Mixture toxicity of the heavy metal copper and the fluoroquinolone antibiotic ciprofloxacin on Microcystis aeruginosa.

Exposure to chemical mixtures is the norm in natural environments. Yet, water quality regulations are still mostly constructed for individual chemicals. However, an important ambition of the European Green Deal is the future implementation of mixture toxicity to address the risks posed by the joint presence of multiple chemicals in aquatic ecosystems (e.g., via a mixture allocation factor, MAF). This study aimed to contribute to this by investigating the ecotoxicity of a series of binary ciprofloxacin‑copper (CIP-Cu) mixtures at environmental realistic concentrations to the cyanobacterium Microcystis aeruginosa, which is the most CIP-sensitive freshwater species identified so far. The two chemicals have distinct modes of action, however, the bioavailability of both is influenced by pH. The toxicity data of CIP-Cu mixtures were analysed to examine if significant interactions exist relative to the concentration addition (CA) and independent action (IA) models. CIP-Cu mixtures behaved antagonistically relative to the CA model, whereas there was no interactive toxicity relative to the IA model, which better describes the CIP-Cu mixture toxicity, in line with expectations based on the distinct modes of action of Cu and CIP. Furthermore, attention was placed to the influence of Cu in the low-effect concentration range (≤ EC10, the 10 % effective concentration) on CIP ecotoxicity, in order to identify potential synergistic effects that may lower the toxic threshold of CIP in the presence of Cu. Across the pH range 7.3-9.0, the presence of '≤ EC10' level of Cu did not affect the EC10 of CIP. Only at pH 8.0, but not at pH 7.3 or 9.0, the addition of an (EC10)/2 level of Cu lowered the 50 % effect concentration (EC50) of CIP from 2.8 to 2.0 μg L. The overall observations confirmed that there are no significant interactive effects between Cu and CIP relative to the IA model. Therefore, regulatory environmental threshold concentrations for CIP derived from single-chemical tests, are protective in situations of co-occurrence with Cu, provided that Cu remains below its own environmental quality standards.