Removal effect of NH4+-N and NO2--N by Chlorella vulgaris and the assimilation pathway of NO2--N
Since the non-ionic ammonia and ionic ammonia can be converted into each other,in aquaculture,ammonia nitrogen(NH4+-N)and nitrite nitrogen(NO2--N)are the key factors that affect the growth and develop-ment of aquatic animals.Therefore,the removal of NH4+-N and NO2--N in aquaculture water is of great signific-ance to ensure the health of aquaculture animals.As we all know,microorganisms and algae play important roles in maintaining the ecological balance of aquaculture ponds.It has been proved that Chlorella had the ability of purifying aquaculture water,with different Chlorella species different removal efficiency on NH4+-N and NO2--N.Chlorella vulgaris is widely used in aquaculture,however,we know little about the removal effect on nitrogen nutrient especially NO2--N by C.vulgaris.Given the shortage of effective ecological control measures in aquacul-ture,it is of great significance to illustrate the removal effect of NH4+-N and NO2--N by C.vulgaris and related influencing factors.Aiming to evaluate the application prospect of C.vulgaris in purifying NH4+-N and NO2--N in water,in the present study,C.vulgaris was taken as the research object and feed wastewater was taken as the cul-ture medium.We firstly detected the cell density of C.vulgaris and the temporal variations of NH4+-N and NO2--N in water under aeration,light,combined light and aeration conditions.Then we analyzed the effects of time(X1),light intensity(X2)or initial C.vulgaris density(X3)on the removal rates of NH4+-N and NO2--N(Y).Finally,we evaluated the removal efficiency of NH4+-N,NO2--N and NO3--N from water by C.vulgaris,and we analyzed the potential pathway of NO2--N assimilation by C.vulgaris.The results showed that C.vulgaris could remove NH4+-N,NO2--N and NO3--N significantly under suitable light conditions.The NH4+-N removal rate reached up to at 18 000 lx(96.23%),and NO2--N removal rate reached up to 99.19%at 9 000 lx.The initial density of C.vulgaris at 2.5×105 cells/mL had the highest removal rates for NH4+-N and NO2--N,accounting 94.92%and 99.05%,respectively.The regression equation of NH4+-N and NO2--N removal rates with treatment time and light intensity was as follows:YNH4+-N=1.189X1+5.79×10-4X2+24.158(R2=0.664),YNO2--N=1.562X1+1.909×10-3X2-26.078(R2=0.762).The regression equation of NH4+-N and NO2--N removal rates with treatment time and initial C.vul-garis density was as follows:YNH4+-N =0.888X1+1.02×10-5X3+32.555(R2=0.408),YNO2--N =1.746X1+1.64×10-5X3-17.250(R2=0.613).The order of nitrogen removal by C.vulgaris was NH4+-N>NO3--N>NO2--N,and the activity of nitrite reductase in C.vulgaris at NH4+-N decline stage was significantly lower than that at NO2--N decline stage.In conclusion,C.vulgaris can significantly reduce the contents of NH4+-N and NO2--N in water,and NO2--N may be reduced to NH4+-N by intracellular nitrite reductase and assimilated by C.vulgaris.These results provide scientific basis for in-situ bioremediation of aquaculture waters.
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