Study of the factors controlling nitrogen fixation in the Subtropical North Atlantic and in the Mediterranean: Experimental manipulation of natural microbial populations on board of Meteor 68/3 and 71/3 expeditions

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Nitrogen fixation is an important process for the global nitrogen cycle because it can balance the biologically available nitrogen lost through denitrification. However, there is ongoing debate concerning which organisms are responsible for, and what environmental factors, control nitrogen fixation. Additionally, Manipulative experiments of natural plankton communities have been conducted during the Meteor 68/2-3 and Meteor 71/3 cruises in order to assess the factors controlling phytoplankton and diazotroph growth and abundance in the Subtropical North Atlantic and the Mediterranean Sea, respectively. One of the fundamental questions in assessing the importance of nitrogen fixation is the paucity of information concerning the diversity and activity of marine diazotrophs in the world's ocean. During the Meteor 68/2-3 and 71/3 expeditions, we collected samples for nitrogen fixation rates and for the analysis ofnifH gene presence and expression. Using Taqman probes specific for the most important diazotroph phylotypes present in our samples, we propose here to obtain quantitative information for the abundance and activity of diazotrophs in the context of manipulative bioassay experiments. This work will provide basic information concerning the diel patterns of and the nutrients limiting nitrogen fixation and primary production in the Subtropical North Atlantic and the Mediterranean.