水稻土中固氮微生物驱动多氯联苯转化的机制研究

Polychlorinated biphenyls (PCBs) are a group of notorious chlorinated persistent organic pollutants. PCBs contamination in the farmland is of great concern due to their bioaccumulation through the food chain and high toxicity to the environment and human health. Development of remediation strategy to tackle PCBs pollution in farmland soils has been listed as one crucial and urgent environmental issue. Nitrogen fixing bacterial strains could promote PCBs transformation through biodegradation and biological nitrogen fixation. However, at the higher and more complicated integrated level, whether diazotroph community could facilitate PCBs transformation in soil remains to be explored. Thus, this project will analyze varied paddy soils around typical PCBs contaminated sites and carry out microcosm experiments. By integrating high-throughput sequencing, stable isotope probing technique, GeoChip and other methods, we will evaluate the diazotroph resources, nitrogen fixing capacity and PCB degradation efficiency in contaminated paddy soils, target the active dual function degradative diazotroph species, and clarify the coupling relationship and molecular mechanism between nitrogen fixation activity and PCB transformation. The results will help to elucidate the contribution of diazotroph community to PCBs transformation in soil, thereby providing scientific evidence for targeted optimization of bioremediation strategies in PCBs contaminated farmland soils.

多氯联苯（PCBs）作为一类优先控制的持久性有机污染物，可经农田土壤累积并传递，进而危及国家粮食安全、环境安全及人体健康。探索环境友好、高效的PCBs污染农田土壤修复方式是亟待解决的重要环境科学问题。固氮微生物纯菌株可通过生物转化、提供氮素等机制促进PCBs降解；然而在土壤微生态系统中，固氮微生物群落促进PCBs转化的机制尚不清楚。本项目拟选取PCBs污染稻田土壤，采用原位土壤调查和室内微宇宙试验，结合高通量测序、稳定性同位素核酸探针标记、微生物功能基因芯片等手段，明确不同污染土壤中固氮微生物群落结构多样性、固氮活性以及对PCBs的降解潜能，挖掘“PCB降解-固氮”双功能微生物资源，探明微生物固氮活性与PCB转化的耦联关系及机制，以从群落水平揭示固氮微生物对土壤PCBs转化的驱动机制，为靶向优化PCBs污染农田土壤的生物修复策略提供科学依据。

生物固氮过程是土壤中生物可利用氮素的主要来源，其在维持陆地生态系统生产力、生物地球化学循环等过程中起到重要作用。然而，在稻田土壤中，固氮微生物群落对有机污染物的响应适应机制及其潜在的生态功能尚不清楚。本项目选取典型电子垃圾拆解区污染稻田土壤，采用原位土壤调查和室内微宇宙试验，分析了长期多氯联苯（PCBs）污染稻田土壤中固氮微生物群落特征、固氮活性、降解潜能及驱动因素，探究了土壤中固氮微生物驱动有机污染物转化的机制。结合高通量测序、乙炔还原法等手段发现，高浓度PCB污染增加了污染稻田土壤中固氮微生物丰度和固氮速率，改变了固氮微生物群落结构和功能；通过随机森林、结构方程模型等分析说明，固氮微生物群落β多样性是影响固氮速率最显著的预测因子；明确了以Bradyrhizobium、Desulfomonile、Cyanobacteria为主的固氮微生物生态簇是影响固氮速率的关键菌群。进一步采用微宇宙培养试验并向稻田土壤中外源添加PCB 52，发现污染土壤中生物固氮活性与PCB 52消减速率之间存在显著正相关；PCBs降解菌和PCBs脱氯菌可能在生物固氮过程相关的PCBs生物降解过程起作用。通过建立稳定性同位素探针标记培养体系，鉴定了污染稻田土壤中的潜在活跃固氮微生物类群共57个。基于上述结果，进一步选取了原位污染土壤并建立微宇宙培养体系，通过外源添加钼酸钠（Mo）和钨酸钠（W）以调控土壤固氮酶活性，研究生物固氮活性对污染土壤中PCBs、多环芳烃（PAHs）消减的影响。结果表明，施加一定浓度Mo（600 ~1200 μg kg-1）能有效促进土壤中PAHs消减，但是对PCBs促进效果不显著。施加Mo能通过提高土壤中PAHs降解基因（PAH-RHDα）的丰度、固氮酶活性和固氮基因丰度，从而促进PAHs的去除。本项目揭示了长期PCBs污染稻田土壤中固氮微生物群落结构及功能特征，明确了土壤固氮活性与有机污染物转化的耦联关系并探究其耦合驱动机制，为靶向优化污染农田土壤中基于生物固氮功能的生物修复策略提供科学依据。