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Nitrate Dynamics and Microbial Activity in Marine Chemistry: ALOHA and K2 Studies

This research investigates nitrate isotopic profiles and microbial community dynamics in marine ecosystems, specifically at the ALOHA and K2 sites. The study employs coupled δ15N and δ18O analysis to explore nitrification and denitrification processes, assessing rates of microbial activity and comparing free-living and particle-attached communities. By examining these dynamics with whole water samples and suspended particles, this research aims to elucidate nitrogen cycling, supply, and export mechanisms, contributing to our understanding of marine biogeochemistry.

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Nitrate Dynamics and Microbial Activity in Marine Chemistry: ALOHA and K2 Studies

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  1. Nitrate Dynamics at ALOHA and K2 Karen Casciotti Woods Hole Oceanographic Institution Department of Marine Chemistry and Geochemistry

  2. VERTIGO Experiments ALOHA 2004 • Nitrate isotopic profiles • Coupled d15N and d18O analysis • Rates of microbial activity • Nitrification Rates • Whole water samples and MULVFS particles • Microbial community analysis: free-living vs. particle attached • Nitrifying and denitrifying bacteria • Suspended particles (MULVFS) • Sinking particles (CLAP traps Hg vs. Formalin preservation) • Hand-picked squiggles, tufts, and puffs K2 2005 • Nitrate isotopic profiles • Coupled d15N and d18O analysis • Microbial community analysis: Nitrifying and denitrifying bacteria • Free-living vs. particle attached

  3. What we can learn from NO3- isotopes? • Supply of N to the surface: NO3-, N2? • N export and remineralization • N cycling/turnover Nitrogen fixation N Norg Assimilation N NH4+ O N NO3- Denitrification Nitrification O O

  4. Subarctic Front Map of Surface Nitrate

  5. Comparison of Basic Chemical and Physical Parameters T O2  S NO3- Si ALOHA = Blue K2 = Red

  6. Nitrate Isotopic Profiles • 10 profiles from ALOHA • 6 profiles from K2 • Analyzed for d15N and d18O using “denitrifier method” • Allows isotopic analysis of previously inaccessible pools of NO3-

  7. The denitrifier method Pseudomonas aureofaciens Inject NO3- or NO2- sample or Pseudomonas chlororaphis Denitrification takes place NO3- NO2- NO N2O N2 Analyze N2O isotopes N2O (g) Sigman et al., 2001. Analytical Chemistry: 15N-NO3- Casciotti et al., 2002. Analytical Chemistry: 18O-NO3-

  8. K2: T, S, NO3-, and PO4-

  9. d15N and d18O-NO3- at K2

  10. Temperature vs. d15N and d18O

  11. d15N and d18O vs. density Thorium excess

  12. d15N and d18O-NO3- at K2

  13. d15N-NO3- at K2 EZD MLD PN T [NO3-] 15NNO3

  14. Fluxes of N at Station K2 NO3- NH4+ Export Remineralization flux NO3- PN Flux, d15N 150 m trap Remineralization flux PN Flux, d15N 300 m trap Remineralization flux PN Flux, d15N 500 m trap

  15. d15N vs. d18O Close-up, station 73 All data

  16. N and O isotope dynamics in nitrate Assimilation d18O-NO3- Initial NO3- d15N-NO3-

  17. N and O isotope dynamics in nitrate Assimilation d18O-NO3- Partially consumed NO3- Produced PN d15N-NO3-

  18. N and O isotope dynamics in nitrate Assimilation d18O-NO3- Partially consumed NO3- Produced PN d15N-NO3-

  19. d15N vs. d18O Close-up, station 73 All data

  20. ALOHA T, S, NO3-, and PO4-

  21. d15N and d18O-NO3- at ALOHA

  22. Fluxes of N at Station ALOHA N2 NO3- NH4+ Export Remineralization flux PN Flux, d15N 150 m trap Remineralization flux PN Flux, d15N 300 m trap Remineralization flux PN Flux, d15N 500 m trap

  23. ALOHA d15N vs. d18O

  24. N and O isotope dynamics in nitrate Fully consumed NO3- Assimilation d18O-NO3- Produced PN d15N-NO3-

  25. N and O isotope dynamics in nitrate Fully consumed NO3- Assimilation d18O-NO3- N2 fixation PN d15N-NO3-

  26. ALOHA d15N vs. d18O

  27. d15N and d18O-NO3- at ALOHA

  28. d15N and d18O-NO3- at ALOHA

  29. d15N and d18O-NO3- at K2 EZD MLD PN Real data from K2 cast 24 15NNO3

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