白蚁及共生微生物木质纤维素水解酶的种类

白蚁是热带生态系统重要的木质纤维素降解者。白蚁种类丰富,可分成高等白蚁和低等白蚁,食性也具有各自特点。白蚁自身可以产生纤维素酶,主要是GHF9的内切葡聚糖酶(EG),也有β-葡萄糖苷酶(GB)。低等白蚁共生的原虫中已发现丰富的纤维素酶基因,属于GHF5,7和45。同时还有其他相关功能基因,如木聚糖酶和果胶类物质水解酶。高等白蚁肠道中没有共生原虫。高等培菌白蚁可以利用共生蚁巢伞属真菌促进木质纤维素降解,真菌可以产生纤维素酶,果胶质水解酶类、木聚糖酶,同时还产生可能与木质素分解相关的一种漆酶,但是从分子水平,关于共生真菌纤维素水解酶的研究还较少。白蚁肠道已分离出许多具有木质纤维素降解能力的菌株,最近的研究也发现了大量细菌纤维素酶基因。白蚁-共生系统丰富的木质纤维素水解酶类为发展生物方法开发纤维素乙醇这一思路提供有价值的资源。     

高效降解木质纤维素的白蚁肠道微生物组

木食性白蚁是自然界木质纤维素的高效降解者,在长期进化过程中白蚁与其肠道微生物组协同作用发展出不同的纤维素降解机制。木食性白蚁具有分别来源于白蚁和共生微生物的两套纤维素酶系统。在低等白蚁中,木质颗粒经过白蚁前、中肠分泌的内源性酶初步消化后,在后肠共生鞭毛虫中被降解为乙酸、二氧化碳和氢。高等木食性白蚁在进化中丢失了鞭毛虫,木质颗粒经白蚁自身分泌的酶初步消化后,在后肠大量共生细菌的帮助下被有效降解。培菌类白蚁利用其菌圃中的蚁巢伞菌和肠道微生物协同作用降解木质纤维素。共生微生物在白蚁的氮素固定与循环、中间产物代谢及纤维素降解等过程中发挥了重要作用。学习和模拟白蚁高效降解木质纤维素的体系,对生物质能源的产业化发展具有积极的意义。     

昆虫与生物质能源利用:一个新的交叉学科前沿

昆虫与生物质能源利用密切相关。这些昆虫包括白蚁类、甲虫类、树蜂类、食叶类水生昆虫、衣鱼类、大蚊类等。它们能在树木、枯枝以及落叶上生活,并具有了相当可观的降解和转化木质纤维素的能力,是自然界中协助进行碳循环的一类重要节肢动物。近几年来,这些昆虫独特的肠道消化能力以及它们的生物质催化转化系统已引起了科学家和研究人员的极大兴趣,希望能通过发现新的降解木质纤维素的酶及酶系统、掌握相关的这些酶的表达和其功能控制基因、并能解开昆虫肠道的消化及其相关机制的谜;更高效的降解和转化植物细胞壁中的碳水化合物并用来生产不同种类的生物能源或生物基材料。目前,对这类昆虫高效降解木质纤维素能力的认识和相关降解机制的研究已发展成为一个与生物质能源应用密切相关的新兴研究领域,成为新的交叉学科前沿。本文将简要讨论这类昆虫消化木质纤维素的几种不同作用机制、共生微生物与昆虫所产生的不同木质纤维素酶以及相互间的协同作用的基础上,还探讨了当前第二代生物质能源研究与开发中所面临的主要挑战、消化木质纤维素类昆虫,特别是白蚁所处的独特地位、潜在的科学和应用价值,以及今后的主要研究方向。     

白蚁及其共生菌来源的4种木质纤维素酶的共表达

天然的木质纤维素材料含有纤维素、半纤维素和木质素等成分。降解天然木质纤维素底物时,需要木质纤维素酶共同作用。近年在木质纤维素酶的相互协同作用方面的研究引起人们的关注,成为一个新的研究热点,文中使用两个不同的共表达载体pETDuet-1和pRSFDuet-1,在大肠杆菌中共表达了白蚁及其肠道微生物来源的β-葡萄糖苷酶、内切β-1,4-葡聚糖酶、漆酶和木聚糖酶这4种木质纤维素酶,经过SDS-PAGE分析得到了与理论值一致的蛋白条带,同时经过酶活验证,这4种蛋白都具有酶活性。以磷酸处理的微晶纤维素(PASC)为底物,测定了共表达酶粗酶液与单独表达酶混合液的协同作用因子,从还原糖的产量上经计算共表达的粗酶液比单独表达酶的混合液对PASC的降解协同作用提高44%;以滤纸和磷酸处理的玉米芯为底物,测定降解协同作用,分别提高了34%和20%。结果表明,共表达酶的降解效率要高于混合的单组分酶液降解效率的总和。     

