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