南海氨氧化与亚硝酸盐氧化功能群的耦联关系及其所介导的碳氮耦合过程研究

Nitrogen, in the bioavailable forms of ammonium and nitrate, is one of the key nutrients in marine waters and may limit primary production especially in coastal systems. Most of the fixed organic nitrogen in the ocean is converted to nitrate by remineralization consisting of ammonification and nitrification. In the two-step process of nitrification, ammonia is oxidized first to nitrite by aerobic ammonia-oxidizing microorganisms and then to nitrate by aerobic nitrite-oxidizing microorganisms. Ammonia-oxidizing Archaea (AOA) are now recognized as major contributors to oceanic nitrification. These AOA are numerically abundant, especially in the deep sea. Despite these new insights, fundamental questions about the marine nitrogen cycle remain open. For example, little is known about nitrite oxidation and nitrite oxidizers in the ocean. Different coupling relationship between ammonia oxidizers and nitrite oxidizers among variant water masses or niches need to be studied for a better understanding of nitrification and distribution of inorganic nitrogen species in the ocean. In addition, nitrifiers are autotrophs, fixing CO2 using ammonia or nitrite as the energy source. Hence they contribute to a significant part of the dark primary production, in particular, in deep waters. This project is focusing on the two coupling relationships: between ammonia oxidizers and nitrite oxidizers; between nitrification and carbon fixation. By comparing different coupling or decoupling relationships among different water masses or niches in the South China Sea, this project, from perspective of marine microbial ecology and molecular ecology, will try to resolve how couplings control nitrification and distribution of inorganic nitrogen species in the ocean and nitrifiers’ stoichiometric impact on both C and N cycles. Overall, this project will make some breakthroughs in theory and methodology.

硝酸盐是海洋中最丰富的生物可利用无机氮，其形成主要是通过硝化作用。作为硝化过程的第一步，氨氧化过程被普遍认为是限速步骤而被长期关注；而亚硝酸盐的氧化过程一直被人们所忽略，研究甚少。硝化作用中这两个过程的耦联在不同水团及水柱的垂直梯度上具备不同的特征以及不同的生态学意义；此外，氨氧化和亚硝酸盐氧化菌均是自养微生物，因此，海洋中广泛分布的硝化微生物的代谢活性势必形成不可忽略的初级生产力组分，尤其是在深海。本项目聚焦于两个耦联关系—氨氧化与亚硝酸盐氧化功能群的耦联关系、硝化过程与固碳过程的耦合关系，从海洋微生物生态学和分子生态学研究水平，在海洋生态系统不同生境（如光区、弱光区、溶氧最底层、深海；河口区、陆架区、寡营养海区；以及微小生境，如颗粒物）之间，对不同硝化微生物种群而导致的不同的耦联关系进行解析，探讨不同的耦联关系如何控制了海洋硝化过程及氮营养盐的分布，以及对海洋C、N循环化学计量的影响。

海洋是地球上最大的氮储库，硝化作用是海洋中最重要的氮循环过程之一，其形成的硝酸盐是海洋中最丰富的生物可利用无机氮。氨氧化和亚硝酸盐氧化功能群所介导的两步硝化过程是实现光能传递到深海被再利用的重要途径，为深海生物圈提供了“新”的有机质，同时积累硝氮。本项目重点解析了两个耦联关系——氨氧化与亚硝酸盐氧化功能群的耦联关系、硝化过程与固碳过程的耦合关系，取得了一系列突破性进展成果。首次在南海及邻近西太平洋海域阐明了主要固氮类群的生物地理学分布及其控制机制。自河口、近岸、外海至深海，开展了碳−氮过程耦合研究，阐明了AOB主导的（尤其在大颗粒物上）硝化作用是珠江口上游氧化亚氮释放的主要来源；填补了中国边缘海及邻近洋区NOB结构和活性研究的空白，表明了AOB和Nitrospira（NOB）在珠江口是硝化速率的主要贡献者，而AOA和Nitrospina（NOB）则是海盆区的主要优势类群，阐释了水团特征因子、营养盐底物浓度及影响底物可得性的环境因子可能是影响河口硝化过程的重要因素；阐明了AOA和NOB在单细胞水平的碳−氮计量学关系，建立了深海硝化速率、固碳速率、硝化类群生物量、生长率及营养盐浓度之间的动态耦合模型，重新估算了全球海洋的黑暗固碳量，即量化了硝化过程对深海生物圈及全球海洋碳循环的贡献和影响。首次定量了陆架侧向输运的颗粒物对深海碳需求的显著贡献，填补了边缘海深海碳供需不平衡的缺口，揭示了侧向传输过程对深海碳、氮生物地球化学循环的重要影响以及对海洋储碳的重要意义。