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New Stable Isotopic Tools for the Marine Nitrogen Cycle

New Stable Isotopic Tools for the Marine Nitrogen Cycle. Karen L. Casciotti WHOI, Department of Marine Chemistry and Geochemistry. Biogeochemical cycles of C, N, P. Gruber and Galloway 2008 Nature 451: 293-296. Increasing Greenhouse Gas Levels. Atmospheric CO 2 at Mauna Loa Observatory.

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New Stable Isotopic Tools for the Marine Nitrogen Cycle

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  1. New Stable Isotopic Tools for the Marine Nitrogen Cycle Karen L. Casciotti WHOI, Department of Marine Chemistry and Geochemistry

  2. Biogeochemical cycles of C, N, P Gruber and Galloway 2008 Nature 451: 293-296

  3. Increasing Greenhouse Gas Levels Atmospheric CO2 at Mauna Loa Observatory AGAGE Atmospheric N2O records N2O has 300x global warming potential per mole than CO2

  4. Oxygen Deficient Zones Oxygen (mL/L) at 250 m ETNP Arabian Sea ETSP WOA05 data plotted in ODV • Highly productive • Sources of N2O • Response to anthropogenic activity--expansion/intensification • Quantitative importance in marine nitrogen budget

  5. Suboxic Nitrogen Cycle N2 N2, N2O Particulate NO3- Surface ocean N DON, NH3 upwelling Deep ocean N2 Anammox Remineralization NH3 NO2- NO3- Nitrification NO Denitrification N2O Burial N2 Major questions…

  6. What role can a microbiologist play in understanding global biogeochemistry?

  7. Stable Isotopes ofNO3-, NO2-, and N2O 14N: 99.6337% 15N: 0.3663% in Air N2 d15N (‰) = [(15N/14Nsample / 15N/14NAIR) -1 ] *1000 16O: 99.76206% 17O: 0.03790% 18O: 0.20004% in Standard Mean Ocean Water d18O (‰) = [(18O/16Osample / 18O/16OSMOW) -1 ] *1000

  8. The Denitrifier Method: Simultaneous N and O Isotopic Analyses Pseudomonas aureofaciens or Pseudomonas chlororaphis Inject NO3- or NO2- sample 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-

  9. Peru Upwelling: d15NO3- 19 18 17 10 16 11 12 15 10 11 12 13 14 16 17 19 13 14

  10. d15NH4+ (substrate) d15NO2- (instantaneous product) d15NO2- (accumulated product) Rayleigh Model for Isotope Fractionation in a Closed System d15Nsubstrate ≈ d15Ninitial substrate - e * ln(f) e e d15Naccumulated product ≈ d15Ninitial substrate + e * f ln(f)/(1-f) BEGINNING END

  11. Product e d15N f*ln(f)/(1-f) Linearized Rayleigh Model Substrate Product Substrate d15N e -ln(f) f = the fraction of initial substrate remaining

  12. Phylogeny of June Ammonia monooxygenase Clones from Sugar Creek, Indiana Stream nitrifier community 67 = 14.2 ± 0.5‰ Casciotti and Voytek, unpublished Nitrosospira tenuis 79 98 = 24.6 ± 1.4‰ 90 60 91 81 Nitrosomonas marina = 14.2 ± 3.6‰ Nitrosomonas sp. C-113a 74 = 19.1 ± 1.2‰ 72 Nitrosomonas eutropha = 32.8 ± 1.7‰ 96 100 Nitrosomonas europaea = 38.2 ± 1.6‰ 98

  13. Potential for variability in isotope effects within a ‘functional group’ Important to know who is there and who is active

  14. Stable Isotope Fractionation at the Cell Level Nitrifier cell periplasm Transport? NH4+ NH4+ expt eeq eeq cytoplasm eamo ehao ediff NH3 NH3 NH2OH NO2- NO2- ediff Diffusion eoverall is dominated by ediff Without transport With transport eoverall is dominated by expt

  15. Potential for variability within a given species Important to know physiological factors that may play a role

  16. d15N variations in NO3- 20 Water column denitrification e  20-30 ‰ 15 Nitrate assimilation e  5-10 ‰ d15NO3- (‰ vs. AIR) 10 Nitrogen fixation d15N 0 ‰ 5 Sedimentary denitrification e  0 ‰ 0 0 0.5 1 1.5 2 [NO3-]/[NO3-]initial Sigman and Casciotti, 2001 EOS

  17. d15N variations in NO3- d15NNO3 d15NNO3 N2 Fixation Low d15N Depth (m) High d15NO3- Depth (m) Denitrification Arabian Sea ETNP d15NN2 d15NN2 Brandes et al. (1998) L&O Gross rates of nitrogen fixation and denitrification are difficult to diagnose.

