OCE-RIG: The chlorophyll 15N proxy for nitrogen cycling and export production: Understanding nitrogen isotope fractionations in chlorophyll biosynthesis

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Overview: In this project, the investigator will use a multidisciplinary approach to understand nitrogen isotope fractionations associated with chlorophyll biosynthesis in cyanobacteria and eukaryotic algae. Nitrogen isotope ratios of chlorophyll degradation products are an important new proxy for surface water nitrogen that is unaltered by diagenesis. Biomarkers derived from chlorophyll in sedimentary samples can be well preserved for hundreds of millions of years, and can therefore act as a tool for understanding ancient marine nitrogen cycling. Additionally, large taxon-specific differences in chlorophyll 15N fractionations allow for the use of 15N offsets between sedimentary chloropigment and bulk nitrogen to reconstruct the contribution of cyanobacteria and eukaryotic algae to export production. In order to use the chloropigment 15N proxy, these differences in isotopic fractionation must be understood. The work uses an isotope box model approach for calculating enzymatic fractionations at each step along the chlorophyll biosynthetic pathway in order to determine where important fractionations occur. Purified metabolic intermediates of chlorophyll biosynthesis will be measured for their isotopic composition. Metabolomics techniques such as kinetic flux profiling will be used to measure intermediate pool sizes and fluxes along pathway branch points. Isotope, concentration, and flux data will be integrated in order to understand the control of expressed fractionations. Intellectual Merit: A better understanding of the controls of 15N fractionations in algal and cyanobacterial chlorophyll will improve the use of a powerful new paleoceanographic proxy. Beyond the obvious contributions of this work to understanding marine nitrogen cycling, this approach provides a new framework for understanding fractionations of organic molecules that are preserved in the sedimentary record. Isotopic analysis of these molecular fossils has great utility in paleoceanography because it allows for linking of information on taxonomic identity of organisms that produce sedimentary organic matter with information about their metabolism or nutrient substrates. The proposed approach can be directly applied to other biomarker systems. The novelty in this approach lies in its combination of analytical techniques from stable isotope geochemistry and metabolomics, an emerging field in biomedical research that has tremendous potential in biogeochemical studies. Broader Impacts: Further development of an important new compound-specific isotope proxy with diverse applications will contribute to our understanding of the modern marine nitrogen cycle, which is important for predicting and mitigating the effects of anthropogenic climate change on marine biogeochemistry. This project will contribute to improving STEM education and increasing the participation of women in science. The proposed work involves training and mentoring of undergraduate interns and a female high school student. In conjunction with this project, the investigator is involved in outreach through ongoing work on oceanography-based elementary school curriculum design. The investigator is also actively involved in efforts to increase retention of women in geosciences by establishing a group for women in her department, and developing a lectureship series to bring prominent female geoscientists to her department for multi-day interactive visits that will focus on both scientific research and mentoring.