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N 2 fixation/New production

1 Department of Ocean, Earth & Atmospheric Sciences Old Dominion University. N 2 fixation/New production. Margie Mulholland. N 2 fixation and C productivity. Ultimately all new production is from N 2 fixation. N limitation of primary productivity in the bulk of the ocean

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N 2 fixation/New production

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  1. 1Department of Ocean, Earth & Atmospheric Sciences Old Dominion University N2 fixation/New production Margie Mulholland

  2. N2 fixation and C productivity • Ultimately all new production is from N2 fixation. N limitation of primary productivity in the bulk of the ocean • Fix new N into N limited aquatic systems to fuel ecosystem production • N losses exceed N inputs in current N budgets – missing N2 fixation relative to denitrification? • Changes in oceanic N inventory over time affects ocean productivity

  3. Global distributions of diazotrophs • How do we know where they are? • Based on what’s known or thought about physiology • Temperature • Nutrient concentrations – absence of N, presence of Fe and P • Ratios – geochemical inventories • Satellites – ocean color and unique cyanobacterial pigments

  4. Factors Potentially Limiting Marine N2 Fixation • Physical: • light (photoautotrophs), • temperature • stability (mixing) • Chemical: O2 (inhibition) • Nutritional: Fe, P, Si, Mo, other metals; presence of DIN • Organic matter supply (heterotrophs)

  5. Temperature - 20 degree isotherm Trichodesmium focused

  6. Nutrient limitation of N2 fixation Moore et al. 2002

  7. N2 fixation and dissolved Fe Figure 3. Seasonal maps showing the potential for nitrogen fixation in the world oceans. Maps were generated by converting global dust iron fluxes to total dissolved iron and using the relationship shown in between log[Fe’] and nitrogen fixation : maximum nitrogen fixation. From Berman-Frank et al. 2001 (Fig. 6).

  8. Global Trichodesmium bloom occurrence • From Westberry and Siegel (2006) • 6-year mean (Sep 1997-Dec 2003)

  9. Quantifying N2 fixation and N2 fixation from blooms • Using 1500 mmol N m-2 d-1 for bloom N2 fixation rates (after Capone et al., 2006) • this depends on rate per colony used • also on density of Tricho per liter • also their physiological state • “Normal” range of water column N2 fixation 15-691 mmol N m-2 d-1 (Mulholland & Lomas 2008, Mulholland et al. 2006) • 1000 mmol N m-2 d-1 during a Richelia bloom (Subramaniam et al. 2008) Global Tricho bloom N2 fix = 8.5 ± 1.2 Tg N yr-1

  10. Total oceanic N2 fixation ~ 100 Tg N yr-1 (Galloway et al. 2004) • Range is 5 – 150 Tmol N yr-1 (Carpenter & Capone 2008) • depends on model used & drivers (which limitations are assumed) • Basin-specific estimates also have order(s) of magnitude variability • Not many measurements – need measurements for models! • Mostly Trichodesmium-based estimates – high variability in rates of N2 fixation (0.1 – 20.4 nmol N col-1 d-1) and variable density, C:N2 fixation (1.2 – 703; is this physiology?) and N release rates (12-74% of recently fixed N2) (Mulholland et al. 2006, Mulholland 2007) Context

  11. Global Geochemical N2 Fixation ~130 TgN/yr (40o S-65o N) [mmol/m2/yr] From Deutsch et al. (2007)

  12. Other pelagic sources of N2 fixation: • Trichodesmium • Diazotrophic diatom associations (e.g., Richelia/Hemiaulus and others) • Coccoid cyanobacteria (groups a, b, and c) • Bacterioplankton-  &  proteobacteria • Copepod gut flora • Archaea

  13. Other diazotrophs might have broader ranges UCYN-A 18 & 25 oC isotherm Crocosphaera Moisander et al. 2010

  14. Global “Cyanobacteria” distribution • Yellow = Synechococcus-like cyanobacteria (SLC) From Alvain et al. (2008)

  15. C Flux from Dinitrogen Fixation Assume flux associated with fixation Deutsch et al. 2007

  16. View with N2 fixation Classical view N2 CO2 Upper euphotic zone - 0 - 100 PON/POC PON/POC NH4+ DIC/NH4+ - 25 - N2 fixation Desert - 50 - Depth(m) NO3-/ DIC NO3-/ DIC PON/ POC PON/ POC - 75 - Lower euphotic - 100 - zone NO3- PON NO3- PON (Chl max) CO2 CO2 More new production = greater export production But does this account for the ecology?

  17. Hood et al. 2000, Mulholland 2007

  18. Trichodesmium Trichodesmium Implications? Does N2 fixation yield stoichiometric drawdown of atmospheric CO2? OR Virus NH4+ & DON & DOC Grazers Microbial Loop Phyto Sinking Ecology matters

  19. Subramaniam et al. 200?

  20. DDA’s? Trichodesmium Implications? Maybe for some N2 fixers? OR Virus NH4+ & DON & DOC Grazers Microbial Loop Phyto Sinking What about picocyanos?

