Journal Article

Doubling of marine dinitrogen-fixation rates based on direct measurements

Schunck, Harald, Baustian, Tina, Lavik, Gaute, LaRoche, Julie, Mohr, Wiebke, Kuypers, Marcel M. M., Wallace, Douglas W. R., Gill, Diana, Großkopf, Tobias, Schmitz, Ruth A.

2012

Overview

A newly developed method of measuring oceanic nitrogen-fixation rates provides significantly higher estimates than a current widely applied technique, and could close gaps in the marine nitrogen budget. A question of nitrogen balance Attempts to produce a balanced marine nitrogen budget on the basis of direct measurements have proved difficult, with nitrogen loss exceeding the gain by dinitrogen (N 2 ) fixation. Here, Julie LaRoche and colleagues report a possible reason for this problem. They suggest that a method widely used to measure oceanic N 2 -fixation rates significantly and variably underestimates the contribution of microorganisms known as diazotrophs. This emerges from comparisons with fixed nitrogen measurements made with a newly developed method in the Atlantic Ocean. If the findings could be extrapolated to other ocean basins, N 2 -fixation rates would be almost double current estimates, a significant narrowing of the gap between nitrogen loss and gain. Biological dinitrogen fixation provides the largest input of nitrogen to the oceans, therefore exerting important control on the ocean's nitrogen inventory and primary productivity 1 , 2 , 3 . Nitrogen-isotope data from ocean sediments suggest that the marine-nitrogen inventory has been balanced for the past 3,000 years (ref. 4 ). Producing a balanced marine-nitrogen budget based on direct measurements has proved difficult, however, with nitrogen loss exceeding the gain from dinitrogen fixation by approximately 200 Tg N yr −1 (refs 5 , 6 ). Here we present data from the Atlantic Ocean and show that the most widely used method of measuring oceanic N 2 -fixation rates 7 underestimates the contribution of N 2 -fixing microorganisms (diazotrophs) relative to a newly developed method 8 . Using molecular techniques to quantify the abundance of specific clades of diazotrophs in parallel with rates of 15 N 2 incorporation into particulate organic matter, we suggest that the difference between N 2 -fixation rates measured with the established method 7 and those measured with the new method 8 can be related to the composition of the diazotrophic community. Our data show that in areas dominated by Trichodesmium , the established method underestimates N 2 -fixation rates by an average of 62%. We also find that the newly developed method yields N 2 -fixation rates more than six times higher than those from the established method when unicellular, symbiotic cyanobacteria and γ-proteobacteria dominate the diazotrophic community. On the basis of average areal rates measured over the Atlantic Ocean, we calculated basin-wide N 2 -fixation rates of 14 ± 1 Tg N yr −1 and 24 ±1 Tg N yr −1 for the established and new methods, respectively. If our findings can be extrapolated to other ocean basins, this suggests that the global marine N 2 -fixation rate derived from direct measurements may increase from 103 ± 8 Tg N yr −1 to 177 ± 8 Tg N yr −1 , and that the contribution of N 2 fixers other than Trichodesmium is much more significant than was previously thought.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/326/41/2535 / 704/158/2446 / Animal and plant ecology / Animal, plant and microbial ecology / Aquatic Organisms - metabolism / Atlantic Ocean / Biological and medical sciences / Carbon fixation / Cyanobacteria - genetics / Cyanobacteria - metabolism / Diatoms - metabolism / Fixation / Fundamental and applied biological sciences. Psychology / Humanities and Social Sciences / Kinetics / letter / Marine / Methods / Microorganisms / multidisciplinary / Nitrogen / Nitrogen - metabolism / Nitrogen fixation / Nitrogen Fixation - physiology / Ocean basins / Oceans / Oxidoreductases - genetics / Particulate organic matter / Proteobacteria - genetics / Proteobacteria - metabolism / Science / Science (multidisciplinary) / Sea water ecosystems / Seawater - chemistry / Synecology / Taq Polymerase - metabolism / Temperature / Tropical Climate