How climate stressors intensify antibiotic toxicity: A histopathological study in zebrafish

As global changes alter aquatic environments, temperature and pH fluctuations affect the behavior and toxicity of emerging contaminants, such as antibiotics. Liver histological analysis in fish reveals significant effects under varying environmental conditions. This study assessed histopathological liver alterations in Danio rerio, after chronic exposure to environmentally relevant concentrations of sulfamethoxazole (SMX), trimethoprim (TRIM), and their mixture (MIX) under environmental scenarios: temperature (26, 28, and 32 ºC), pH (6.5, 7.5, and 9.0), and combined scenario (28 ºC + pH 9.0). Liver alterations were evaluated using qualitative and semi-quantitative approaches, and the total liver histopathological index (LI) was quantified. An Independent Action (IA) model was used to integrate the effects of temperature and pH with antibiotic exposure, revealing how these factors interact and affect liver histopathology. Results of qualitative analysis showed circulatory (e.g., increased sinusoidal space), regressive (e.g., necrosis, and hepatocellular degeneration), and progressive (e.g., hepatocytes nuclear hypertrophy) changes in all scenarios tested. Semi-quantitative analysis revealed: i) rising temperature intensified LI, even without antibiotics; ii) at 32 ºC, SMX caused severe lesions, indicating a temperature-dependent rise in LI; iii) LI increased for all antibiotics at pH 7.5, and for TRIM and MIX at pH 9.0; iv) no significant alterations were observed under combined scenario. Histological lesions were observed across all antibiotic treatments and scenarios, reflecting sustained adverse impacts. IA model showed synergistic effects for SMX and TRIM, with increased liver damage under combined stressors, while MIX showed antagonistic effects, with lower damage than expected. These findings highlight the susceptibility of fish liver to antibiotics, particularly under altered temperature or pH conditions. Overall, our findings demonstrate that environmental stressors act as active drivers of antibiotic toxicity, reinforcing the importance of multi-stressor approaches for ecological risk assessment under climate change scenarios.