| The shrimp aquaculture industry has developed rapidly in China,and its scale and production have ranked first in the world for many years.However,the extensive culture mode restricted the sustainable development of shrimp culture.Ammonia nitrogen and nitrite nitrogen in aquaculture water are the main toxic substances,and their effective removal is the key to water purification of shrimp aquaculture systems.Microalgae and bacteria are two kinds of important microorganisms that regulate aquatic ecosystem.There is a very complex relationship between them from symbiosis to parasitism,and they play an important role in ecological restoration of aquatic environment.Nitrifying microorganisms are aerobic autotrophic microorganisms,including ammonia oxidizing bacteria（AOB）,ammonia oxidizing archaea（AOA）and nitrite oxidizing bacteria（NOB）.AOB and AOA convert ammonia nitrogen into nitrite nitrogen,and NOB converts nitrite nitrogen into nitrate nitrogen,thereby reducing the damage of ammonia nitrogen and nitrite nitrogen to aquatic organisms.Microalgae can assimilate nitrogen and phosphorus nutrients,and as natural feed,it can improve the immunity and survival rate of aquatic animals.At present,the algal species used in aquaculture systems mainly include Chlorella sp.,Scenedesmus sp.and Spirulina sp.In this study,aiming at the removal of nitrogen and phosphorus nutrients,single factor and response surface experiments were used to study the optimization of coculture conditions（algal-bacteria ratio,p H,light intensity,light time）between C.proteinosa and nitrifying microorganisms,and to determine the effects on water quality and shrimp growth in Litopenaeus vannamei aquaculture system of algal-bacteria coculture and the immobilization.The following results were obtained from the study.（1）The results of single factor experiments showed that the coculture of Chlorella pyrenoidosa and nitrifying microorganisms had obvious synergistic effects.Under the conditions of algae-bacteria ratio of 2:1,p H 8.0,light intensity of 6000 lux,and light time of 18.0 h,the growth of C.pyrenoidosa in the coculture was the best,and after 14 days of culture,the removal rates of ammonia nitrogen,nitrate nitrogen and total phosphorus were 99.64%,94.38%and 98.99%.（2）By establishing a response surface model,the effects of the interaction of each two factors on the concentrations of ammonia nitrogen,nitrite nitrogen and total phosphorus were determined.The results showed that the interaction of algae-bacteria ratio and p H,algae-bacteria ratio and light time,and p H and light intensity had significant effects on ammonia nitrogen concentration,and the interaction of p H and light intensity had the greatest effect;the interaction of algae-bacteria ratio and light time had a significant effect on nitrite nitrogen;the interaction of algae-bacteria ratio and p H,p H and light intensity had a significant effect on total phosphorus concentration,and the interaction between algae-bacteria ratio and p H had the greatest effect.（3）The optimal culture conditions obtained by the response surface model were:algal-bacteria ratio of 2:1,p H 7.8,light intensity of 6109 lux,light time of 17.6 h,and under these conditions,the predicted ammonia nitrogen concentration is 0.00 mg·L-1,nitrite nitrogen concentration.was 0.03 mg·L-1,and the total phosphorus concentration was 0.19 mg·L-1.Through the verification experiment,under this condition,the concentrations of ammonia nitrogen,nitrite nitrogen and total phosphorus were 0.00 mg L-1,0.04 mg L-1and 0.19 mg L-1,respectively,and the absolute errors of the predicted values were all less than 6%.It shows that the model can be used to predict the removal effect of nitrogen and phosphorus by algal-bacteria coculture.（4）The C.pyrenoidosa and nitrifying microorganism coculture of suspended state,adsorbed immobilization by polyurethane sponge and embedded immobilization by sodium alginate were added to the culture system of Litopenaeus vannamei for a40-day experiment.The results showed that the suspended and immobilized algae-bacteria cocultures had significant differences in the water purification effect of the L.vannamei culture system,the nitrogen and phosphorus removal effect is the best among them in the polyurethane sponge immobilized algae-bacteria system.During the whole culture cycle,nitrogen and phosphorus did not increase significantly,and the average concentrations of ammonia nitrogen,nitrite nitrogen,nitrate nitrogen and total phosphorus were 0.02 mg L-1,0.24 mg L-1,15.87 mg L-1,and 0.04 mg L-1,respectively;the peaks were 0.03 mg L-1,0.42 mg L-1,26.73 mg L-1and 0.07 mg L-1,respectively.（5）There were differences between the growth conditions of L.vannamei in the suspended and immobilized algal-bacteria coculture systems.The average weights of L.vannamei in the blank system,suspended algae-bacteria system,polyurethane sponge immobilized algae-bacteria system and sodium alginate immobilized algae-bacteria system were 5.13±0.15 g,6.38±0.44 g,8.27±0.18 g and 7.83 g±0.14 g,respectively;the survival rates were 66.67%,73.33%,86.67%and 80.00%,respectively. |
