Characterization of the diazotroph communities in the Subtropical North Atlantic (M60/5)

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The importance of nitrogen fixation in the marine nitrogen cycle has been underestimated until recently (Gruber and Sarmiento, 1997). 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 60/5 cruise, we collected samples for the analysis of nifH gene presence and expression. Using Taqman probes specific for various diazotrophs phylotypes present in our North Atlantic samples, we propose here to analyze the Meteor 60 samples to obtain quantitative data from nutrient addition bioassay experiments. In addition, we propose to characterize the dominant 16 S and 18 S rDNA phylotypes of the planktonic microbial community present during these incubations. The bacterioplankton phylotypes will be characterized both by TRFLP analysis of 16S rDNA while the eukaryotic component will be characterized mainly by cloning and sequencing of the 18 S rDNA. The clarity of the response from the bioassay experiments conducted during the Meteor 60/5 justifies a more complete analysis of the microbial community structure in the various treatments, using molecular techniques.