白蚁肠道共生体的纤维素代谢体系

白蚁是大陆生态系统中木质纤维素降解的生力军,其肠道共生系统纤维素酶对纤维素的消化起到了关键的作用.本文概述了白蚁自身及其肠道共生微生物的纤维素水解系统的特点、相互关系以及相互作用的研究进展.     

黄翅大白蚁后肠优势菌大白蚁营发酵菌的全基因组序列分析

【目的】营发酵单胞菌属Dysgonomonas是黄翅大白蚁后肠的第二优势微生物。前期研究中,我们从黄翅大白蚁后肠分离出一种命名为大白蚁营发酵菌的新菌。为深入了解大白蚁营发酵菌在宿主白蚁体内发挥的作用和功能,有必要解析大白蚁营发酵菌的基因组序列信息。【方法】使用Illumina Miseq测序平台获取该菌的全基因组序列,将其全基因组序列经过注释的基因蛋白质序列提交COG和KEGG数据库进行BLASTp比对分析,确定该菌潜在的重要酶类和代谢途径,并对个别纤维素酶活进行检测。【结果】大白蚁营发酵菌整个基因组大小为4655756 bp,GC含量为38.54%,DDBJ数据库登录号为BBXL01000001–BBXL01000078。生物信息学分析结果表明菌株大白蚁营发酵菌具有多个木质纤维素降解酶基因,且具备完整的木质纤维素降解和乙酸、乳酸生成通路。此外发现该菌株中存在与氮源代谢和抵御病原体相关的基因。【结论】本研究首次解析大白蚁营发酵菌的全基因组序列,了解其基因组基本特征,初步探讨了该菌降解木质纤维素的过程,为细菌协助宿主白蚁降解木质纤维素提供了理论基础,同时为该菌可能参与宿主白蚁氮源代谢和抵御病原体入侵提供了依据。     

培菌白蚁菌圃微生物降解木质纤维素的研究进展

白蚁及其共生微生物协同降解植物细胞壁的机理一直被世界各国科学家所关注。培菌白蚁作为高等白蚁,相比低等食木白蚁具有更多样化的食性,其利用外共生系统“菌圃”,对多种植物材料进行处理。本文综述了菌圃微生物降解木质纤维素的研究进展,以期为深入研究菌圃中木质纤维素降解过程及其机制,并挖掘利用菌圃降解木质纤维素的能力及仿生模拟菌圃开发新的生物质利用系统提供参考。培 菌白蚁在其巢内利用由植物材料修建的多孔海绵状结构——“菌圃”来培养共生真菌鸡枞菌Termitomyces spp.,形成了独特的木质纤维素食物降解和消化策略,使木质纤维素在培菌白蚁及其共生微生物协同作用下被逐步降解。幼年工蚁取食菌圃上的共生真菌菌丝组成的小白球和老年工蚁觅得食物并排出粪便堆积到菌圃上成为上层菌圃。这一过程中,被幼年工蚁取食的共生真菌释放木质素降解酶对包裹在植物多糖外部的木质素屏障进行解聚。菌圃微生物(包括共生真菌)对解聚的木质素基团进一步降解,将多糖长链或主链剪切成短链,使菌圃基质自下而上被逐步降解。最后下层的老熟菌圃被老年工蚁取食,其中肠的内源酶系及后肠微生物将这些短链进一步剪切和利用。因此,蚁巢菌圃及其微生物是培菌白蚁高效转化利用木质纤维素的基础。化学层面的研究表明,菌圃能够实现对植物次生物质解毒和植 物纤维化学结构解构。对共生真菌相关酶系的研究显示可能其在菌圃的植物纤维化学结构和植物次生物质的降解中发挥了作用,但不同属共生真菌间其效率和具体功能不尽相同。而菌圃中的细菌是否发挥了作用和哪些细菌类群发挥了作用等仍有待进一步的研究。相比于低等食木白蚁利用其后肠共生微生物降解木质纤维素,培菌白蚁利用菌圃降解木质纤维素具有非厌氧和能处理多种类型食物两大优势,仿生模拟菌圃降解木质纤维素的机制对林地表面枯枝落叶的资源化利用具有重要意义。     

