Unraveling the Oxygen Secrets of Nitrogen Removal in Estuaries: Insights from Chesapeake Bay
- Ray Sullivan
- 3 days ago
- 3 min read
Nitrogen is vital for marine life, but excessive human inputs have caused coastal problems like eutrophication and hypoxia, low oxygen conditions. Denitrification is a critical process that converts excess bioavailable nitrogen back to nitrogen gas (N2), helping to mitigate these issues.
Denitrification isn't a simple conversion. It's a multi-step bacterial process, starting with nitrate (NO3-) reduction to nitrite (NO2-), then nitrite reduction to nitric oxide (NO), followed by nitrous oxide (N2O), and finally to nitrogen gas (N2). Nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting substance, is an intermediate product in this pathway. The balance between N2O production and its subsequent reduction to N2 is critical for controlling N2O emissions from aquatic environments.
Denitrification is typically considered an anaerobic process, strongly influenced by oxygen levels. While it was thought to occur below ~6 μM oxygen and that different steps might have varying oxygen sensitivities (with N2O reduction being the least tolerant), the precise thresholds and sensitivities in complex natural environments remained uncertain.
Bess Ward's Lab at Princeton University conducted this research in the Chesapeake Bay – the largest estuary in the United States, which experiences seasonal hypoxia. Their research provides direct measurements of how oxygen affects the different steps of denitrification. Using 15N stable isotope tracers and manipulating oxygen concentrations in water samples from various depths, the researchers measured the rates of nitrate (NO3-) reduction to nitrite (NO2-), nitrite reduction to nitrous oxide N2O, and nitrite reduction to N2 gas.
A key finding was that, in Chesapeake Bay, all measured denitrification steps showed remarkably similar sensitivities to changes in oxygen concentration. The average oxygen inhibition factor, where the rate decreases to 37% of maximum, was around 1 μM O2 for all three steps. This contrasts with observations in some marine oxygen minimum zones (OMZs) where different steps had varying sensitivities.
While the absolute rates of N2O production decreased with increasing oxygen, the yield of N2O from denitrification (N2O relative to N2O + N2) significantly increased with increasing oxygen. This yield rose from about 1.6% near zero oxygen to 100% when oxygen was above 5 μM. This suggests that while denitrification slows down in general with oxygen, the final step producing N2 is particularly inhibited at higher oxygen levels. Variations in N2O yield across depths might be linked to the composition and "modularity" of the microbial community.
Comparing Chesapeake Bay to other aquatic environments, the lab noted differences; for instance, nitrate reduction to nitrite appeared more sensitive to oxygen in Chesapeake Bay than in some marine OMZs, while N2 production from nitrite seemed less sensitive. These differences might be due to factors like the presence of particles, adaptation to seasonal vs permanent low oxygen, and nutrient availability.
These findings have significant implications for computer models simulating nitrogen cycling. The study suggests that current Chesapeake Bay models might overestimate nitrogen removal via denitrification in the presence of oxygen because they use parameters that are less sensitive to oxygen than observed. This highlights the need for developing specific model parameters for different environments like estuaries and marine OMZs.
By providing the first quantification of oxygen sensitivity across different denitrification steps in estuarine waters, this research improves our ability to understand nitrogen removal and constrain N2O emissions in coastal areas affected by hypoxia.
Tang W, Fortin SG, Intrator N, Lee JA, Kunes MA, Jayakumar A, Haynes SJ, Oleynik S, Sigman DM, Ward BB. Similar Oxygen Sensitivities of Different Steps of Denitrification in Estuarine Waters. Environ Sci Technol. 2025 Apr 15;59(14):7165-7175. doi: 10.1021/acs.est.5c02248. Epub 2025 Apr 4. PMID: 40184320.