Collaborative Research: The Potential Importance of Intracellular Nitrate Cycling in the Nitrogen Cycle in Marine Sediments

氮是生命的基本元素。因此，需要了解控制氮循环的速率和过程，以了解海洋生态系统动态。氮以多种化学形式存在，将一种形式转化为另一种形式的过程主要由细菌进行。然而，新的数据表明，储存在更复杂的生物细胞内的硝酸盐是海洋沉积物中硝酸盐的主要来源。这种细胞内的硝酸盐还没有得到充分的测量和整合到氮循环的整体框架。研究人员将在拟议的研究中解决这一问题，以更好地了解和修订海洋中的氮库存和流量。这些信息可用于增进对全球碳循环和气候变化反馈控制的理解。一名研究生将接受使用工具和技术解决复杂海洋系统问题的培训，三名本科生研究人员将参与这项工作。这个研究小组将通过以下方式扩大这项研究的影响：1）与美国国家科学院合作;缅因州教学和学习创新中心和扩大参与研究所制作和分发有关这项研究及其对环境的影响的教育视频;和2）领导夏季研讨会，指导高中教师与缅因州研究所海湾合作开发海洋科学课程。影响海洋沉积物中氮循环的关键过程是反硝化和厌氧氨氧化，导致生物有效氮的净损失，以及异化硝酸盐还原为铵（DNRA），将硝酸盐从反硝化作用中转移回系统。传统上认为，这些过程是由缺乏细胞内硝酸盐的细菌控制的，并由孔隙水硝酸盐池驱动。然而，最近的数据表明，大型细胞内硝酸盐池广泛存在于海洋沉积物中，真核微生物，特别是底栖有孔虫和硅藻，可能是这些池的原因。该团队假设，需要充分考虑这种细胞内硝酸盐池的大小和通量率，以准确测量和了解海洋沉积物中的氮循环。细胞内硝酸盐对目前对海洋氮循环的理解提出了重大的方法和概念挑战。研究人员提出了两个目标：1）测量缅因州海湾从透光潮下带到黑暗大陆架沉积物的深度梯度上海洋沉积物中细胞内硝酸盐循环和其他主要氮循环途径的速率;（二）通过对从拟议研究地点分离的不同微生物真核细胞进行生理学实验，进一步检查对细胞内硝酸盐循环的控制，细胞内硝酸盐池大小和循环速率。他们提出了一种新的策略，使用氮-15标记的硝酸盐尖峰实验来测量细胞内硝酸盐在海洋沉积物中氮循环总体速率中的作用。通过拟议的工作，他们的目标是开发一个模型系统，以了解和整合细胞内硝酸盐循环到海洋沉积物中的氮循环更广泛。这一奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Nitrogen is an essential element for life. Thus, knowledge of the rates and processes that control nitrogen cycling are needed to understand marine ecosystem dynamics. Nitrogen exists in many chemical forms, and the processes that convert one form to another are mostly carried out by bacteria. However, new data suggest that nitrate stored inside the cells of more complex organisms accounts for a major pool of nitrate in marine sediments. This intracellular nitrate has not been adequately measured and integrated into an overall framework of nitrogen cycling. The investigators will address this issue in the proposed research to better understand and revise nitrogen inventories and flows in the oceans. This information can be used to increase understanding of controls on global carbon cycling and climate change feedbacks. One graduate student will be trained in using tools and techniques to solve complex marine systems issues, and three undergraduate researchers will participate in the work. The team will expand the impact of this research by 1) collaborating with the U. Maine Center for Innovation in Teaching and Learning and the Institute for Broadening Participation to produce and distribute educational videos about this research and its implications for the environment; and 2) leading summer workshops to mentor high school teachers to develop ocean sciences curricula in partnership with the Gulf of Maine Institute. Key processes that influence nitrogen cycling in marine sediments are denitrification and anammox, which lead to a net loss of biologically available nitrogen, and dissimilatory nitrate reduction to ammonium (DNRA), which diverts nitrate away from denitrification back into the system. It has traditionally been assumed that these processes are controlled by bacteria lacking intracellular nitrate and driven by porewater nitrate pools. However, more recent data indicates that large intracellular nitrate pools are widespread in marine sediments and that eukaryotic microbes, particularly benthic foraminifera and diatoms, may be responsible for these pools. The team hypothesizes that the size and flux rate of this intracellular nitrate pool needs to be sufficiently accounted for to accurately measure and understand the nitrogen cycle in marine sediments. Intracellular nitrate presents significant methodological and conceptual challenges to current understanding of the marine nitrogen cycle. The investigators propose two objectives: 1) To measure rates of intracellular nitrate cycling and other major nitrogen cycling pathways in marine sediments across a depth gradient from photic subtidal to dark continental shelf sediments in the Gulf of Maine; 2) To further examine controls on intracellular nitrate cycling by conducting physiological experiments on different microbial eukaryotic cells isolated from the proposed study sites to measure their intracellular nitrate pool sizes and cycling rates. They propose a new strategy using nitrogen-15 labeled nitrate spike experiments to measure the role that intracellular nitrate plays in overall rates of nitrogen cycling in marine sediments. Through the proposed work, they aim to develop a model system to understand and integrate intracellular nitrate cycling into the nitrogen cycle in marine sediments more broadly.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.