Influences of two antibiotic contaminants on the production, release and toxicity of microcystins.

The influences of spiramycin and amoxicillin on the algal growth, production and release of target microcystins (MCs), MC-LR, MC-RR and MC-YR, in Microcystis aeruginosa were investigated through the seven-day exposure test. Spiramycin were more toxic to M. aeruginosa than amoxicillin according to their 50 percent effective concentrations (EC(50)) in algal growth, which were 1.15 and 8.03 μg/l, respectively. At environmentally relevant concentrations of 100 ng/l-1 μg/l, spiramycin reduced the total MC content per algal cell and inhibited the algal growth, while exposure to amoxicillin led to increases in the total MC content per algal cell and the percentage of extracellular MCs, without affecting the algal growth. Toxicity of MCs in combination with each antibiotic was assessed in the luminescent bacteria test using the toxic unit (TU) approach. The 50 percent effective concentrations for the mixtures (EC(50mix)) were 0.56 TU and 0.48 TU for MCs in combination with spiramycin and amoxicillin, respectively, indicating a synergistic interaction between MCs and each antibiotic (EC(50mix)<1TU). After seven-day exposure to 100 ng/l-1 μg/l of antibiotics, spiramycin-treated algal media and amoxicillin-treated algal media showed significantly lower (p<0.05) and higher (p<0.05) inhibition on the luminescence of Photobacterium phosphoreum, respectively, compared with the untreated algal medium. These results indicated that the toxicity of MCs were alleviated by spiramycin and enhanced by amoxicillin, and the latter effect would increase threats to the aquatic environment.