| The identification of new feeds or the development of new additives that improve resource utilization efficiency to expand the feed resource base has an important impact on the sustainable development of aquafeeds.As an extremely rich,fast growing and renewable biological resource in the ocean,algae has the potential as an unconventional resource.China is rich in algae resources,especially kelp.In 2019,the annual output of kelp in China has exceeded 2.5 million tons,ranking the first in the world.So,we tend to take kelp with easy availability,rich nutrition as the research objects.We compared the nutritional quality changes of kelp before and after processing.Then,different processed kelp products were added to the diet of channel catfish to study the effect of kelp on its growth,and the mechanism of kelp was analyzed by transcriptome sequencing method to comprehensively evaluate kelp as a feed raw material.The research contents were as follows:1.Changes of nutritional components of kelp before and after preparation.First,we used the ultra-fine grinding and enzymatic hydrolysis as the processing methods to break the cell wall of kelp.Then,three kinds of kelp products were finished including broken kelp(BK),enzymatic hydrolysis kelp(KEH),and kelp enzymatic hydrolysis extract(KEHE).The wall-breaking rate of the kelp product obtained by ultra-fine grinding reached 54.22%～79.85%.The content of reducing sugar of kelp products by enzymatic hydrolysis was increased by 32.59%.The composition analysis results showed that the main component of kelp is carbohydrates(57.75%),while it contains moderate protein(9.31%)and low fat(2.13%).There was no significant difference in moisture,crude fat and ash among kelp,broken kelp and enzymatic hydrolysis kelp,but the crude protein content of broken kelp was significantly higher than the other two kelp products.Compared with enzymatic hydrolysis kelp,broken kelp had more advantages in terms of total amino acid content and retention rate of I,Ca,K,Mg,Fe,Cu,Zn,Mn mineral content and other nutrients.The pigment content did not change significantly after processing.The water,total carbohydrates and crude protein of kelp enzymatic hydrolysis extract were significantly lower than the other three products(P<0.05),and the crude ash content was significantly higher.2.The effect of kelp products on the growth of channel catfish.The kelp products prepared above were used as test materials.The Ictalurus punctatus(channel catfish)with an average initial weight of(27.19±0.17)g were taken as the experimental object,and the diet of channel catfish without kelp products was taken as the control group S In addition,0.5%(K5),1%(K10),1.5%(K15)kelp,0.5%(BK5),1%(BK10),1.5%(BK15)broken kelp,0.5%(KEH5),1%(KEH10),1.5%(KEH15)enzymatically hydrolyzed kelp,and 0.05%(KEHE0.5),0.1%(KEHE1.0),0.15%(KEHE1.5)Enzymatic hydrolysis of kelp extract added in channel catfish diets were used as kelp product groups.After 60 days of cage culture,the effects of different kelp products on growth performance,body composition,serum biochemical indexes,hormone levels,digestive enzyme activities and intestinal tissue structure of channel catfish were studied.The results showed that the specific growth rate of channel catfish was increased by 1.30%～5.22%,the feed coefficient was decreased by 0%～10.42%,and the fat efficiency and protein efficiency were significantly improved.The growth speed of channel catfish fed with BK5 broken kelp diet was the best amone them.Aiming at the specific growth rate,the appropriate addition amount of each kelp product was obtained through a one-variable quadratic regression analysis as kelp 1.14%,broken kelp 0.91%,enzymatic hydrolysis kelp 2.65%,kelp enzymatic hydrolysis extract 0.09%.The results showed that superfine grinding method was more suitable for kelp processing.The four kelp products had no significant effects on the body composition and body indexes of channel catfish.However,after consuming the kelp product diet,the blood glucose level of channel catfish had a tendency to increase,while the CHOL,TG,HDL,and LDL of each group had a downward trend,and the high-addition group had a more obvious decrease.Among them,K10,BK5,BK15,KEH15,KEHE10,LDL decreased significantly in KEHE15 group.There was no significant difference in AST,ALT,GLO and ALB contents Kelp products could also increase the serum GH,IGF-1 and T4 levels of channel catfish,while reducing INS to varying degrees.Kelp