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NITROGEN FIXATION IN THE EQUATORIAL PACIFIC: RATES, DIVERSITY, NUTRIENT LIMITATION

NITROGEN FIXATION IN THE EQUATORIAL PACIFIC: RATES, DIVERSITY, NUTRIENT LIMITATION AND BIOGEOCHEMICAL IMPLICATIONS. EUC-Fe Kilo Moana Cruise 0625 Data Workshop, Seattle. Sophie Bonnet WiSE Postdoctoral Fellow University of Southern California - Los Angeles.

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NITROGEN FIXATION IN THE EQUATORIAL PACIFIC: RATES, DIVERSITY, NUTRIENT LIMITATION

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  1. NITROGEN FIXATION IN THE EQUATORIAL PACIFIC: RATES, DIVERSITY, NUTRIENT LIMITATION AND BIOGEOCHEMICAL IMPLICATIONS EUC-Fe Kilo Moana Cruise 0625 Data Workshop, Seattle Sophie Bonnet WiSE Postdoctoral Fellow University of Southern California - Los Angeles Doug Capone Lab, Department of Biological Sciences 18-19 September 2007

  2. NITROGEN FIXATION: WHY DO WE CARE? N2 Endless source of nitrogen atmosphere ocean N2 + 8H + + 8e- 2 NH3+ H2 NH3 ‘New’ Nitrogen Prokaryotes Sustains ‘new’ primary production Diazotrophs (prokaryotes)

  3. NITROGEN FIXATION: WHY DO WE CARE? ‘New’ Nitrogen Prokaryotes Sustains ‘new’ primary production ≥ NO3- from diapycnal diffusion (Capone et al., 2005) Up to 50% (Karl et al., 1997)

  4. EVOLUTION IN THE PERCEPTION OF N2 FIXATION Large cyanobacteria (> 10µm) - Trichodesmium Explain only 40-59% of nitrogen fixation predited by geochemical methods (N*, stable isotopes) (Mahaffey et al., 2005) Small cyanobacteria (< 10µm) - Crocosphaera  Contribution ≥ Trichodesmium(Zehr et al., 2001)  Sustain 10% of global NPP (Montoya et al., 2004)  Very few data on these organisms (postdoc)

  5. N2 FIXATION IN THE EQUATORIAL PACIFIC N2 fixation suggested in the warm pool: - delta 15N settling particles (Yoshikawa et al., 2005) - total organic nitrogen measurements (Hansell and Feely, 2000)  No direct measurements Surface [nitrate] gradient (S. Pal) Surface DFe gradient (L. Ossiander)

  6. PROTOCOLE Labelling technique: 15N2 (Montoya et al. 1996) 15N (atom %) Europa Integracontinuous flow mass spectrometer Incubation in presence of 15N2gas Gentle filtrations (2 size fractions)

  7. PROTOCOLE • Equatorial stations • Vertical profiles (50% - 10% - 1% - 0.1% surface irradiance) • Each depth: • total N2 fixation (2 replicates) • size fractionnation (2 replicates per fraction) • > 10µm • < 10µm

  8. TRACE METAL CLEAN CONDITIONS!!! N2 NITROGENASE N2 + 8H + + 8e- 2 NH3+ H2 NH3 Fe-Mo protein (7 Fe atoms) Diazotrophs: 10-fold higher Fe requirements than than non diazotrophic phytopk Station 25 TMC versus non TMC conditions Culture: Fe limits growth and N2 fixation of Trichodesmium (Rueter 1988; Pearl et al., 1992) Field: Fe limits nitrogen fixation in the North tropical Atlantic (Mills et al., 2004)

  9. TRACE METAL CLEAN CONDITIONS!!! • Trace metal clean sampling AND incubation • Go flo (in the bubble) • Clean tubes (Suprapur HCl) • Clean bottles (Suprapur HCl)

  10. Before….. After…..

  11. RESULTS (Total N2 Fixation - nmol N.l-1.h-1) Mean = 0.71 nmol N.l-1.d-1 Mean = 2.3 nmol N.l-1.d-1

  12. RESULTS (Size fractionnation) = 25% of Total N2 fixation > 10µm Adrian Marchetti

  13. RESULTS (Size fractionnation) = 25% of Total N2 fixation > 10µm Characterisation of the assemblage by molecular tools = 75% of Total N2 fixation < 10µm

  14. RESULTS - Molecular data Collaboration Isabelle Biegala - IRD Marseille (France) 3 size fractions >10µm 3-10µm 0.2-3µm TSA-FISH technique: whole cell specific fluorescent probes hybridization (Biegala et al. 2003, 2005). Abundance of unicellular diazotroph in each size fraction

