地下河口微生物厌氧硝化作用驱动的耦合脱氮过程和作用机制

Humans are modifying global nitrogen (N) cycle at an alarming rate. The release of reactive nitrogen（Nr）into the environment drives eutrophication and hypoxia, which are the greatest threats to coastal ecosystems worldwide. Significant Nr contamination of coastal aquifers may arise from agricultural runoff, septic tank discharge and landfill leachate. Transport of contaminated groundwater via submarine subterranean estuary (STE) is a major source of Nr to the coastal zone and an overlooked driver of eutrophication. STE, a region of the coastal aquifer where mixing between discharging terrestrial water and marine water, has critical roles in the biogeochemical processes due to steep gradient and rapid variation of redox states driven by tide. Recent studies of the biogeochemical processes have suggested the STE as a hotspot of removing Nr associated with permeable nature of sandy sediments..Preliminary results from a STE in the Pear River Estuary showed that there was substantial N2 production in ferruginous and sulfidic zones where neither nitrite nor nitrate was detectable, suggesting anoxic nitrification (anNTR) occurred by oxidation of ammonium in the absence of oxygen using other common chemical oxidants such as metal oxides (namely, Fe and Mn) or sulfate. anNTR could provide nitrite for anammox and denitrification, serve as a vital link to N2 production, and attenuate N loads discharging from a subterranean estuary (STE). Therefore, coupled anoxic nitrification to anammox or denitrification in coastal groundwater may be a major unrecognized sink for fixed nitrogen at the land-sea interface. In addition to coastal groundwater, the coupled N cycle has potential importance in anoxic environments. This proposal focuses on the STE because geochemical conditions there appear optimal for the proposed reactions to occur, and our preliminary data show strong evidence for the coupled processes of nitrogen removal. The proposed work uses a combined geochemical, 15N isotope tracer and microbiological approach to evaluate environmental controls on this new tpye N cycle as well as to estimate its contribution to reduction of fixed N fluxes to the coastal ocean. Four approaches are proposed: (1) Field characterization of anoxic nitrification reactions and associated microbial communities in a subterranean estuary; (2) Laboratory incubation experiments to identify hotspots of the cryptic N cycle; (3) Controlled microcosm experiments to determine geochemical controls on anoxic nitrification; and (4) in situ assessment of anoxic nitrification to estimate the importance of the cryptic N cycle in a coastal aquifer. The proposed project would change the current recognition of the biogeochemical nitrogen cycle in anoxic environments and proved a pattern of microbial nitrogen loss in anoxic environments. At same time, this study will allow us to better understand interactions among the nitrogen, metals, and sulfur cycles in coastal environments under the ongoing human modification.

地下河口（STE）是在海陆交界面形成的地下淡水输入与海水入侵的混合地带，由于其氧化还原条件的快速变化，成为生物地球化学反应的活跃区。前期研究结果表明，在STE中富含铁、锰和硫化物的无氧层，虽然没有检测到硝态氮，但却有过量氮气产生，推测存在一种厌氧硝化作用（anNTR），为厌氧氨氧化（Anammox）和反硝化（DNF）过程的发生提供了有效的底物，形成anNTR-DNF/Anammox耦合脱氮过程。本项目拟运用地球化学、同位素示踪和分子生态学的综合技术方法，深入研究这种耦合脱氮过程的发生机制以及与环境因素之间的相互作用关系，分析这种耦合氮循环过程的反应热点，并通过原位实验估算其对近海氮通量减少的贡献。研究结果将会提高目前对无氧环境中氮素生物地球化学循环过程的认知，阐明无氧环境中微生物脱氮的一种新机制，揭示人类活动影响下的陆-海界面STE系统在氮素循环平衡中的生态功能。

地下河口（STE）是在海陆交界面形成的地下淡水输入与海水入侵的混合地带，由于其氧化还原条件的快速变化，成为生物地球化学反应的活跃区。项目采用同位素地球化学和分子生态学方法结合，探究了高渗透性和低渗透性两种典型地下河口氮去除的过程机制。结果表明，地下河口是个强的氮汇区域，反硝化和厌氧氨氧化作用在地下河口氮素去除中发挥关键作用，其中反硝化过程发挥主导作用，期中有氧-无氧界面具有高的微生物多样性，且代谢类型丰富，是反硝化和厌氧氨氧化作用发生的热点。创新性将地下水水文学、同位素地球化学和微生物分子生态学技术结合，多角度、系统地探讨了地下水和海水交互作用影响地下河口氮素去除的分子机制，厘清反硝化和厌氧氨氧化过程对地下河口氮素去除的速率和贡献比例，地下河口通过反硝化作用有效地将输入到地下河口的氮素去除，降低地下水中氮素对近海海域的污染。采用实验室发展的氮去除功能基因高通量测序分析方法，对功能基因（反硝化nirS、nosZ和厌氧氨氧化16S rRNA基因）进行了高通量测序分析，发现海水和地下水的交换作用影响了微生物的群落结构和丰度。项目研究揭示了地下河口的脱氮功能和机制，揭示了地下河口对近海生态系统氮素净化的贡献，提高了目前对无氧环境中氮素生物地球化学循环过程的认知。项目研究共计发表学术论文33篇，部分研究成果获得海洋工程自然科学一等奖。