Award: OCE-1732246

Nitrogen is an essential nutrient for the plants, algae and phytoplankton that are at the base of the marine food chain. While most essential plant nutrients originate from weathering of rock, the reactive forms of nitrogen that are used by photosynthetic organsims are ultimately introduced to the environment by bacteria that can convert inert N2 gas into ammonia – a process called "nitrogen fixation." In the ocean, it remains unclear how much total nitrogen fixation occurs, where it occurs, and which groups of bacteria contribute to the process. Until recently, nitrogen fixation was thought to occurr largely in subtropical and tropical sufrace waters of the ocean. However, a number of studies have cast doubt on this paradigm, with reports of nitrogen fixation and nitrogen fixing organisms in deeper ocean waters, in oxygen deficient waters at the continental margins, in coastal regions, and in the polar oceans. Thus, the spatial distribution and magintude of global ocean N2 fixation is being re-examined. In many of the studies contributing to this pardigm shift, however, the reported N2 fixation rates tend to be very low, approaching, or perhaps exceeding, the limit of detection of the incubation methods used to estimate nitrogen fixation. In this project, we thus sought to clarify the lower limitis of detection and of quantification of the incubation method to measure N2 fixation, in order to provide guidelines to the community to ensure that published estmates are valid and robust. Specifically, N2 measurments of fixation rates from on-deck bottle incubations involve mass spectrometric analyses ofthe stable isotopes of nitrogen (15N and 14N) in N2 gas samples and in samples of plankton particles. These measurments require specialized equipment and rigorous standardization. Here we examined best practices for these measurements on differnt mass spectrometric instruments. In a manuscript intended for publication (to be submitted for review in September in 2018), we enunciate best practices for isotopic analyses of N2 gas and plankton particles, based on our observations. We anticipate that this manuscript will be of great value to the community to help standardize practices, allowing for inter-comparison of N2 fixation rate estimates among group, and instilling confidence in the validity of N2 fixation rate estimates thus generated. Last Modified: 07/17/2018 Submitted by: Julie Granger