Effects Of Nitrogen Sources On MCs Synthesis And Nitrogen Metabolism Of Microcystis Aeruginosa

In recent years,with the aggravation of eutrophication in water,the frequent occurrence of cyanobacteria blooms not only endangers the safety of animals and plants in freshwater environment,but also threatens human health.exploring the production and regulation of microcystins has become a hot topic in the environmental field.However,the current studies pay more attention to the effects of total nitrogen on the growth and toxin synthesis of Microcystis aeruginosa in the environment,ignoring the role of nitrogen forms in it,and the field monitoring and indoor simulation of Microcystis aeruginosa are mainly focused on rivers and lakes.However,there are few reports on the effects of different nitrogen sources on Microcystis aeruginosa in aquaculture water.Therefore,in this study,Microcystis aeruginosa(FACHB-905)was inoculated into the medium with glutamic acid(Glu),ammonium nitrogen(NH₄Cl),nitrate nitrogen(NaNO₃)and urea(CH₄N₂O)as the only added nitrogen source through indoor culture experiment.Two concentration gradients were set,the low concentration of glutamic acid was 2.94 mg/L,the high concentration was 14.7 mg/L,and the low concentration of other nitrogen sources was0.2 mg/L.The high concentration was 0.5 mg/L,and BG-11 culture medium was used as blank control.The effects of different concentrations and forms of nitrogen sources on the growth,MCs synthesis and nitrogen metabolism of Microcystis aeruginosa were studied,and the relationship between growth and MCs synthesis and nitrogen metabolism genes was preliminarily explored.The main results are as follows:?All nitrogen sources could promote the growth of Microcystis aeruginosa and increase the related chlorophyll fluorescence parameters.From high to low,the growth-promoting effects of each nitrogen source group were as follows:blank control group>0.5 mg/L NaNO₃>0.5 mg/L NH₄Cl>2.94 mg/L Glu>0.2 mg/L NaNO₃>0.2 mg/L NH₄Cl>14.7 mg/L Glu>0.5 mg/L CH₄N₂O>0.2 mg/L CH₄N₂O.?All nitrogen sources could promote the synthesis of MCs in Microcystis aeruginosa,the intracellular MCs content was the highest and the extracellular MCs content was the lowest in the blank control group,the intracellular MCs content in algae treated with NaNO₃and NH₄Cl was significantly higher than that of Glu and CH₄N₂O,and the extracellular MCs content of algae was significantly lower than that of Glu and CH₄N₂O.With the increase of nitrogen source concentration,the intracellular MCs content increased and the extracellular MCs content decreased in NaNO₃ and NH₄Cl treated groups,but there was no significant change in extracellular and extracellular MCs content in Glu and CH₄N₂O treated groups.The expression of MCs synthesis genes mcy A and mcy D in each nitrogen source group was consistent with the change trend of MCs content in algae cells.Combined with the RDA analysis of MCs and its synthesis genes,it was found that MCs had a significant effect on the expression of toxin synthesis genes in algae cells,indicating that nitrogen sources could not only promote the synthesis of MCs,but also affect the expression of toxin synthesis genes.?The utilization of Microcystis aeruginosa to each nitrogen source group was NaNO₃?NH₄Cl>CH₄N₂O>blank control group;the expression of nitrogen metabolism genes Gln B,Pip X and Ntc A in each nitrogen source group showed an upward trend,except Glu,nitrogen metabolism genes in other nitrogen source groups increased with the increase of nitrogen source concentration,and the maximum values appeared in 0.5 mg/L treatment group.Combined with RDA analysis,it was found that MCs and its synthetic genes had significant effects on the expression of nitrogen metabolism genes in algae cells,indicating that MCs was involved in the process of nitrogen metabolism in algae cells.?The genes related to MCs synthesis and nitrogen metabolism were closely related to the cell density(cells),chlorophyll a content(chla),maximum light energy conversion efficiency(Fv/Fm)and electron transfer rate(ETR)of Microcystis aeruginosa,and each growth index had significant effects on the gene expression.Combined with the RDA analysis of MCs content,MCs synthesis and nitrogen metabolism genes,it was found that the growth indexes had significant effects on the gene expression.Nitrogen sources may affect the release of MCs and the expression of related genes by affecting the growth of algae cells.