The main objective of the proposed research is the study of the impact of atmospheric inputs of nutrients (phosphorus, iron) on the biological production in oligotrophic marine systems. In particular, we will focus on the impact of atmospheric input - in particular from the Sahara- on the nitrogen fixation, which is suspected to be limited by one or both nutrients. This work will be based on an experimental approach and a field study (oceanographic campaign). Cultures of Trichodesmium will be performed at the host institution, in seawater and fertilization by Fe and or P will be done in order to simulate the atmospheric input : various parameters will be measured as the N_2 fixation and the chl a concentration. These experiments will allow to quantify and parameterize the response of Trichodesmium in term of increase of the N_2 fixation after an input of a limited nutrient. On board the R/V Meteor in the tropical Atlantic ocean (13 October to 16 November 2002), experiments of incubations of oligotrophic seawater following various scenario of limitation will be performed (see details in tablel). The tropical Atlantic Ocean, lying downwind of the Sahara dust-generation region is the major deposition region in the world ocean for terrestrial dust. These quasi in site fertilizations will allow to respond to the question : "Can sporadic Saharan dust inputs lead to an increase of the N_2 fixation, in this part of ocean". This campaign is part of the international SOLAS program (Surface Ocean Lower Atmosphere Study).