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

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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.