藻型湖湾中硫酸盐还原条件下异养生物固氮过程特征及机理

Negative effects of microbial sulfate reduction on eutrophic lakes have been concerned with increasing attention. In addition to promoting phosphorus release and black odor production, sulfate reduction also often induces the occurrence of heterotrophic biological nitrogen fixation. Heterotrophic biological nitrogen fixation is an important source of nitrogen source, and the increase of nitrogen source can promote the growth of cyanobacteria bloom as well as the release of algal toxin. Therefore, investigation about heterotrophic biological nitrogen fixation process under sulfate reduction will improve the understanding of mechanisms related to cyanobacteria bloom occurrence and maintenance in eutrophic lakes, and also help develop the technology for cyanobacterial bloom prevention. A novel idea to study the heterotrophic biological nitrogen fixation in shallow lakes by understanding the development of anoxic zone in the lake bay and the structure of microbial community for sulfate reduction is proposed. One heavy-contaminated bay in Taihu Lake is selected as the research site, and the main contents in the project include the investigation of temporal and spatial characteristics of sulfate reduction process and heterotrophic nitrogen fixation; the determination of main factors influencing heterotrophic nitrogen fixation process under sulfate reduction for organic matter decomposition; and the application of metagenomic technology to analyze the microbial communities associated with heterotrophic nitrogen fixation in lakes. Based on the studies mentioned above, this project will obtain the mechanism for heterotrophic nitrogen fixation under sulfate reduction in shallow freshwater lakes. The project has theoretical significance for the understanding of the nitrogen cycle in lakes, and also has important application in pollution treatment and management of lakes.

硫酸盐还原过程对富营养化湖泊环境的危害已日益受到重视。除了促进磷释放和黑臭物质产生，硫酸盐还原还往往诱导异养生物固氮的发生。异养生物固氮是水体氮源补充的重要途径，而氮源增加会促进湖泊蓝藻水华生长和藻毒素释放。因此，研究湖泊中硫酸盐还原条件下的异养生物固氮过程，有助于认识湖泊蓝藻水华爆发和维持的内在机理，也有利于蓝藻水华预防的技术和工艺的研发。本项目提出通过研究湖湾缺氧区发展及参与硫酸盐还原的微生物群落结构来认知浅水湖泊异养生物固氮机理的一种新思路。以太湖中藻型湖湾区为研究对象，研究水体中硫酸盐还原过程与异养生物固氮的时空特征；分析硫酸盐还原条件下异养生物固氮的影响因素；再应用宏基因组技术分析异养生物固氮的微生物群落，从而获得浅水湖泊有机质硫酸盐还原分解条件下异养生物固氮发生机理。本项目对于认识湖泊氮循环方面有重要理论意义，同时在湖泊环境治理和管理方面也有明显的应用价值。

生物固氮是生物可利用氮的重要来源之一，海洋环境中硫酸盐还原过程能够强烈耦合氮的固定，然而富营养淡水湖泊中硫酸盐还原、生物固氮以及蓝藻水华的潜在关系尚不清楚。本研究选择蓝藻水华暴发频发的大型浅水湖泊太湖为主要研究区域，通过野外实地采样和室内实验模拟相结合，探讨了不同湖湾沉积物的异养生物固氮特征。对比蓝藻水华区和沉水植物分布区的沉积物异养生物固氮潜力，表明蓝藻水华区的沉积物具有更高的固氮微生物多样性及固氮酶活性，蓝藻水华暴发能增强沉积物中的异养生物固氮。根据为期一年采样所获得的多点位的固氮速率，借助于ArcGIS软件平台，藻型生境梅梁湾区域内表层（10 cm）沉积物的年固氮量估算为107.51±5.19 t，平均速率为2.47±0.12 mg m−2 d−1, 甚至高于部分海洋环境。跨年度野外调查发现，梅梁湾的沉积物中具有较高的固氮微生物多样性，硫酸盐还原微生物是固氮微生物群落的组分之一；基于nifH基因序列的簇划分，淡水湖泊固氮微生物明显区别于海洋环境。此外，藻型湖湾沉积物的异养生物固氮具有显著的季节变化，平均固氮速率的最大值和最小值分别出现在2月和8月的前后，这也与海洋中的生物固氮季节模式完全相反。运用多元统计方法分析，沉积物的固氮速率与nifH基因丰度、硫酸盐、三价铁以及硝酸盐显著正相关，与溶解态硫化物、二价铁呈显著负相关。室内模拟研究表明，沉积物的生物固氮能够快速响应有机物的分解，其中蓝藻生物量的分解对固氮有显著促进作用；作为固氮酶复合体组成的必需元素，钼元素的添加快速提高了沉积物的固氮酶活性。进而结合不同氧化还原条件下的生物固氮潜力比较，明确了还原态硫化物通过影响钼的形态成为调控梅梁湾沉积物固氮的关键因子，从而揭示了蓝藻水华发生区域沉积物的生物固氮与硫/铁元素循环关联机制，为深入了解湖泊生物固氮特征及湖泊管理提供了科学依据。