Dietary exposure to sulfamethazine, nanoplastics and their binary mixture disrupts the spermatogenesis of marine medaka (Oryzias melastigma)

In the coastal environment, the co-occurrence of antibiotic and nanoplastic pollution is common. Investigating their individual and combined toxicity to marine organisms is of great necessity. In the present study, the reproductive toxicity of sulfamethazine (SMZ) and nanoplastics (polystyrene, PS) via the dietary route on the spermatogenesis of marine medaka (Oryzias melastigma) was examined. After 30 d of dietary exposure, SMZ alone decreased the gonadosomatic index (GSI) value (∼35%) and the proportion of undifferentiated type A spermatogonia (Aund) (∼40%), probably by disrupting the testicular sex hormone production, the spermatogenesis-related growth factor network and the balance of apoptosis. Individual exposure to PS did not affect the GSI value or the proportions of germ cells at different developmental stages, but dysregulated the expression of several spermatogenesis-related genes. Interestingly, the presence of PS alleviated the decreased GSI value caused by SMZ. This alleviation effect was achieved by enhancing the spermatogonia differentiation instead of reversing the suppressed self-renewal of Aund, suggesting that the mixture of PS and SMZ could cause reproductive effects in a different way. These findings expand our knowledge of threats of ubiquitous antibiotic and nanoplastic pollution to fish reproduction and population.