Combined Exposure to Ciprofloxacin and Polystyrene Microplastics: Responses of Aquatic Organisms and Electroactive Bacteria

Combined pollution by microplastics and antibiotics is an increasing environmental concern due to their widespread occurrence in aquatic ecosystems and their potential interactions with biological communities. This study evaluated the acute toxicity of individual and combined toxicity of polystyrene microplastics (PS, 2–5 µm) and ciprofloxacin (CPX) on the electroactive bacteria Geobacter sulfurreducens, commonly used in bioelectrochemical systems (BES) for wastewater treatment, and three aquatic model organisms representing different trophic levels: Aliivibrio fischeri, Raphidocelis subcapitata and Daphnia magna. Acute ecotoxicological assays were carried out to calculate the Effective Concentration (EC). Combined toxicity was assessed using the combination index (CI)-isobologram method to identify synergistic, additive, or antagonistic interactions.

For G. sulfurreducens and A. fischeri, EC50 values for mixtures were below 1 mg L⁻¹ and 10 mg L⁻¹, respectively. In D. magna the values were also below 1 mg L⁻¹. In R. subcapitata the EC50 value was generally higher, below 10 mg L⁻¹, showing lower sensitivity. Synergistic effects were mainly observed in bacterial assays and for R. subcapitata the interaction shifted to moderate antagonism as the level of inhibition increased. For D. magna, very strong antagonism was observed, while at higher levels of immobilisation the interaction shifted towards moderate antagonism.

Overall, the sensitivity of the tested organisms followed the order: Aliivibrio fischeri > Geobacter sulfurreducens > Daphnia magna > Raphidocelis subcapitata. The mixture of CPX with 2 µm PS produced the strongest toxic effects, particularly in bacterial systems, suggesting that smaller microplastics enhance contaminant stress more efficiently than larger particles.