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

Project description from PI:
The project entitled, “Collaborative Research: Dinitrogen fixation rates and diazotrophic communities in contrasting oxygen regimes of the Eastern Pacific Ocean” will involve two cruises in the Eastern Tropical Pacific Ocean, one in the Eastern Tropical South Pacific (ETSP) and the other in the Eastern Tropical North Pacific (ETNP). During cruises we will measure water column hydrography, light, nutrient concentrations (total dissolved nitrogen, ammonium, nitrate, nitrite, urea, dissolved free amino acids, oxygen, and phosphate), and rates of primary productivity and dinitrogen (N2) fixation in detailed depth profiles at stations within and adjacent to oxygen minimum zones in the ETSP (2015) and the ETNP (2016). In addition, we will measure the abundance and expression of nifH genes, concentrations of particulate carbon (C) and nitrogen (N) and the natural abundance of 13C and 15N in particles, chlorophyll a, and cyanobacterial and heterotrophic bacterial abundance. These measurements will be compared with rates of dissolved N uptake (NO3-, NO2-, NH4+, urea, and amino acids). We will conduct experimental incubations wherein we examine the effect of organic C additions on N2 fixation rates.

Description from NSF award 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.

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.