The Effect Of External Nitrogen-phosphorus Ratio On The Intracellular Nitrogen And Phosphorus Composition In Microcystis Cells And Its Feedback

External input with a large amount of nitrogen and phosphorus will lead to eutrophication of lakes,resulting in cyanobacteria bloom disasters.In recent years,the feedback effect of cyanobacteria on the exogenous input with nitrogen and phosphorus has gradually attracted attention.The absorption and transformation of inorganic nitrogen and phosphorus during the proliferation of cyanobacteria will reduce the biologically available nitrogen and phosphorus in water to a very low level.However,the process of cyanobacteria changing the ratio of intracellular carbon,nitrogen and phosphorus responding to different exogenous input with nitrogen and phosphorus is still less researched.This study carried out a field survey on the cyanobacteria gathering area of Dianshan Lake to study the response relationship between the intracellular nitrogen and phosphorus composition in Microcystis cells and the dynamic change of the nitrogen and phosphorus ratio of water,and then further explored the effect of external input with different nitrogen-phosphorus ratio on the proliferation,nutrient absorption ability and the composition of intracellular carbon,nitrogen and phosphorus in Microcystis cells through laboratory experiments.Based on the above results,a technical idea of using cyanobacteria to absorb nutrients in tail water and then collecting cyanobacteria for resource utilization is proposed.The main conclusions of this article are as follows:(1)The increase of nitrogen-phosphorus ratio in external input promotes the absorption of nitrogen and phosphorus by Microcystis.Field Microcystis are cultivated in the laboratory and continuously added with low concentrations of nutrients,when the phosphorus is limited(the nitrogen and phosphorus ratio in external input is 60:1),the nitrogen removal rate(89.4%)and phosphorus removal rate(97.1%)are both the highest.Thus Field Microcystis can fully absorb nitrogen and phosphorus until the nutrient content in water is extremely low.When the environment is under nitrogen limitation(the nitrogen and phosphorus ratio in external input is 8:1),the absorption capacity of Field Microcystis for both nitrogen and phosphorus is weakened,resulting in the remaining 20%～30% of phosphorus content in the water.(2)The increase of nitrogen-phosphorus ratio in external input will increase the absorption nitrogen-phosphorus ratio of Microcystis.The nitrogen-phosphorus ratio absorbed by Microcystis is positively correlated with the input nitrogen-phosphorus ratio,which also makes the intracellular nitrogen-phosphorus ratio in Microcystis cells gradually approach the exogenous nitrogen-phosphorus ratio.The field observation results of Dianshan Lake also confirmed this.It was found in the field observations that the Microcystis at the point with the highest nitrogen-phosphorus ratio(N:P=16:1)in Dianshan Lake had the highest intracellular nitrogen-phosphorus ratio(N:P=31:1).On the other hand,there is a certain hysteresis in the response of Microcystis nitrogen and phosphorus absorption capacity to the change of input nitrogen-phosphorus ratio,which is manifested in the fact that the change range of absorption nitrogen-phosphorus ratio of Microcystis is smaller than the input nitrogen-phosphorus ratio,which will play a certain buffering effect on the change of the nitrogen-phosphorus ratio in water.The outlet nitrogen-phosphorus ratios of the N:P～16:1 group and the N:P～32:1 group 8:1and 28:1 respectively,which are significantly lower than the input nitrogen-phosphorus ratio.(3)The input of high nitrogen-phosphorus ratio will promote the absorption of nutrients by Microcystis,and increase the intracellular nitrogen-phosphorus ratio.But when the external nitrogen-phosphorus ratio is low in input,the intracellular carbohydrate content will increase.The intracellular carbohydrates in the N:P～8:1group are twice that of the N:P～32:1 group,which causing better resource utilization efficiency of Microcystis.On this basis,this study used Microcystis to purify lowpolluted water.The system can operate stably under the conditions of input nitrogenphosphorus ratio of 16:1 and hydraulic retention time of 1 day,and the removal rate of nitrogen and phosphorus both are higher than 95%.At the same time,the intracellular carbohydrate content of Microcystis effluent is more than 50%,which can be directly collected for subsequent resource utilization.This research initially reveals the relationship between the nitrogen and phosphorus absorption capacity and the intracellular composition of Microcystis with the exogenous input nitrogen-phosphorus ratio.The research results are helpful for in-depth understanding of the influence of exogenous nitrogen and phosphorus input on the intracellular carbon,nitrogen and phosphorus content in Microcystis cells and its feedback on water quality,which further provide a theoretical basis for the development of new technologies for the nutrient removal and resource utilization based on cyanobacteria.It provides new technical ideas for the control of nitrogen pollution and cyanobacteria blooms.