products could also enhance the body’s antioxidant capacity by improving the activity of channel catfish serum SOD,CAT,and GSH.Kelp products could increase the activity of channel catfish pepsin to a certain extent,but will reduce the activity of intestinal and stomach lipase.What’s more,Kelp products could increase the height of the channel catfish intestinal folds,the thickness of the muscle layer,improve the neatness,compactness and thickness of the striated border,which could improve the structure of the intestine and improve the digestion and absorption of the intestine ability and increase the number of intestinal crypts.However,the number of intestinal crypts increased.3.Transcriptome analysis of growth regulation of channel catfish by kelp products.After the feeding experiment,we selected S,K10,BK5,KEH15,KEHE1.0 groups,randomly dissected the three-tailed fish,and collected five gene samples of its tentacles,brain,pituitary,liver,and stomach for transcriptome sequencing.The Q30 of all samples was above 93.77%,the comparison efficiency was above 89.35%,so we could infer that the transcriptome data was reliable.Comparing K,BK,KEH,KEHE and S in turn,1752,1723,2035,and 2848 differential genes were obtained in tentacles tissue;1306,2331,1691,and 1600 differential genes were obtained in brain tissue;2922,3103 in pituitary tissue were obtained,3151,3561 differential genes;liver tissues got 1796,1343,1726,1322 differential genes;gastrointestinal tissues got 2437,3228,2553,3889 differential genes.The results of GO functional classification analysis showed that the antenna differential genes were annotated to the extracellular region,metabolic processes,signal sensor activities,etc.The brain differential genes were annotated to the extracellular region,phylogeny,transcription factor activity,etc.The pituitary differential genes were annotated to plasma membrane,neuron projection development,structural molecular activity,etc.Liver differential genes are annotated to mitochondria,lipid catabolism,aspartate endopeptidase activity,etc.Gastrointestinal differential genes are annotated to extracellular Areas,carbohydrate metabolism,oxidoreductase activity,etc.The results of KEGG pathway analysis indicated that the tentacles differential genes were significantly enriched in disease immune-related pathways,the brain and pituitary differential genes were enriched in signal pathways such as neuroactive ligand receptor interactions,and the liver tissue differential genes were enriched in steroid organisms.In the pathways such as synthesis and FOXO signaling pathways,gastrointestinal tissue differential genes were enriched in the PPAR signaling pathway and other glycan degradation pathways.Ten differentially expressed genes were selected for real-time fluorescent quantitative PCR verification,and the Q-PCR results were basically consistent with the transcriptome sequencing results.The results of the analysis of the different genes showed that kelp had no significant effect on taste receptor and conduction,but it could inhibit POMC expression by increasing GABA receptor expression,and the appetite regulating factors receptors such as KISS1R,ADRA2A,HTDR6 and up regulated DRD4 could promote the appetite of the body.Kelp could also regulate the growth and development of channel catfish by up regulating the expression of TRHR and serum T4 level.In addition,kelp could activate P13K-AKT pathway through IGF-1,phosphorylate FoxO3,down regulate the expression of downstream CCNB1,CCNG2 and CDKN1A,inhibit cell apoptosis,maintain liver health to play its normal metabolic function,and play a positive role in promoting growth.After all,kelp could improve the expression level of gale,AGA,FUCA1,FUCA2,GLB1,HEXB,MAN2B1,GLAT,GNPDA2 and HK1 in the digestive tract of catfish,and improve the ability of intestinal absorption of carbohydrate.However,kelp would down regulate the expression of FABP2 and CD36,reduce the activity of gastrointestinal enzymes,inhibit the intestinal transport of fatty acids and the absorption and metabolism.In general,the results of this experiment showed that cell crushing is a more suitable way of kelp feed processing.Kelp could improve the growth performance of channel catfish,and its growth regulation was mainly through feeding,hormone regulation based on thyroid hormone,digestion and absorption of intestinal carbohydrate,liver health and other aspects. |
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