  18. Coupled d15N and d18O analyses of nitrate N isotopes O isotopes Nitrogen fixation Denitrification N2 N2 Denitrification Norg Norg Assimilation Assimilation NO3- NO3- Nitrification Nitrification NH4+ NH4+ O2, H2O

  19. Coupled N and O isotopes of NO3- 20 15 d18O-NO3- (‰ vs. VSMOW) 10 Denitrification 15e/ 18e  1 (Granger et al., 2006) Nitrate assimilation 15e/ 18e  1 5 (Granger et al., 2004) Deep Ocean 0 0 5 10 15 20 d15N-NO3- (‰ vs. AIR)

  20. Coupled N and O isotopes of NO3- 20 15 d18O-NO3- (‰ vs. VSMOW) 10 5 Remineralization/Nitrification 0 0 5 10 15 20 d15N-NO3- (‰ vs. AIR)

  21. Coupled N and O isotopes of NO3- 20 15 d18O-NO3- (‰ vs. VSMOW) 10 N2 fixation/ Nitrification D(15,18) 5 Remineralization/Nitrification 0 0 5 10 15 20 d15N-NO3- (‰ vs. AIR) D(15,18) = (d15N - d15Ndeep) - 15e/18e * (d18O - d18Odeep) Sigman et al., 2005

  22. Coupled N and O isotopes of NO3- 20 15 d18O-NO3- (‰ vs. VSMOW) 10 N2 fixation/ Nitrification D(15,18) 5 Remineralization/Nitrification 0 0 5 10 15 20 d15N-NO3- (‰ vs. AIR) D(15,18) = (d15N - d15Ndeep) - 15e/18e * (d18O - d18Odeep) Sigman et al., 2005

  23. Nitrate d15N and d18O deviations 20 N2 Fixation Low d15N Remineralized PN from Nitrogen fixation 15 Nitrite reoxidation High d15NO3- d18O-NO3- (‰ vs. VSMOW) Nitrite reoxidation 10 Denitrification Denitrification 15e/ 18e  1 5 0 0 5 10 15 20 NO3- NO2- N2 d15N-NO3- (‰ vs. AIR) D(15,18) = (d15N - d15Ndeep) - 15e/18e * (d18O - d18Odeep) Sigman et al., 2005

  24. ETNP NO3- isotope studies Map from Cline and Kaplan, 1975 Brandes et al. 1998 Cline and Kaplan 1975 This study Voss et al. 2001 Sutka et al., 2004 Sigman et al. 2003, 2005 Altabet et al., 1999

  25. The Denitrifier Method: Simultaneous N and O Isotopic Analyses Pseudomonas aureofaciens or Pseudomonas chlororaphis Inject NO3- or NO2- sample 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-

  26. The Denitrifier Method: Simultaneous N and O Isotopic Analyses Pseudomonas aureofaciens or Pseudomonas chlororaphis Inject NO3- or NO2- sample Denitrification takes place NO3- NO2- NO N2O N2 Granger et al., 2006 Analyze N2O isotopes N2O (g) Sigman et al., 2001. Analytical Chemistry: 15N-NO3- Casciotti et al., 2002. Analytical Chemistry: 18O-NO3-

  27. Nitrite isotope subtraction Pretreatment Ascorbate NO2- Isotopic analysis Denitrifier Azide Denitrifier d15N, d18O (NO3- + NO2-) d15N, d18O (NO2-) d15N, d18O (NO3-) 2 Subtract d15N, d18O (NO3-) 1 Compare

  28. ETNP Nitrate Isotope Data d15N d18O Casciotti and McIlvin, 2007 Marine Chemistry

  29. ETNP Nitrate and Nitrite Isotopes d15N (‰), D(15,18) (‰), and [NO2-] (mM) d15NNO3 d15NNO2+NO3 d15NNO2 D(15,18) D(15,18) = (d15N - d15Ndeep) - 15e/18e * (d18O - d18Odeep) Casciotti and McIlvin, 2007 Marine Chemistry

  30. ETNP Nitrate and Nitrite Isotopes d15N (‰), D(15,18) (‰), and [NO2-] (mM) d15NNO3 d15NNO2+NO3 d15NNO2 D(15,18) Casciotti and McIlvin, 2007 Marine Chemistry

