高等食木锯白蚁肠道共生细菌降解木质素的功能与协同机制

Lignin present in biomass is highly recalcitrant biopolymer which presents a key obstacle for degradation of plant biomass. The lignin structure modification occurred in the wood-feeding higher termite gut system might be an essential processing to eventually overcome the biomass recalcitrance for carbohydrate utilizations. However, we know little about the lignin degradation mechanisms and its processing characterizations in this digestive system. The role of intestinal symbiotic bacteria in lignin degradation is still ambiguous. Our previous studies showed that Microcerotermes sp., a widely distributed wood-feeding higher termite in the south of China, has a variety of symbiotic bacteria that can degrade lignin in its digestive system , which may have a synergistic mechanism with the endogenous oxidases of termites. This proposed grant would scope the whole digestive system of Microcerotermes sp. to investigate the composition and distribution of symbiotic bacteria that can degrade lignin and to identify the profiles of enzyme system involved, by using high-throughput sequencing technology. The characterizations of the chemical changes of biomass particles within various gut compartments would be observed by various chemical analysis tools, to indicate the synergistic collaboration between symbiotic bacteria and endogenous ligninases of termites in the lignin deconstruction processing. This systematic investigation will indeed shed a light on synergistic mechanism of lignin degradation in wood-feeding higher termite, and discover new promising lignin-degrading enzymes, which will potentially apply to a biomimetic approach towards biorefinery industry.

高等食木白蚁消化系统对食材中木质素致密结构的代谢是其突破木质纤维素抗降解屏障的关键。然而，高等食木白蚁木质素代谢过程与机制尚不明确，其肠道共生细菌在木质素降解中的作用仍存在争议。我们前期研究表明，锯白蚁是我国南方广泛分布的高等食木白蚁，其肠道系统存在多种共生细菌，并具有显著的木质素降解功能，有可能与白蚁内源性的木质素酶系存在一个协同作用机制。基于这一假设，本项目拟以锯白蚁为研究对象，开展以下研究：一是通过高通量测序等技术手段，明确锯白蚁消化系统降解修饰木质素的肠道细菌组成、分布及关键酶系构成；二是通过进一步的化学分析手段，系统解析木质素大分子代谢过程中，白蚁肠道共生细菌与白蚁内源性木质素降解酶系进行协同与互补的特征，完整揭示肠道共生细菌与白蚁内源性酶系的协同作用机制。其研究结果将为木质素代谢过程的仿生应用以及开发新型生物预处理酶系资源提供重要的理论依据与技术基础。

高等食木白蚁是高效的木质纤维素生物转化系统。本研究以我国南方广泛分布的高等食木锯白蚁为研究对象，通过16SrRNA高通量测序明确了肠道各区段（前肠、中肠、后肠）细菌群落多样性组成。研究发现，后肠共生细菌的物种丰度与多样性显著高于前肠和中肠，而细菌物种组成差异表现在前肠丰度最高的是厚壁菌门和变形菌门，中肠丰度最高的是厚壁菌门和螺旋体门，后肠丰度最高的为螺旋体门和拟杆菌门。进一步通过宏转录组和转录组测序技术，分别解析了白蚁肠道共生细菌及宿主肠道组织木质纤维素降解酶系构成，发现其酶系构成与丰度存在差异，肠道共生细菌的木质素酶基因量显著低于宿主，而纤维素酶与半纤维素酶基因量显著高于宿主，肠道细菌与宿主协同高效降解木质纤维素。通过体外培养方法，本研究共获得67株菌株，分属4个门（变形菌门、放线菌门、拟杆菌门和厚壁菌门）17个属40个种，其中62.6%的菌株能够同时降解木质素、纤维素和半纤维素，链霉菌属与沙雷氏菌属细菌丰度最高且产酶能力强。从已获得的菌株中选取高效菌株Streptomyces sp. MS-S2对其产酶条件进行优化，并研究其生物转化农作物秸秆的过程与特征。在15g/L的小麦秸秆作为碳源，1.5%浓度的酵母粉作为氮源，pH8.0，培养温度30℃的最佳产酶条件下，菌株可以直接水解小麦秸秆产还原糖，并进一步发酵产生10.8g/L的生物乙醇。本研究明确了高等食木锯白蚁肠道细菌组成、分布及其在木质素、纤维素与半纤维素降解中发挥的重要作用，解析了肠道共生细菌及宿主肠道组织协同互作高效降解木质纤维素的特征，揭示了锯白蚁肠道共生细菌在生物质资源化利用中具有潜在的应用价值，为木质纤维素代谢过程的仿生应用以及固体废弃物的资源化利用提供重要的理论依据与技术基础。