| This experiment was conduct to study the issues as follows:1. The effect of adjusting C/N ratio on the culture of Litopenaeus vannamei, i.e. heterotrophic bacteria, nitrogen cycle bacteria, zooplankton, water quality and shrimp growth performance;2. The response of the bacterial growth to the C/N ratio and the inhibition of probiotic bacteria on the harmful bacteria.3. How to improve the technology of adjusting C/N ratio according to the cellulose decomposing of cellulose-degrading bacterium. The primary results were listed below.1. Effects of carbohydrate addition on shrimp culture systemThe present study investigated the effects of carbohydrate addition on the water quality, carbon/nitrogen ratios, densities of total bacteria, nitrogen cycle bacteria and zooplankton, and shrimp growth performance in a zero-water exchange system for Litopenaeus vannamei intensive culture. Sucrose was added as the carbohydrate into the water at five levels, i.e.,0,25%,50%,75%and100%of the theoretical adding quantity. Our results demonstrated that carbohydrate addition, especially75%and100%of the theoretical quantity significantly reduced the concentrations of total ammonia nitrogen and nitrite nitrogen during the late culture period in the water (P<0.05). Carbohydrate addition did not significantly affect the concentration of nitrate nitrogen, chlorophyll-a, COD and BOD. In addition, carbohydrate addition significantly increased carbon/nitrogen ratios and total bacterial densities of the water (P<0.05), whereas did not affect those of the sediment on the whole. We also found that the densities of nitrogen cycle bacteria (ammonia-oxidizing, nitrite-oxidizing and denitrifying bacteria) in both the water and the sediment of the carbohydrate-added treatments were obviously affected compared to those of the control treatments. Zooplankton densities in the water were determined to be significantly increased by carbohydrate addition (P<0.05). The parameters for shrimp growth performance including the yield, survival and feed conversion ratio demonstrated that75%of the theoretical quantity was more suitable for L. vannamei culture. Conclusively, carbohydrate addition into zero-water exchange systems for L. vannamei intensive culture can effectively improve the water quality, bacterial activities and zooplankton growth, consequently resulting in the better growth performance.2. Isolation and identification of several bacteria in the culture system of Litopenaeus vannametIn this study, we isolated several bacteria from different shrimp culture system, i.e. the north coast ground-water culture system, the mid coast seawater culture system and the south coast seawater culture system, respectively. We identified these bacteria using chemical method preliminarily, and further isolated the strain which has more significant response to the C/N ratio. Six types of bacteria were isolated, i.e. Bacillus, lactic acid bacteria, Vibrio, ammonia-oxidizing, nitrite-oxidizing and denitrifying bacteria, respectively. According to the16S rDNA sequence analysis, they were indentified as Bacillus sp., Enterococcus sp., Vibrio sp., Nitrosomonas sp., Nitrobacter sp. and Pseudoalteromonas sp., respectively. The bacteria isolated in this part were prepared for the study of the response of bacteria growth to the C/N ratio and the futher work.3. The effect of C:N ratio of the water on the growth and competition of bacteriaFrom the Litopenaeus vannamei culture ponds, the12strains of Bacillus, lactic acid bacteria and Vibrio were isolated which performed better growth in the conditions of carbon-limited (C:N=2) or nitrogen-limited (C:N=20). The levels of C:N ratio of the water (C:NW=2,5,10,15,20) were set to determine its effects on the growth, competition and the cell C:N ratio of the bacteria. It was found that the appropriate C:NW for the growth of Bacillus, lactic acid bacteria, Vibrio and denitrifying bacteria were C:Nw=15, C:Nw=10, C:Nw=5or10and C:Nw=10, respectively; and the highest values were determined as6.7×107,2.6×107,5.1×107and2.6×107cells/ml, respectively. The C:NW did not significantly effect the growth of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria. After24h culturing of Bacillus and Vibrio together in the treatments with C:Nw=10,15and20, the densities of Bacillus reached2.5×10’cells/ml, significantly higher than those of Vibrio (<2.0×107cells/ml;<0.05), and the experiment under the imitated culture environment had the same result with the media environment. The inhibition of lactic acid bacteria extracellular products on the growth of Vibrio were significantly higher in the treatments with C:Nw=10and15than that in the other treatments (P<0.05). The C:N ratios of the body composition of Bacillus, lactic acid bacteria and Vibrio were increased with the C:NW, in range of5.41-7.27,3.92-5.07and5.44-15.35, respectively. Conclusively, adjusting the C:N ratio in the water above10with carbohydrate addition not only improved the growth of Bacillus and lactic acid bacteria, but also inhibited the Vibrio growth.4. Isolation and carbohydrate-hydrolyzing-capacity study of cellulose-degrading bacteriumIn this study, through isolation from the Litopenaeus vannamei culture environment and selective enrichment culture, a cellulose-degrading bacterium, strain GS-2was isolated. Clear zones which showed cellulose hydrolysis were41.7mm in diameter, and the ratio to bacteria growth diameter was15.6. The enzyme activity of exoglucanase, endoglucanase and B-Glucosidases were8.03,7.43and3.92U/ml, respectively. According to the16S rDNA sequence analysis, it was indentified as Bacillus pumilus. The optimum pH of the initial culture substrate for the enzyme production was8, an optimum temperature30℃and an optimum culture time30h. The decomposed efficiency of strain GS-2was highest with the substrate of Avicel, which was21.24%in the culture medium. Futhermore, the production of decomposing Avicel could significantly increase the dissolved carbon concentration and C/N ratio (217.33mg/1;59.27) of the environment, and improve the growth of Bacillus. Conclusively, adding the cellulose-degrading bacterium and nature cellulose into the culture system can keep the cellulose decomposing continuous, provide more carbon source and increase the C/N ratio, and improve the growth of Bacillus and the water quality. |
