Water Treatment Effect Of Biofloc With Different Particle Size And Its Short-term Stress On Juveniles Of Lateolabraxjaponicus

Due to the increasing concerns for environmental protection in recent years,the aquaculture industry also aims at addressing these concerns in the recent decade.Biofloc culture technology has gradually become a hot spot of aquaculture due to its environmental friendly advantages.However,there are still many problems in the development of biofloc culture mode,such as carbon source selection,C/N ratio,total suspended solid particles treatment and breeding object selection,etc.The basic theory and practical application of the biofloc technology system still require further fine-tuning and research.In this study,bioflocs were cultured under different shear stress conditions to explore the morphological changes,particle size distribution,water treatment effect and the effects of different shear stress on carbon metabolism of bioflocs and to analyze the effects of different particle sizes of the bioflocs on the short-term stress of juveniles of Japaneseseaperch.The changes in physiological and biochemical indices were used to evaluate the tolerance of the fish juveniles to bioflocs of different particle sizes.1. Study on the morphology,particle size and water treatment effect of biofloc under different shear forcesNine self-made conical reactors were divided into three groups according to the stirring intensity:low speed group?300r/min?,speed group?600 r/min?and high speed group?900 r/min?.After 21 days of culture,mature and stable biofloc were formed and rapid transformation tests were carried out.The results showed that the biofloc structure of low speed group is loose and the edge is fuzzy,the biofloc structure of medium and high speed group is compact and the edge is neat..Also,the biofloc size decreases with the increase of shear force and the biofloc size and proportion are inversely proportional to the shear force.In the process of biofloc culture,the water quality changes revealed that each reaction group had a good removal effect for NH₃^--N and NO₂^--N.Additionally,the rapid transformation tests revealed that each group could complete the removal of NO₂^--N between 6-10 h.However,there was no significant difference in the removal rate of NO₂^--N in the early stage of rapid transformation?6-8h?and the removal rate for NO₂^--N in the low speed group?0.55mg/L·h?was significantly higher than the other two groups.The results of this experiment show that mature biofloc can be formed under different shear forces.Overall,the results showed that there were some differences in the morphology and particle size distribution of bioflocs due to the different shear forces but there was no significant difference in water treatment capacity between groups.2. Study on carbon source metabolism of bioflocs under different shear stressNine self-made conical reactors were divided into three groups according to the stirring intensity:low speed group?300 r/min?,Medium speed group?600 r/min?and high speed group?900 r/min?.After 21 days of culture,mature and stable biofloc were formed and Biolog-ECO test was carried out.The results showed that the microbial community diversity of biofloc was better than that of the other two groups at low rotating speed.Also,the microorganisms in each group showed higher utilization rate of amino acids and the lowest utilization rate of phenolic acids in terms of carbon source preference.Additionally,there were 19 kinds of carbon sources whose main component 1 correlation coefficient was more than 0.5 and whose main component 2 correlation coefficient was more than 0.5.Notably,there were 13 species of carbon sources.?-D-lactose,?-methyl-d-glucoside,?-hydroxybutyric acid,itaconic acid and phenylethylamine,which can be specifically utilized by microorganisms in biofloc of different treatment groups.The results showed that the biofloc cultured under low shear stress had rich microbial diversity and high utilization of carbon sources.3. Study on the effects of short-term stress of different particle sizes of biofloc on juveniles of JapaneseseaperchAccording to the biofloc size,the Japaneseseaperch juveniles were divided into three groups:small particle size group?SG?,medium particle size group?MG?,large particle size group?LG?and clear water control group?CG?.The results showed that the biofloc with different size screened by standard sieve accounted for more than 50%in each experimental group;50.98%in SG group,49.34%in mg group,and 59.11%in LG Group.Notably,the gills of Japaneseseaperch juveniles were damaged by biofloc of different sizes and the wearing of small size bioflocs were of serious magnitude.The changes of blood glucose and cortisol content in small size bioflocs were significantly different from those of the other three groups?P<0.05?and the contents of total protein,total cholesterol and triglyceride were significantly different at different time points?P<0.05?.Also,alkaline phosphatase?ALP?and lysozyme?AYS?activities showed a trend of constant fluctuation.The alkaline phosphatase activity of the SG group was always in a downward trend and the trend of superoxide dismutase?SOD?and catalase?CAT?activities of the SG group and MG group were the same,which was in an upward trend.The changes in CG group were not significant,while that of LG group remained unchanged after being reduced to a low level in the early stage.The results showed that the short-term stress on gill,blood biochemistry and immune enzyme activity of Japaneseseaperch juveniles were the most affected by small particle sizes of bioflocs.