IMAGE ABOVE: Weiyi Tang鈥檚 research group studies how global ocean ecosystem (particularly the biogeochemical cycling of elements) will change under human impacts and climate change, and characterizes how these changes will affect our planet. Courtesy of Weiyi Tang.
"Nitrogen is a major nutrient that regulates marine primary production in a large portion of the ocean. However, nitrogen input from human-related activities causes an excessive amount of nutrients (eutrophication) and, as a consequence, a reduced amount of oxygen (hypoxia) in estuarine and coastal waters. Eutrophication and deoxygenation are some of the most concerning environmental issues facing humanity. When oxygen is depleted, a process known as denitrification converts fixed nitrogen into gaseous end products (nitrate or nitrite reduction to nitric oxide, nitrous oxide, and dinitrogen gas), that reduces the amount of bioavailable nitrogen. Meanwhile, denitrification produces N2O, a powerful greenhouse gas that also depletes ozone. Since hypoxia has been observed in many estuarine and coastal waters (including the West Florida Shelf) and deoxygenation is projected to expand globally, characterization of the oxygen sensitivity of denitrification can inform how nitrogen availability and N2O emissions in estuarine and coastal environments may change under anthropogenic perturbations and future climate. However, the knowledge of oxygen sensitivities of denitrification in these environments is limited."
IMAGE ABOVE: The response of different denitrification steps to oxygen concentration changes in estuarine waters versus in open ocean oxygen minimum zones. Courtesy of Weiyi Tang.
"We filled this knowledge gap by measuring oxygen sensitivities of various steps of denitrification in Chesapeake Bay, one of the largest estuaries in the world. We found rates of all denitrification steps increased under decreasing oxygen while the percentage of N2O production decreased with decreasing oxygen in Chesapeake Bay. We then compared results obtained from Chesapeake Bay with previously measured oxygen sensitivities of denitrification in marine oxygen minimum zones that are regions with lowest oxygen concentrations such as North and South Eastern Tropical Pacific. We found that the first step of denitrification (nitrate reduction to nitrite) was generally more sensitive to oxygen in Chesapeake Bay than in marine oxygen minimum zones, which was opposite to the case for other steps of denitrification like nitrite reduction to N2 (see the figure above). The different oxygen sensitivities of denitrification steps between Chesapeake Bay and marine oxygen minimum zones may help to explain why some denitrification intermediates (e.g., nitrite and N2O) accumulate in marine oxygen minimum zones but less so in Chesapeake Bay. The difference in oxygen sensitivities of denitrification across aquatic environments may be driven by the adaptation of denitrifying microbes to different oxygen conditions and the availabilities of fixed nitrogen (e.g., nitrate and nitrite) versus organic matter. Moving forward, we plan to incorporate our newly derived oxygen sensitivities of denitrification into biogeochemical models in order to constrain the estimates of nitrogen availability and N2O emissions in global estuarine and coastal environments experiencing hypoxia or deoxygenation."