Resistance of cyanobacteria Microcystis aeruginosa to erythromycin with multiple exposure

Here we report a set of experiments in which water blooming cyanobacteria Microcystis aeruginosa was repeatedly exposed to erythromycin. Growth inhibition increased with increasing erythromycin concentration (1-150 μg/L) upon first exposure. Maximum inhibition rate (76.06%), occurred under 150 μg/L erythromycin. Moreover, 96-h 50% effective concentration (EC₅₀) was 22.97 μg/L, indicating that the growth of M. aeruginosa was affected by erythromycin under common environmental concentrations. Photosynthesis was hindered by chlorophyll and photosystem II limitations. Malondialdehyde, reactive oxygen species, and superoxide dismutase contents increased significantly under certain concentrations of erythromycin, but superoxide dismutase was suppressed by 150 μg/L erythromycin. Synthesis of intracellular and extracellular microcystins was promoted by 10-60 and by 20-60 μg/L erythromycin, respectively, but both were inhibited by 100-150 μg/L. Principal component analysis and Pearson's correlation revealed the accumulation of reactive oxygen species as the dominant mechanism of erythromycin toxicity to cells. M. aeruginosa repeatedly subjected to erythromycin exposure showed obvious resistance against the antibiotic, especially when treated twice with 60 μg/L erythromycin. The 96-h EC₅₀ was 81.29 μg/L. As compared to the first exposure to erythromycin, photosynthetic and antioxidant activities increased, while growth inhibition and oxidation stress decreased upon multiple exposures. Production and release of microcystins were enhanced by repeated exposure to the antibiotic. Thus, erythromycin persistence in water should be examined, as repeated exposure may lead to serious environmental and human health hazards.