Toxic effects of long-term dual or single exposure to oxytetracycline and arsenic on Xenopus tropicalis living in duck wastewater

Direct discharge of aquaculture wastewater may have toxic effects, due to the presence of heavy metals, antibiotics, and even resistant pathogens, but little attention has been given. Here, tanks simulating a wild ecosystem were built to study the effects of long-term exposure to duck wastewater containing oxytetracycline (OTC) and/or arsenic (As) on the growth, physiological function, and gut microbiota evolution of Xenopus tropicalis. The results showed that duck wastewater had no apparent impact on X. tropicalis, but the impact increased significantly (P < 0.05) with exposure to OTC and/or As, especially the impact on body weight and growth rate. Biochemical indicators revealed varying degrees of oxidative stress damage, hepatotoxicity (inflammation, necrosis, and sinusoids), and collagen fibrosis of X. tropicalis in all treated groups after 72 days of exposure, which indirectly inhibited X. tropicalis growth. Moreover, 16S rDNA amplicon sequencing results showed that the gut microbiota structure and metabolic function were perturbed after chronic exposure, which might be the leading cause of growth inhibition. Interestingly, the abundance of intestinal resistance genes (RGs) increased with exposure time owing to the combined direct and indirect effects of stress factors in duck wastewater. Moreover, once the RGs were expressed, the resistance persisted for at least 24 days, especially that conferred by tetA. These results provide evidence of the toxic effects of DW containing OTC (0.1-4.0 mg/L) and/or As (0.3-3.5 µg/L) on amphibians and indicate that it is vital to limit the usage of heavy metals and antibiotics on farms to control the biotoxicity of wastewater.