Phytotoxicity Of Oxytetracycline To Lettuce And Its Photolysis In Metal Ion Solutions

Oxytetracycline (OTC) is one of broad-spectrum antibiotics, which is widely used for the treatment of infectious diseases in livestock and fish. It can enter water and soil through various pathways, which can cause potential ecotoxicological effects, such as development of antibiotic-resistant genes. Therefore, the study of environment behavior and ecotoxicological effects of OTC is very important.In order to study the uptake, phytotoxicity and the risk of OTC by plant, a series of experiments were conducted in hydroponic system using lettuce as a model species. It was found that the growth of lettuce root was inhibited at the OTC concentration of 1 mg/L. When the concentration of OTC increased to higher than 5 mg/L, both root and foliage exhibited phytotoxicity response. Furthermore, the water and trace elements (Fe, Cu, Zn, Mn) uptake by lettuce were inhibited as well. The effects of pH (OTC = 1 mg/L; pH = 5-8) were also studied. It was shown that pH could affect the uptake of OTC, while the influence on the lettuce growth was insignificant. In this study, it was also set up the HPLC method to measure the residues of OTC in lettuce, celery and pakchoi cabbage. The recovery of OTC was from 66.8% to 82.9%, and the RSD values of replicates were less than 10%. The results indicated that the method is accurate and stable.Photolysis is one of the important pathways for degradation of OTC in nature water. The effects of different metal ions on the photolysis rate of OTC were studied at different pH values. The photolysis rate of OTC was accelerated with addition of Ca²⁺, Mg²⁺, Zn²⁺ and Cu²⁺ at pH=7.0. However, when pH value was adjusted to 8.5, the situation was opposite to pH = 7 with addition of Ca²⁺ and Mg²⁺. According to the spectrometry analysis, the influence of metal ions on the photolysis of OTC was probably related to the complexation between them. In the photolysis process, OTC could generate ¹O₂ and O₂^? at pH = 8.5 and in pH = 7.0 metal ion solutions. The calculation results of photolysis contribution ratio of direct and indirect (¹O₂ and O₂^?) showed that indirect (¹O₂ and O₂^?) photolysis was significant for accelerating the photolysis rate. In sea water, the changes of OTC photolysis rate coincided with OTC in addition of Ca²⁺and Mg²⁺. Therefore, the photolysis of OTC is influenced by Ca²⁺and Mg²⁺ in sea water.