Effects Of Ozone On Biofloc Performance And Litopenaeus Vannamei Culture

In the late stage of biofloc culture of Litopenaeus vannamei,there are some problems in the water body,such as high concentration of organic pollutants,large number of Vibrio per unit water body and large consumption of dissolved oxygen.It is easy to accumulate inorganic pollutants such as ammonia nitrogen and nitrite nitrogen in water,and organic pollutants such as chemical oxygen demand（COD）and total organic carbon（TOC）increase significantly.The outbreak of Vibrio parahaemolyticus leads to the deterioration of the body of Litopenaeus vannamei and the decrease of resistance to harmful bacteria such as Vibrio,causing large-scale infection and a significant decline in production.In order to improve the current situation of water quality deterioration and excessive death of Litopenaeus vannamei due to Vibrio parahaemolyticus infection in the biofloc system,this paper studies the effect of ozone on the improvement of water quality and nitrogen treatment performance of biofloc,the killing effect on Vibrio parahaemolyticus and the effect on the growth performance of Litopenaeus vannamei,so as to provide reference for improving the application of ozone in the biofloc system.The research is divided into the following three parts:1.The effects of ozone on water quality and nitrogen treatment of flocs with different TSS concentrations were studied.In order to study the effects of ozone on water quality and nitrogen treatment capacity at different TSS concentrations,bioflocs with TSS（Total Suapended Solids,TSS）of 500mg/L and 3000mg/L were selected as experimental objects.In this study,the control group A without ozone and the four experimental groups B,C,D and E with ozone concentrations of 1mg/L,2mg/L,4mg/L and 6mg/L were set up.The results showed that the ammonia nitrogen in the water was increased by 540.3%（P<0.05）in the TSS500mg/L floc group compared with the control group.In addition,ozone had high efficiency in the removal of nitrite nitrogen.In the flocs of TSS 500 mg/L,group B increased by 80.6%（P<0.05）,while group C,D and E decreased by 42.04%（P<0.05）.In the flocs of TSS 3000mg/L,group B increased by 157.9%（P<0.05）,group C increased by 26.3%（P<0.05）,while group D and group E decreased by 73.7%（P<0.05）.In addition,ozone had a significant effect on the sedimentation of flocs.In the flocs with TSS500mg/L,the C,D,and E groups decreased by 9.6%,4.6%,and 28.8%,respectively,and the A group was significantly reduced（P<0.05）.In the flocs of TSS 3000mg/L,each group was significantly lower than that of group A（P<0.05）,followed by 7.5%,13.4%,14.8%,21.5%;After ozone injection,the COD_Mn of flocs decreased significantly（P<0.05）.In the two TSS concentration flocs,group B decreased by 4.9%and 1.1%respectively,group C decreased by 9.6%and 2.1%respectively,group D decreased by14.6%and 4.9%respectively,and group E decreased by 28.8%and 9.5%respectively.The ammonia nitrogen conversion rate of each group was in the range of0.21～0.29mg/g·h and 0.59～0.71mg/g·h.Among them,the group B of TSS 50mg/L flocs was 0.71±0.01mg/g·h,and the group E of TSS 3000 mg/L flocs was 0.21±0.02mg/g·h,which was significantly different from other experimental groups（P<0.05）.2.The killing effect of ozone on Vibrio parahaemolyticus in Litopenaeus vannamei culture system and the effect on immune enzyme activity of Litopenaeus vannameiIn order to study the killing effect of ozone on Vibrio parahaemolyticus in Litopenaeus vannamei culture system and the effect of shrimp immune enzyme activity,the following results were obtained:（1）In the test of killing Vibrio parahaemolyticus,Vibrio parahaemolyticus was added to make the initial concentration of Vibrio parahaemolyticus in the flocs 10⁷CFU/ml.The ozone dose was set to five doses of 0mg/L,1mg/L,2mg/L,4mg/L,and 6mg/L,which were set as A,B,C,D,and E groups.The results showed that when the ozone dose was 6mg/L,the sterilization rates in the flocs of500mg/L and 3000mg/L were 99.96%and 99.2%,respectively.（2）The median lethal concentration of V.parahaemolyticus in the flocs was 7.9×10⁵ CFU/ml within 24 hours.（3）Vibrio parahaemolyticus with a semi-lethal concentration of 24 hours was injected into the bio-flocs of TSS 500mg/L.Group A was the control group without Vibrio and ozone,and group B was added Vibrio without ozone.In group C,vibrio was added and6mg/L ozone was used to sterilize.In group D,only ozone was introduced.The results showed that the copy number of tox R gene in hepatopancreas of shrimp in group B without ozone sterilization was about 7.8×10⁶±5.6×10⁵copies/ul DNA after 12 hours of Vibrio parahaemolyticus infection.After 24 hours,the copy number of tox R gene increased to about 4.0×10⁷±9.1×10⁵copies/ul DNA,an increase of about 409.8%.In contrast,the tox R gene was almost not detected in group C after ozone sterilization;there was no significant difference in the three immune enzyme activities of catalase（Catalase,CAT）,malondialdehyde（Malonaldehyde,MDA）and superoxide dismutase（Superoxide dismutase,SOD）between the control group and the control group（P>0.05）.3.Study on the dosage and effect of ozone in biofloc Litopenaeus vannamei culture systemIn order to study the effect of ozone on the growth performance of flocs and Litopenaeus vannamei,group A was set to not add ozone during the culture of Litopenaeus vannamei,and the ozone feeding ratios of group B,C and D were set to5g/kg,15g/kg and 30g/kg,respectively.The results showed that the ammonia nitrogen and nitrite nitrogen in each group were maintained at a low level during the breeding period.There were no significant differences in weight gain rate,specific growth rate and feed coefficient among groups（P>0.05）.There was a significant difference in the survival rate between group A and group B（P<0.05）,and group A>group C>group D>group B.