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Stable Isotopes and Microbial Biogeochemistry

Karen Casciotti Woods Hole Oceanographic Institution. Stable Isotopes and Microbial Biogeochemistry. Part 2: Isotopic fractionation and natural abundance level isotope variations. Marine Nitrogen Cycle. Atmospheric deposition and continental inputs. CO 2. Atmospheric N 2. N 2 O. N 2 O.

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Stable Isotopes and Microbial Biogeochemistry

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  1. Karen Casciotti Woods Hole Oceanographic Institution Stable Isotopes and Microbial Biogeochemistry Part 2: Isotopic fractionation and natural abundance level isotope variations.

  2. Marine Nitrogen Cycle Atmospheric deposition and continental inputs CO2 Atmospheric N2 N2O N2O Dissolved N2 + NH Particulate 4 DON Nitrogen Fixation N - Assimilation NO 3 Surface ocean Vertical mixing/ Deep ocean upwelling Nitrification NO3- + Remineralization NH 4 Sedimentary N2O Denitrification Water column Denitrification Sediment Burial

  3. Summary from Lecture 1 N2 • Isotopes provide a sensitive tracer of reactions mediated by the microbial community • SIP can be used to identify metabolic capabilities of uncultured organisms • Taxon-specific metabolic rates and activities can be determined by cell sorting and/or RNA capture techniques NH4+ 15NO3- Cell sorting Or RNA capture

  4. Sampling the ocean

  5. Overview of Lecture 2 • Applications of natural abundance isotopes to microbial (nitrogen) biogeochemistry: • Isotopic fractionation • Natural variations in isotope ratios--a record of microbial, chemical, and physical processes • Applications and interpretations • Example 1: Global N budgets • Example 2: Sources of N to the euphotic zone

  6. Notation • 15N: the isotope, its molar amount or concentration • 15F: Fractional abundance = 15N/(14N+15N) • 15Atom % = 15F x 100% • 15R: Isotope ratio = 15N/14N range: 0.0036581 to 0.0038236 (approx.) precision: 0.0000007 • d15N: delta value (‰) = (15R/15Rstd -1) x 1000 range: -5 to 40‰ (approx.) precision: 0.2‰ • ak: Kinetic fractionation factor = 14k/15k range: 0.985 to 1.040 (approx.) • ek: Kinetic isotope effect (‰) = (ak -1) x 1000 range: -15 to +40‰

  7. Stable Isotopic Fractionation 14k 14NO3- 14NO2- 15k 15NO3- 15NO2- ak = 14k/15k = 1 => no fractionation > 1 => normal fractionation: preference for 14N < 1 => inverse fractionation: preference for 15N

  8. Why fractionate? NO3- NO2- - - 16O 16O - 16O 14N 15N 14N 16O 16O 16O 16O 18O 16O 14NO3- 15NO3- N16O218O-

  9. Stable Isotopic Fractionation R 12 light 3 heavy No fractionation: p (heavy) = 4/20 0.25 11 light 4 heavy ‘Normal fractionation’: p (heavy) < 4/20 0.36 16 light 4 heavy 13 light 2 heavy ‘Inverse fractionation’: p (heavy) > 4/20 R = 4/16 = 0.25 0.15 Remove 5 out of 20; 15 remain

  10. Stable Isotopic Fractionation:Nitrate reduction NO3- NO2- Substrate (NO3-) Normal fractionation 75 Reaction progress 50 d15N 25 0 Product (NO2-) Normal fractionation 1.0 0.8 0.6 0.4 0.2 0.0 f = [NO3-]/[NO3-]0

  11. Stable Isotopic Fractionation:Nitrogen fixation N2 Norg 1 Substrate (N2) No fractionation 0 Product (Norg) No fractionation -1 d15NNO3 Reaction progress -2 1.0 0.8 0.6 0.4 0.2 0.0 f = [NO2-]/[NO2-]0

  12. Stable Isotopic Fractionation:Nitrite oxidation NO2- NO3- Product (NO3-) Inverse fractionation 25 0 -25 d15NNO3 Reaction progress Substrate (NO2-) Inverse fractionation -50 1.0 0.8 0.6 0.4 0.2 0.0 f = [NO2-]/[NO2-]0 Casciotti, submitted

