The combined toxicity of binary mixtures of antibiotics against the cyanobacterium Microcystis is dose-dependent: insight from a theoretical nonlinear combined toxicity assessment method

The entry of antibiotics into aquatic ecosystems has a serious impact. Antibiotics usually exist as mixtures in natural water bodies. Therefore, it is particularly important to evaluate the mixed toxicity of antibiotic mixtures. The study of the combined toxicity of binary mixtures of antibiotics is the basis for exploring the mixed toxicity ofmultiple antibiotics. In this investigation, Microcystis aeruginosa (M. aeruginosa) was used as the test organism, and a theoretical nonlinear combined toxicity assessment method was adopted to evaluate the effects of binary mixtures of antibiotics consisting of tetracycline (TC), sulfadiazine (SD), and sulfamethoxazole (SMX) on cell growth, enzymatic activity, and gene expression. The median lethal concentrations of TC, SD, and SMX to M. aeruginosa were 0.52 mg L-1, 1.65mg L-1, and 0.71 mg L-1, respectively. The results from the theoretical nonlinear combined toxicity assessment method showed that SD + TC was synergistic at low concentrations and antagonistic at high concentrations, while the combinations of SMX + SD and SMX + TC were synergistic. The determination of enzymatic activity and gene expression indicated that the antibiotics could inhibit the growth of M. aeruginosa by destroying the cell membrane structure, inhibiting photosynthesis, impeding the cell division process and the electron transfer process, and destroying the molecular structure of proteins and DNA. Different combinations of antibiotics have different degrees of damage to the antioxidant system and cell membrane self-repair function of M. aeruginosa, which are the reasons for the different combined toxicity effects.