海草及其根际微生物协同适应西沙岛礁寡营养环境的生态策略研究

Xisha Islands is the complete and most distinctive ecosystem, which play an crucial role in protecting biodiversity and ecological resources. Seagrass can promote the calcification of coral reef, slow the degradation of coral reef and accelerate the restoration coral reef ecosystems. This project aims at investigating the ecological coadaptation strategies of Seagrass and its rhizospheric microbes in tropical Coral Reef Islands basing on the prospective of the synchronous and real-time responses and adaptation seagrasses and their rhizospheric microbes. This study consists of both field investigation and simulation experiment. Molecular techniques including Geochip, high-throughput sequencing and real-time PCR are involved to investigate exploring the relationship between the growth status of seagrasses and structure and function of rhizospheric microbe community, and identifying the microbial species which exert the crucial role in helping seagrasses living in the coral reef ecosystems. Result of the nitrogen isotope experiment will reveal the contribution of fixed nitrogen by microbe to the seagrasses growth and their allocation in plants. One of the most important aspects of this project is to study the synchronous and real-time response of the seagrasses and their rhizospheric microbes by nitrogen-addtion experiment. The physiological and ecological parameters of seagrass will be measured. At the same time, the abundance, diversity and real-time expression of key enzymes (nitrogen fixation, ammonia oxidation and denitrification) involved in nitrogen cycles at DNA and RNA level will be analyzed. It will elucidate the nitrogen feedback regulation mechanism of seagrasses and their rhizospheric microorganisms.　The results of this project will contribute to sustain the stable development and ecological function of both the natural and artificial coral reef Islands in South China Sea.

西沙岛礁是我国最完整最具有代表性的大洋型岛礁，在生物多样性保护和生态资源供给方面发挥着重要作用。海草能够促进珊瑚的钙化、减缓珊瑚礁的衰退并加速其生态系统的恢复。本项目拟从海草及其根际微生物同步响应和适应的角度出发，研究海草及其根际微生物协同适应西沙岛礁寡营养环境的生态策略。通过将野外监测和模拟实验相结合，应用基因芯片、高通量测序和荧光定量等技术对根际微生物的群落结构与功能进行研究，探讨海草生长状态与根际微生物的关系并识别对海草适应岛礁环境起关键作用的微生物类群；利用氮同位素技术明确生物固氮对海草的氮素营养贡献及其在海草体内的分配特性；开展氮添加实验，分析海草在生理生态水平上和氮循环重要的微生物类群(固氮、氨氧化和反硝化)在DNA、RNA水平的同步响应与适应特征，揭示海草及其根际微生物的氮素反馈调节机制。本研究将对我国南海自然珊瑚岛礁和人工吹填岛礁的稳固发展和生态功能的维持有重要的指导意义。

目前全球珊瑚礁生态系统普遍处于衰退状态，而海洋酸化会降低珊瑚的钙化能力、阻碍珊瑚的生长，进一步加剧了珊瑚礁的衰退。珊瑚礁环境中的海草能够在一定程度上缓解酸化对珊瑚的影响、保护并促进珊瑚的生长。由于珊瑚礁生态系统通常是寡营养且氮限制环境，所以明确海草在这种环境下采取何种生存策略以保持高生产力具有重要的生态学意义。基于此，本项目以西沙岛礁为主要区域，通过野外现场和室内培养实验相结合，调查监测了海草的分布、种类和环境因子等,厘清了海草根际微生物群落组成、功能及其与海草的协同进化关系、明确了氮循环重要过程的微生物类群、阐明了海草及其根际微生物对不同氮素添加的响应模式并解析了海草根际微生物与海草植株互作效应等。获得了以下的主要研究结果：海草根际优势微生物种类分别为变形菌、拟杆菌和浮霉菌，谱系alpha多样性显著性差异仅存在西沙群岛4种不同海草之间而物种alpha多样性显著性差异在不同海草种类和不同生态系统间均均存在；海草根际微生物存在核心微生物群并与沉积物中的TN、TOC、TP和AP显著相关；与海南岛新村的海草相比，西沙海草根际微生物氮循环关键过程(如固氮、氨氧化、硝化和反硝化)微生物的丰度更高；海草不同部位的固氮微生物群落呈现显著微生境特异性；海草根际氨氧化微生物的群落结构呈现显著的地理分布特异性，氨氧化古菌多样性高而氨氧化细菌丰度更高；硝酸盐的添加可以有效地提升海草泰来草对光的利用能力，而铵盐的添加使泰来草对光强更为敏感；海草的定殖会提高高温处理后海草沉积物的多样性、塑造和改善海草根际沉积物微生物群落组成；不同种类的海草与其根际微生物群落存在协同进化的关系；综上结果明确了海草及其根际微生物的互作并协同适应寡营养岛礁环境的生存策略，为进一步研究我国南海岛礁海草床对全球气候变化的响应与反馈提供了重要的科学数据和技术支撑。