Cellular and transcriptional responses in Microcystis aeruginosa exposed to two antibiotic contaminants.

The responses of Microcystis aeruginosa under exposure to spiramycin and amoxicillin were investigated on both cellular and genetic levels through a 7-day exposure test. Algal growth was inhibited by spiramycin while promoted by amoxicillin at test concentrations of 0.6-1.8 μg L(-1), indicating a higher toxicity of spiramycin than amoxicillin. During the whole exposure period, the chlorophyll a content and expression levels of psbA, psaB, and rbcL were significantly inhibited by spiramycin at test concentrations of 1.2 and 1.8 μg L(-1) (p < 0.05) and stimulated by 0.6-1.8 μg L(-1) of amoxicillin (p < 0.05), with respective decreases of up to 26, 75, 72, and 82% compared to the control and respective increases of 20, 70, 135, and 55%. During the 4 to 7 days of exposure, the microcystin-LR content and expression levels of mcyB and grpE were reduced by up to 66, 47, and 72% in spiramycin-treated algal cells, respectively, and stimulated by up to 1.3-, 1.4-, and 1.5-folds in amoxicillin-treated algal cells, respectively. Elevated recA expression was only observed in 1.2 and 1.8 μg L(-1) of spiramycin-treated algal cells, indicating severe DNA damage due to the high toxicity. Target antibiotics were suspected to regulate the growth and microcystin-production in M. aeruginosa via the photosynthesis system.