AT26-26

Ship

Cruise Party

Departure: Dec 31, 2014

Arrival: Jan 23, 2015

Operations Area

Science Objectives

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 fix N₂ (diazotrophs). Diazotrophs can stimulate biological production via the introduction of new N into otherwise N-depleted oceanic systems.  Recent work suggests that planktonic diazotrophs are geographically more widely distributed than previously thought.  Recent studies suggest that there is active N₂ fixation in relatively warm (14-23^oC) aphotic oxygenated pelagic waters and in aphotic waters within OMZs. Because the volume of aphotic water in the ocean is large, if N₂ fixation is widely distributed at sub-euphotic depths, this could result in a dramatic upward revision of global N inputs via this process. However, at present there are few measurements of rates and we know little about how vertical chemical and physical gradients affect N₂ fixation and the diazotrophic communities mediating these N inputs.

Nitrogen loss from the ocean, via pelagic marine denitrification, occurs primarily in oceanic OMZs and, like N₂ fixation, is accomplished by a diverse group of microbes that occupy these regions. The juxtaposition of N₂ fixation and denitrification (including anammox) has not been widely examined. Heterotrophic diazotrophy has now been observed in areas within and adjacent to oxygen minimum zones where concentrations of dissolved inorganic N (DIN) are high. In fact, if DIN is not inhibitory to active N₂ fixation of the resident diazotrophs, oxygen minimum zones should be havens for diazotrophic activity because nitrogenase, the enzyme complex that mediates N₂ fixation, is extremely sensitive to oxygen and many diazotrophic organisms employ physiological or behavioral strategies for removing or avoiding oxygen.  Expression of nifH was previously observed in the Arabian Sea OMZ suggesting active N₂ fixation occurs there. In addition, N₂ fixation and proteobacterial nifH phylotypes, were detected in the low oxygen, nitrate-rich, waters of coastal California and in the OMZ region of the Eastern Tropical South Pacific where it co-occurred spatially and temporally with denitrification, refuting the paradigm of spatial and/or temporal uncoupling between the two processes. 

While aphotic N₂ fixation could substantially increase our estimates of depth-integrated rates of oceanic N₂ fixation and therefore oceanic N inventories, there are still very few measurements of N₂ fixation from oxic or suboxic aphotic waters and these measurements are poorly resolved with respect to vertical chemical and biological gradients in the ocean. It is imperative that we understand how diazotrophy varies along these gradients so that we can better predict and model the distribution of marine N₂ fixation past, present, and future. The expansion of oxygen minimum zones will undoubtedly affect the marine N and C cycles through the expansion of oceanic N losses from denitrification. However, here we hypothesize that this may be offset by N inputs from N₂ fixation by diazotrophic communities that thrive at low oxygen concentrations.

We propose to examine N₂ fixation rates and nifH gene diversity in the context of light, nutrient, and oxygen gradients (and necessarily temperature gradients) along vertical profiles that penetrate into to the ETNP and ETSP OMZs.  These oceanic realms have contrasting surface productivity which may control rates of microbial growth and processes at depth.  We will compare rates of N₂ fixation and diazotrophic community composition in vertical profiles within the OMZs to those in water masses adjacent to OMZs.  Rates will be measured using stable isotope tracer techniques that account for slow gas dissolution and that we have already applied successfully in the ETNP; we will continue to refine those methods as part of this project.  We will compare rate measurements of N₂ fixation with the abundance and expression of nifH genes and nirS genes as a proxy for active denitrification in the region to better understand the juxtaposition of these two processes in association with OMZs. 

The overarching questions that we will address are:

1. What is the contribution of diazotrophy to the total productivity in the euphotic zone of the ETP?
2. Is N₂ fixation occurring in and above the ETP OMZs and if so, how do the rates compare to N₂ fixation rates in euphotic and aphotic N₂ fixation in adjacent oxic waters?
3. How does the community composition of N₂ fixing microbes vary with respect to the vertical gradients of light, oxygen, and dissolved inorganic N concentrations with depth?
4. What is the contribution of heterotrophic N₂ fixation to depth integrated N₂ fixation in the ETP both in and adjacent to the OMZs?
5. Is the rate of N₂ fixation (N inputs) within and above the ETP OMZs enough to partially offset denitrification (N losses) from these regions? 

Science Activities

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 (this cruise) 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.  Collaborators will measure rates of denitrification and measure the natural abundance of stable isotopes in nitrate.

We will examine the distribution of N₂ fixation and the diversity of diazotrophs with respect to vertical gradients in oxygen, light, and dissolved N, and spatial gradients of productivity in the mesotrophic Eastern Tropical South Pacific (ETSP), one of the most productive oceanic regions on Earth, and the more oligotrophic Eastern Tropical North Pacific Oceans (ETNP).  We will to make detailed vertical profiles (e.g., 20 depths sampled over several days) of N₂ fixation and the abundance and activity of diazotrophs with respect to chemical and biological gradients in the ETNP and ETSP both within and adjacent the OMZs.

Additional Info

Stations:

Supporting documentation:

Shipboard Equipment

Shipboard Communication

CTD/Water Sampling

Critical CTD Sensors

MET Sensors

Sample Storage

Navigation

Navigation

Winches

Wire use and application

Standard Oceanographic Cables

Portable Vans

Checklist

Chile (Arica)

Primary Agent:
Vasile Tudoran Transport
819 Ohio Ave.
Long Beach, CA 90804

Contact: Vasile Tudoran
Phone: (562) 882-5590       
Fax: (562) 434-9800
Email : vtudoran@aol.com  

Checklist