| A large amount of lignocellulosic biomass is produced globally every year,mainly including agricultural,forestry and related industrial wastes.The biomass can theoretically be converted into a variety of bioenergy products as well as biomassbased products for different uses.However,in lignocellulosic structures,cellulose,hemicellulose and lignin are closely bonded through covalent bonds such as hydrogen bonds,forming biomass recalcitrance,which hinders the efficient conversion and utilization of lignocellulosic biomass.In recent years,people’s low-value treatment of biomass has brought serious pollution problems.Lignocellulosic solid waste has become a major global environmental management problem.It is one of the most potential approaches to solve the problem of biomass conversion to study and utilize natural high efficiency bioconversion system.Termites are one of the most efficient lignocellulosic biological transformation systems in nature,and wood-feeding higher termites are the species at the top of the termite evolutionary tree.It is of great scientific value and application significance to systematically study and analyze their biological transformation system.It has been shown that the success of termites in degrading lignocellulose is largely dependent on their complex and abundant gut symbiotic microorganisms.However,there is still a lack of systematic and in-depth understanding of the diversity of gut symbiotic bacteria of wood-feeding higher termite,and their unique biological transformation mechanism.In this study,Microcerotermes sp.,a higher wood-feeding termite widely distributed in southern China,was investigated.The composition and functional characteristics of the gut bacteria of termites were systematically analyzed and understood by non-culturable(16S r RNA sequencing and macrotranscriptomic analysis)and culturable(enrichment and screening of intestinal microorganisms)methods.The main objectives of this thesis are as follows: first,to systematically understand the composition and diversity of symbiotic bacteria in various intestinal segments(foregut,midgut and hindgut)of Microcerotermes sp.The second is to analyze the composition and function of intestinal bacterial enzyme of termites,and to search for lignocellulosic degradation-related enzyme system genes.Third,the potential application value of gut bacteria from termites in crop straw biotransformation was evaluated.The main research results of this paper are summarized as follows:1.The bacterial composition and function in each intestinal segment of Microcerotermes sp.was analyzed using 16 S r RNA high-throughput sequencing method.It was found that the intestinal symbiotic bacteria of termites belonged to 27 phyla,of which 10 phyla were distributed in all the intestinal segments of termites,but the composition of intestinal bacteria in different segments was different.Firmicutes(84.7%)and Proteobacteria(5.3%)were the most abundant bacteria in foregut.The highest abundance in midgut were Firmicutes(72.1%)and Spirochaetes(10.5%),while the richest bacteria in hindgut was Spirochaetes(61.0%)and Bacteroidetes(12.9%).At the same time,the bacterial diversity index of hindgut was significantly higher than that of foregut and midgut.In addition,principal component analysis(PCo A)found that the Bray-Curtis distance and weighted unifrac distance of the bacterial community between different gut segments were greater than those between the repeats.These results indicated that there were significant differences in the composition and diversity of bacteria in each gut segment of termites.In the prediction analysis of intestinal bacterial function by Tax4 Fun,we found that the relative abundance of xenobiotics biodegradation and metabolic pathways related to lignin degradation was significantly higher in the foregut and midgut than in the hindgut.In addition,the relative abundance of carbohydrate metabolism,amino acid metabolism and lipid metabolism in foregut and midgut was also significantly higher than that in hindgut.The relative abundance of energy metabolism,cofactor and vitamin metabolism,cell motility,glycan biosynthesis and metabolism in the hindgut was higher than that in the foregut and midgut.It can be seen that the function of bacteria in each region of the gut of termites was also different.2.Using metatranscriptomics to analyze the gut bacteria of Microcerotermes sp.,we found that at the transcript level,the dominant bacteria were Spirochaetes(47.54%),Firmicutes(17.03%),Proteobacteria(6.75%)and Fibrobacteres(17.03%).The annotation of KEGG and egg NOG databases showed that the main function of gut bacteria of termites was carbohydrate metabolism and energy metabolism.Further,through the functional annotation of CAZy,a professional database of carbohydrate active enzymes,we found that there were many digestive enzyme genes in the gut bacteria of Microcerotermes sp.,among which the glycoside hydrolase family(GH)accounted for 60.23%,followed by the glycosyltransferase family(GT)29.26%.These results indicated that gut bacteria of Microcerotermes sp.played an important role in the degradation of lignocellulose.Subsequently,the lignocellulosic degradation-related enzyme series genes were further screened and analyzed,and the composition and origin of related enzyme series genes were analyzed.A total of 779 clusters were found to be related to lignocellulosic degrading enzyme systems.In addition to the enzyme system genes related to cellulose,hemicellulose and lignin,some oligosaccharides in lignocellulosic,such as mannan,arabinose and rhamnose-related invertase genes were also found,suggesting that intestinal bacteria of termites contain a large number of lignocellulosic degrading enzyme genes.By analyzing the sources of these lignocellulose-degrading enzyme genes,we found that these lignocellulose-related enzyme genes were mainly derived from Treponema,Chitinispirillum and Fibrobacter,which not only demonstrated the ability of these gut bacteria to degrade lignocellulose,but also provided the research basis for screening gut lignocellulose-degrading bacteria of Microcerotermes sp.3.In vitro enrichment culture,straw degrading bacterial consortium JUBRS and JUBCS were screened from the gut tract of Microcerotermes sp.,and their degradation ability to rice straw and corn straw was tested.The 15-day degradation rate of rice straw by JUBRS was 36.55%,while that of corn straw by JUBCS was22.55%.The enzymes involved in the degradation of cellulose and hemicellulose was tested,Endo-glucanase activity in JUBRS was up to 0.627 U/m L,exo-glucanase activity was 0.263 U/m L,xylanase activity was 4.10 U/m L,and endo-glucanase activity in JUBCS was 0.221 U/m L,exo-dexanase activity was 0.207 U/m L and xylanase activity was 3.17 U/m L.The analysis of the main components of lignocellulose in rice straw and corn straw before and after degradation showed that the lignocellulose in rice straw and corn straw changed,and the cellulose content of rice straw decreased by 28.92%,the cellulose content of corn straw decreased by32.38%,and the lignin content of rice straw decreased by 58.07%.Hemicellulose content decreased by 17.33%,while lignin content and hemicellulose content of corn straw decreased by 13.22% and 28.71%.Scanning electron microscopy(SEM)analysis showed that the microstructures of straw treated with microflora were significantly damaged.In addition,thermogravimetric analysis detected changes in thermodynamic properties of straw before and after degradation,indicating changes in straw structure.Further analysis and fermentation of reducing sugar products after straw degradation showed that the highest reducing sugar content was 96.97 mg/L from rice straw treatment and 103.33 mg/L from corn straw treatment.The highest bioethanol yield was 149.97 mg/L in rice straw treatment solution and 157.62 mg/L in corn structure treatment solution.These results indicate the potential of these two bacteria in the biotransformation of lignocellulosic solid wastes.In conclusion,the results of this study indicate that there are differences in the composition and distribution of symbiotic bacteria in each gut segment of Microcerotermes sp.,and their functions are also focused.A large number of lignocellulose-related enzyme genes were found in gut bacteria of termite,suggesting that intestinal bacteria play an important role in lignocellulosic degradation.The gut system of Microcerotermes sp.contains abundant bacterial resources that can degrade lignocellulose.Its unique lignocellulose transformation and metabolic mechanism can provide a new idea for biotransformation.This study further systematically elucidates the mechanism of lignocellulosic degradation by the wood-feeding higher termites Microcerotermes sp.,and provides an important theoretical basis and a possible new approach for solving the difficult problem of efficient bioconversion of lignocellulosic. 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