纤维素酶与木质纤维素生物降解转化的研究进展

利用纤维素酶将预处理后的秸秆降解成可发酵性单糖,然后发酵生产所需的液体燃料及化工产品的技术,对于我国解决能源、环境、人口就业等难题有着巨大的积极影响。在木质纤维素生物降解转化工艺中,减少纤维素酶用量及提高酶解效率是降低木质纤维素降解成本的关键。纤维素酶系和木质纤维素酶水解技术的改进需要深入了解纤维素酶系统的组成及其协同作用、纤维素酶的结构与功能以及纤维素酶的生产技术。将就以上几个方面的研究进展进行讨论,并深入探讨了纤维素酶糖化能力的评价方法。     

白蚁肠道微生物研究方法概述

白蚁是木质纤维素的主要降解者,在森林生态系统碳氮循环过程中发挥着重要作用。白蚁肠道共生微生物主要包括原生生物、细菌、古菌和真菌。在白蚁对木质纤维素进行降解、发酵,从而产生乙酸、氢气和甲烷以及对氮的固定过程中,白蚁肠道共生微生物起着重要的作用。本文对白蚁肠道微生物的研究方法进行总结,概述了各种方法的优缺点,同时对肠道微生物的研究进展进行了总结,以期为白蚁肠道微生物的进一步研究和利用提供参考。     

β-木糖苷酶及其在纤维素乙醇生产中的应用研究进展

将木质纤维素类生物质生物转化生产液体燃料,如纤维素乙醇和大宗化学品,对缓解当前人类社会面临的能源和资源危机以及保护环境具有重要意义。半纤维素是木质纤维素类生物质的主要组成成分之一,它的生物降解转化对实现木质纤维素生物炼制意义重大。由于半纤维素糖种类的多样性和半纤维素结构的复杂性,需要一个复杂的半纤维素酶系才能完成对半纤维素的有效降解。除了木聚糖酶等以外,β-木糖苷酶也是半纤维素酶系的主要组分。在半纤维素降解过程中,β-木糖苷酶将木聚糖酶的水解产物木寡糖和木二糖水解为木糖,不仅在木聚糖的彻底降解过程中起着重要作用,而且可以缓解木寡糖对木聚糖酶和纤维素酶的抑制作用。该文综述了目前在β-木糖苷酶方面的研究进展,包括β-木糖苷酶的分类、酶学性质、酶结构及其催化机制、基因的克隆与表达等,并对β-木糖苷酶在纤维素乙醇生产中的应用情况进行了简述。     

Leucoagaricus gongylophorus Produces Diverse Enzymes for the Degradation of Recalcitrant Plant Polymers in Leaf-Cutter Ant Fungus Gardens

Plants represent a large reservoir of organic carbon comprised primarily of recalcitrant polymers that most metazoans are unable to deconstruct. Many herbivores gain access to nutrients in this material indirectly by associating with microbial symbionts, and leaf-cutter ants are a paradigmatic example. These ants use fresh foliar biomass as manure to cultivate gardens composed primarily of Leucoagaricus gongylophorus, a basidiomycetous fungus that produces specialized hyphal swellings that serve as a food source for the host ant colony. Although leaf-cutter ants are conspicuous herbivores that contribute substantially to carbon turnover in Neotropical ecosystems, the process through which plant biomass is degraded in their fungus gardens is not well understood. Here we present the first draft genome of L. gongylophorus, and, using genomic and metaproteomic tools, we investigate its role in lignocellulose degradation in the gardens of both Atta cephalotes and Acromyrmex echinatior leaf-cutter ants. We show that L. gongylophorus produces a diversity of lignocellulases in ant gardens and is likely the primary driver of plant biomass degradation in these ecosystems. We also show that this fungus produces distinct sets of lignocellulases throughout the different stages of biomass degradation, including numerous cellulases and laccases that likely play an important role in lignocellulose degradation. Our study provides a detailed analysis of plant biomass degradation in leaf-cutter ant fungus gardens and insight into the enzymes underlying the symbiosis between these dominant herbivores and their obligate fungal cultivar.     

