Effects Of Ammonia Nitrogen And Nitrate Nitrogen On The Growth And Toxin Production Of Microcystis Aeruginosa And Nostoc And Their Mechanism

With the intensification of human activities,the concentration of nutrients in natural water increases rapidly,water eutrophication is becoming more and more serious,and harmful cyanobacteria bloom occurs frequently.Harmful cyanobacteria releases a large amount of cyanobacteria toxin to water,which endangers the health of aquatic organisms and seriously affects the safety of aquatic ecosystem.Microcystins are cyclic heptadeptides with the highest content,the widest distribution and the strongest toxicity in natural water.At present,Microcystins Pollution in freshwater has become one of the environmental problems of widespread concern all over the world.In this study,Microcystis aeruginosa and Nostoc were taken as the research objects to investigate the effects of different concentrations of NH₄Cl and Na NO₃ on the growth and toxin production of cyanobacteria,analyze the changes of intracellular and extracellular microcystin（MC-LR）concentrations in each nitrogen source treatment group,and deeply explore the change of light conditions and nitrogen sources on the growth and MC-LR release of co-culture system of cyanophage and Nostoc.The main results of the study are as follows:1.Nitrate nitrogen was more easily absorbed and utilized by Microcystis aeruginosa and Nostoc,and the algae density increased rapidly with the increase of nitrate nitrogen concentration;High concentration of ammonia nitrogen（16 mg/L）inhibited the growth of Microcystis aeruginosa,and low concentration of ammonia nitrogen promoted the growth of Microcystis aeruginosa.When the concentration of ammonia nitrogen was 8 mg/L,the growth effect of Microcystis aeruginosa was the best.On the contrary,ammonia nitrogen inhibited the growth of Candida.When the nitrogen concentration was 16 mg/L and cultured for 15 days,the dry weight of Nostoc in ammonia nitrogen treatment group was 0.077 g/L,while that in nitrate nitrogen treatment group was 0.402 g/L.Therefore,compared with ammonia nitrogen,nitrate nitrogen is more conducive to promote the growth and reproduction of cyanobacteria2.The synthesis of MC-LR in single cell Microcystis aeruginosa increased with the increase of nitrate concentration.When the nitrate concentration was 16 mg/L,the highest content was 11.07 fg/cell.It decreased with the increase of ammonia nitrogen concentration.When the ammonia nitrogen concentration was 2 mg/L,the maximum value of single cell MC-LR was 12.59 fg/cell,and the content of single cell MC-LR in ammonia nitrogen treatment group was higher than that in nitrate nitrogen treatment group.The two nitrogen sources had no significant effect on the synthesis and release of MC-LR by Nostoc,and the addition of nitrogen source may reduce the content of intracellular MC-LR.In each nitrogen treatment group,the synthesis and release of MC-LR by Microcystis aeruginosa were significantly higher than that of Nostoc.3.Cyanophage Yong M can rapidly lyse Nostoc within 48 hours,control the population density of Nostoc and reduce the release of MC-LR.The cyanophage infecting Nostoc has a rhythmic effect.Light promoted the infection effect of Cyanophage and accelerated the lysis rate of Nostoc,and the 24-hour light intensity leads to the rapid decrease of absorbance of Nostoc after 20 hours of culture,while dark conditions inhibited the infection of cyanophage.Under light/dark conditions,the content of MC-LR decreased,and the content of MC-LR in dark treatment group was less than that in light treatment group;The addition of different concentrations of ammonia nitrogen and nitrate nitrogen had no significant effect on the infection of Nostoc by cyanophage,but could reduce the release of MC-LR.The effects of different concentrations of ammonia nitrogen and nitrate nitrogen on the growth of Microcystis aeruginosa and Nostoc were clarified.The changes of MC-LR in algae cells and water were revealed,and the effects of light/dark conditions and nitrogen sources on the cyanophage-Nostoc co-culture system were explored.