Assessing the diversity and activity of diazotrophs in the tropical and Subtropical Atlantic

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The importance of nitrogen fixation in the marine nitrogen cycle has probably been underestimated until recently. One of the fundamental questions in assessing the importance of nitrogen fixation is the paucity of information concerning the diversity of marine diazotrophs in the world's ocean. This question is being addressed here by looking at the sequence diversity of nifH genes in DNA extracted from filtered seawater samples collected during Meteor 55. Confirmation that the nifH genes from diverse clades of diazotrophs are indeed expressed in situ will be seeked via reverse transcription and polymerase chain reaction (RT-PCR) analysis of RNA extracted from parallel samples. Samples were also taken from 4 of 7 nutrient addition bioassay experiments designed to address experimentally the question of nutrient limitation of primary productivity and nitrogen fixation. The presence and expression of the nifH genes will be studied in the samples from the different bioassays, in order to assess whether or not the addition of certain nutrients favoured the growth of diazotrophs within the plankton community. The results will be analyzed in the context of the bioassay results and other measurements collected during the cruise that are relevant to nitrogen fixation and atmospheric dust deposition.