Award: OCE-1554474

Award Title: CAREER: The biological nitrogen isotope systematics of ammonium consumption and production
Funding Source: NSF Division of Ocean Sciences (NSF OCE)
Program Manager: Henrietta N. Edmonds

Outcomes Report

Nitrogen is an essential element for life. It’s availability at the surface ocean influences the fertility of ecosystems. The naturally occurring isotope ratios of nitrogen (15N/14N) are measured at the surface ocean as they provide insights into the sources and cycling of N. To infer processes from isotope ratios requires a prior knowledge of how different biological processes influence the isotope ratios. To this end, we conducted laboratory experiments to determine how marine phytoplankton fractionated 15N and 14N isotopes of ammonium, an important compound in the surface ocean. We observed that phytoplankton cease fractionating ammonium isotopes at the low concentrations germane to the surface ocean. We also infer that fractionation during assimilation occurs during transport, due to the deprotonation of ammonium. We can now interpret isotope ratios of ammonium in the environment. Pioneering measurements at the surface of the Southern Ocean reveal relatively low isotope ratios, corroborating our finding that assimilation does not fractionate the N isotope ratios. I developed a service-learning course for undergraduate seniors, wherein students were involved in research to determine the causes of water quality impairment in local coves, in collaboration with a group of citizen scientists (Clean Up Sounds and Harbor). The course is highly popular, introducing students to laboratory and field methods, as well as data analysis, interpretation and presentation. Last Modified: 06/29/2022 Submitted by: Julie Granger
DatasetLatest Version DateCurrent State
Nitrogen isotope fractionation for ammonium assimilation by marine phytoplankton (Biological Nitrogen Isotope Fractionation project)2021-11-15Data not available
Volume-dependent offsets in NO3- N and O isotope ratios of reference materials (Biological Nitrogen Isotope Fractionation project)2021-11-16Final no updates expected
ẟ15N (ẟ18O) scale contraction was calculated as the percent deviation of the difference between measured ẟ15N (ẟ18O) values of IAEA-NO3 and USGS-34 from the true difference (Biological Nitrogen Isotope Fractionation project)2021-11-16Final no updates expected
Effects of sample volume on N2O recovery and isotopic analysis (Biological Nitrogen Isotope Fractionation project)2021-11-16Final no updates expected
Volume effects on blanks originating from the bacterial concentrates, equilibration with atmospheric N2O, and NO3- contamination of the water into which the standards were diluted from 2017-2020 (Biological Nitrogen Isotope Fractionation project)2021-11-16Final no updates expected
Analyses of nitrate reference solutions in 18O-labeled water with the denitrifier method (Biological Nitrogen Isotope Fractionation project)2021-11-29Final no updates expected
O exchange with water during denitrification with the denitrifier method (Biological Nitrogen Isotope Fractionation project)2021-12-01Final no updates expected
Simulation of δ15N and δ18O scale contraction, given the observation of the fraction of O atom exchange with water and blanks (Biological Nitrogen Isotope Fractionation project)2021-12-01Final no updates expected
Simulation of the volume-dependent changes in ẟ15N and ẟ18O for IAEA-NO3 and USGS-34 reference materials (Biological Nitrogen Isotope Fractionation project)2021-12-01Final no updates expected
2024-03-08Preliminary and in progress
Nitrate 15N/14N measurements in two adjacent mesoscale eddies in the North Pacific Subtropical Gyre from water samples collected on R/V Kilo Moana cruise KM1709 in June-July 20172025-01-13Final no updates expected

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Principal Investigator: Julie Granger (University of Connecticut)