Pharmaceutical cocktails in aquatic ecosystems: unveiling the hidden ecotoxicological threats to Microcystis aeruginosa.

Pharmaceutical compounds (PhACs), continuously released into the environment, represent a growing ecotoxicological threat to aquatic ecosystems. While the toxicity of individual pharmaceuticals has been extensively investigated, the effects of complex mixtures, the so-called "cocktail effect", remain poorly understood, particularly for primary producers such as cyanobacteria. This study evaluates the individual and combined effects of four commonly detected pharmaceuticals in aquatic environments: acetaminophen (APAP), atenolol (ATN), carbamazepine (CBZ), and diazepam (DZP), using the model cyanobacterium Microcystis aeruginosa over a 25-day exposure period at environmentally relevant concentrations (0.01-545 µg/L). The results demonstrate a clear dose-dependent toxicity of the individual compounds (biomass reduced by 85-91%; chlorophyll reduced by 54-70%). Exposure to lower concentrations exhibited moderate reductions in chlorophyll (0.23-0.81 mg/L to 4.90-5.96 mg/L). The most remarkable results are the amplification of toxicity at combined low concentration (low-mix group caused 52% growth inhibition compared to only 14-24% of single pharmaceuticals at the same low doses). The high-dose mixture had the greatest effect on M. aeruginosa by 58.7% growth inhibition. Multi-biomarker analyses indicated significant changes in 1) esterase activity (60.7% decrease), 2) microcystin-LC production (fivefold increase from 0.55 to 2.90 µg/L), and 3) genotoxic effects (IF > 1.5), as evidenced by the SOS chromotest particularly in treatments containing CBZ. Overall, the multi-biomarker responses indicate that pharmaceutical mixtures overwhelm cellular defenses, inducing amplified stress responses even at environmentally realistic concentrations. These findings highlight the urgent need to incorporate mixture toxicity into risk assessment frameworks.