The Biological Effects And Toxic Mechanisms Of Two Mixed Antibiotics In Microcystis Aeruginosa

Cyanobacterial blooms occur frequently in freshwater ecosystems, leading to release of cyanotoxins from toxic cyanobacterial strains. Among the toxic cyanobacterial species, Microcystis aeruginosa is of great concern due to its wide distribution and its ability to produce microcystins (MCs). In recent years, antibiotic contaminants are discharged into aquatic environments, which are likely to interact with cyanobacteria. Control methods obtained from experiments which only focused on Microcystis often could not achieve expected effect when applied in complex aquatic environment. Therefore, investigations were required to reveal the biological effects and action mechanisms of co-exsiting antibiotics in M. aeruginosa. Since various antibiotic contaminants are simultaneously discharged into aquatic environments, mixed antibiotics were selected as the target pollutants in the present study.M. aeruginosa was selected as the target cyanobacterial species, and spiramycin (SP) and amoxicillin (AM) were the target antibiotics. The combined effects of two mixed antibiotics on algae growth, photosynthesis, MC-production and MC-toxicity were studied. The degradation of target antibiotics by M. aeruginosa were also investigated. Further, the action mechanisms of mixed antibiotics in M. aeruginosa were interpreted. Results showed that the two mixed antibiotics affected the growth of M. aeruginosa. When SP and AM were mixed according to the ratio of their EC50 values (SP:AM=1:7), the algae growth rate showed a negative correlation with the test concentrations after seven-day exposure to 2-15μg/L of mixed antibiotics. When SP and AM were mixed at equivalent ratio (SP:AM=1:1), the algae growth was stimulated at low concentrations (1-4μg/L), and was inhibited at higher test concentration (5-10μg/L). Alterations in expression levels of photosynthesis-related genes (psb A, psaB and rbcL) and chlorophyl-la contents suggested that mixed antibiotics interfered the photosynthesis system. Antioxidant responses were initiated in M. aeruginosa following exposure to mixed antibiotics, which were related with mixture ratios of mixed antibiotics and algae growth phase. Variations in the activities of SOD, CAT, POD were in response to the damage of reactive oxygen species. The increased contents of MDA, as well as expression levels of recA showed that the two mixed antibiotics caused damage to M. aeruginosa.Increased expression of mcyB suggested that mixed antibiotics stimulated the production of MCs. Furthermore, the release of target MCs also increased. MC-LR was the major kind of MC variants released by M. aeruginosa, which accounted for up to 84.53% of the total MC-production. The luminescent bacteria test indicated that the combined toxicity of MCs together with each antibiotic or mixed antibiotics was synergistic, which suggested that the toxicity of MCs was enhanced by antibiotics. Degradation of SP and AM coupled to the increased levels of GST were also observed in M. aeruginosa, and the degradation percentages were 12.3%～12.5% and 30.5%～31%, respectively. The degradation percentages remained stable at varying concentrations and different mixture ratios. Results indicated that low concentrations of mixed antibiotics could intensify Microcystis pollution and enhance the toxicity of MCs. Therefore, it is necessary to consider co-existing antibiotic pollutants during the control of Microcystis pollution.