  13. Notation • 15N: the isotope, its molar amount or concentration • 15F: Fractional abundance = 15N/(14N+15N) • 15Atom % = 15F x 100% • 15R: Isotope ratio = 15N/14N range: 0.0036581 to 0.0038236 (approx.) precision: 0.0000007 • d15N: delta value (‰) = (15R/15Rstd -1) x 1000 range: -5 to 40‰ (approx.) precision: 0.2‰ • ak: Kinetic fractionation factor = 14k/15k range: 0.985 to 1.040 (approx.) • ek: Kinetic isotope effect (‰) = (ak -1) x 1000 range: -15 to +40‰

  14. Microbial Nitrogen Cycle Nitrogen fixation N2O Denitrification N2 e ≈ 0‰ NO Anammox e ≈ 15‰ e = ? e < 5‰ e ≈ 20-30‰ Assimilation Org NH3 NO2- NO3- e ≈ 5-10‰ Nitrite oxidation e ≈ 10-20‰ e ≈ -15‰ Nitrification NH2OH Ammonia oxidation e ≈ 15-38‰ Oxidation state (-III) (-I) (0) (I) (II) (III) (V)

  15. Nitrate isotope variations in seawater d15NO3- Depth (m) Casciotti et al., 2007, Casciotti unpublished, Sigman et al., 2005

  16. Overview of Lecture 2 • Applications of natural abundance isotopes to microbial (nitrogen) biogeochemistry: • Isotopic fractionation • Natural variations in isotope ratios--a record of microbial, chemical, and physical processes • Applications and interpretations • Example 1: Global N budgets • Example 2: Sources of N to the euphotic zone

  17. Inputs and Outputs of Fixed N Atmospheric Deposition Continental Runoff N2 fixation NH4+, NO3-, DON Norg Water column Denitrification N2 Anammox N2 Sediment Burial Sedimentary Denitrification N2 Norg

  18. Inputs and Outputs of Fixed N Approaches to flux determination: • Direct or indirect rate measurements • Geochemical modeling • N:P Stoichiometry • Isotopes Denitrification Nitrogen fixation

  19. Nitrogen Budgets Is the N cycle in balance? Maybe not! Is it a moving target? What are the consequences?

  20. Marine N Isotope Budget Sigman, Karsh, and Casciotti, EOS in press

  21. Steady State Model for Ocean N Budget Sedimentary Denitrification dSD~ dNO3- eSD N losses dout~ 0‰ Mout NO3- N inputs din~ 0‰ Min dNO3= 5‰ Water column Denitrification dSD~ dNO3- eSD • N isotope budget insensitive to rate of N2 fixation, but very sensitive to the ratio of water column to sediment denitrification. • Suggest that given estimated fluxes of WC denit (75 Tg N yr-1), sedimentary denitrification should be ~ 280 Tg N yr-1. • Sum of sink terms far outweighs the estimated rate of N2 fixation (~125 Tg N yr-1)! • Shorter residence time for marine N. Brandes and Devol, 2002

  22. Global N budgets • Still out of balance! • Are we underestimating N2 fixation rates? • Are we overestimating water column denitrification? • Are we under/over estimating edenit? • What is the role of anammox? • Sedimentary nitrogen fixation? • Can we refine the budget using different isotope studies? • Can we really assume steady state?

  23. P* estimate of Nitrogen fixation rates Deutsch et al. (2007) Nature 445: 163-167

  24. New/Export Production Atmospheric deposition and continental inputs N2 CO2 NH4+ DON N2 Biomass NO3- Surface ocean NO3- Export of C, N Deep ocean

  25. Pacific Surface Nitrate 20 15 Nitrate (mM) 10 5 0 WOA05 data from NODC

  26. Approaches for quantifying the N2 fixation flux • Scaled microscopic enumeration: • 0.14 ± 0.07 mmol N m-2 d-1 (Karl et al., 1997) • Molecular analyses: • Abundance and diversity; qualitative activity • Extrapolated instantaneous rate measurement: • 0.08 ± 0.05 mmol N m-2 d-1 (Karl et al., 1997) • 0.07 ± 0.02 mmol N m-2 d-1 (Montoya et al., 2004) • Satellite observation: • ~ 5x1015 mmoles N d-1 (Westberry and Siegel, 2006) • Geochemical measurements • Nutrient stoichiometry: • 0.09 ± 0.04 mmol N m-2 d-1 (Karl et al., 1997) • 0.11 ± 0.02 mmol N m-2 d-1 (Deutsch et al., 2001) • Isotope balance: • 0.14 mmol N m-2 d-1 (Karl et al., 1997) • 0.13 ± 0.05 mmol N m-2 d-1 (Dore et al., 2002)

