Species-specific bioaccumulation and biotransformation of antibiotics shape differential toxicological effects in a...

The widespread occurrence of antibiotics in aquatic ecosystems poses a significant ecological threat, yet the mechanisms underlying the differential sensitivity of species remain poorly understood, limiting the accuracy of risk assessments. Here, we investigated the bioaccumulation, biotransformation, and toxicological effects of four typical antibiotics, i.e., sulfamethoxazole (SMX), ciprofloxacin (CIP), oxytetracycline (OTC), and azithromycin (AZM) each at 1, 10, 100 μg/L, in a flow-through freshwater microcosm containing duckweeds, snails, and zebrafish for 45 days. Our results revealed distinct bioaccumulation patterns of antibiotics: duckweeds accumulated the highest levels of the antibiotics facilitated by their ionized form, while accumulation in the animals was governed by hydrophobicity and acid dissociation characteristics. By employing an integrated suspect and non-target screening approach, we identified a total of 33 transformation products (TPs), 18 of which are newly reported. Duckweeds produced the most diverse TP suite, generating unique Phase I (e.g., hydroxylation) and Phase II (e.g., glycosylation and sulfation) TPs, notably including several highly toxic TPs of CIP and OTC. Most of the TPs in the animals were a subset of those observed in duckweeds. These differential bioaccumulation and biotransformation patterns were associated with toxicological outcomes. In duckweeds, oxidative stress correlated with both accumulated parent antibiotics and toxic TPs. Snails displayed minimal physiological effects, while fish experienced oxidative stress from parent antibiotic accumulation and a gut microbiota dysbiosis associated with biomass increase. Our findings elucidate that the species-specific bioaccumulation and biotransformation of antibiotics are key factors shaping differential toxicological effects, providing critical mechanistic insights for refining ecological risk assessment.