Influence Of Magnetic Effect On Dissimilatory Nitrate Reduction To Ammonium(DNRA)

With the development of society,more and more anthropogenic nitrogen is being imported into the environment,constantly disrupting the global balance of the nitrogen cycle.Nitrate transformation in ecosystems has raised concern for scholars.In recent years,dissimilatory nitrate reduction to ammonium(DNRA)received more attention.However,the contribution of DNRA to the global nitrogen cycle is highly controversial.Up to now,the effects of various factors to DNRA in activated sludge and other nitrogen removal systems were not clear.Moreover,the interaction mechanisms between factors are currently needed to establish DNRA-enriched reactors to further investigate the conversion pathways.In this paper,in order to achieve the successful application of electromagnetic effect in DNRA process,the effects of electromagnetic and magnetic field strengths on DNRA start-up and long-term operation were investigated to determine the appropriate electromagnetic and magnetic field strengths and to explore the mechanism of their effects.The main conclusions obtained are as follows(1)The study proved that the electromagnetic waves can effectively improve the activity of DNRA,but the start-up time of DNRA will vary with the increase of intensity.Based on the water quality data,the EMF irradiated group started later than the control group.Rck started to produce ammonia earlier than R1 with the application of 0.04 μT EMF,while R1 was earlier than R2 with 0.06 μT EMF.With the increase of EMF intensity,the start-up process of ammonia production by DNRA bacteria in the reactor was prolonged for a period of time.However,the rate of ammonia rise was greatly shortened with the increase of electromagnetic wave intensity.The time from initial to 60%nitrate conversion was almost the same among all three reactors,but the average conversion rate of ammonia nitrogen in R1 and R2 was higher than that in Rck.Electromagnetic waves may both facilitate the DNRA process and increase the stability of system.qPCR results revealed that the abundance of the DNRA functional gene nrfA increased continuously.The nrfA of high abundance in the R1 reactor irradiated by 0.04μT electromagnetic waves was maintained.(2)This survey also demonstrated that suitable magnetic field strength(40 mT)could effectively shorten the time of DNRA start-up process,but did not have a significant effect on the nitrate conversion efficiency of DNRA.The magnetic field most likely only affected the enrichment rate of DNRA bacteria.After 120 days of domestication process,DNRA in the three reactors were started successfully.M1 reactor under the influence of 40mT magnetic field intensity started up successfully first,followed by the control group with a start-up time of 77 days,and the slowest start-up was the reactor under the influence of 100mT intensity with a start-up time of 93 days.(3)Based on the KEGG functional perdiction,the abundance of nitrite reductase,a key functional enzyme of the DNRA process,increased substantially.The higher the electromagnetic wave intensity,the more obvious the promotion of this key enzyme,indicating that electromagnetic waves can promote the activity of this enzyme in the appropriate range.40 mT magnetic field promoted this key enzyme significantly.It implied that the suitable intensity of magnetic field promoted the activity of the enzyme.(4)The experiment of pure bacteria confirmed that the 0.04 μT electromagnetic wave promoted the ability of Bacteroidetes fragilis to convert nitrite to ammonia.And the effect was gradually significant with time.