Antibiotic contaminants reduced the treatment efficiency of UV-C on Microcystis aeruginosa through hormesis.

Antibiotic contaminants exert stimulatory hormetic effects in cyanobacteria at low (ng L-1) concentrations, which may interfere with the control of cyanobacterial bloom in aquatic environments exhibiting combined pollution. This study investigated the influence of a mixture of four popular antibiotics (sulfamethoxazole, amoxicillin, ciprofloxacin, and tetracycline) during the application of UV-C irradiation for controlling the bloom of Microcystis aeruginosa. In the absence of antibiotics, 100-500 mJ cm-2 UV-C irradiation reduced cell density, growth rate, chlorophyll a content, Fv/Fm value and microcystin concentration in M. aeruginosa in a dose-dependent manner through the downregulation of proteins related to cell division, chlorophyll synthesis, photosynthesis and microcystin synthesis. UV-C irradiation stimulated microcystin release through the upregulation of the microcystin release regulatory protein (mcyH). The presence of 40 ng L-1 antibiotic mixture during UV-C treatment significantly reduced (p < 0.05) the treatment efficiency of 100-300 mJ cm-2 UV-C on microcystin concentration, while 80 and 160 ng L-1 antibiotic mixture significantly reduced (p < 0.05) the treatment efficiency of 100-500 mJ cm-2 UV-C on cell density and microcystin concentration. The antibiotic mixture alleviated the toxicity of UV-C on M. aeruginosa through a significant stimulation of photosynthetic activity (p < 0.05) and the upregulation of proteins involved in photosynthesis, biosynthesis, protein expression, and DNA repair. Microcystin release in UV-C-treated cyanobacterial cells was further stimulated by the antibiotic mixture through the upregulation of mcyH and four ATP-binding cassette transport proteins. The interference effects of antibiotic contaminants should be fully considered when UV-C is applied to control cyanobacterial bloom in antibiotic-polluted environments. In order to eliminate the interference effects of antibiotics, the concentration of each target antibiotic is suggested to be controlled below 5 ng L-1 before the application of UV-C irradiation.