  15. RESULTS - molecular data Micro- fraction Nano- fraction Pico- fraction Dominance of pico diazotrophs

  16. RESULTS - molecular data Free picodiazotrophs (station 18) Symbiosis with dinoflagelates (stations 10 & 27) Tricho (station 27) Pictures from Isabelle Biegala

  17. RESULTS - molecular data • Summary • 75% of N2 fixation occurs in the small size fraction • molecular data : small size fraction = pico diazotrophs (free or attached) • Cf lagon noumea (Biegala et al., in rev) micro nano pico

  18. BIOGEOCHEMICAL IMPLICATIONS Areal rates: 32 to 268 µmol N.m-2.d-1 HNLC waters Warm pool ‘New’ N provided by N2 fixation = 2% of the vertical transport of nitrate (Carr et al., 1995) ‘New’ N provided by N2 fixation = 70% of the vertical transport of nitrate (Lewis et al. 1986) Contribution of N2 fixation to PP = 20% (up to 70% at station 14)  if we consider ‘New’ PP Contribution of N2 fixation to PP = 0.8% PP data from Veronica Lance

  19. Different species / different ecological niches? > 10µm < 10µm

  20. Different species / different ecological niches? • What controls N2 fixation in the ocean? • P = Sañudo-Wilhelmy et al. 2001; Dyhrman et al. 2002; Krauk et al. 2006; Sohm & Capone 2006 • Fe = Rueter 1988; Pearl et al., 1992; Mills et al., 2004 • Temperature = Staal et al., 2003; Breitbarth et al., 2006 •  CO2= Hutchins et al., 2007 Different controlling factors for different diazotrophs in the open ocean. Nature of limitation of diazotrophy changes over time and space

  21. Trichodesmium dominates at high DFe levels > 10µm [DFe] DFe data from L. Ossiander

  22. Different species / different ecological niches NITROGENASE Fe-Mo protein (7 Fe atoms) Unicellulars diazotrophs Trichodesmium High cellular Fe quotas = 30 µmol Fe : mol C (Berman-Franck et al., 2001) ? Higher S/V (x4)  outcompet Tricho at low Fe? Fe limits N2 fixation in 75% of the global ocean!!!

  23. The role of Fe on unicellular diazotrophs (Post-doc) Crocosphaera watsonii (WH 8501) Bonnet, Unpublished data Cellular quotas = 0.3 µmol Fe : mol C (Berman-Franck et al., 2007) 100 times lower than Tricho’s

  24. Different species / different ecological niches > 10µm > 10µm [DFe] DFe data from L. Ossiander

  25. IS IT NECESSARY TO WORK UNDER TMC CONDITIONS? Why? Fe unintended fertil? (DFe Lia) The 2 distributions are significantly different (p<0.05) Overestimation by a factor of 2 in non TMC conditions !!!!!!

  26. RESULTS FROM Fe FERTILISATIONS Grow out exp: A. Marchetti, V. Lance, D. Varela, S. Bonnet Grow out exp: S. Bonnet • Different combinations of nutrients added: • Fe • P • N • Si • deepwater

  27. Grow out exp No effect of Fe , P and FeP additions on N2 fixation Units: nmol.l-1.d-1

  28. WHY WORKING UNDER TMC CONDITIONS? Why do we observe such differences in TMC versus non TMC? Hypothesis: other metals exert a control on N2 fixation Preliminary results (Bonnet, Capone, Panzeca, Sanudo-Wilhelmy, Webb) The role of cobalt on N2 fixation Crocosphaera watsonii (WH 8501) [Co] [Co]

  29. WHY WORKING UNDER TMC CONDITIONS? Produce large quantities of B12 Crocosphaera watsonii (WH 8501) 2 fmol.cell-1 (Bonnet, Panzeca) Vitamin B12 [Co]

  30. The role of Cobalt B12 synthesis = N expensive process C63H88CoN14O14P • Dual ecological function of diazotrophs in N-depleted environments? • Source of ‘new’ N (sustains ‘new’ PP) • Source of B12 for eukaryotes

  31. SUMMARY • First rates measured in the eq pac: N2 fixation well established • E-W gradient (very high rates close to PNG) • - Minor biogeochemical importance in HNLC waters • - Major importance in the warm pool (up to 70% of PP) • The small size fraction represents 75% of total N2 fixation • Dominance picodiazotrophs - importance of symbiosis • ‘Necessity’ to work under trace metal clean conditions Article in prep. For L&0. Bonnet, S., Biegala, I., Capone, D.G., Ossiander, L., Dutrieux, P., Murray, J. Nitrogen fixation in the Equatorial Pacific Ocean.

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