  31. Model Interpretation of D(15,18) 20 Mixing with shallower water Remineralized PN from Nitrogen fixation 15 Remineralization of diazotroph biomass d18O-NO3- (‰ vs. VSMOW) Nitrate reduction Nitrite reoxidation 10 Denitrification (or Anammox) NO3- NO2- Denitrification 15e/ 18e  1 Nitrite “reoxidation” 5 0 Mixing with deeper water 0 5 10 15 20 d15N-NO3- (‰ vs. AIR) Based on Sigman et al. 2005 model

  32. Model Results NO3- NO2- N2 Casciotti and McIlvin, 2007 Marine Chemistry

  33. ETNP Nitrate and Nitrite Isotopes d15N (‰), D(15,18) (‰), and [NO2-] (mM) d15NNO3 d15NNO2+NO3 d15NNO2 D(15,18) Nitrite reoxidation going on…additional information needed

  34. Nitrite Oxidation in the ETSP Nitrite oxidation rate nmol/L/day [NO2-] mmol/L Lipschultz et al., 1990: Showed nitrite oxidation below SNM and associated w/PNM but little at the top of the OMZ in the ETSP Nitrate reduction rate nmol/L/day [O2] mmol/L

  35. ETNP Samples d15N (‰), D(15,18) (‰), and [NO2-] (mM) d15NNO3 d15NNO2+NO3 d15NNO2 D(15,18)

  36. NO3- NO3- 15eNR NO3- d15N d15N NO2- NO3- NO2- 15eNO Initial condition Nitrate reduction Nitrite oxidation Isotope effects for nitrite reactions NO3- 15eNR NO3- NO2- N2 NO2- 15eNIR Initial condition Nitrate reduction Nitrite reduction

  37. Nitrite reduction by (de)nitrifying bacteria 14NO2-14NO 15NO2-15NO Casciotti, Sigman, and Ward, unpublished

  38. NO3- NO3- 15eNR NO3- d15N d15N NO2- NO3- NO2- 15eNO Initial condition Nitrate reduction Nitrite oxidation Isotope effects for nitrite reactions NO3- 15eNR NO3- NO2- NO2- 15eNIR N2 Initial condition Nitrate reduction Nitrite reduction

  39. Nitrite oxidation by Nitrococcus mobilis NO2- NO3- “Inverse isotope effect” 14NO2-14NO3- d15NNO3 15NO2-15NO3- d15NNO2

  40. d15N d15N Isotope effects for nitrite reactions NO3- NO3- NO3- 15eNR 15eNR NO3- NO3- NO2- NO3- NO2- 15eNIR NO2- 15eNO NO2- N2 Initial condition Nitrate reduction Nitrite reduction Initial condition Nitrate reduction Nitrite oxidation

  41. ETNP Samples d15N (‰), D(15,18) (‰), and [NO2-] (mM)

  42. Implications for interpretation of D(15,18) • Now required nitrite oxidation rate less than 0.9 mM/yr and oxidation/reduction ratio ~0.7 • Can’t rule out a contribution from N2 fixation, but… • Many indicators consistent with nitrite oxidation: • Location of D(15,18) minimum • Low nitrite d15N • Large nitrate/nitrite d15N offset • Inverse isotope effect for nitrite oxidation • Strength of D(15,18) minimum

  43. Implications for interpretation of D(15,18) • Now required nitrite oxidation rate less than 0.9 mM/yr and oxidation/reduction ratio ~0.7 • Can’t rule out a contribution from N2 fixation, but… • Many indicators consistent with nitrite oxidation: • Location of D(15,18) minimum • Low nitrite d15N • Large nitrate/nitrite d15N offset • Inverse isotope effect for nitrite oxidation • Strength of D(15,18) minimum

  44. Suboxic Nitrogen Cycle N2 N2, N2O Particulate NO3- Surface ocean N DON, NH3 upwelling Deep ocean N2 Anammox Remineralization NH3 NO2- NO3- Nitrification NO Denitrification N2O Burial N2

  45. Ongoing and Future Work • Oxygen isotopic systematics of ammonia oxidation • Nitrogen and oxygen isotopic systematics of nitrite oxidation • Interpret coupled N and O isotopic signatures from nitrate and nitrite using more sophisticated models • Extend analysis to additional ODZs (Arabian Sea; Benguela; GEOTRACES)

  46. Acknowledgements Matthew McIlvin Danny Sigman ETNP work Carrie Tuit Penzance Assistant Scientist Endowed Support NSF Chemical Oceanography Nitrite Oxidizer work Carly Buchwald NSF Chemical Oceanography

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