  27. HOT sinking d15NPN Deep NO3- N2 Dore et al., 2002 study d15N = ((15N/14N)sample/(15N/14N)std)-1)*1000

  28. HOT Sinking PN flux at 150 m Dore et al., 2002 study

  29. HOT sinking d15NPN Flux-weighted average d15NPN Dore et al., 2002 study

  30. Euphotic Zone Nitrogen Isotope Balance Nitrogen fixation (1) FN2fix + FNO3= FPN surface NO3- PN (2) FN2fix*d15NN2fix + FNO3*d15NNO3 = FPN*d15NPN 150 m (3) FN2fix/FPN = (d15NPN- d15NNO3)/(d15NN2fix- d15NNO3) Nitrate flux Export flux (Dore et al., 2002)

  31. Sample Types MULVFS (suspended particles) CTD Rosette (nitrate samples) NBST’s (sinking particles) 0 150 300 500 1000 3000 (Buesseler, Valdes, et al.) (Bishop)

  32. Euphotic Zone Mass and Isotope Balance FN2fix; d15NN2fix 0 NO3- PN 150 FNO3,150;d15NNO3,150 FPN,150;d15NPN,150

  33. VERTIGO Nitrate Isotopic Data Casciotti et al., 2008 in press DSRII

  34. VERTIGO Particulate N Isotopic Data Casciotti et al., 2008 in press DSRII

  35. Euphotic Zone Mass and Isotope Balance FN2fix = ? d15NN2fix = 0 ± 1 ‰ 0 NO3- PN 150 FNO3,150 = ? d15NNO3,150 = ? FPN,150 = 0.18 ±0.04 mmol m-2 d-1 d15NPN,150 = 2.5 ± 0.4‰

  36. VERTIGO Nitrate Isotopic Data Scenarios: 1) Diffusive flux a) 150-250 m b) 150-300 m 2) Pulse event a) reaching 200 m b) reaching 300 m Casciotti et al., 2008 in press DSRII

  37. VERTIGO Nitrate Isotopic Data Scenarios: 1) Diffusive flux a) 150-250 m: 3.0 ± 0.3‰ b) 150-300 m: 3.7 ± 0.3‰ ((d[15N]/dz ÷ 15Rstd )-1 ) *1000 d[14N]/dz Casciotti et al., 2008 in press DSRII

  38. VERTIGO Nitrate Isotopic Data Scenarios: 1) Diffusive flux a) 150-250 m: 3.0 ± 0.3‰ b) 150-300 m: 3.7 ± 0.3‰ 2) Pulse event a) reaching 200 m: 2.5 ± 0.5‰ b) reaching 300 m: 4.5 ± 0.5‰ Casciotti et al., 2008 in press DSRII

  39. Euphotic Zone Nitrogen Isotope Balance Nitrogen fixation (1) FN2fix + FNO3 = FPN surface NO3- PN (2) FN2fix*d15NN2fix + FNO3*d15NNO3 = FPN*d15NPN 150 m (3) FN2fix/FPN = (d15NPN - d15NNO3)/(d15NN2fix- d15NNO3) Nitrate flux Export flux

  40. PN flux during VERTIGO HOT Sinking PN flux at 150 m

  41. d15NPN during VERTIGO HOT sinking d15NPN

  42. But why the low d15NNO3?

  43. ALOHA Physical Properties (150m) (300m) (500m) (1000m) (>3000m) Casciotti et al., 2008 in press DSRII

  44. Mixing Curves for [NO3-] and d15NNO3 NPDW NPIW (500m) NPDW NPBW SSMW (300m) NPIW (500m) SSMW (300m) STSMW (150m) STSMW (150m) Casciotti et al., 2008 in press DSRII

  45. Scenario 1: 14N trapping by remineralization Low 15N/14N Lower 15N/14N Higher 15N/14N Lower 15N/14N Higher 15N/14N

  46. VERTIGO Particulate N Isotopic Data Casciotti et al., 2008 in press DSRII

  47. Modeled d15NNO3 Distribution Glover and Casciotti, unpublished

  48. Scenario 2: Long-term accumulation of N fixation signal NO3- uptake Low 15N/14N N2 fixation Low 15N/14N Low 15N/14N Lower 15N/14N Low 15N/14N High 15N/14N

  49. ALOHA 15N Mass Balance

  50. ALOHA 15N Mass Balance

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