Collaborative Research: Dinitrogen fixation rates and diazotrophic communities in contrasting oxygen regimes of the Eastern Pacific Ocean

Old Dominion University Research Foundation, Norfolk VA

Investigators

Abstract

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.