The oxidative stress response of oxytetracycline in the ciliate Pseudocohnilembus persalinus

Oxytetracycline (OTC) is commonly employed in fish farms to prevent bacterial infections in China, and because of their widely and intensive use, the potential harmful effects on organisms in aquatic environment are of great concern. Ciliates play an important role in aquatic food webs as secondary producers, and Pseudocohnilembus persalinus, is one kind of them which are easily found in fish farms, surviving in polluted water. Therefore, using P. persalinus as experimental models, this study investigated the effects of oxytetracycline (OTC) on the growth, antioxidant system and morphological damage in pollution-resistant ciliates species. Our results showed that the 96-h EC50 values for OTC of P. persalinus was 21.38 mg L−1. The increased level of SOD and GSH during 96 h OTC stress was related to an adaptive response under oxidative stress induced in ciliates. Additionally, sod1, sod2 and sod3 exhibited a significant increased expression level compared to control group at 24 h treatment, indicating a promoting of dense system in ciliates at this exposure time. However, only sod1 and sod2 showed raised expression level at 48 h stress, showing the different sensitive of gene isoforms to some extent. With OTC treatment, damage of regular wrinkles, shrunk, twisted on the cell surface, even forming cyst of scuticociliatid ciliate cells were firstly observed by SEM (scanning electron microscope) in this study. Overall, physiological, molecular and morphological information on the toxicological studies of ciliates and more information on possibility of ciliates as indicators of contamination were provided in this study.