Table of Contents

• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Supplemental Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

Trial A test of the dissolution method

Inocula of 15N2-enriched water were prepared according to either of two protocols outlined by Klawonn et al. (2015). 
In a first Trial A, respective 1.9 mL of 15N2 gas aliquots (Cambridge Isotope Laboratories, Lot #I-21065) were injected into crimped-sealed 120 mL glass serum vials filled with deionized water. To dissolve the 15N2 bubble, each of the two serum vials was vortexed for 5 minutes. Two subsamples of each inoculum were dispensed into Exetainers™ with a peristaltic pump for analysis on the MIMS. An aliquot of each inoculum (5 % vol/vol) was then dispensed in replicate 160 mL serum incubation bottles containing air-equilibrated deionized water (Trials A1-A4), which were then crimped-sealed. Following homogenization, triplicate subsamples of each incubation were collected in Exetainers™ for MIMS analysis. The 15N atom % of the inocula and of the corresponding incubations were measured by MIMS at the University of Connecticut (Bay Instruments) and computed as follows: 

Equation 4: 

Equation S2: 

= 0.7299 % Equation S3b

= 0.0013 % Equation S3c
 

BCO-DMO Processing Notes:
- table was extracted from original spreadsheet.
- added conventional header with dataset name, PI name, version date
- modified parameter names to conform with BCO-DMO naming conventions

NSF Award Abstract:
The availability of nitrogen is required to support the growth and production of organisms living in the surface of our global ocean. This element can be scarce. To alleviate this scarcity, a special class of bacteria and archaea, called nitrogen fixers, can derive the nitrogen needed for growth from nitrogen gas. This project would carefully examine one specific method for measuring nitrogen fixation that has been used recently to suggest the occurrence of small amounts of nitrogen fixation in subsurface ocean waters. If these reports are verified, then a revision of our understanding of the marine nitrogen cycle may be needed. The Ocean Carbon and Biogeochemistry program will be used as a platform to develop community consensus for best practices in nitrogen fixation measurements and detection of diversity, activity, and abundances of the organisms responsible. In addition, a session will be organized in a future national/international conference to communicate with the broader scientific community while developing these best practices.

The goal of this study is to conduct a thorough examination of potential experimental and analytical errors inherent to the 15N2-tracer nitrogen fixation method, in tandem with comprehensive molecular measurements, in mesopelagic ocean waters. Samples will be collected and experimental work conducted on a cruise transect in the North Atlantic Ocean, followed by analytical work in the laboratory. The specific aims of this study are to (1) determine the minimum quantifiable rates of 15N2 fixation based on incubations of mesopelagic waters via characterization of sources of experimental and analytical error, and (2) seek evidence of presence and expression of nitrogen fixation genes via comprehensive molecular approaches on corresponding samples. The range of detectable rates and diazotroph activity from the measurements made in this study will be informative for the understanding of the importance of nitrogen fixation in the oceanic nitrogen budget.