Environmentally relevant concentrations of the antibiotic azithromycin enhance the toxicity of the cyanobacterium Microcystis aeruginosa on the water flea Daphnia similis.

Artificial eutrophication has degraded water bodies worldwide, promoting harmful cyanobacterial blooms (CyanoHABs). In addition to nutrient loading, antibiotics are increasingly being introduced into aquatic environments, exacerbating water pollution, and the impact on biota. Among antibiotics, azithromycin (AZT) has displayed a rise in environmental occurrence following its increased global consumption during the COVID-19 pandemic. Therefore, we assessed the effect of environmentally relevant concentrations of AZT (≤3 μg l-1) on the interaction between the toxic cyanobacterium Microcystis aeruginosa and the freshwater zooplankton Daphnia similis. The acute toxicity of both the cyanobacterium and antibiotic on Daphnia was determined, followed by an herbivory and a life-table assay to assess behavioral and sublethal effects under single and combined exposures. The 96-hour LC₅₀ and EC₅₀ for AZT were 35.2 and 29.1 mg l-1, respectively. The risk quotient for D. similis was 0.098, indicating a low ecological risk (RQ < 0.1). Regarding M. aeruginosa, 96-hour LC₅₀ and EC₅₀ values were 713 and 303 μgC l-1, respectively, with RQ values >700, indicating a high ecological risk. Environmental levels of AZT did not affect Daphnia, while the cyanobacterium inhibited zooplankter feeding and life-table responses. However, combined exposure to AZT and toxic M. aeruginosa resulted in a greater decrease in survival and overall population fitness, besides an increased cyanobacterial ingestion. These findings suggest that environmental concentrations of AZT may intensify the impacts of toxic cyanobacteria on zooplankton. This highlights the need for further research into the mechanisms underlying the enhanced toxicity of cyanobacteria promoted by antibiotics at environmental levels.