Effects Of Chlorella On Aquaculture Water Quality And Microbial Community Of Tilapia

In this study,the effects of different concentrations of Chlorella pyrenoidosa on growth performance,intestinal and gill microbial community structure,nitrogen and other water quality indexes of tilapia were studied,the most suitable concentration of Chlorella pyrenoidosa in tilapia culture water was determined.This experiment was added the initial concentration of 10⁹ cell/L Chlorella pyrenoidosa in aquaculture ponds,with Tilapia（Oreochromis niloticus）as the research object,choosing the initial weight of 14.18±0.93 g healthy tilapia,then randomly divided into control group（NC）,the low concentration group（LC）,the concentration（MC）and high concentration group（HC）four groups,each group of three repeat,each repeat 30 fish in 1000L circular culture barrels.The breeding experiment was carried out outdoors.The tilapia was feeded by using the basic feed during the culture period,the feeding amount was3%of the body weight of the fish,and the adding amount of Chlorella was 0m L,50m L,500m L and 5000m L,the breeding period was 50 days.In terms of the effect of chlorella on tilapia culture water,the contents of nitrogen and phosphorus in tilapia culture water could be reduced by spraying chlorella in the water,and the removal rate of three forms nitrogen was NH₄⁺-N>NO₂^--N>NO₃^--N,and the concentration of three forms nitrogen in water was significantly decreased in high concentration group（P<0.05）;The addition of chlorella could significantly reduce the concentration of TP in water（P<0.05）,the degradation effect of TN and COD_Mn was not significant（P<0.05）;The content of Chl.a in water body increased with the increase of chlorella addition concentration,at the later period,the difference between each group was obvious,and the concentration of Chl.a in HC group was significantly higher than NC group（P<0.05）;The comprehensive nutritional status index was used to measure the eutrophication degree of water.Chlorella can reduce the eutrophication level of aquaculture water,but there exist no significant difference among treatment groups（P<0.05）.The addition of chlorella could decrease the diversity and richness of bacterial community in tilapia culture water,decreasing the diversity of bacterial community in intestinal tract,increasing the richness of bacterial community,and the diversity and richness of bacteria on gills was increased.At the phylum classification level,the dominant bacteria in water are Proteobacteria,Actinobacteria,Bacteroidetes and Cyanobacteria;the dominant bacteria in the intestinal tract of tilapia are Proteobacteria,Actinobacteria,Bacteroidetes and Fusobacteria;Proteobacteria,Actinobacteria,Bacteroidetes and Fusobacteria are the dominant bacteria on the gills of tilapia.Chlorella could increase the relative abundance of Proteobacteria in culture water,Actinobacteria in intestinal tract,Actinobacteria in gills and Fusobacteria.The relative abundance of Bacteroidetes,Cyanobacteria,Fusobacteria,and Proteobacteria on the gills decreased with the increase of chlorella supplemental level.At the level of genus classification,the addition of chlorella could change the distribution of relative abundance of microbial community in water,gut and gill of tilapia.Phytocyanobacteria and Acidobacteria were the dominant bacteria phylum in the water.Mycobacterium,Cetobacteria,Bacillus and Romboutsia were relatively abundant in the intestinal canal of tilapia,while Methylbacillus,Mycobacterium,Cetobacteria and 2013Ark 19i were more abundant on the gills of tilapia.Mycobacterium and Cetobacillus were common bacteria in the gut and gill of tilapia,and accounted for a higher proportion in each sample.The relative abundance of Cetobacteria and Mycobacterium in the gut and gills of tilapia were increased by adding chlorella.The relative abundance of Cetobacteria in the water was directly proportional to the amount of chlorella added.Through PICRUSt 2 function prediction analysis,it was found that adding chlorella can change the intestinal and gill on energy metabolism,protein and amino acid metabolism enzyme abundance of metabolic enzymes in the intestinal lipid and amino acid transport and metabolism,carbohydrate transport and metabolism,energy production and conversion of related gene functional groups increases,it may have rich protein decomposition,transshipment and metabolic enzyme genes. the weight gaining rate（WGR）,specific growth rate（SGR）and absolute growth rate of body weight of tilapia could be increased by using chlorella in water,and the values of HC group and NC group were significantly different（P<0.05）,there was no significant difference between the other two groups and NC group;There were no significant differences in initial mean weight and survival rate among treatment groups（P>0.05）,but there were significant differences in the final mean body weight（P<0.05）,and the addition rate in HC group was higher than that in NC group.The total length and body length of tilapia were also significantly increased by adding chlorella in HC group,and there were significant differences between HC group and NC group at the later stage of culture（P<0.05）.The addition of chlorella could significantly reduce the total amount of sediment,the contents of nitrogen and phosphorus and the organic pollution index,besides,the effect of high concentration group（HC）was the most significant（P<0.05）.