Study On The Effect Of DO On The Nitrogen Removal Performance Of Biological Filter And The Characteristics Of Salt-tolerant Aerobic Denitrifying Bacteria

Recirculating Aquaculture System?RAS?has significant advantages?intensive,high efficiency and environmental protection?,which is an important trend for the development of aquaculture in the future.Biological filter,as the core unit of water treatment of circulating aquaculture system,mainly depends on the biofilm attached to the surface of the filler,can control the water quality,and plays an important role in achieving intensive high-density culture of the circulating water system.The circulating aquaculture system needs to maintain a high dissolved oxygen state to achieve large-scale and high-density aquaculture.Therefore,the dissolved oxygen?DO?concentration has become a key factor that affects the performance of the biofilter and further affects the water treatment efficiency of the circulating aquaculture system.In addition,the traditional biological filter can effectively remove NH₄⁺-N and NO₃^--N,which are more harmful to the aquaculture organisms in the aquaculture water body,but it cannot effectively remove the NO3^--N,resulting in a large amount of NO3^--N accumulation in the water body,resulting in a large accumulation of NO3^--N in the cultured water.The stable operation of the circulating water aquaculture system and the growth and development of aquaculture organisms cause serious harm.Therefore,the effect of DO concentration on the nitrogen removal performance and microbial community structure of the biological filter was studied.At the same time,a new biological nitrogen removal technology was sought to improve the nitrogen removal performance of the biological filter and solve the problem of nitrate accumulation,thereby improving the efficiency of the industrial circulating water culture system has very important significance.This article explores different DOs by setting three different DO conditions(HDO:10?12 mg·L^-1,MDO:6?8 10?12 mg·L^-1,LDO:2?4 10?12 mg·L^-1)and running for 30days.Effect of concentration on nitrogen removal performance and microbial community structure of biofilter.Several strains of highly efficient salt-tolerant aerobic denitrifying bacteria were screened,and their aerobic denitrification performance,nitrogen balance analysis,and aerobic denitrification functional genes were studied.The main experimental results are as follows:?1?Influence of dissolved oxygen concentration on the nitrogen removal performance of the biofilter:during the operation of the system,three different dissolved oxygen treatment groups can effectively remove ammonia nitrogen,and the removal rates are all above 60%.Among them,the HDO treatment group(DO:10-12 mg·L^-1)has the highest ammonia nitrogen removal rate,and the ammonia nitrogen removal rate can be maintained above 80%during the system operation,while the LDO treatment group(DO:0-2 mg·L^-1),ammonia nitrogen removal rate is the lowest.The results indicate that higher dissolved oxygen concentration promotes the growth of ammonia-oxidizing bacteria and the progress of nitrification.In the three treatment groups,the concentrations of NO₂^--N and NO₃^--N all increased first,then decreased,and finally stabilized.For the treatment groups with different dissolved oxygen concentrations,the groups of high dissolved oxygen,the accumulation of NO₂^--N was severe.The accumulation rates of NO₂^--N and NO₃^--N were lower than those of the HDO and MDO treatment groups.In the initial stage of the system operation?0d-15d?of the LDO treatment group,ammonia oxidizing bacteria?AOB?dominated the system,converting ammonia nitrogen into NO₂^--N,while the dissolved oxygen concentration in the system was low and nitrite oxidizing bacteria?NOB?.The growth lag,the oxidation rate of NO₂^--N is slow,so the phenomenon of gradual accumulation of NO₂^--N appears.After 28 days of system operation,the concentration of NO₂^--N began to decrease and the concentration of NO₃^--N increased.?2?Effect of DO concentration on microbial community structure:OTU cluster analysis showed that the number of OTUs shared by the three groups of samples was 558,accounting for 91.6%of all OTUs.Among them,the MDO processing group has the most OTUs,accounting for 94.9%of the total OTUs,and the largest number of unique OTUs.According to the analysis results of the Alpha diversity index,the species abundance in the MDO treatment group is the lowest but the species diversity is the highest.Beta diversity analysis results show that different DO concentrations have an effect on the microbial community composition,which makes the species composition vary greatly.The results of microbial community analysis showed that the microorganisms of the three different DO treatment groups were mainly distributed in 10 gates.The bacterial community structure of the different treatment groups had a certain similarity in composition,but the relative abundances of various bacterial groups were different.Among the three treatment groups,Proteobacteria?Proteobacteria?dominates the biofilm samples of the different treatment groups?53–62%?,and the other relatively abundant abundances are Parcubacteria?7-14?%),Acidobacteria?2-8%?,Bacteroidetes?6-12%?,Planctomycetes?7-10%?,of which Nitrospirae,Proteobacteria and Acidobacteria are related to system nitrification and denitrification.The relative abundance is higher in the high-DO treatment group and lower in the LDO treatment group.Because the main function of Nitrospirae is to oxidize NO₂^--N to NO₃^--N,which is the main functional bacterium in the system of nitrosation,it may be related to the relatively good removal effect of NH₄⁺-N in the HDO treatment group.?3?Study on aerobic denitrification mechanism:a novel salt-tolerant aerobic denitrifier was isolated from the marine recirculating aquaculture systems?RASs?.Identified as Halomonas sp.by morphological observation and 16SrDNA analysis,named Halomonas sp.AD9.We studied the aerobic denitrification performance under different conditions,nitrogen balance analysis and identification of napA and narG functional genes.The results showed that the optimal culture conditions of the bacterium were temperature30?,shaking speed 150 rpm,C/N=10.Under optimal culture conditions,when NO₃^--N(initial concentration 101.9 mg·L^-1)was used as the sole nitrogen source,the removal rate of NO₃^--N at 24h could reach 94.60%,and the total nitrogen removal rate at 48h was74.2%.Nitrogen balance analysis showed that 21.7%of NO₃^--N was converted into intracellular nitrogen,and 3.3%of NO₃^--N was converted into other nitrification products?NO₂^--N,NH₄⁺-N,organic nitrogen?.The identification of functional genes indicates that the strain AD9 contained the napA gene and did not obtained the narG gene,indicating that the strain has aerobic denitrification ability.The aerobic denitrification strain Halomonas sp.AD9 will provide some help for the industrialized recirculating aquaculture systems?RASs?to solve the problem of NO₃^--N accumulation.