Dietary‐driven variation effects on the symbiotic flagellate protist communities of the subterranean termite Reticulitermes grassei Clément

The ability of subterranean termites to digest lignocellulose relies not only on their digestive tract physiology, but also on the symbiotic relationships established with flagellate protists and bacteria. The objective of this work was to test the possible effect of different cellulose‐based diets on the community structure (species richness and other diversity metrics) of the flagellate protists of the subterranean termite Reticulitermes grassei. Termites belonging to the same colony were subjected to six different diets (natural diet, maritime pine wood, European beech, thermally modified European beech, cellulose powder and starvation), and their flagellate protist community was evaluated after the trials. All non‐treated sound woods produced similar flagellate protist communities that were more diverse and of high evenness (low dominance). On the contrary, flagellate protist communities from cellulose‐fed termites and starving termites were considered to be significantly different from all non‐treated woods; they were less diverse and some morphotypes became dominant as a consequence of flagellate protist communities having suffered major adaptations to these diets. The flagellate protist communities of untreated beech and thermally modified beech‐fed termites were considered to be significantly different in terms of abundance and morphotype diversity. This may be caused by a decrease in lignocellulose quality available for termites and from an interference of thermally treated wood with the chemical stability of the termite hindgut. Our study suggests that as a consequence of the strong division of labour among these protists to accomplish the intricate process of lignocellulose digestion, termite symbiotic flagellate protist communities are a dynamic assemblage able to adapt to different conditions and diets. This study is important for the community‐level alteration approach, and it is the first study to investigate the effects of thermally modified wood on the flagellate protist communities of subterranean termites.     

Multipartite symbioses in fungus-growing termites (Blattodea: Termitidae,Macrotermitinae) for the degradation of lignocellulose

Fungus-growing termites are among the most successful herbivorous animals and improve crop productivity and soil fertility. A range of symbiotic organisms can be found inside their nests. However, interactions of termites with these symbionts are poorly understood. This review provides detailed information on the role of multipartite symbioses (between termitophiles, termites, fungi, and bacteria) in fungus-growing termites for lignocellulose degradation. The specific functions of each component in the symbiotic system are also discussed. Based on previous studies, we argue that the enzymatic contribution from the host, fungus, and bacteria greatly facilitates the decomposition of complex polysaccharide plant materials. The host–termitophile interaction protects the termite nest from natural enemies and maintains the stability of the microenvironment inside the colony.     

Termite-Microbe Symbiotic System and Its Efficient Degradation of Lignocellulose

Termites thrive in the tropics and play an important role in lignocellulose degradation. This ability depends mainly on intestine microbes in the gut, but most of them are so-called unculturable microbes, which can not be cultivated by traditional culture methods. The recent development of molecular approaches such as the PCR method has made it possible to access the enormous numbers of unculturable microbes in the gut of termites.

This review explains our research on the ecological role of the termite, the termite-microbe symbiotic system, and the functions of lignocellulose degradation using various molecular methods. In the future, new technologies such as genomics should make it possible to analyze and utilize unculturable microbial resources in natural environments.     

Phylogenetic Analysis of Cellulolytic Enzyme Genes from Representative Lineages of Termites and a Related Cockroach

The relationship between xylophagous termites and the protists resident in their hindguts is a textbook example of symbiosis. The essential steps of lignocellulose degradation handled by these protists allow the host termites to thrive on a wood diet. There has never been a comprehensive analysis of lignocellulose degradation by protists, however, as it has proven difficult to establish these symbionts in pure culture. The trends in lignocellulose degradation during the evolution of the host lineage are also largely unknown. To clarify these points without any cultivation technique, we performed meta-expressed sequence tag (EST) analysis of cDNA libraries originating from symbiotic protistan communities in four termite species and a wood-feeding cockroach. Our results reveal the establishment of a degradation system with multiple enzymes at the ancestral stage of termite-protistan symbiosis, especially GHF5 and 7. According to our phylogenetic analyses, the enzymes comprising the protistan lignocellulose degradation system are coded not only by genes innate to the protists, but also genes acquired by the protists via lateral transfer from bacteria. This gives us a fresh perspective from which to understand the evolutionary dynamics of symbiosis.