THE NO3-/-O2 RESPIRATION RATIO OF THE DEEP SEDIMENTARY BIOSPHERE IN THE PACIFIC GYRES
Organic matter produced in the ocean has an average C/N ratio of 106:16 (the Redfield ratio). However, during transport to the seafloor, N is preferentially respired. This results in depletion of the N relative to C in the organic matter that fuels subseafloor microbial communities. It has also been argued that preferential depletion of organic N occurs in sediment. This depletion may force sedimentary microorganisms to reduce nitrate or fix dinitrogen, which requires the expenditure of a
... enditure of a significant amount of additional energy in oxic sediment. Thus the availability of organic nitrogen may place a fundamental control on energy budget and ecosystem of the sedimentary microbial life. We test this possibility by determining the NO 3 -/-O 2 respiration ratio at three sites in the Pacific gyres. We created a diffusion-reaction model and used existing dissolved oxygen and nitrate profiles from interstitial water to determine the best-fit NO 3 -/-O 2 respiration ratio. This model has an advantage over linear correlation analysis, because it explicitly considers respiration as a function of depth, and uses the curvature of the concentration profiles to determine the microbial reactions. The down-core profiles of dissolved oxygen and nitrate reflect the net production of nitrate and consumption of oxygen due to microbial aerobic respiration. The curvature in the profiles reveals that organic nitrogen is not depleted in sedimentary organic matter respired. Dissolved nitrate and oxygen concentrations from all sites are linearly correlated with a NO 3 -/O 2 ratio of -0.098 ± 0.005. The bestfit NO 3 -/-O 2 respiration ratios calculated by the one-zone model, assuming a constant respiration ratio in the entire sediment column at each site, are between 0.089 to 0.100. These ratios are indistinguishable from the Redfield NO 3 -/-O 2 ratio of 0.094 determined in the ocean. We also consider a model to determine whether the respiration ratio varies with depth. There is no clear indication that the respiration ratio varies with sediment depth. This indicates that sedimentary microbes utilize organic carbon and nitrogen with a C/N ratio that is indistinguishable from the Redfield ratio, even though the C/N ratio of the bulk organic matter might be different due to the preferential degradation. This might be controlled by the nutrient requirements of microbial communities. iv ACKNOWLEDGMENTS I would like to express my deepest appreciation to my major advisor, Art Spivack, for his continual guidance and support during the accomplishment of this Master's thesis. His encouragement and faith in me was extremely helpful during the last two and a half years. I will always remember his patience in helping me develop the model, explaining new knowledge, and correcting my English grammatical errors. I would like to thank Steven D'Hondt, whose C-DEBI grant (NSF-OCE 0939564) funded part of my thesis and who gave me constructive suggestions and wise insights in my research. His support motivated me to make more progress. I am very grateful to my committee member David Fastovsky for his enthusiasm and support in my Master's thesis. I greatly thank the scientific parties of IODP Expedition 329 and Knorr 195(III), who measured all the oxygen and nitrate data. Without their data, I would not be able to work on this project. I also thank NSF and C-DEBI for funding for this project.