In the trophic transfer experiment, the biomagnification factor from D. magna to C. obscuripes was lower than 0.032 based on the limit of quantification (LOQ), suggesting negligible biomagnification and a tendency toward trophic dilution. Despite the limited transfer efficiency, C. obscuripes fed with azithromycin-contaminated prey showed a small but statistically significant reduction in final body length (P = 0.048), suggesting a possible sublethal response under the tested dietary exposure conditions. Collectively, these results suggest that azithromycin can accumulate efficiently in primary producers but is unlikely to biomagnify along the simplified freshwater food chain tested here, while potential sublethal responses at higher trophic levels require further attention under environmentally relevant conditions.