Dietary Taurine Intake Affects Growth And Taurine Synthesis Regulation In Trachinotus Ovatus

The golden pompano,Trachinotus ovatus（Linnaeus 1758）,belongs to Carangidae and Trachinotus,and is classified as a warm water,upper migratory fish with an annual production of 160 000 tons.It has become an important economic aquaculture fish in the coastal areas of South China.However,in the process of aquaculture,the high feed coefficient and the increasing price of fish meal lead to the increase of aquaculture cost.Substitution of fish meal with cheap and widely distributed plant protein is an important way to maintain the sustainable development of the golden pompano industry.Some studies have shown that too much plant protein replacement will lead to the lack of taurine in the feed,resulting in decreased growth performance of fish,decreased antioxidant immunity and other adverse effects.Therefore,it is necessary to explore the physiological effects of exogenous taurine on fish.The purpose of this study was to investigate the effects of varying dietary taurine levels on the growth,antioxidant and immune capacity,and anabolism of taurine in golden pompano,hoping to provide references for optimizing feed and taurine synthesis regulation mechanism.The diets were formulated with varying concentrations of taurine(1.3,4.4,7.4,10.5 and 12.7 g·kg^-1),other ingredients included imported fish meal,fermented soybean meal,corn protein flour.Seven hundred and fifty T.ovatus with an average body weight of（80.0±0.5）g were randomly allocated into 5groups（150 fish in each group,divided into 3 replicates）and the experiment lasted 56 d.The experimental results were as follows:1.In this experiment,exogenous taurine intake significantly improved the growth performance of T.ovatus.The specific growth rate and weight gain rate peaked at 10.5 g kg^-1（P<0.05）;The feeding rate of the experimental group was significantly higher than that of the T0 group（P<0.05）,there was no significant difference in feed conversion ratio among all groups（P>0.05）.Hepatosomatic index of the T3 group was significantly higher than that of the T0 group（P<0.05）,there was no significant differences in viscerosomatic index and condition factor among all groups（P>0.05）.The crude protein and crude lipid contents of whole body in the T2,T3 and T4 groups were significantly higher than those in the T0 group,and reached the highest value when the taurine content was 12.7 g·kg^-1（P<0.05）;The water content of whole body in the T3 and T4 groups was significantly lower than that in the T0 group（P<0.05）;There was no significant difference in ash content among all groups.Taurine can significantly affected the antioxidant and immune capacity,and the activities of superoxide dismutase,glutathione peroxidase in serum increased with the increase of taurine content,and the superoxide dismutase and glutathione peroxidase activities reached the highest when the taurine content was 10.5g·kg^-1（P<0.05）;The activity of catalase in the T3 and T4 groups was significantly higher than that in the T0 group（P<0.05）;The content of serum malondialdehyde decreased with the increase of feed taurine,the content of malondialdehyde in the T3 and T4 groups was significantly lower than that in the T0 group（P<0.05）.With the intake of exogenous taurine,lysozyme activity in the T2,T3 and T4 groups was significantly higher than that in the T0 group（P<0.05）;The contents of C4 and immunoglobulin（A,G）in the T3 group were significantly higher than those in T0 group（P<0.05）;There were no significant differences in the content of C3 and immunoglobulin M among all groups（P>0.05）.Combined with the effects of taurine on the growth performance,antioxidant and immune ability of golden pompano,when the addition of taurine of 10.0 g·kg^-1 can significantly improve the growth,antioxidation and immunity of golden pompano,to achieve the best growth requirements.2.The cDNA sequences of ToCBS,ToCSE,ToCDO,ToCSAD and ToADO from T.ovatus were obtained for the first time.ToCBS cDNA has a sequence length of 2290 base pairs（bp）,and its open reading frame（ORF）contains 1776bp and encodes 591 amino acids.ToCSE cDNA has a sequence length of 2463 base pairs（bp）,and its open reading frame（ORF）contains 1230bp and encodes 409 amino acids.ToCDO cDNA has a sequence length of 1070 base pairs（bp）,and its open reading frame（ORF）contains 606bp and encodes 201 amino