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Lecture 19 The Ocean Nitrogen Cycle

Lecture 19 The Ocean Nitrogen Cycle. Denitrification Reactions Distributions Nitrogen Fixation Reactions Distributions. Main Ocean Source of N Nitrogen Fixation Enzyme catalyzed reduction of N 2 N 2 + 8H + + 8e - + 16 ATP → 2NH 3 + H 2 + 16 ADP + 16P i

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Lecture 19 The Ocean Nitrogen Cycle

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  1. Lecture 19 The Ocean Nitrogen Cycle Denitrification Reactions Distributions Nitrogen Fixation Reactions Distributions

  2. Main Ocean Source of N Nitrogen Fixation Enzyme catalyzed reduction of N2 N2 + 8H+ + 8e- + 16 ATP → 2NH3 + H2 + 16 ADP + 16Pi Mediated by a two protein (Fe and Fe-Mo) complex called nitrogenase Inactivated when exposed to O2

  3. Main Ocean Sink of N Fixed Nitrogen (NO3-, NO2-, NH4+) is converted to N2 in low oxygen zones of the ocean Two Pathways Denitrification ( <2 to 10 mM O2): 2 NO3- + organic matter → N2 Anammox (<2 mM O2) NH4+ + NO2- → N2 + H2O

  4. Schematic of Ocean Nitrogen Cycle Gruber (2005) Nature 436, 786

  5. Global distribution of O2 at the depth of the oxygen minimum Gruber and Sarmiento, 1997

  6. Coupling of N sources and sinks (Deutsch et al, 2007, Nature, 445, 163) Capone and Knapp (2007) Nature, 445, 159

  7. What is N*? The solid line shows the linear equation P = 1/16 N + 0.345 (equivalent to N* = 0) Values to the right have positive N*, to the left have negative N* N* is defined as N* = [NO3] – 16 x [PO4] +2.9 PO4 versus Nitrate (GEOSECS data) Insert shows the effect of nitrification, photosynthesis, N2 fixation and denitrification.

  8. N* is defined as N* = [NO3] – 16 x [PO4] +2.9

  9. N* at 200m in the Pacific (Gruber and Sarmiento, 1997)

  10. Nitrogen Cycle w/ anammox Kuypers (2003) Nature 422: 608-611.

  11. Why is this important for chemical oceanography? What controls ocean C, N, P? g ≈ 1.0 Mass Balance for whole ocean: C/ t = VRCR – f B CS = 0; CD = CD VU = VD = VMIX Negative Feedback Control: if VMIX↑ VUCD↑ B ↑ f B ↑ (assumes f will be constant!) assume VRCR  then CD↓ (because total ocean balance VUCD↓ has changed; sink > source) B ↓ The nutrient concentration of the deep ocean will adjust so that the fraction of B preserved in the sediments equals river input! CS CD if VMIX = m y-1 and C = mol m-3 flux = mol m-2 y-1

  12. Nitrogen species: NO3- ; NO2- ; N2O; N2 ; NH4+ (V) (III) (I) (0) (-III) Nitrogen Isotopes: 14N 99.634% 15N 0.366% Isotopic Composition: ‰ The standard is atmospheric N2

  13. Fractionation factors , where e is the isotopic enrichment factor Fractionation Heavier stable isotope forms stronger bond. Microbial Enzymes break light isotope bonds more easily. Reactants become heavier (enriched) (e.g. NO3-→ N2) Products become lighter (depleted) Partial versus total reaction (products have same values as reactants)

  14. The Global Nitrogen Budget-one example (Brandes et al, 2002)

  15. % of Export Production (as N) at HOT derived from N2 Fixation (N-P mass balance model of Karl et al (1997) Nature 388, p. 533)

  16. The Global Nitrogen Budget-one example (Brandes et al, 2002)

  17. Spatial coupling of N2 fixation and denitrification (Deutsch et al, 2007)

  18. Downcore records of 15N-orgN from several sites High values of 15N-OrgN suggest more extensive denitrification Deutsch et al, 2004)

  19. Deutsch et al, 2004

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