Stanford Earth Undergraduate Research Symposium
Summer research projects from the 2020 SESUR cohort
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
Abstract:
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).
Click HERE to view my poster and click HERE to view my presentation.
Hello, Isabelle!
Great presentation!
I did have one question: Given that one of your stations was located on the coast, could fertilizer runoff have impacted your study?
Hello, Isabelle!
Amazing presentation!
I did have a question about your data. Could it be possible for fertilizer runoff to affect your collection sites near and further from the coast?
Thank you!
Chris N.
Hi! That is a great question. We do find higher nitrogen levels near the coast due to many factors including runoff. However, the PNM still exists in these regions (although it may look a bit different in profile), signaling that it is a distinct entity in and of itself.
Nice work Isabelle!
It sounds like this project was to look at the controls of the nitrite max. At what depth were the water samples taken? Was it at PNM? How do you think the results would change if the water was collected higher in the water column?
Also, I’m not sure if you answered your question – who is controlling it? You did a nice job setting up the point that the nitrifers and the phytoplankton may impact it. Do you think you know enough to answer it? I like how you laid out the next steps.
– Jenny
Hi! The samples were taken at varying depths depending on where the PNM was that those stations. The water was sampled to determine the location of the PNM and then water was extracted from those depths. For the data in my presentation, the samples were taken from 30 and 70m, both respective PNM depths. As for your second question, it is a great one, one that I am still asking myself. We do not know enough to answer it. The PNM is extremely complicated so drawing any conclusions is very difficult. Additionally, different PNMs are likely controlled in different manners – some more by phytoplankton and others more by nitrifiers. We hope that the model we built will provide more clarity on the topic.
Great work, Isabelle! Super presentation, and very interesting results! I love that your model was able to explain the high light results but not the low light or dark experiments. Sounds like you might have found evidence for a different combination of processes occurring in different light conditions.
Thank you!
Awesome presentation! Any idea as to why the model couldn’t predict the high light conditions?
Hi Patrick! Yes, I think that our model did not fit the high light conditions because, here, the processes and rates are unique. Often, high light conditions causes differential photo-inhibition in nitrifiers as well as release as a protection mechanism in phytoplankton. In addition, light affects phytoplankton growth rates. It likely leads to a different environment with different nitrogen cycling.
Hi Isabelle — fantastic presentation! It’s clear you’ve worked hard to make this highly technical topic accessible to a wide audience, and it has absolutely paid off. The topic is really interesting, and I’m curious about the sensitivity to light. Is “high light” defined as the photic zone? The ETNP and ETSP appear coincident with stratocumulus clouds that inhibit sunlight during much of the year (these clouds are projected by some studies to disappear with global warming). Might they affect upper ocean N cycling via light limitation? Or is the effect of clouds on light availability too weak?
Hi Tyler. Thanks for the question… super interesting! Light levels in our experiments and in much of the field are defined in terms of PAR (photosynthetic available radiation) relative to the surface value. At the base of the euphotic zone where the PNM lies, it is usually 1% PAR. Our high light conditions were around 10-20% PAR if I am remembering correctly. I personally have never considered the effects of clouds, but I would conjecture that the effects of clouds would likely be too weak to have a large impact on marine nitrogen cycling. Additionally, despite the presence of clouds, phytoplankton and other creatures still have enough light to thrive. I definitely want to look more into this, and I bet that large scale changes in cloud cover could impact the cycling! Thanks for the new avenue of research!
Hi Isabelle!
Great job on your presentation! Can you explain more about the environmental factors that created the PNM and why nitrite isn’t as abundant in other areas of the ocean in comparison to the PNM?
Hi! The PNM is created by an imbalance in the nitrite cycling processes. In most of the ocean, nitrite levels hover around zero because the processes that produce and consume this intermediate are tightly coupled; once nitrite is produced, it is consumed. However, in the PNM, these processes become decoupled, leading to a build-up of nitrite. The exact mechanisms are at the root of my research questions and have not fully been answered. However, we speculate environmental factors – namely light and nitrate availability – impact biological nitrite-cycling processes. For instance, light has been shown to trigger differential photoinhibition in nitrifiers, meaning that the microorganisms that consume produce nitrite may still function normally while those that consume it may be inhibited by light. This is one theory to explain the PNM. Many more exist, and we are trying to clarify which combinations of biological processes and environmental factors produce PNMs around the world. Hope that helps!
Hi Isabelle — great work! How might the mixture of processes have changed under the low light/dark conditions? Do you think more nitrification (as opposed to phytoplankton assimilation/nitrite excretion) is occurring in these experiments?