How much nitrogen is fixed in the ocean?

Helmholtz Centre for Ocean Research in Kiel: ...Of course scientists like it when the results of measurements fit with each other. However, when they carry out measurements in nature and compare their values, the results are rarely “smooth”. A contemporary example is the ocean’s nitrogen budget. Here, the question is: how much nitrogen is being fixed in the ocean and how much is released? “The answer to this question is important to predicting future climate development. All organisms need fixed nitrogen in order to build genetic material and biomass”, explains Professor Julie LaRoche from the GEOMAR | Helmholtz Centre for Ocean Research Kiel.

Despite scientific efforts, the nitrogen budget suffers from an apparent dilemma. The analysis of ocean sediment as a long-term climate archive has shown that the amounts of fixed nitrogen equaled those of released nitrogen for the past 3000 years. However, modern measurements in the ocean demonstrate that the amounts of released nitrogen exceed the amounts of nitrogen being fixed. These results leave a “gap” in the nitrogen budget and show inconsistencies between past, long-term reconstruction and short-term measurements.

In 2010, the GEOMAR microbiologist Wiebke Mohr pointed out that these inconsistencies could be partially due to the methods widely used to measure modern biological nitrogen fixation. Following this finding, scientists from GEOMAR, Christian-Albrechts-University of Kiel (CAU), Max Planck Institute for Marine Microbiology (MPI) Bremen and Dalhousie University in Halifax (Canada) tested a new approach in the Atlantic Ocean which had been suggested by Mohr: The results of the study are now presented in the international journal “Nature”.

...The “gap” in the nitrogen budget, however, cannot be entirely closed even with improved measurements of nitrogen fixation. “This is also due to the fact that we still don’t know all the microorganisms responsible for all these processes”, Groβkopf says. Professor Ruth Schmitz-Streit, microbiologist at the CAU and co-author of the “Nature” article, adds: “Within the Collaborative Research Centre 754 we just identified seven new clusters of nitrogen-fixing microorganisms, and many species are still waiting to be found.”...

One incubation chamber on RV POLARTSTERN. In this chamber the samples are prepared for later measurements. Photo: T. Baustian, GEOMAR