N 2 fixation/New production Margie Mulholland 1 Department of Ocean, Earth & Atmospheric Sciences Old Dominion University

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 Fix new N into N limited aquatic systems to fuel ecosystem production N losses exceed N inputs in current N budgets – missing N 2 fixation relative to denitrification? Changes in oceanic N inventory over time affects ocean productivity

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

Factors Potentially Limiting Marine N 2 Fixation Physical: –light (photoautotrophs), –temperature –stability (mixing) Chemical: O 2 (inhibition) Nutritional: Fe, P, Si, Mo, other metals; presence of DIN Organic matter supply (heterotrophs)

Temperature - 20 degree isotherm Trichodesmium focused

Moore et al Nutrient limitation of N 2 fixation

N 2 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 (Fig. 6).

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

Quantifying N 2 fixation and N 2 fixation from blooms Using 1500 mmol N m -2 d -1 for bloom N 2 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 N 2 fixation 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 N 2 fix = 8.5 ± 1.2 Tg N yr -1

Total oceanic N 2 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 N 2 fixation (0.1 – 20.4 nmol N col -1 d -1 ) and variable density, C:N 2 fixation (1.2 – 703; is this physiology?) and N release rates (12-74% of recently fixed N 2 ) (Mulholland et al. 2006, Mulholland 2007) Context

~130 TgN/yr (40 o S-65 o N) From Deutsch et al. (2007) Global Geochemical N 2 Fixation [mmol/m 2 /yr]

Trichodesmium Diazotrophic diatom associations (e.g., Richelia/Hemiaulus and others) Coccoid cyanobacteria (groups a, b, and c) Bacterioplankton-  &  proteobacteria Copepod gut flora Archaea Other pelagic sources of N 2 fixation:

Moisander et al UCYN-A Crocosphaera 18 & 25 o C isotherm Other diazotrophs might have broader ranges

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

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

Lower euphotic zone (Chl max) PONNO 3 - Upper euphotic zone N 2 fixation Classical view View with N 2 fixation Desert PON/ POC DIC/NH 4 + PON/POC Depth(m) CO 2 NO 3 -/ DIC PONNO 3 - NH 4 + CO 2 N2N2 PON/POC NO 3 -/ DIC PON/ POC More new production = greater export production But does this account for the ecology?

Hood et al. 2000, Mulholland 2007

Implications? Does N 2 fixation yield stoichiometric drawdown of atmospheric CO 2 ? Sinking Trichodesmium Grazers Trichodesmium OR Microbial Loop Phyto NH 4 + & DON & DOC Virus Ecology matters

Subramaniam et al. 200?

Implications? Maybe for some N2 fixers? Sinking DDA’s? Grazers Trichodesmium OR Microbial Loop Phyto NH 4 + & DON & DOC Virus What about picocyanos?

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 N 2 fixation net heterotrophy release of recently fixed N 2 microbial remineralization high DOM flux Little particle flux Developed microbial loop

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

Results from CliVEC High N 2 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

High rates of coastal N2 fixation in coastal Atlantic

N 2 fixation rates were not correlated with temperature Higher rates in colder water Areal and volumetric rates comparable to oceanic rates High rates of areally integrated N 2 fixation rates (Aug 2009)

Range of 17.0 – 715  mol N m -2 d -1 N 2 fixation rates were not confined to warm waters

High N 2 fixation not focused in high Chl areas Depth integrated N 2 fixation  mol m -2 d -1 on Chl Aug 09

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

Annual rates of N inputs due to N 2 fixation Seasonally and between o N, an area 6.4% of the North Atlantic continental shelf Use our average of  mol 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 N 2 fixation Estimates of N 2 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 N 2 fixation???

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

2.5 – 3.5 x 10 7 nif gene copies for UCYNA; very high! High Tricho nif abundance and DDAs in – 6.5 x 10 6 nif gene copies for UCYNA Tricho detected UCYNA & DDAs Who and where were the diazotrophs?

Range of 11.4 –  mol m -2 d -1 Tricho quantifiable UCYN-AHemi/Richelia UCYN-A UCYN-A are dominant UCYN-A were the dominant diazotroph detected with qPCR (x10 6 gene copies) Among highest gene copies observed

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

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

Summary The mid-Atlantic shelf harbors a diverse group of diazotrophs which fix N 2 at high rates (bacteria have low  15 N 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 N 2 fixation occurs and the physiological capacity and limitations of diazotrophic organisms.

Implications for C N 2 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 CO 2 ?

Future Increased N 2 fixation - ~ % with doubled pCO 2 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

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?

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 N 2 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

Needs Where is N 2 fixation? Where isn’t it? (e.g., eastern Med – models predict N 2 fixation; Yogev et al. 2011, Mulholland & Capone 2009) What N 2 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

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)