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Award Detail

Doing Business As Name:Old Dominion University Research Foundation
  • Margaret R Mulholland
  • (757) 683-3972
Award Date:01/14/2014
Estimated Total Award Amount: $ 501,142
Funds Obligated to Date: $ 501,142
  • FY 2014=$501,142
Start Date:09/01/2014
End Date:08/31/2017
Transaction Type:Grant
Awarding Agency Code:4900
Funding Agency Code:4900
CFDA Number:47.050
Primary Program Source:040100 NSF RESEARCH & RELATED ACTIVIT
Award Title or Description:Collaborative Research: Dinitrogen fixation rates and diazotrophic communities in contrasting oxygen regimes of the Eastern Pacific Ocean
Federal Award ID Number:1356056
DUNS ID:077945947
Program Officer:
  • Michael Sieracki
  • (703) 292-7585

Awardee Location

Street:4111 Monarch Way
Awardee Cong. District:03

Primary Place of Performance

Organization Name:Old Dominion University
Cong. District:03

Abstract at Time of Award

There are three major open ocean oxygen minimum zones (OMZs) in the world: the Eastern Tropical North Pacific (ETNP), the Eastern Tropical South Pacific (ETSP), and the Arabian Sea. OMZs 
are important areas of denitrification (including anammox) and represent a significant loss 
of fixed nitrogen (N) from the ocean. However, multiple lines of evidence have recently indicated that N inputs via dinitrogen (N2) fixation and denitrification may be more closely coupled in space than previously suggested. Despite the geochemical inferences regarding the location and magnitude of N2 fixation that might be associated with OMZs and the importance of these regions for removing fixed N from the ocean, it is thought that OMZs do not harbour diazotrophs. In a preliminary study, nifH genes and their expression were detected from within the Arabian
 Sea and ETNP OMZs and active N2 fixation was measured in the ETNP OMZ, confirming that N2 fixation occurs in oxygen deficient waters. The PIs propose to measure N2 fixation and the 
diversity of diazotrophic communities with respect to vertical gradients of oxygen, light, 
and dissolved nitrogen (N) concentrations. They will compare these detailed vertical profiles
 with similar profiles made in fully oxic waters adjacent to the OMZs. In addition, they will 
compare and contrast two very different OMZ regions; one that includes some of the most productive oceanic waters on Earth (ETSP), and another that is far less productive (ETNP). As part of
 this project, they will acquire a better understanding of where N2 fixation occurs with respect 
to areas of active denitrification and the microbes involved in these processes. Armed with 
this more comprehensive understanding of the vertical distribution of N2 fixation and active diazotrophic communities with respect to chemical and biological gradients in OMZ waters,
 they will garner a more realistic view of the N cycle within these regions and a better understanding 
of depth-integrated rates of N2 fixation that include oxic and anoxic aphotic waters. Intellectual Merit: Despite the geochemical inferences regarding the location and magnitude of N2 fixation and
its juxtaposition with denitrification within OMZs, there are few rate estimates and biological 
data supporting these conjectures. The biological evidence we do have suggests that diazotrophs are active within the Arabian Sea and ETNP OMZs and that there is measureable N2 fixation 
within the ETNP OMZ. The proposed research will allow the PIs to establish the contribution of diazotrophy to N inventories within and adjacent to two expansive OMZ regions. The ability 
to reconcile oceanic N budgets and construct accurate biogeochemical models is currently limited by the geographical paucity of rate measurements from diverse oceanic environments. In addition, while the PIs have measured active N2 fixation in aphotic waters, they lack the depth resolution 
to include the expansive sub-euphotic oceanic realm in depth-integrated estimates of oceanic 
N2 fixation anywhere. Through their ongoing collaboration with Ward and Devol, who have measured N losses associated with denitrification in this OMZ, the PIs will be able to construct an "end-to-end" view of the N cycle in this globally important system. The molecular data generated from 
this project will not only shed light on the active clades that contribute to oceanic 
N inputs through N2 fixation and N loss through denitrification, but also give insights 
into the relative distributions and activity of diazotophs and dentrifiers along vertical 
gradients of oxygen, light, and dissolved N. Broader Impacts: Results will contribute to our understanding of controls on marine N2 fixation; variations 
in diazotrophy along vertical gradients of light, oxygen, and nutrient concentrations; and
 the balance between N inputs and losses from OMZs. This project will
 be potentially transformative in shaping our view of the oceanic N budget, both the input and output ends. The project will also contribute to education on a number of levels. Undergraduate and graduate students at Old Dominion University and Princeton will have opportunities to gain valuable research experience using state of the art research facilities both on land and at 
sea. In addition, we will create a video classroom to communicate with elementary and middle school students during the cruises to the ETNP and ETSP and keep the scientific community and general public updated through blogs.

Publications Produced as a Result of this Research

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Widner, B., C. Mordy, and M. R. Mulholland "Cyanate distribution and uptake above and within the Eastern Tropical South Pacific oxygen deficient zone." Limnology and Oceanography, v., 2017, p..

