Stimulation effects of ciprofloxacin and sulphamethoxazole in Microcystis aeruginosa and isobaric tag for relative and absolute quantitation‐based screening of antibiotic targets

Antibiotics are normally regarded as safe to aquatic ecosystems when their contamination concentrations are lower than the toxic threshold. This study observed the hazard of ciprofloxacin, sulphamethoxazole and their binary mixture to the aquatic environment at environmentally relevant concentrations lower than the toxic threshold, due to the stimulation on the bloom of Microcystis aeruginosa. The enhanced growth of M. aeruginosa, coupled with elevated photosynthesis activity, was exerted by 50-200 ng/L of ciprofloxacin, 100-200 ng/L of sulphamethoxazole and 20-100 ng/L of the binary antibiotic mixture. Stimulated production and release of microcystins were observed at even lower concentrations. The upregulation of transcription-related proteins, cell division-related proteins, a gas vesicle protein, a global nitrogen regulator (ntcA), two microcystin synthetases (mcyC and mcyH) and ATP-binding cassette transporters provided direct proteomic evidence for the regulation of target antibiotics on M. aeruginosa bloom. Cytochrome P450 was an essential component involved in stress responses and antibiotic biodegradation. Proteomic responses to antibiotic exposure presented a shift in the energy metabolism of M. aeruginosa towards the excitation of photosynthesis, an increase of carbohydrate biosynthesis and the inhibition of carbohydrate catabolism. Superoxide dismutase, enolase and D1 protein were candidate target proteins of different antibiotics in M. aeruginosa. The antibiotic mixture showed a greater hazard than single antibiotics, and a safe threshold of 5 ng/L was suggested for each target antibiotic under the coexistence condition.