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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 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 NO3/-O2 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 NO3/-O2 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 NO3/O2 ratio of -0.098 ± 0.005. The bestfit NO3/-O2 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.

THE NO3-/-O2 RESPIRATION RATIO OF THE DEEP SEDIMENTARY BIOSPHERE IN THE PACIFIC GYRES