Effects Of Dietary Phosphorus Levels On Growth,Body Composition And Lipid Metabolism Of Lateolabrax Japonicus Reared In Freshwater

Phosphorus is an essential mineral element for fish.The main source of phosphorus in cultured fish is feed.Low phosphorus intake can lead3 to fish slow growth,higher mortality,fat deposition and so on.Fish could excrete phosphorus into the water when intaking excessive phosphorus,which causes the waste of nutrient elements,increases the cost of dietary,and also causes environmental pollution in aquaculture.Therefore,it is very important to study the appropriate phosphorus requirement of fish.In this experiment,the suitable demand for phosphorus in dietary of freshwater reared Lateolabrax japonicus was studied with K₂HPO₄ and NaH₂PO₄ as the Phosphorus source,which provided theoretical basis for the proper addition of phosphorus in commercial dietary.In addition,in order to study the effect of dietary phosphorus level on the metabolism of fish body lipid and its pathway,we have studied the effects of different phosphorus levels on lipid metabolism enzyme activity,liver histology and expression of related genes in liver of L.japonicus.The main results showed following:1.Effects of dietary phosphorus levels on growth,body composition and apparent digestibility of nutrients in L.japonicusThe dietary available phosphorus were 0.48%,0.69%,0.89%,1.10%,1.28%,1.51%and1.77%,respectively.After these 7 groups of feed were used to breed L.japonicus?initial weight of 4.26±0.03 g?for 10 weeks,the growth,body composition and apparent digestibility and other indexes of fish were analyzed.The results of WG,SGR,PER and PPV in each group showed that the growth and the protein deposition reached the maximum when the phosphorus level in diet was 0.89%.The results of FCR,HSI,VSI,and liver fat content of each group found that the FCR of the 0.89%phosphorus levels adding was significantly lower than that of other groups.The HSI was the minimum in the 1.10%phosphorus level group,and the liver fat content was in the 0.89%phosphorus level group.The body composition study showed that the crude lipid decreased significantly with the increasing of phosphorus levels.However,the crude protein,ash,calcium and phosphorus significantly increased.The results showed that phosphorus could improve the growth performance of L.japonicus.Based on broken-linear regression analysis for weight gain rate against dietary phosphorus levels,the optimum requirement of L.japonicus for dietary phosphorus was0.72%of dry diet.With the determination of liver lipid content,the requirement for effective phosphorus of L.japonicus was 0.89%.The dietary with low phosphorus?0.48%?caused excessive deposition of body lipid in fish.The dietary of 0.89%phosphorus level significantly decreased the activity of AST and ALT in serum.With the increasing of dietary phosphorus levels,the liver phosphorus increased significantly?P<0.05?.The apparent digestibility results showed that the apparent digestibility of dry matter increased significantly with the dietary phosphorus supplementation?P<0.05?,while the apparent digestibility of calcium and phosphorus were on the contrary?P<0.05?.The content results of mineral elements in the vertebra showed that dietary phosphorus level increased the content of calcium,phosphorus and copper.When the phosphorus level was 0.89%,the content of calcium and phosphorus was significantly higher than that of the 0.48%group.The content of copper in the 1.10%groups was significantly higher than that of the 0.48%groups,however,the content of zinc,iron,magnesium and manganese decreased significantly?P<0.05?.In conclusion,the liver damage was reduced when the dietary phosphorus was 0.89%.Dry matter apparent digestibility and bone mineral content increased with the dietary phosphorus level increasing,indicating that bone mineralization ability was enhanced.2.Effects of dietary phosphorus levels on liver enzyme Activities,fatty acid composition and histology of L.japonicusIt was found that dietary phosphorus supplementation significantly decreased the content of TG and T-CHO?P<0.05?.The activity of HL,LPL and total lipase increased with the dietary phosphorus supplementation and then decreased.The data of fatty acid composition in the liver showed that the addition of 1.10%phosphorus significantly increased the proportion of HUFA,significantly reducing the proportion of SFA and MUFA?P<0.05?.In the fish liver histology,the liver cell demarcation of 0.48%low phosphorus groups was not obvious,and the vacuolization of cells was more serious and contains more fat drops.With the dietary phosphorus supplementation,the degree of vacuolation of hepatocytes was reduced.The results showed that the appropriate level of phosphorus could alleviate the excessive deposition of liver fat and increase the content of HUFA in the liver.In conclusion,when the dietary phosphorus was more than 0.89%,liver fat deposition was less.3.Effects of dietary phosphorus levels on the expression of genes related to lipid metabolism in L.japonicusThe expression of lipid metabolism related genes showed that the expression of CYP27B1,CYP24,HSL,FAS,ACC-2 and SREBP-1 showed a trend of first decreasing and then increasing with the dietary phosphorus supplementation?P<0.05?.The opposite was true for LPL?PPAR??CPT-1?P<0.05?.The expression of VDR and AMPK increased with the dietary phosphorus supplementation?P<0.05?.The results of this part of experiment showed that dietary phosphorus affected the expression of liver lipid metabolism related genes.When the dietary phosphorus was 0.48%,the expression of lipoclasis related genes was down-regulated,and the fat synthesis related genes was up-regulated.In summary,0.89%phosphorus was beneficial for bone mineralization and more than0.89%phosphorus level could increase the content of HUFA and alleviate the deposition of liver fat.0.48%phosphorus level diet could down regulate of genes related to lipoclasis and up regulate of fat-synthesis-related genes.The phosphorus requirement of seabass cultured in freshwater aquaculture is 0.72%?effective phosphorus?based on the weight gain rate.The quanity of available phosphorus was 0.89%with the determination index of liver lipid content.