南海环礁潟湖中关键氮循环过程和对海洋酸化的响应

Lagoons associated with coral atoll are very unique ecosystems, and they usually have a very special water chemistry and unique microbial community composition compared to the surrounding ocean. Lagoons are more vulnerable to anthropogenic and natural environmental changes, such as ocean acidification due to their relative small volume and lack of exchange with the open ocean. Strong stratification is formed in the water body of the closed lagoon, with each layer harboring a unique microbial community. However, the high rates of microbial nitrogen processes in different water layers, which support the high productivity of the ecosystem, are by far largely unknown, although nitrogen is usually the limited nutrient critical to the ecosystem. Ocean acidification is known to be able to affect the rates of nitrogen fixation, nitrification and denitrification, but its impact on the overall nitrogen cycle is still not clear. This project will focus on the closed and open lagoons in the South China Sea, using metatranscriptomic approach to study the microbes involved in nitrogen cycle and isotopic 15N2 method to measure the nitrogen fixing rate in the surface water. At the same time, incubation experiments with artificial acidification will be carried out to study the impact of the ocean acidification on the microbial mediated nitrogen cycle and nitrogen fixing rate in the lagoons. Results generated from this project will help to elucidate the microbial communities and processes that are responsible for the nitrogen metabolisms in different types of lagoons and their responses to the global climate change, the difference and interaction between the lagoons and the surrounding open oceans, as well as the contribution of the tropical lagoon ecosystems to global oceanic nitrogen cycle.

环礁潟湖是寡营养高生产力的珊瑚礁生态系统，因水动力弱、水体小、垂直分层明显等特征，与外围海域有明显不同。潟湖内每层具有独特的水化特征，并形成相应的微生物群落。但作为系统主要限制营养盐的氮素在不同水层中的循环过程、速率、特异性还不清楚。另一方面，海洋酸化会影响到生物固氮、硝化、反硝化等速率，但其对整个氮循环的影响还缺乏了解。本项目拟以南海封闭型和开放型环礁潟湖为切入口，利用宏转录组学研究微生物介导的氮循环过程，并开展同位素实验测定表层水体中的生物固氮速率，来研究该系统内氮素的地球化学循环过程、相关微生物功能组群、生物固氮速率，以及与外围海域的异同。同时结合酸化模拟培养实验，来阐明海洋酸化背景下环礁潟湖生态系统中微生物氮循环过程、固氮率的响应机制，以期增加对气候变化下海洋氮循环过程、速率演变规律的认识。

该项目采用了原位微宇宙酸化培养实验结合宏基因组和宏转录组手段，研究了南海西沙潟湖水体微生物对海洋酸化的响应机制。目前获得了不同类型环礁潟湖中微生物类群的群落组成和代谢特征，揭示了酸化能显著影响封闭珊瑚礁系统中各微生物类群的丰度、生长速率及其介导的氮循环关键环节，验证了封闭型潟湖体系更易受到海洋酸化扰动的科学假设。此外，通过比较宏转录组分析，表明了原核微生物在酸化的初期更多表现为胁迫响应，后期为催化活性等细胞适应性响应；证实了酸化导致原核生物的硝化和硝酸盐还原过程的上调，并促进了微型真核生物碳循环相关基因的富集，特别是与光合作用有关的碳代谢基因。我们的研究结果初步揭示了海洋酸化背景下我国西沙海域珊瑚岛礁微生物群落的演变规律及对海洋酸化的响应机制。