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

氮是生命所必需的元素。因此，需要掌握控制氮循环的速率和过程的知识，以了解海洋生态系统的动态。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，也没有整合到氮循环的整体框架中。调查人员将在拟议的研究中处理这一问题，以更好地了解和修订海洋中的氮清单和流动。这些信息可以用来增加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。该团队将通过以下方式扩大这项研究的影响：1)与缅因州大学教学创新中心和研究所合作，扩大参与，制作和分发关于这项研究及其对环境影响的教育视频；2)领导夏季研讨会，指导高中教师与缅因湾研究所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，这导致生物有效氮的净损失，以及异化硝酸盐还原为氨(DNRA)，它将硝酸盐从反硝化转移到系统中。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在海洋沉积物中广泛存在，真核微生物，特别是底栖有孔虫和硅藻可能是造成这些池的原因。该团队假设，需要充分考虑这个细胞内硝酸盐池的大小和通量速率，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学和概念性挑战。研究人员提出了两个目标：1)测量从缅因湾从光潮下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；2)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实验，测量它们的细胞内硝酸盐池大小和循环速率，进一步检查对细胞内硝酸盐循环的控制。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总的氮循环速率中所起的作用。通过拟议的工作，他们的目标是开发一个模型系统，以更广泛地理解细胞内硝酸盐循环并将其纳入海洋沉积物中的氮循环。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.