acids.ToCSAD cDNA has a sequence length of 1882 base pairs（bp）,and its open reading frame（ORF）contains 1527bp and encodes 508 amino acids.ToADO cDNA has a sequence length of 2405 base pairs（bp）,and its open reading frame（ORF）contains 762bp and encodes 253 amino acids.Tissue expression analysis showed:ToCBS mRNA expression was highest in the brain and liver.ToCSE mRNA expression was the highest in the brain and liver.ToCDO mRNA expression was the highest in liver.ToCSAD mRNA expression was higher in kidney,liver and intestines.The expression of ToADO mRNA was higher in the heart,kidney and liver.The taurine content in whole fish,liver,heart,eye,kidney,brain,muscle and gill significantly increased with the increase in dietary taurine（P<0.05）.The activity of CSE,CDO,CSAD and ADO decreased with the increase in dietary taurine（P<0.05）.Taurine intake had no significant impact on CBS activity（P>0.05）.The gene expression of ToCSE,ToCDO,ToCSAD and ToADO exhibited a downward trend with an increase of taurine（P<0.05）.Compared with the control group,the expressions of ToCBS and ToCDO were significantly down-regulated in the T2,T3 and T4 groups（P<0.05）.The expression of ToCSE in the T3 and T4 groups was significantly lower than that in the control group and T1 group,and the expression of ToCSE in the T3 group was the lowest（P<0.05）.The expression of ToCSAD gene was gradually decreased with the increase of dietary taurine content,and the expression level was the lowest in the T4 group（P<0.05）.The expression of ToADO gene in each experimental group was lower than that in the control group（P<0.05）.In addition,the recombinant CDO protein was obtained by prokaryotic expression system.In summary,there are two key enzymes for taurine synthesis in the main pathways of T.ovatus,which have the ability of endogenous taurine synthesis,and taurine can be synthesized through ADO and CDO-CSAD pathways,and the synthetic expression is high in liver and kidney.Exogenous taurine can affect tissue taurine deposition,enzyme activity and gene expression of related enzymes,and regulate endogenous taurine synthesis.3.The intestinal microflora was analyzed by 16S rRNA sequencing technology,a total of 87 707 sequences were obtained in the intestine of T.ovatus.The number of OTUs that can be annotated to the database was 5 130（95.32%）.At the phylum level,Proteobacteria,Tenericutes,and Spirochaetes were the predominant phylum;At the genus level,the dominant bacteria included Mycoplasma,Brevinema,and Methylobacterium.Dominant microflora often affects the composition and structure of the host microorganisms.Appropriate taurine addition reduced the relative abundance of the Proteobacteria and Spirochetes,and increased the relative abundance of the Tenericutes and Bacteroides.The abundance of Mycoplasma was significantly reduced,the abundance of Brachyspira was significantly increased,and the abundance of Methylobacterium also showed an increasing trend.Alpha diversity analysis showed that the intestinal microbial abundance of the experimental group decreased significantly with the increase in feed taurine content.The ACE and Chao1 index of the T1,T2,and T3groups were significantly lower than those of the T0 group,the Shannon and Simpson index in the T2 group were the lowest（P<0.05）.Beta diversity analysis showed that T1,T2,and T3 groups tended to cluster together with high species similarity,the distance of samples between T1 and T4 groups was far and they tended to be distributed discretely.Exogenous taurine has a significant effect on intestinal immune gene expression of T.ovatus.With the increase of taurine content,the expression of ToTLR-1,ToTLR-2,ToTNF-αand ToIL-1βgenes in experimental group decreased significantly（P<0.05）;The expression of ToNFkBP65 gene in the T1,T2 and T4 groups was significantly lower than that in the control group（P<0.05）;The expression of To IL-10 gene in the T3 and T4 groups was significantly higher than that in the control group（P<0.05）.In conclusion,taurine plays an important physiological function in the intestine of golden pompano,and has significant effects on intestinal microflora and immune function.Taurine can change the relative abundance and structure of intestinal flora,which can lead to the increase and decrease of dominant bacteria.Taurine can regulate the intestinal TLRS/NF-κB signaling pathway,and improve the intestinal immune capacity.