New method detects much higher rates of N2-fixation in the oceans
Microorganisms responsible for nitrogen-fixation rates in the oceans (called N2-fixers) are essential to marine productivity. A new study by German and Canadian researchers, including faculty members from Dalhousie University’s Biology and Oceanography departments, reports new measurements of N2-fixation rates that are 1.5-10 times higher than previous measurements. The study is published in the journal Nature on the same day as Germany’s Chancellor, Angela Merkel, and the Canadian Minister of Science and Technology, Gary Goodyear, witness the signing of a new agreement for joint, bilateral study of the Atlantic and Arctic Oceans.
Marine productivity ultimately depends on phytoplankton, tiny microscopic plants that grow in the sunlit surface waters of the ocean. Phytoplankton, like other plants, require nutrients in addition to sunlight. However, fixed nitrogen concentrations are often extremely low in the surface ocean and can limit phytoplankton growth. There is plenty of nitrogen gas dissolved in seawater, but most organisms simply cannot use it. Only a special group of microbes, called N2-fixers, can reduce the N2 gas to ammonium. These specialized microorganisms act like a “fertilizer factory” for the ocean.
The N2-fixers' fertilizing action takes place far away from Canadian waters, in places such as the tropical and sub-tropical ocean off West Africa. However, the nutrients produced are recycled and transported far from where the N2-fixation takes place and contribute to the biological productivity of the entire North Atlantic. An understanding of marine N2-fixation rates, including the ability to accurately measure it within the ocean, is crucial for assessing the future of ocean productivity.
Dr. LaRoche and co-workers compared field measurements made with a long-standing, traditional method that had been identified recently in laboratory studies to underestimate N2 fixation rates, with rates measured using a recently developed, improved technique. They found that N2-fixation rates in the Atlantic Ocean measured with the improved method were 1.5-10 times higher than measurements using the traditional approach. The authors propose that extrapolation of these data to other oceans would result in a doubling of our estimate of oceanic N2-fixation.
An involvement of the community of researchers working on N2-fixation worldwide will be essential to assess whether or not prior measurements of this important rate can be corrected. The researchers must also agree on a practical, standard method that will eliminate the current biases in N2-fixation rate measurements.
"The magnitude of underestimation with the prior method was seen to depend on which species of N2-fixers were present in the water sample: this may turn out to be even more significant than our revision to the actual fixed nitrogen budget of the ocean. The new method aims to circumvent these problems." -- Dr. Douglas Wallace, Canada Excellence Research Chair (CERC) in Ocean Science and Technology, Scientific Director at the Halifax Marine Research Institute, and co-author of the study.
"This study is an important reminder that measurement methods have to be carefully and critically tested before widespread application and should also be regularly re-evaluated." -- Dr. Julie LaRoche, lead researcher and co-author of the study, Department of Oceanography.
Trichodesium, a type of N2-fixer.
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