May
have an impact on climate change
Duke University
Nitrogen fixation is surprisingly
high in coastal waters and may play a larger role than expected in carbon
dioxide (CO2) uptake in these waters, a new study led by Duke University
scientists shows.
Knowing where and when nitrogen
fixation is occurring will help scientists better quantify coastal waters’
ability to absorb and store carbon dioxide and aid in future climate
predictions.
“Past models suggested most nitrogen
fixation occurred in the open ocean. We found the opposite: Rates are actually
higher in coastal areas. They’ve just been mostly overlooked until now,” said
Nicolas Cassar, professor of biogeochemistry at Duke’s Nicholas School of the
Environment, who was senior author on the study.
Most of this overlooked activity is
being driven by microbes known as cyanobacterial diazotrophs that live in
coastal waters, the study shows.
The tiny organisms take in nitrogen gas (N2) and convert it into ammonia (NH3), a form of nitrogen that phytoplankton, the base of the marine food web, can use as food. This in turn fuels photosynthesis by the phytoplankton and the uptake of CO2.
“This means that we have to revisit
the global distribution of marine N2 fixation and re-evaluate its role in the
coastal carbon cycle,” said Weiyi Tang, a Ph.D. student in Earth and Ocean
Sciences at Duke’s Nicholas School, who conducted the research as part of his
doctoral dissertation.
Cassar and Tang published their
peer-reviewed study Feb. 19 in Nature Communications.
Their findings are based on
thousands of seawater samples collected from across 6,000 kilometers of the
western North Atlantic during two 10-day research cruises in 2015 and 2016.
Collecting that unprecedented volume
of data in such a short amount of time at sea was possible, Cassar explained,
because his lab team has developed an instrument that allows them to do
near-continuous real-time analysis of N2 fixation. They use a method called
FARACAS, or flow-through incubation acetylene reduction assays by cavity
ring-down laser absorption spectroscopy.
Previously, the only way scientists
had to quantify marine nitrogen fixation, which tends to be very episodic in
nature, was to sporadically collect samples, perhaps twice daily, in locations
where their research vessel stopped. This was like reading only two chapters of
a book instead of all of them, Cassar said. It left gaps in the narrative.
“Our new instrument lets us collect
near-continuous samples over broad expanses of the ocean while the ship is
moving, which makes it a much better tool for assisting researchers. It’s like
a biogeochemical compass,” he said. “While each data point is not fully
independent at this resolution, we can collect more data on one cruise than you
find in the existing literature.”
A meta-analysis of the existing
literature that Tang subsequently conducted shows that nitrogen fixation is
likely significant in other coastal regions worldwide, too, not just in the
western North Atlantic.
“Nitrogen fixation is one of the most
important processes governing life on Earth and in the oceans. Developing this
new technique to measure nitrogen fixation is already changing our
understanding of where and when it happens,” said Hedy Edmonds, program
director at the National Science Foundation (NSF) Division of Ocean Sciences,
which funded the research through a CAREER grant (#1350710) to Cassar.
Additional support for the research
came from a Link Foundation Ocean Engineering & Instrumentation Fellowship
to Tang.
Seaver Wang, a doctoral student in
Earth and Ocean Sciences at Duke’s Nicholas School, co-authored the paper.
Other co-authors were from Dalhousie
University; Vrije Universiteit; the University of North Carolina at Chapel
Hill; Laboratoire des Sciences de l’Environment Marin at the Institut
Universitaire Européen de la Mer; and the Instituto de Oceanografia y Cambio
Global.
CITATION: “Revisiting the
Distribution of Oceanic N2 Fixation and Estimating Diazotrophic Contribution to
Marine Production,” Weiyi Tang, Seaver Wang, Debany Fonseca-Batista, Scott
Gifford, Aridane G. Gonzalez, Morgane Gallinari, Hélène Planquette, Géraldine
Sarthou and Nicolas Cassar. Nature Communications, Feb. 19, 2019.
DOI: 10.1038/s41467-019-08640-0