Stress Response Of Microcystis Aeruginosa To Quinolone Antibiotics

Antibiotic residues in aquatic ecosystems may have potential adverse effects on aquatic organisms and human health.Quinolone antibiotics are widely used in the treatment of infectious diseases of respiratory and urinary systems because of their strong antibacterial ability,good pharmacokinetic performance,high bioavailability,low price and no cross-resistance with many antibiotics.China is a big user of antibiotics.Due to the relatively low removal efficiency of quinolones antibiotics in wastewater treatment,the impact on the surrounding environment is quite significant.As primary producers,microalgae exist widely in rivers and lakes and are the basis for the survival of other organisms.As a prokaryotic organism,cyanobacteria are generally more sensitive to antibiotics than other eukaryotic microalgae,among which Microcystis aeruginosa is a typical bloom cyanobacteria.In order to assess the environmental risks of residual quinolone antibiotics on aquatic ecosystems,levofloxacin and ciprofloxacin were selected as research objects in this paper to analyze the growth inhibition effect and mechanism of the two antibiotics on Microcystis aeruginosa under different exposure forms from the physiological and biochemical characteristics of microalgae growth,photosynthesis,oxidative stress,etc.In order to understand the effect of quinolone antibiotics on microalgae more scientifically and comprehensively.The main research results of this paper are as follows:（1）The toxic effects of levofloxacin and ciprofloxacin on Microcystis aeruginosa under acute exposure were compared.The results showed that the two quinolone antibiotics showed certain growth inhibition on Microcystis aeruginosa,among which Levofloxacin 96h-EC₅₀ was 1.702 mg/L,ciprofloxacin 96h-EC₅₀ was 5.647 mg/L.Levofloxacin is more toxic to algal cells.With the increase of antibiotic concentration,the inhibitory effect on algae growth and photosynthetic pigment increased,and the soluble protein content decreased gradually.The content of malondialdehyde（MDA）increased with the increase of concentration,and the experimental group was significantly higher than the control group（P<0.01）.At the same time,the activities of two antioxidant enzymes,superoxide dismutase（SOD）and catalase（CAT）,increased first and then decreased with the increase of antibiotic concentration,indicating that the two antibiotics had a serious oxidative stress effect on Microcystis aeruginosa.（2）The toxic effects of levofloxacin and ciprofloxacin on Microcystis aeruginosa were studied under equal concentration and equal toxicity combined exposure.The results showed that the equal concentration of 96h-EC₅₀ was 0.06 TU（Levofloxacin0.06 mg/L,ciprofloxacin 0.06 mg/L）,and the equal toxic concentration of 96h-EC₅₀was 0.09 TU（Levofloxacin 0.09 mg/L,ciprofloxacin 0.27 mg/L）.Both combined toxicities were synergistic.The combined exposure of the two antibiotics showed significant inhibition on algae growth and photosynthesis.The combination of antibiotics can lead to the accumulation of reactive oxygen species（ROS）in algal cells and cause lipid peroxidation damage,which leads to the increase of MDA content as a by-product,damages the structure and function of cell membrane,and impedes the transport of nutrients in algal cells.The antioxidant system of algal cells is stimulated by the excessive accumulation of ROS,which leads to a significant increase in the activity of SOD and CAT,thus enhancing their ability to resist external stress.（3）The effect of Levofloxacin on Microcystis aeruginosa under long-term exposure was studied,and it was found that Levofloxacin significantly inhibited the growth of microalgae,and the toxic effect increased with the extension of exposure time.Levofloxacin also inhibited chlorophyll and carotene synthesis under long-term exposure,reducing the ability of algal cells to absorb and transform light energy.Long-term exposure leads to the occurrence of lipid peroxidation,the significant increase of MDA content,the activation of antioxidant system,and the promotion of SOD and CAT synthesis.