太湖水华暴发中后期藻附生菌硝化/反硝化研究

Cyanobacterial blooms have afflicted lakes all over the world over the past few decades. While tremendous efforts have been made to understand the effects of nutrients on the abundance and composition of phytoplankton in lakes and the potentially harmful effects of cyanobacterial blooms, it is less well known whether phytoplankton can change the nutrient dynamics in lake ecosystems. Assimilation has been recognized to be the critical first step in nitrogen removal from waterbody, although it is less known about the next step that leads to long-term storage or permanent removal from aquatic ecosystem. Our previous studies have shown that algal bloom can form a micro-niche for coupled nitrification/denitrification. However, whether algal epiphytic nitrification / denitrification occurs or not as well as its nitrogen removal efficiency, there is a lack of evidence in practice. Thus, in this research project, the epiphytic nitrification / denitrification during the middle and later stage of algal bloom will be investigated in laboratory microcosms and field enclosure experiments to explore the fate of nitrogen algae assimilated. Moreover, the correlation between the bloom size and the subsequent total nitrogen removal will be researched to reveal the nitrogen removal mechanism during algal bloom, and thereby provide a theoretical basis for the effective control of the lake eutrophication.

近几十年来，蓝藻水华已成为世界范围内一个重点关注的问题。目前，大量的研究工作都集中在营养盐对水华暴发的作用机理，藻华的潜在危害以及藻华的控制方法等，而关于藻华对营养盐的反作用机制方面的研究甚少。藻的同化作用一般被认为是氮素从水体去除的第一步，但此后真正从水体中脱除的途径和方式还缺乏大量的研究。前期研究表明，藻华暴发可以形成有利于耦合硝化反硝化作用的微生境，从而提高湖泊的脱氮作用，但是藻附生菌硝化/反硝化作用是否发生以及其对氮素的去除效率尚缺乏实践证据。本研究项目将通过室内模拟和野外试验对藻华暴发中后期湖泊中附生菌硝化/反硝化作用进行研究，探索藻类所吸收氮素的最终去向，并通过对藻华暴发规模及后续氮素的总去除量的相关关系研究，揭示藻华暴发过程中的氮去除机制，从而为有效治理湖泊富营养化提供理论依据。

一般认为，沉积物是浅水湖泊中氮素去除的主要场所，然而强烈的附生硝化和反硝化作用已经在许多水生态系统中被观察到。前期研究发现藻华暴发可以形成有利于硝化和反硝化作用的微生境，但是实际湖泊中蓝藻附生菌硝化/反硝化作用是否发生以及其对氮素的去除效率尚缺乏实践证据。. 本项目主要采用湖泊原位调查、野外模拟试验和室内模拟试验等方法，围绕蓝藻及其附生菌的硝化和反硝化潜力的影响因素、藻华暴发和衰亡过程中附生菌和浮游细菌的群落结构、硝化和反硝化等功能基因、溶解性有机质和各形态氮素等变化过程进行研究，所获得的重要结果、关键数据及科学意义可归纳如下：1. 蓝藻附生菌及浮游细菌具有极强的硝化和反硝化能力，其潜在硝化潜力和潜在反硝化能力分别达到7 mg/L/d和0.5 mg/L/d；2. 藻源类蛋白物质是附生菌和浮游细菌反硝化主要碳源；3. 藻附生菌和浮游细菌中参与硝化的细菌主要是微小杆菌属和马赛菌属，而假单胞菌和黄质菌属是参与反硝化的优势菌；4. 蓝藻水华的暴发可促进硝化细菌的增殖，而衰亡则对反硝化细菌生长有利；5. 在氮充足或氮限制的条件下，附生或浮游硝化细菌均可在与蓝藻竞争氨氮的过程中逐渐占据优势；6. 蓝藻+水+硝酸盐氮的野外模拟试验从化学和生物学角度证明了藻华暴发水体可以发生耦合硝化-反硝化作用；7. 处于暴发期的蓝藻是影响湖泊N2O产生的重要因素；8. 高浓度蓝藻和硝酸盐氮的体系中，硝酸盐氮的去除速率达到0.56 mg/L/d，这为合理利用打捞的蓝藻提供了新的方法和思路。综上，本研究证明了藻华暴发中后期水体中存在着耦合硝化-反硝化脱氮作用，从而加深了对湖泊氮素循环的了解。