Jayakumar, A., B. X. Chang, B. Widner, P. W. Bernhardt, M. R. Mulholland, and B. B. Ward. "Biological Nitrogen Fixation in the Oxygen Minimum Region of the Eastern Tropical North Pacific Ocean" ISME Journal, v., 2016, p..

Xin Sun, Qixing Ji, Amal Jayakumar and Bess B. Ward "Dependence of nitrite oxidation on nitrite and oxygen in low-oxygen seawater" Geophys. Res. Let., v.44, 2017, p.7883. doi:10.1002/2017GL074355 

Jayakumar, A., B. X. Chang, B. Widner, P. W. Bernhardt, M. R. Mulholland, and B. B. Ward "Biological Nitrogen Fixation in the Oxygen Minimum Region of the Eastern Tropical North Pacific Ocean" ISME Journal, v., 2017, p.. doi:10.1038/ismej.2017.97 

Project Outcomes Report


This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

In much of the world ocean, the bioavailability of dissolved nitrogen (N) limits primary production in surface waters. While dinitrogen (N2) is abundant in marine waters, it is biologically unavailable to all but certain groups of prokaryotic marine organisms that are able to fix N2 (diazotrophs).  Diazotrophs can stimulate biological production via the introduction of new N into otherwise N-depleted oceanic systems. We now know that diazotrophs inhabit many oceanic domains but we still know little about their distribution with respect to environmental gradients in chemical, biological and physical properties.  This project examined rates of N2 fixation and the abundance and diversity of diazotrophs in surface and deep waters with respect to gradients in oxygen, light, nutrient concentrations, as well as with respect to contrasting productivity regimes in the Eastern Tropical Pacific Ocean where there are also profound losses of oceanic N due to denitrification.

Specifically, the major objective of this project was to examine the juxtaposition of N inputs to the ocean from N2 fixation and N losses from the ocean via denitrification.  Because N losses from the ocean occur primarily in areas devoid of oxygen, we tested our hypotheses in the Eastern Tropical North and South Pacific where there are vast oxygen deficient zones.

The field observations were made during two month-long cruises, one to the Eastern Tropical South Pacific in January 2015 and the other to the Eastern Tropical North Pacific in April 2016.  During cruises we occupied stations along transects across the major oxygen deficient zones both nearshore and offshore to establish how N2 fixation and the abundance and diversity of diazotrophs vary with respect to environmental gradients in the nearshore and offshore environments.  To do this, we used specialized sampling techniques to prevent oxygen contamination of samples and developed a method to more accurately estimate N2 fixation rates using gas addition experiments. We also conducted rigorous error analysis to better evaluate the limit of detection of N2 fixation rates in natural systems where rates are often low. 

We found that rates of N2 fixation were highest in surface waters, however, the dominant diazotrophs there were not always photoautotrophs.  In aphotic waters, oxic and oxygen deficient, N2 fixation rates were generally low but highly variable and often near the limit of analytical detection. Because detection limits are sample specific, our low rates promulgated us to undertake detailed error analysis along with additional environmental sample analyses.  We determined that many of the previous measurements of N2 fixation from aphotic waters were likely at or near the limit of analytical detection.  Although rates of aphotic N2 fixation were generally low, we found that additions of amino acids stimulated N2 fixation suggesting that heterotrophic diazotrophs may be limited by the production of fresh organic matter for growth.  Molecular analyses revealed that nifH gene copy abundance correlated well with N2 fixation rates throughout the water column. 

Despite contrasting productivity regimes, we found that rates of N2 fixation were comparable in the Eastern Tropical North and South Pacific Oceans with higher rates observed in surface waters at the nearshore stations where primary productivity was also higher.

In addition to our measurements of N2 fixation rates we discovered that cyanate was denitrified by marine microbes in oxygen deficient waters in the Eastern Tropical North and South Pacific Oceans.  While not a part of this project, we used the cruise opportunities to expand our measurements of cyanate concentrations in the sea and examine rates of cyanate uptake and cyanammox. We also measured nitrite oxidation, an essential step in the nitrogen cycle, during the 2016 cruise to the Eastern Tropical North Pacific.  We found that nitrite oxidation was sensitive to oxygen and dependent on nitrite supply.  On our cruises we also facilitated research by groups from Instituto del Mar del Perú, University of Concepcion, University of Southern California, Stanford University, Monterey Bay Aquarium Research Institute, Xiamen University, and Universidad Autonoma de Baja California.

In summary, our research demonstrates significant rates of N2 fixation in surface waters, particularly in coastal waters, consistent with studies in the North Atlantic Ocean.  Rates of aphotic N2 fixation were generally low but often detectable using our improved methods.  Similarly, rates of N2 fixation within oxygen deficient zones were low but variable.  These rates were stimulated by organic matter enrichment suggesting microbes were limited by the provision of fresh organic matter.  Our research will help resolve biogeochemical and oceanic productivity models and contribute to our ability to constrain the marine N budget.


Last Modified: 11/30/2017
Modified by: Margaret R Mulholland

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