Adsorption of antibiotics onto graphene oxide imparts their antagonistic effects on Synechocystis sp.: model development and proteomic analysis

The interactions between graphene oxide (GO) and antibiotics play an important role in determining their combined toxicity. However, the related information is lacking, especially for aquatic organisms. Here, we found that the combined toxicity of GO and antibiotics to a cyanobacterium Synechocystis sp. PCC 6803 was dependent on their interplays. GO exhibited an additive effect with florfenicol (FLO) but an antagonistic effect with erythromycin (ETM), ofloxacin (OFL), or chlorotetracycline (CTC). Furthermore, the adsorption of antibiotics onto GO resulted in their antagonistic effects. An adsorption-isobolographic model to predict the combined effect of GO and antibiotics (gamma) was developed by combining the adsorption isotherm and isobolographic method. The model was validated by the good agreement between the theoretical and experimental values of gamma, and can also be used to predict the combined toxicity of carbon nanotubes and tetracycline. Proteomic analysis further revealed that the exposure to antibiotics upregulated the pathway of the ribosome but downregulated the pathway of oxidative phosphorylation, which were attenuated by the addition of GO in the co-exposure groups. Compared with the single exposure groups of antibiotics, the addition of GO downregulated the proteins related to perceiving and transmitting the signals of hyperosmotic stress, explaining the increased membrane permeability in the combined exposure groups. This study highlights the significance of pollutant adsorption onto GO in determining their combined toxicity, and calls for further studies on other types of engineered or naturally occurring carbon-based nanomaterials.