  21. The fate of new N in tropical systems Alternative hypotheses: 1. New N from nitrate net autotrophy sinking of large cells grazing by large copepods Large particle flux No microbial loop 2. New N from N2 fixation net heterotrophy release of recently fixed N2 microbial remineralization high DOM flux Little particle flux Developed microbial loop

  22. Trophic transfer of fixed N • Correlation between timing and magnitude of blooms of Karenia brevis and Trichodesmium spp. in GOM and coastal Atlantic

  23. Results from CliVEC • High N2 fixation rates in unexpected places (coastal systems) • Diverse diazotrophs in coastal systems Other observations: • Chl a not always well-correlated with productivity • C and N productivity are not well correlated

  24. High rates of coastal N2 fixation in coastal Atlantic

  25. High rates of areally integrated N2 fixation rates (Aug 2009) • N2 fixation rates were not correlated with temperature • Higher rates in colder water • Areal and volumetric rates comparable to oceanic rates

  26. N2 fixation rates were not confined to warm waters Range of 17.0 – 715 mmol N m-2 d-1

  27. High N2 fixation not focused in high Chl areas Aug 09 Depth integrated N2 fixation mmol m-2 d-1 on Chl

  28. Areal rates of N2 fixation • Areal rates 36.7 to 340 mmol N m-2 d-1 in Summer and 32.7 to 199.6 mmol N m-2 d-1 in Fall • Areal rates for tropical and subtropical oceans range from 3.7 to 703 mmol N m-2 d-1 • Areal rates for tropical North Atlantic Ocean average 239 mmol N m-2 d-1 • English channel (2 sites) 350 mmol N m-2 d-1 in summer

  29. Annual rates of N inputs due to N2 fixation • Seasonally and between 35-45oN, an area 6.4% of the North Atlantic continental shelf • Use our average of 135.1 mmol N m-2 d-1 • N input is 0.02 Tmol N y -1, the amount previously calculated for the entire North Atlantic continental shelf (Nixon et al. 1996) • If this rate applies for the whole shelf then 0.31 Tmol y -1 is input from shelf N2 fixation • Estimates of N2 fixation for the entire N Atlantic basin are 0.15 to 6.4 Tmol N y -1 • This is about 10% of the estimated N removal due to denitrification for the same area • How widespread is coastal N2 fixation???

  30. Distribution of nif groups - qPCR Group a – UCYN-A Group b (DNQ) Trichodesmium Hemi/Richelia Unknown

  31. 4.3 – 6.5 x 106nif gene copies for UCYNA UCYNA & DDAs 104 - 106 Tricho detected 2.5 – 3.5 x 107nif gene copies for UCYNA; very high! High Tricho nif abundance and DDAs in 2006 Who and where were the diazotrophs?

  32. UCYN-A are dominant • UCYN-A were the dominant diazotroph detected with qPCR (x106 gene copies) • Among highest gene copies observed UCYN-A Tricho quantifiable UCYN-A Hemi/Richelia Range of 11.4 – 275.2 mmol m-2 d-1

  33. Primary productivity (mmol C m-2 d-1) and SST (not correlated to N2 fixation) Aug 09 Depth integrated C fixation on SST; Range 11.2 – 209.4 mmol m-2 d-1

  34. Primary productivity (mmol C m-2 d-1) and Chl Aug 09 Depth integrated C fixation mmol m-2 d-1 on Chl Range 11.2 – 209.4

  35. Summary • The mid-Atlantic shelf harbors a diverse group of diazotrophs which fix N2 at high rates (bacteria have low d15N there although diazotrophs were not observed in Meador study) • Coastal areas are largely excluded from geochemical estimates on the grounds that they are nutrient replete. • We are really in our infancy in understanding where N2 fixation occurs and the physiological capacity and limitations of diazotrophic organisms.

  36. Implications for C • N2 fixation related to new production and fluxes • New production underestimated Many other complications: • But UCYN-A are photoheterotrophs so may not fix or draw down C • Food web interactions • Future scenarios with high CO2?

  37. Future • Increased N2 fixation - ~25 - 50% with doubled pCO2 affecting ecosystems and C export • Increases in Tricho and other cyanos ranges with warming • Increased N release fueling regenerated production • Which taxa will respond? • In food webs and biogeochemical function and ocean C uptake, not all phytoplankton are equal Hutchins, Mulholland, & Fu 2009

  38. Other diazotrophs • Yes for Crocosphaera • Working on others • Effect of other climate change variables on diazotrophy? Stratification? etc • Expanding ranges of diazotrophs and cyanos in general? • Other changes in phytoplankton communities in general?

  39. The Future • Difficult to assess changes over time because of variability and we don’t know the physiological diversity of most diazotrophs (Tricho won’t grow at 180ppm) • More measurements from the coastal region here and elsewhere to better resolve variability in C and N2 fixation • Better understanding of the physiology of the diverse group of diazotrophs and their limits and tolerances to advise models • Need to understand mechanisms to be predictive

  40. Needs • Where is N2 fixation? Where isn’t it? (e.g., eastern Med – models predict N2 fixation; Yogev et al. 2011, Mulholland & Capone 2009) • What N2 fixers are where? What is their physiology and role in ecosystem production? • Relationship of diazotrophic production with export (grazing on diazos) • Relationship of diazotrophs within ecosystems – ecosystem balance (net auto vs. net hetero) • Ecosystem shifts over time associated with climate variability and change • Nitrogen:carbon relationship

  41. Physiology • Important • Can’t capture it all in models • What are the most important physiological characteristics to get right? • To describe the past? • To project the future? (the goal)

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