Analysis Of Water Purification Effect And Bacterial Community Structure Of Vannamei Shrimp Culture System Based On Polyurethane Nitrification Biofilm

China’s shrimp aquaculture industry scale and production has been the world’s first for many years.However,the traditional pond culture,high pond culture will not only pollute the water environment to varying degrees,damaging the aquaculture ecosystem,and can exacerbate the occurrence of disease,posing a catastrophic threat to the aquaculture industry.Therefore,the development of water-saving,energy-saving,high-yield,environmentally friendly farming model is an effective way to promote the sustainable and healthy development of the aquaculture industry,in which new technologies for water quality control is one of the main directions for the development of shrimp aquaculture.Ammonia and nitrite produced by the decomposition of residual bait,excrement and biological debris during the farming process are the main toxic substances and removal targets of farmed water bodies,which are mainly removed by biological methods.In the treatment process Ammonia-oxidizing microorganisms（AOA and AOB）convert ammonia to nitrite,and Nitrite-Oxidizing Bacteria（NOB）further convert nitrite to nitrate.As nitrifying microorganisms are chemoautotrophic bacteria,with aerobic,slow reproduction rate,long generation time and attached growth,biofilm water treatment process is often used in recirculating water aquaculture system,and the system start-up is relatively slow,especially the seawater aquaculture system usually takes 40～60 days to complete the start-up process.In this study,we used polyurethane fillers as biofilm carriers,and compared the effects of artificially and naturally hung polyurethane fillers and different dosing ratios of polyurethane fillers in the water quality control of Litopenaeus vannamei culture system,observed the morphology of biofilm and sorbents of polyurethane fillers by scanning electron microscopy,and analyzed the structure and diversity of bacterial communities of biofilm and sorbents by high-throughput sequencing.The following results were obtained from the study.（1）Differences in water purification effects of natural and artificial polyurethane fillers on shrimp culture systems in Vannamei.The NH₄⁺-N in both natural and artificial polyurethane filled systems was maintained at low concentrations(less than 0.3 mg·L^-1)with mean concentrations of 0.07±0.06 mg·L^-1and 0.01±0.01 mg·L^-1,respectively;peak NO₂^--N concentrations were 12.21 mg·L^-1and 0.87 mg·L^-1,with mean concentrations of3.82±3.33 mg·L^-1and 0.32±0.17 mg·L^-1,respectively;the mean values of turbidity were0.98±0.79 NTU and 2.11±1.46 NTU,respectively.（2）The increase of polyurethane filler feeding ratio in the culture system can improve the water purification effect.5%（v/v）artificially hung film filler feeding ratio culture system compared with 2.5%（v/v）system,NH₄⁺-N,NO₂^--N and turbidity decreased 62.5%,68.7%and 36.0%,respectively,and NO₃^--N concentration increased25.8%.After increasing the casting density from 800/m3 to 1200/m3 in a culture system with artificially hung film filler casting ratios of 2.5%（v/v）and 5%（v/v）,The NH₄⁺-N concentrations increased by 84.6%and 12.5%,respectively;the NO₂^--N concentrations increased by 107.8%and 10.8%,respectively;the NO₃^--N concentration decreased by 15.1%in the system with 2.5%（v/v）artificially hung film filler dosing,while NO₃^--N concentrations increased by 9.1%in the system with 5%（v/v）artificially hung film filler dosing.（3）Different treatment methods and seeding densities had significant effects on the specific nitrification rate of polyurethane biofilm.The SAORs of naturally hung polyurethane filler,artificially hung polyurethane filler,2.5%（v/v）artificially hung filler feeding ratio,5%（v/v）artificially hung filler feeding ratio,and artificially hung filler mesh bag without built-in aeration culture system were 0.11 mg·（L·h·g）^-1,0.13mg·（L·h·g）^-1,0.25 mg·（L·h·g）^-1,0.15 mg·（L·h·g）^-1,0.12 mg·（L·h·g）^-1;SNOR was 0.47mg·（L·h·g）^-1,0.78 mg·（L·h·g）^-1,1.67 mg·（L·h·g）^-1,0.98 mg·（L·h·g）^-1,0.74mg·（L·h·g）^-1.The results of the significance test showed that there was no significant difference in the SAOR of the artificially hung polyurethane biofilm but the SNOR increased significantly,and both SAOR and SNOR increased significantly after increasing the polyurethane dosing ratio,and there was no significant change in the SAOR,but the SNOR increased significantly under the built-in aeration treatment of the mesh bag;The SAOR and SNOR of the polyurethane filler increased with the increase of the dosing density.（4）Polyurethane filler can effectively adsorb suspended matter in the culture water,and its adsorption capacity can be regenerated by regular cleaning of the filler.2.5%（v/v）and 5%（v/v）of the artificial hanging film filler feeding ratio of the culture system turbidity is 4.83±2.80 NTU and 3.09±2.12 NTU,respectively.By regularly cleaning the polyurethane filler,the turbidity of the breeding system can be reduced by 3.94～8.96NTU.（5）There were differences in the growth of Vannamei shrimp in different polyurethane filler treatment and feeding density culture systems.The survival rates of natural,artificial,2.5%（v/v）artificial,5%（v/v）artificial,and unbuilt aeration systems were 65.0%,90.3%,83.6%,88.6%,and 80.7%,respectively,with average weights of7.6±0.21 g,9.2±0.13 g,8.4±0.18 g,8.6±0.11 g,8.3±0.24 g,and 8.3±0.24 g,respectively.In the 2.5%（v/v）artificial film filler dosing ratio culture system,the survival rate of Vannamei shrimp in the 800/m³and 1200/m³culture system was 88.5%and 83.6%,respectively,and the average weight was 8.9±0.16 g and 8.4±0.18 g,respectively.The survival rates of Vannamei shrimp in the 800/m³and 1200/m³culture systems were90.3%and 88.6%with an average weight of 9.2±0.13 g and 8.6±0.11 g,respectively;in the 5%（v/v）artificial film filler dosing ratio system.（6）High-throughput sequencing was used to analyze the structure and diversity of the bacterial community of polyurethane filler.The results showed that the highest bacterial community diversity was found in the samples with artificially hung film cleaning to remove adsorbents from the pores of the filler,while the lowest bacterial community diversity was found in the samples with artificially hung film without any treatment,and the highest bacterial community abundance was found in the samples with artificially hung film without use,while the lowest bacterial community abundance was found in the samples with natural film without any treatment.Proteobacteria,Bacteroidota and Chloroflexi were the dominant phyla in the different treatment systems;Five genera with denitrification function were detected in all samples,namely Desulfuromonadia,Pseudomonas、SM1A02、Caldilineaceaea and Nitrosomonas.The greatest differences in the structural composition of bacterial communities and the abundance of bacterial community members were found between the unused samples and the other groups of samples after artificial hanging;the smallest differences in the structural composition of bacterial communities were found between the pore sorbents of the polyurethane filler removed by washing and the untreated polyurethane samples in the same culture system.