Title: Who Makes Nitrite In The Primary Nitrite Maximum?: Investigating Nitrite In The Upper Ocean Using Stable Isotopes
Authors: Isabelle Pilson, Nicole Travis, Dr. Karen Casciotti, and the Casciotti Lab
The Primary Nitrite Maximum, or PNM, represents a thin layer at the base of the ocean’s euphotic zone where nitrite (NO2-) concentrations rise to detectable levels, creating a ubiquitous spike. Despite having documented the feature in marine profiles around the globe, oceanographers do not yet understand the biological processes and environmental influences that create and control it. The purpose of this research project is two-fold: to clarify the relative contribution of phytoplankton and microbial nitrifiers to NO2- production and consumption in the PNM as well as determine the impact of key environmental factors on NO2- release. To do so, the researchers first manipulated light and nitrate levels of ocean water samples from the Primary Nitrite Maximum at two sites in the Eastern Tropical North Pacific (ETNP). The concentrations of nitrate (NO3-) and nitrite were measured over time. Then, using the denitrifier method and a mass spectrometer, the experimenters obtained stable N and O isotope data. Through the lens of microbial isotope fractionation, laboratory isotopic values provide insight into the biological sources of the marine N compounds. Finally, the research group developed an Excel model based on first-order rate constants and Rayleigh isotope fractionation to simulate marine nitrogen processing in the PNM and further interpret the experimental results. Side-by-side analysis of measured and modeled concentrations and isotope values highlights the complex nature of the Primary Nitrite Maximum. With initial parameters matching each condition, the model largely predicted nitrate levels and isotope readings over time across both ETNP locations. Additionally, in high light situations, it trended with or fit all real measurements – concentrations and ẟ15N values of NO3- and NO2-. However, in low light and dark conditions, the model failed to coincide with experimental measurements of nitrite – [NO2-] and ẟ15N -NO2-. The results suggest that NO3- cycling at all irradiances and overall nitrogen cycling in high light can be explained with rate constants and substrate concentrations. Yet, diminishing light levels significantly impact nitrite processing, particularly that of NO2-. In the future, a light parameter will be added to the model to adjust the rates of both phytoplankton and nitrifier nitrite metabolics. Such a change would account for potential increased NO2- release by phytoplankton and light inhibition of nitrifiers as irradiance decreases. Additional replications of the field manipulations will also amplify the dataset, leading to a more robust model and complete understanding of the PNM. Knowledge of this kind promises to shed light on the ecological function of the PNM as well as its potential link to climate change and natural production of the potent greenhouse gas, nitrous oxide (N2O).