浅水湖泊富营养化过程中氮与磷循环的偶联机制研究

Lake eutrophication has greatly constrained economic developments and social progresses in China, whose key driving element (nitrogen or phosphorus) remains unclear. In this project, using the methods of ecological investigations in large scales and enclosure experiments combined with algal cultures and molecular biotechnologies, the biogeochemical cycling processes of nutrients were described, together with their microbial basis and responding modes by phytoplankton communities, in urban lakes of Wuhan city and distinct areas of Lake Chaohu with different trophic status. The objective of this study is to reveal the mechanisms behind the coupling between the cycling pathways of nitrogen and phosphorus. In details, enrichments of organic nitrogen and ammonia would enhance sediment phosphorus release through anaerobic dissociation, subsidizing bioavailable nitrogen and phosphorus into water column in parallel, which resulted in algal bloom. In turn, further increases in phosphorus loading would strengthen the coupling between nitrification and denitrification thereby substantially removing nitrogen from the lakes, leading to the apparent coexistence of the limitation by nitrogen and phosphorus and the shifting between them. Based on these demonstrations, the function of nitrate as an oxygenant that promotes phosphate sorption on sediments could be further explored to improve ecotechnology for sediment remediation in eutrophic lakes. Therefore, the performance of this project will greatly facilitate the ecologists to have a deep insight into the generality of eutrophication theory, and as a consequence, give the primary goal for nutrient controlling both in general and with the stage specific emphases to combat eutrophication. Finally the principles of sediment remediation by oxidation in eutrophic lakes could be further clarified.

湖泊富营养化已严重制约了我国的经济发展与社会进步，其关键驱动因素氮与磷作用的单一性和主导性仍不甚明晰。本项研究以武汉市沿岸带土地利用方式与营养状态迥异的湖泊和巢湖不同湖区为实验对象，采用大尺度生态调查、围隔实验、室内模拟、微生物和藻类培养与分子生物学鉴定相结合的方法，系统描述营养状态明显不同的湖泊中氮与磷循环的生物地球化学过程及其微生物学基础和浮游植物响应模式，揭示湖泊氮与磷循环途径相互偶联的生物化学机制，即有机氮和氨输入可借厌氧机理促进沉积物磷的释放，导致生物可利用性磷的补给，促发水华，磷的进一步富集则加速硝化与反硝化过程的耦合，引起氮的减少，从而促成氮磷关键调控作用在表观上的共存与交替。在上述基础上发掘硝酸盐的氧化性有利于沉积物磷固定的功能。项目的实施将有助于深入阐明富营养化理论所具有的一般性，并据此提出富营养化过程中营养控制的首要目标与分阶段侧重点，同时丰富沉积物氧化修复技术的原理。

湖泊富营养化已成为目前我国面临的最为严重的水环境问题之一，其本质在于水中氮和磷的富集导致浮游植物的大量生长甚至引起水华，而氮和磷循环的交互作用及其对水华过程的影响机制仍不甚明晰。以武汉市53个湖泊以及巢湖为实验对象，并辅之以三峡水库、汉江和养殖池塘，系统分析了不同季节水和沉积物的理化参数以及浮游植物的密度与组成，检测了沉积物硝化速率和反硝化速率，并借分子生物学方法解析了沉积物氨氧化古菌、氨氧化细菌和反硝化细菌的的数量与组成，并用统计方法探讨和归纳了上述变量之间的关系，以期深入揭示富营养化过程中生物可利用性氮和磷的变化模式，验证氮富集导致磷释放且磷富集促进氮去除从而为固氮蓝藻水华提供适宜营养组成的科学假设。结果表明，由蛋白质、碳水化合物和脂类等构成的有机质（尤其是蛋白质）的酶促分解，加速了厌氧状态的形成。此外，蛋白质的分解引起铵的积累，促进需氧的硝化作用，进一步强化了由脱氢酶活性表征的厌氧状态，引起氧化态铁的还原，与其结合的磷亦随之释放。再者，碳和氮的富集将诱导磷酸酶活性，促进磷的释放。磷的补给极大地有利于硝化菌和反硝化菌的生长，同时改变其群落结构，使之有利于各自功能的优化和偶联，从而实现生物可利用性氮的有效去除，且最终导致以缺氮为特征的氮和磷营养的失衡，为固氮蓝藻的水华提供了适宜的营养结构。当磷和碳进一步富集时，生物可利用性氮更趋缺乏，而溶解有机态氮更趋丰富，胞外氨肽酶介导的有机氮的周转时间和酶的米氏常数均与N/P显著正相关，生物可利用性氮的再生因酶促反应速度的提高和底物亲和力的改善而显著增强。氨肽酶的米氏常数与磷酸酶的米氏常数显著负相关，且与铵浓度显著正相关，故为调节有机氮周转效率的关键因素。以绿藻、硅藻、裸藻、隐藻、甲藻为主的浮游植物对氨肽酶的周转效率具有重要贡献，并以其介导的生物可利用性氮的再生作为补充氮缺乏的另一重要途径，最终导致不具备上述功能的鱼腥藻的优势地位的消失。