Research On The Interaction Between Spiramycin And Microcystis Aeruginosa At Different Nitrogen Levels

Cyanobacterial blooms are regulated by various environmental factors, among which nitrogen is usually considered as the dominant one. Recent studies found that cyanobacteria were sensitive to antibiotic pollutants, due to their prokaryotic structure. Antibiotic pollutants are likely to be emerging environmental factors regulating cyanobacterial blooms.But the combined regulation effect and mechanism of environmental factors and antibiotic pollutants on cyanobacterial blooms remained unclear. In this study, an antibiotic spiramycin, a model species of cyanobacteria (Microcystis aeruginosa) and nitrogen nutrient were selected as research targets. The interaction effect and mechanism between spiramycin and M. aeruginosa at different nitrogen levels were studied through a 7-day exposure test.Nitrogen concentration (0.5-50 mg/L) significantly affected (p< 0.05) the toxicity of spiramycin in M. aeruginosa. Lowest toxicity of spiramycin was observed at a nitrogen level of 0.5 mg/L. In this case,0.1μg/L of spiramycin caused hormesis effects. Nitrogen and spiramycin significantly interacted with each other (p< 0.05) on the synthesis of chlorophyll-a and protein, the production and release of microcystins as well as the expression of ntcA gene and mcyB gene in M. aeruginosa. Nitrogen significantly affected the regulation of spiramycin on algal growth (p< 0.05) via protein synthesis. The photosynthesis system including chlorophyll-a, the psbA gene, and the rbcL gene participated in stress responses to spiramycin. Nitrogen significantly regulated (p<0.05) the effects of spiramycin on the growth and antioxidant responses of M. aeruginosa as well as the biodegradation of spiramycin by M. aeruginosa. At a nitrogen level of 0.5 mg/L, the activities of superoxide dismutase and catalase were stimulated by 0.1μg/L and 0.4μg/L of spiramycin to protect algal cells from oxidative damage, resulting in alleviated toxicity of spiramycin and low malondialdehyde content in M. aeruginosa. The catalase activity was inhibited by 0.4μg/L of spiramycin at higher nitrogen levels of 5-50 mg/L, leading to significant growth inhibition (p<0.05) and higher malondialdehyde content through accumulation of hydrogen peroxide. Stimulated glutathione content and glutathione S-transferase activity were coupled to the biodegradation of spiramycin in M. aeruginosa. The 7-day biodegradation percentage of spiramycin varied from 8.9 to 29.6%,which was enhanced by increased nitrogen concentration and decreased spiramycin concentration. According to the luminescent bacteria test,0.1μg/L of spiramycin increased the harm of M. aeruginosa by stimulating the growth and the production and release of microcystins at a nitrogen level of 0.5 mg/L.0.4μg/L of spiramycin alleviated the harm of M. aeruginosa at higher nitrogen levels of 5-50 mg/L by inhibiting growth, the production of microcystins and the expression of ntcA and mcyB.The nitrogen-dependent effects of spiramycin should be considered in the control of M. aeruginosa bloom in the presence of spiramycin. Low concentration of coexisting spiramycin should be considered during the control of M. aeruginosa bloom, especially under nitrogen deficient condition.